Ion separators

Abstract

An ion separation device 10 comprises a plurality of plates, each of the plates having one or more apertures 11 therethrough and at least one electrode 12 around each of the one or more apertures. Th

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2410202.2, which was filed on 12 July 2024. The entire contents of this application are incorporated herein by reference.. FIELD OF THE INVENTION The present invention relates generally to techniques for separating ions according to a physicochemical property such as ion mobility or mass to charge ratio. BACKGROUND An ion mobility separator (IMS) is a known device for separating ions according to their mobility through a gas. An example of such an IMS device is a drift tube IMS device. These devices have an ion trap that pulses a packet of ions into a drift tube that has a background gas therein. A static DC electric field is maintained along the drift tube so as to urge the ions through the gas from the upstream end, near the ion trap, to a downstream end. Ions of different mobility will have different transit times through the gas to the exit of the drift tube and hence are separated according to their mobility. Travelling wave IMS devices are also known. In these devices a DC potential is repeatedly travelled along the drift tube so as to urge the ions in the downstream direction towards the exit of the drift tube, rather than providing a static DC electric field along the drift tube for urging the ions. Ions having different mobilities are urged downstream by different amounts each time that they are passed by a travelling DC potential. As such, the travelling DC potentials cause the ions to become separated and exit the IMS device at different times based on their mobility. However, as such conventional IMS devices trap all of the ions in a relatively small volume prior to pulsing them into the drift tube to be separated, these devices suffer from space-charge effects because the ions are trapped with a relatively high concentration of charge. Such space-charge effects reduce the mobility resolution of IMS devices and may also cause ion losses. Other types of IMS devices are known that separate ions according to ion mobility within an ion trap and then release the ions from the ion trap in order of mobility. These devices trap the ions and then provide opposing forces on the ions such that they separate out along the trapping region according to their mobility. For example, a gas flow may urge the ions in a first direction and a static DC gradient may urge the ions in a second, opposite direction so as to cause the ions to separate according to mobility within the ion trapping region. The gradient of the static DC gradient may then be progressively reduced such that ions elute from the trapping region in order of ion mobility, i.e. ions of relatively low mobility elute first, followed by progressively higher mobility ions as the DC gradient is progressively reduced. However, it is still desired to provide to provide ion separation devices that have an even higher space-charge capacity. SUMMARY From a first aspect the present invention provides an ion separation device comprising: a plurality of plates, wherein each of the plates has one or more apertures therethrough and at least one electrode around each of the one or more apertures, and wherein the plates are arranged so that the one or more apertures and electrodes are aligned to form a plurality of ion channels that extend through each of the plates; and one or more voltage supplies for applying voltages to the electrodes for causing ions to be separated in the plurality of channels according to mass to charge ratio or mobility. The plates may be arranged with their major surfaces parallel to each other such that the ion channels extend along the ion separation device, through the plates, along axes that are substantially orthogonal to the major surfaces of the plates. Said one or more apertures in each plate may be a plurality of discrete apertures, and the plates may be arranged so that the apertures in the plates are aligned to form said plurality of ion channels that extend through each of the plates. Said one or more apertures in each plate may comprises an aperture having said at least one electrode around, wherein the aperture and at least one electrode are shaped so as to define the plurality of ion channels through the plates. The aperture in each plate may comprise a plurality of enlarged aperture portions that are separated from each other by smaller aperture portions, and the at least one electrode may be arranged around the aperture so as to form said plurality of ion channels which extend through the plurality of respective enlarged aperture portions in each of the plates. The aperture in each plate may comprises a plurality of substantially circular aperture portions that are separated from each other by smaller aperture portions so as to form the aperture. The shape of the aperture in each plate may correspond to the shape that would be formed by making a plurality of substantially circular apertures in the plate, where adjacent ones of these circular apertures only partially overlap with each other or touch each other at their sides. This embodiment has the advantage that the gaps between the ion channels can be reduced, thus allowing more ion channels in an ion separation device of a given volume. Each plate may have a further aperture, or further aperture portion, so that the plates define a further ion channel; and the ion separation device may be configured to transmit ions through said further ion channel substantially without separating them according to mobility or mass to charge ratio. For example, the further aperture may be a discrete aperture that only has the further ion channel passing therethrough. Alternatively, one of the apertures in each plate (and its respective one or more electrodes) may be shaped such that the further ion channel passes therethrough as well as one or more of said plurality of ion channels. Each plate may be a printed circuit board (PCB) having said one or more apertures therethrough, wherein the one or more electrodes are conductive traces on the PCBs. The conductive traces may be arranged around the one or more apertures on one or both major surfaces of each PCB, and / or the conductive traces may extend around the wall between the major surfaces of the PCB. The PCBs may further include conductive traces for supplying voltages from the one or more voltage supplies to the electrodes. The ion separation device may comprise at least one voltage supply for supplying at least one RF voltage to the electrodes on the plates so as to radially confine ions within the ion channels. The one or more voltage supplies may comprise: a first voltage supply configured to apply voltages to the electrodes on the plates such that an electric potential repeatedly travels along each ion channel for urging ions in one direction along that ion channel; and a second voltage supply configured to provide different voltages to electrodes at different respective positions along each ion channel so as to provide an electric field for urging ions in an opposite direction to said one direction, and / or a gas supply for flowing gas along each ion channel for urging ions in an opposite direction to said one direction, for causing ions to be separated according to mass to charge ratio or mobility. For example DC voltages may be transiently applied to axially successive electrodes of each ion channel at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion channel. These travelling DC potentials urge the ions in said one direction as they pass the ions. DC voltages may also applied to electrodes of each ion channel so as to provide a DC electric field that urges the ions in the opposite direction. The gas flow may be used in combination with the electric field so as to enhance the resolution of the ion separation device. Adding the gas flow in this manner increases the axial length of the ion channel over which a given range of mobilities or mass to charge ratios reside and therefore provides a higher resolution. Alternatively, substantially no gas flow may be provided along the ion channels. This is useful in arrangements in which it is not practical to maintain a gas flow through the device, e.g. because the cross-sectional area inside the IMS device is relatively large and this would require the vacuum chamber in which the ion separation device is located to be pumped at a high rate. The ion separation device may be configured to cause ions to elute from each ion channel in an order according to mobility or mass to charge ratio by: (i) controlling said first voltage supply so as to progressively vary the speed and / or amplitude of the electric potential that repeatedly travels along that ion channel; and / or (ii) controlling said second voltage supply so as to progressively vary the electric field maintained along that ion channel. The ion separation device may be configured such that all of the plurality of ion channels simultaneously separate the ions in the same manner. For example, the electric potentials may be travelled simultaneously in the same direction along all of the ion channels, against an opposing force due to the electric field and / or gas flow. Alternatively, the electric potentials may be travelled along one or more of the ion channels in a first direction against an opposing force due to an electric field and / or gas flow, whilst the electric potentials are travelled along one or more other of the ion channels in a second opposite direction against an opposing force due to an electric field and / or gas flow. The ion separation device may be configured such that all of the ion channels simultaneously separate ions by mobility. Alternatively, the ion separation device may be configured such that all of the ion channels simultaneously separate ions by mass to charge ratio. Alternatively, the ion separation device may be configured such that one or more of the ion channels separate ions by mobility, whilst one or more other of the ion channels separate ions by mass to charge ratio. Said plurality of ion channels may comprise at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine or at least ten ion channels. The device may comprise an ion funnel arranged at the downstream end of the ion channels so as to receive and funnel ions exiting the ion channels. The ion funnel may comprise: a plurality of plates arranged with their major surfaces parallel to each other, wherein each of the plates has an aperture therein that is surrounded by one or more electrode, wherein the apertures become progressively smaller in the downstream direction and are aligned with each other to form an ion channel therethrough; and at least one voltage supply for applying voltages to these electrodes for radially confining ions within the ion funnel and for driving ions downstream through the ion funnel. An RF voltage supply supplies RF voltages to the electrodes on the plates of the ion funnel so as to radially confine the ions within the ion funnel. For example, opposite phases of an RF voltage supply may be applied to axially alternate electrodes of the ion funnel in order to produce a pseudo-potential that confines the ions radially within the ion funnel. Different DC voltages may also be applied to different electrodes of the ion funnel so as to provide a potential difference across the ion funnel that urges the ions through the ion funnel in the downstream direction. Alternatively, or additionally, DC voltages could be transiently applied to axially successive electrodes of the ion funnel at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion funnel in the downstream direction in order to urge the ions downstream through the ion funnel. The plurality of plates in the ion funnel may be PCBs having said apertures therethrough, and the one or more electrodes may be conductive traces on the PCBs. The conductive traces may be arranged around the one or more apertures on one or both major surfaces of each PCB, and / or the conductive traces may extend around the wall between the major surfaces of the PCB. The PCBs may further include conductive traces for supplying voltages from the at least one voltage supply to the electrodes. The apertures in the plates of the ion funnel may have different shapes at different axial positions along the ion funnel. For example, the apertures that are arranged over an axial length of the ion funnel that extends from the upstream end of the ion funnel may be elongated, such as being rectangular or oval, and the apertures that are arranged over an axial length of the ion funnel that extends to the downstream end of the ion funnel may be circular. The shapes of the apertures may progressively change along the length of the ion funnel from the shape at the upstream end to the shape at the downstream end. The aperture in each of at least some of the plates in the ion funnel may be an elongated aperture, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodes are provided on opposing sides of the aperture that extend between said opposing ends, and wherein the at least one voltage supply is configured to apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes for urging ions towards the centre of the elongated aperture. Maintaining the first and second electrodes at a higher DC potential than the third and fourth electrodes assists in urging ions towards the central axis of the ion funnel. One or more RF voltage may be applied to all of the electrodes on a given plate so as to radially confine the ions. For example, the same RF voltage may be applied to all of the electrodes on each plate, with the electrodes on different plates being supplied with different phases of the RF voltage. DC voltages may also be applied to the electrodes on the plates so as to urge ions downstream through the ion funnel. The ion funnel may comprise plates having said elongated apertures and said first to fourth electrodes at its upstream end, and plates at its downstream end that each have a single continuous electrode surrounding an aperture. Such an arrangement may provide a combination of ion funnelling and transmission to a downstream ion-optical device that is improved over a conventional ion funnel electrode arrangement. The device may comprise an ion distribution region at the upstream end of the ion separation device for receiving a beam of ions and expanding or deflecting the beam of ions such that ions are distributed over the entrances of said plurality of ion channels. The ion distribution region may comprise: a plurality of plates, each of which has one or more aperture therein; wherein the plates are arranged with their major surfaces parallel to each other and at least one electrode is provided around the circumference of each of the apertures; and a voltage supply for applying voltages to these electrodes for radially confining ions. The plates of the ion distribution region may be PCBs having said apertures therethrough, and the one or more electrodes may be conductive traces on the PCBs. The conductive traces may be arranged around the one or more apertures on one or both major surfaces of each PCB, and / or the conductive traces may extend around the wall between the major surfaces of the PCB. The PCBs may further include conductive traces for supplying voltages from the voltage supply to the electrodes. The apertures and electrodes in the plates of the ion distribution region may be arranged and configured so as to divide the incoming ion beam into multiple ion beams. The ion distribution region may comprise: an ion dispersing portion at an upstream end thereof that includes one or more first plates, each of which has a first, elongated aperture therethrough and one or more electrodes arranged around the elongated aperture so as to form a single first ion channel through the first plates; and an ion channelling portion arranged downstream of the ion dispersing portion and that includes one or more second plates, wherein each of the one or more second plates has a second, elongated aperture therethrough and one or more electrodes arranged around the one or more apertures that are configured so as to receive ions from the first ion channel and confine them in multiple separate ion channels; wherein each of the plurality of ion channels for separating ions is arranged to receive ions from a respective one of the multiple separate ion channels of the ion distribution region. The edge of the aperture in each of the one or more second plates may have a plurality of curved regions for forming the multiple ion channels. The ion separation device comprises a voltage supply for supplying an RF voltage to the electrodes on the plates of the ion distribution region so as to radially confine the ions therein. The ion separation device may also comprises a voltage supply for supplying DC voltages to the electrodes of the ion distribution region so as to urge the ions through the ion distribution region in the downstream direction. The device may be configured to vary voltages that are applied to the electrodes in the ion distribution region so as to alternate between an ion accumulation mode in which ions are trapped in the ion distribution region and an ion ejection mode in which ions are urged from the ion distribution region into the plurality of ion channels. The device may be configured to separate ions according to mass to charge ratio or mobility in the ion distribution region during the ion accumulation mode and / or the ion ejection mode. The ion separation device may be configured to separate ions according to mass to charge ratio or mobility by: applying voltages to electrodes in the ion distribution region such that an electric potential repeatedly travels along the ion distribution region for urging ions in a first direction; and applying different voltages to electrodes at different respective positions along the ion distribution region so as to provide an electric field for urging ions in an opposite direction to said first direction, and / or flowing gas along the ion distribution region for urging ions in an opposite direction to said first direction. The first direction may either be the upstream or downstream direction. The ion separation device may be configured to separate ion in, and elute ions from, the plurality of ion channels whilst the ion distribution region is being operated in the ion accumulation mode. The magnitude of the electric field in the ion distribution region may have a maximum value that is less than or equal to the minimum value of the magnitude of the electric field in the plurality of ion channels. This enables the ions to be trapped and separated in the ion distribution region using travelling potentials that have relatively low amplitudes (and / or high velocities). This minimises the instantaneous force exerted by the travelling potentials on the ions in the radial direction. The magnitude of this force is directly related to the maximum position of the ions in the radial direction and hence the maximum charge capacity of the device. Also, as the ions are separated in the relatively low field, high space-charge capacity ion distribution region before entering the ion channels, this enables a reduction in the time that the ions are required to spend in the higher fields and voltages of the ion channels in order to become separated. This therefore minimises ion losses due to insufficient RF confinement and also minimises heating of the ions from the DC travelling potentials in the ion channels. Although embodiments have been described in which the ion separation device has plates having the plurality of ion channels therethrough, it is contemplated that the plates may instead have a single, relatively wide ion channel therethrough and electrodes configured to urge the ions towards the central axis through the device. Accordingly, from a second aspect the present invention provides an ion separation device comprising: a plurality of plates, wherein each of the plates has an elongated aperture therethrough, wherein the plates are arranged so that the apertures are aligned to form an ion channel that extends through each of the plates, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodes are provided on opposing sides of the aperture that extend between the opposing ends; and at least one voltage supply configured to: apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes; and apply voltages to the electrodes on the plurality of plates so as to separate ions according to mass to charge ratio or mobility along the ion channel. Maintaining the first and second electrodes at a higher DC potential than the third and fourth electrodes assists in urging ions towards the central axis of the device. One or more RF voltage may be applied to all of the electrodes on a given plate so as to radially confine the ions. For example, the same RF voltage may be applied to all four electrodes on each plate, with the electrodes on different plates being supplied with different phases of the RF voltage. DC voltages are also applied to the electrodes on the plurality of plates so as to perform the ion separation and elution processes in the ion channel in a corresponding manner to that described in relation to the first aspect of the invention. The second aspect of the present invention may have any of the features described in relation to the first aspect of the invention, except that the plurality of ion channels described in relation to the first aspect need not be provided. For example, each plate may have a further aperture and at least one electrode surrounding it so that the plates define a further ion channel, wherein the ion separation device is configured to transmit ions through said further ion channel substantially without separating then according to mobility or mass to charge ratio. The present invention also provides a mobility and / or mass spectrometer comprising an ion separation device as described herein. Accordingly, the present invention provides a mobility and / or mass spectrometer comprising: an ion separation device as described above; and an ion detector for detecting the separated ions, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. The spectrometer may comprise a mass analyser downstream of the ion separation device that includes said ion detector. The mass analyser detects the separated ions, or ions derived therefrom, so as to determine their mass to charge ratios. The spectrometer may also associate mobilities with the detected ions based on their time of detection by the mass analyser, e.g. relative to the start of an elution cycle from the ion separation device. The mass analyser may be a time of flight mass analyser. The present invention also provides a method of separating ions comprising using an ion separation device as described herein. Accordingly, the present invention provides a method of separating ions according to mobility or mass to charge ratio comprising: providing an ion separation device as described above; and applying voltages to electrodes in the ion separation device so as to separate ions in the ion channel(s) according to mobility or mass to charge ratio. The present invention also provides a method of mobility and / or mass spectrometry comprising: separating ions according to mobility and / or mass to charge ratio using a method as described above; and detecting the separated ions, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. The step of detecting the separated ions, or ions derived therefrom, may be performed by a mass analyser downstream of the ion separation device. The mass analyser detects the separated ions, or ions derived therefrom, so as to determine their mass to charge ratios. The spectrometer may also associate mobilities with the detected ions based on their time of detection by the mass analyser, e.g. relative to the start of an elution cycle from the ion separation device. The mass analyser may be a time of flight mass analyser. The ions may be trapped in the ion separation device as they are separated along the ion separation device according to mobility or mass to charge ratio; and the method may comprise: controlling the ion separation device such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during an elution period; and controlling a mass filter downstream of the ion separation device such that the mass filter only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities from the ion separation device. The mass filter may be a quadrupole mass filter. Ions that are transmitted by the mass filter, or ions derived therefrom, are detected by an ion detector, such as an ion detector of a mass analyser as has been described above. For example, the ion that are transmitted by the mass filter may be fragmented or reacted to produce product ions that are detected. The mass to charge ratio and / or mobility of the detected ions may be determined by the spectrometer. From a third aspect the present invention provides an ion filter for filtering ions according to mass to charge ratio or mobility, comprising: a first plurality of electrodes arranged for guiding ions along a first ion channel having a longitudinal axis therethrough; an ion entrance for introducing ions into said first ion channel in a direction orthogonal to the longitudinal axis; a first voltage supply configured to apply voltages to the first plurality of electrodes such that an electric potential repeatedly travels along the first ion channel for urging ions in a first direction along the first ion channel; and a second voltage supply configured to provide different voltages to said electrodes at different respective positions along the first ion channel so as to provide an electric field for urging ions in a second direction along the ion channel that is opposite to said first direction, and / or a first gas supply for providing a gas flow that urges ions in the second direction along the first ion channel; wherein the voltages applied by the first voltage supply, and the voltages applied by the second voltage supply and / or the gas flow, are such that ions in the first ion channel will be separated according to mass to charge ratio or mobility. As the ions are introduced into said first ion channel in a direction orthogonal to the longitudinal axis, ions may be introduced into the region of the ion filter where the ions are separated regardless of the separation forces acting along the longitudinal axis. For example, ions may be introduced through the ion entrance into the first ion channel substantially continuously, whereas known ion separation devices tend to alternate between an ion accumulation phase in which ions can enter the device and an analytical phase where ions cannot enter the device. Preferably, substantially no gas flow is provided along the first ion channel. This is useful in arrangements in which it is not practical to maintain a gas flow through the ion filter, e.g. because the cross-sectional area inside it is relatively large and this would require the vacuum chamber in which the ion filter is located to be pumped at a high rate. However, less preferred embodiments are contemplated in which the gas flow is provided in the same direction that the electric field urges the ions so as to enhance the resolution of the ion filter. The ion filter may further comprise a first ion exit for transmitting ions out of the ion filter; wherein the voltages applied by the first voltage supply, and the voltages applied by the second voltage supply and / or the gas flow, are such that either: (a) ions having a mass to charge ratio or mobility that is above a first threshold value are urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio or mobility that is below the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter; or(b) ions having a mass to charge ratio or mobility that is below a first threshold value are urged to move along the ion channel from its first end to its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio or mobility that is above the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. The ion filter is preferably part of a mass and / or mobility spectrometer that also comprises an ion-optical device or ion detector arranged to receive ions exiting said first ion exit. The ion-optical device may be an ion guide or a mass analyser. The ion filter may comprise a second ion exit arranged such that ions that are urged to move along the ion channel towards its first end pass out of the second ion exit, such that they are filtered out by the ion filter. Ions that pass out of the second ion exit and not transmitted by into the ion-optical device. Said separation of the ions may cause some of the ions to pass to a second end of the first ion channel; and the ion filter may comprise: a second plurality of electrodes arranged for guiding ions along a second ion channel having a longitudinal axis therethrough, wherein the second ion channel has an ion entrance adjacent the second end of the first ion channel for receiving ions therefrom; a third voltage supply configured to apply voltages to the second plurality of electrodes such that an electric potential repeatedly travels along the second ion channel for urging ions in a third direction along the second ion channel; and a fourth voltage supply configured to provide different voltages to said electrodes at different respective positions along the second ion channel so as to provide an electric field for urging ions in a fourth direction along the second ion channel that is opposite to said third direction, and / or a second gas supply for providing a second gas flow that urges ions along the second ion channel in said fourth direction; wherein the voltages applied by the third voltage supply, and the voltages applied by the fourth voltage supply and / or the second gas flow, are such that ions in the second ion channel will be separated according to mass to charge ratio or mobility. The first and third directions may be the same direction, and the second and fourth directions may be the same direction. Alternatively, the first and third directions may be opposite directions, and the second and fourth directions may be opposite directions. The first and second gas flows may be the same gas flow, or different gas flows (e.g. different speeds or different types or compositions of gases). The ion entrance of the second ion channel may be in a side of the second ion channel that is adjacent to the second end of the first ion channel for receiving ions that exit the side of the first ion channel. The first and second ion channels may be in a side-by-side arrangement, such that their lengths overlap. This enables the ion filter to be relatively compact. The longitudinal axes of the first and second ion channels may be parallel and offset from each other in a direction orthogonal to the axes. The ion filter may be configured to eject ions from the second end of the first ion channel in a direction orthogonal to its longitudinal axis, and receive ions into the second ion channel in a direction orthogonal to its longitudinal axis. The voltages applied by the third voltage supply, and the voltages applied by the fourth voltage supply and / or the second gas flow, may be such that either: (c) ions having a mass to charge ratio or mobility that is above a second threshold value that is different to said first threshold value are urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio or mobility that is below the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter; or (d) ions having a mass to charge ratio or mobility that is below a second threshold value that is different to said first threshold value are urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio or mobility that is above the second threshold value are urged to move along the ion second channel in the opposite direction such that they are filtered out by the ion filter. For example, ions having a mass to charge ratio that is above a first threshold value may be urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio below the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. Ions having a mass to charge ratio that is below a second threshold value that is different to said first threshold value may be urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio or mobility that is above the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter. The second threshold value may be higher than the first threshold value, such that the ion filter functions as a bandpass filter. Alternatively, ions having a mass to charge ratio that is below a first threshold value may be urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio above the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. Ions having a mass to charge ratio that is above a second threshold value that is different to said first threshold value may be urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio that is below the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter. The second threshold value may be lower than the first threshold value, such that the ion filter functions as a bandpass filter. Alternatively, ions having a mobility that is above a first threshold value may be urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mobility below the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. Ions having a mobility that is below a second threshold value that is different to said first threshold value may be urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mobility that is above the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter. The second threshold value may be higher than the first threshold value, such that the ion filter functions as a bandpass filter. Alternatively, ions having a mobility that is below a first threshold value may be urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mobility above the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. Ions having a mobility that is above a second threshold value that is different to said first threshold value may be urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mobility that is below the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter. The second threshold value may be lower than the first threshold value, such that the ion filter functions as a bandpass filter. The mass filter may comprise a third ion exit arranged such that ions that are urged to move along the second ion channel in said opposite direction pass out of the third exit, such that they are filtered out by the ion filter. These ions are not transmitted into the ion-optical device mentioned above. The ion filter may be configured to eject ions from the first end of the second ion channel in a direction orthogonal to the longitudinal axis of the second ion channel. For example, the ion exit of the ion filter may be at the first end of the second ion channel. The ion filter may comprise one or more AC and / or DC voltage supply for applying one or more RF and / or DC voltage to the electrodes forming the first and / or second ion channels so as to confine ions orthogonally relative to their respective longitudinal axes. The first voltage supply, and the second voltage supply and / or first gas supply, may be configured to be operated in: (i) an ion accumulation mode so as to cause ions having different mass to charge ratios or mobilities to be confined at different respective locations along a trapping region of the first ion channel; and then (ii) an elution mode in which ions are caused to elute from the trapping region towards the ion entrance to the second ion channel in an order according to mass to charge ratio or mobility. The elution mode may be performed by controlling said first voltage supply so as to progressively vary the speed and / or amplitude of the electric potential that repeatedly travels along the first ion channel; and / or (ii) controlling said second voltage supply so as to progressively vary the electric field maintained along that ion channel and / or to vary the gas flow from the first gas supply. The ion filter may be configured to repeatedly alternate between the ion accumulation mode and the elution mode. The third voltage supply, and the fourth voltage supply and / or second gas supply, may be configured to be operated in: (i) an ion accumulation mode so as to cause ions having different mass to charge ratios or mobilities to be confined at different respective locations along a trapping region of the second ion channel; and then (ii) an elution mode in which ions are caused to elute from the trapping region towards the ion exit in an order according to mass to charge ratio or mobility. The ion filter may be configured to repeatedly alternate between these ion accumulation and elution modes. The elution mode may be performed by controlling said third voltage supply so as to progressively vary the speed and / or amplitude of the electric potential that repeatedly travels along the second ion channel; and / or (ii) controlling said fourth voltage supply so as to progressively vary the electric field maintained along that ion channel, and / or to vary the gas flow from the second gas supply. The ion filter may comprise an ion gate between the first and second ion channels, wherein the ion filter is configured to synchronise the opening and closing of the ion gate relative to the start of the elution mode of the first ion channel such that only ions having a restricted range of mass to charge ratios or mobilities are transmitted by the ion gate from the first ion channel to the second ion channel. The first plurality of electrodes forming the first ion channel may be arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the first ion channel orthogonally to its longitudinal axis; and / or the second plurality of electrodes forming the second ion channel may be arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the second ion channel orthogonally to its longitudinal axis. Accordingly, the ion filter may have one or more voltage supply configured to apply voltages to the electrodes of the first and / or second ion channels, in the bypass mode, so as to cause ions to pass straight through these channels in a direction orthogonal to their longitudinal axes. The ions may pass in this direction to the ion exit of the ion filter. The ions may substantially not be separated by mobility and mass to charge ratio in the bypass mode. The ion filter may comprise arrays of electrodes between which the ions travel, wherein each array comprises a plurality of rows of electrodes and a plurality of orthogonal columns of electrodes, and wherein each row and each column comprises a plurality of electrodes; and wherein the first and / or second ion channels are formed by these arrays of electrodes. The arrays of electrodes may be planar arrays. Although ions have been described as being transmitted into the first ion channel in a direction that is orthogonal to its longitudinal axis, this need not be the case. Accordingly, from a fourth aspect the present invention provides an ion filter for filtering ions according to mass to charge ratio or mobility, comprising: a first plurality of electrodes arranged for guiding ions in a first direction along a first ion channel having an ion entrance at a first end for receiving ions and an ion exit in a side thereof at its second end; a second plurality of electrodes arranged for guiding ions in a second direction opposite to the first direction along a second ion channel having an ion entrance in a side thereof that is adjacent to the ion exit of the first ion channel for receiving ions from the first ion channel; one or more first voltage supply configured to apply voltages to the electrodes of each of the first and second ion channels such that an electric potential repeatedly travels along each of the first and second ion channel for urging ions in one direction along each of those ion channels; and one or more second voltage supply configured to provide different voltages to the electrodes of each of the first and second ion channels at different respective positions along each of the first and second ion channels so as to provide an electric field for urging ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel, and / or one or more gas supply for providing one or more gas flow that urges ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel; wherein the voltages applied by the one or more first voltage supply, and the voltages applied by the one or more second voltage supply and / or the one or more gas flows, are such that ions in the first and second ion channels will be separated according to mass to charge ratio or mobility. The ion filter described in relation to the fourth aspect of the present invention may have any of the features described above in relation to the third aspect of the invention, except that the ions need not be transmitted into the first ion channel in a direction that is orthogonal to its longitudinal axis. The ion filter may comprise a voltage supply configured to apply voltages to the electrodes of the first and second ion channels so as to transfer ions from the exit of the first ion channel to the entrance of the second ion channel, in a direction orthogonal to their longitudinal axes. The side-by-side arrangement of the ion channels, such that their lengths overlap, enables the ion filter according to the fourth aspect of the invention to be relatively compact. It also enables the ion filter to be easily operated in the bypass mode described above, e.g. in embodiments in which ions enter the first ion channel and exit the second ion channel orthogonally to their longitudinal axes. Accordingly, the first plurality of electrodes forming the first ion channel may be arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the first ion channel orthogonally to its longitudinal axis; and wherein the second plurality of electrodes forming the second ion channel are arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the second ion channel orthogonally to its longitudinal axis. Accordingly, the ion filter may have one or more voltage supply configured to apply voltages to the electrodes of the first and / or second ion channels, in the bypass mode, so as to cause ions to pass straight through these channels in a direction orthogonal to their longitudinal axes. The ions may pass in this direction to the ion exit of the ion filter. The ions may substantially not be separated by mobility and mass to charge ratio in the bypass mode. The present invention also provides a mobility and / or mass spectrometer comprising an ion filter as described herein. Accordingly, the present invention provides a mobility and / or mass spectrometer comprising: an ion filter as described above; and an ion detector for detecting ions transmitted by the ion filter, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. The spectrometer may comprise a mass analyser downstream of the ion filter that includes said ion detector. The mass analyser detects the ions transmitted by the ion filter, or ions derived therefrom, so as to determine their mass to charge ratios. The mass analyser may be a time of flight mass analyser. The present invention also provides a mass spectrometer comprising: an ion filter as described above; a mass filter arranged downstream of a first ion exit of the ion filter; and control circuitry configured to: control the ion filter such that ions having different mass to charge ratios or mobilities elute from the first exit of the ion filter at different respective times during an elution period; and control the mass filter such that it only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities. The mass filter may be a quadrupole mass filter. Ions that are transmitted by the mass filter, or ions derived therefrom, are detected by an ion detector, such as an ion detector in a mass analyser as has been described above. For example, the ion that are transmitted by the mass filter may be fragmented or reacted to produce product ions that are detected. The mass to charge ratio and / or mobility of the detected ions may be determined by the spectrometer. The present invention also provides a method of filtering ions comprising using an ion filter as described herein. Accordingly, the present invention provides a method of filtering ions according to mobility or mass to charge ratio comprising: providing an ion filter as described above; and applying voltages to the electrodes in the ion filter so as to separate ions in the ion channel(s) according to mobility or mass to charge ratio and thus transmit ions having a restricted range of mobility or mass to charge ratio and filter out other ions. The present invention also provides a method of mobility and / or mass spectrometry comprising: filtering ions according to mobility and / or mass to charge ratio using a method as described above; and detecting the ions transmitted by the ion filter, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. The step of detecting the ions, or ions derived therefrom, may be performed by a mass analyser downstream of the ion filter. The mass analyser detects the ions so as to determine their mass to charge ratios. The mass analyser may be a time of flight mass analyser. The present invention also provides a method of mass spectrometry comprising: providing an ion filter as described above; providing a mass filter downstream of a first ion exit of the ion filter; controlling the ion filter such that ions having different mass to charge ratios or mobilities elute from the first exit of the ion filter at different respective times during an elution period; and controlling the mass filter such that it only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities. The mass filter may be a quadrupole mass filter. Ions that are transmitted by the mass filter, or ions derived therefrom, are detected by an ion detector, such as an ion detector in a mass analyser as has been described above. For example, the ion that are transmitted by the mass filter may be fragmented or reacted to produce product ions that are detected. The mass to charge ratio and / or mobility of the detected ions may be determined by the spectrometer. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figs. 1A-1C show views of a known ion mobility separator; Figs. 2A-2D illustrate an ion separation device according to an embodiment of the present invention in which a plurality of apertures are provided in each plate so as to define a plurality of ion channels; Fig. 3 illustrates an ion separation device according to another embodiment of the present invention in which a single aperture is provided in each plate that define a plurality of ion channels; Fig. 4A shows the same embodiment as Fig. 2B except also having an ion funnel, whereas Figs. 4B and 4C show examples of plates that may be used to form the ion funnel; Figs. 5A-5D show an embodiment of an ion separation device having an ion distribution region at the upstream end, whereas Fig. 5E shows a schematic of an embodiment comprising three ion distribution regions; Fig. 6 shows another embodiment of an ion separation device, which has a single, relatively wide ion channel therethrough; Fig. 7 shows one of the plates of an embodiment that corresponds to Fig. 6, except wherein the ion channel is duplicated and there is also a bypass ion channel; Figs. 8A-8F show an embodiment of an ion filter according the present invention; Fig. 9 shows another embodiment of the ion filter; and Figs. 10A-10B show cross sectional view through an embodiment of the ion filter. DETAILED DESCRIPTION Figs. 1A-1C show views of a known ion mobility separator (IMS) device 1 having an entrance electrode 2, a series of intermediate electrodes 3 that form an ion guide and an exit electrode 4. The electrodes are apertured such that ions can pass through them and opposite phases 5,8 of an RF voltage supply are applied to axially alternate electrodes of the ion guide in order to produce a pseudo-potential that confines ions radially within the IMS device. DC voltages are transiently applied to axially successive electrodes of the ion guide at successive respective times so as to provide DC potentials 6, as shown in Fig. 1B, that repeatedly travel along the length of the ion guide in the downstream direction. Ions 7 are transferred into the IMS device and the travelling DC potentials urge the ions in the downstream direction as they pass the ions. A DC voltage supply applies voltages to electrodes of the ion guide so as to provide a DC electric field that urges the ions along the IMS device in the upstream direction. The magnitude of the DC electric field varies as a function of position along the axial length of the IMS device such that when the ions are driven against this electric field by the travelling DC potentials the ions separate out along the axis of the IMS device according to their ion mobility. Ions having different mobilities are axially confined at different respective axial equilibrium positions along the IMS device where the force on them due to the electric field is counter-balanced with the time-averaged force on them due to the travelling DC potentials. When the ions have been separated according to mobility within the IMS device, the ions are caused to elute from the exit of the IMS device by progressively decreasing the magnitude of the DC electric field. However, it has been realised that the space-charge capacity of the IMS device can sometimes be exceeded, causing ions to be lost when the IMS device is filled to a relatively high charge density lost. Additionally, when the IMS device is filled to a relatively high charge density, space-charge effects within the IMS device result in the mobility resolution of the device being distorted or compromised. The present invention provides an ion separation device which, according to a first set of embodiments, comprises a plurality of plate electrodes through which the ions travel. Figs. 2A-2D illustrate an ion separation device according to an embodiment of the present invention. The ion separation device comprises a plurality of plates 10, each of which has a plurality of apertures 11 therein. The plates are arranged with their major surfaces parallel to each other and such that the plurality of apertures in each plate are aligned the plurality of apertures in each of the other plates, so as to form a respective plurality of ion channels through the plates. An electrode 12 is provided around the circumference of each of the apertures 11 and the ion separation device comprises one or more voltage supply for applying voltages to these electrodes for radially confining ions within each of the ion channels and for separating the ions in the ion channels, as will be discussed in more detail below. The plates 10 may be printed circuit boards (PCBs) having the apertures 11 therein. The electrodes 12 may be conductive traces on the PCBs that are arranged around the apertures 11 on one or both major surfaces of the PCB. The conductive trace forming each electrode may additionally, or alternatively, extend around the wall of the aperture (i.e. around the wall that is between the major surfaces of the PCB). The PCBs also have conductive traces for supplying the voltages from the one or more voltage supplies to the electrodes. Fig. 2B shows a view of the ion separation device of Fig. 2A in the x-z plane, at a location that extends through the apertures 11. As can be seen, the apertures 11 form a plurality of elongated ion channels 13 that extend axially in the z-direction. In use, ions are supplied into the ion channels 13 through the apertures 11 in the upstream end of the ion separation device. An RF voltage supply supplies RF voltages to the electrodes 12 on the plates 10 so as to radially confine the ions within each ion channel 13. For example, opposite phases of an RF voltage supply may be applied to axially alternate electrodes in each ion channel in order to produce a pseudo-potential that confines the ions radially within each channel. DC voltages are transiently applied to axially successive electrodes 12 of each ion channel 13 at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion channel in the downstream direction. These travelling DC potentials urge the ions in the downstream direction as they pass the ions. DC voltages are also applied to electrodes 12 of each ion channel so as to provide a DC electric field that urges the ions in the upstream direction. The operation of one of the ion channels will now be described, although it will be appreciated that each of the ion channels may be operated in the same way, e.g. simultaneously. During an ion trapping phase, the magnitude of the DC electric field varies as a function of position along the axial length of the ion channel 13 such that when the ions are driven against this electric field by the travelling DC potentials the ions separate out along the axial length of the ion channel according to ion mobility (or mass to charge ratio, depending on the operating conditions of the ion separation device as will be described further below). Ions having different values of mobility are axially confined at different respective axial equilibrium positions along the ion channel where the force on them due to the electric field is counter-balanced with the time-averaged force on them due to the travelling DC potentials. Fig. 2C illustrates an example of the electric field profile along one of the ion channels during the ion trapping phase. As can be seen, the ion channel has a trapping region 14 within which the magnitude of the electric field increases as a function of position along the ion channel in the downstream direction. Optionally, at the end of the trapping region there is a further region 15 along which the magnitude of the electric field is substantially constant, which will be referred to herein as the analytical region since the separation of the ions increases in this region as they elute from the channel. Fig. 2C also illustrates the DC travelling potentials 16 that move along the ion channel in the downstream direction, through the trapping region and optionally also through the analytical region. Fig. 2C also shows that ions 17 of different mobility are trapped at different equilibrium positions along the ion channel, the different mobilities being illustrated by the different sized circles. The ratio of maximum to minimum DC field amplitude in the trapping region dictates the ion mobility range trapped in that region. This ratio can be adjusted to accommodate the specific analysis or application being performed such that ions of interest are axially spread over as much of the trapping range as possible, thus maximizing space-charge capacity of the device. Once the ions have been separated according to mobility within the ion separation device, the ions may be caused to elute from the exit of the device by progressively decreasing the magnitude of the DC electric field, as shown in Fig. 2D. Fig. 2D shows the same example as in Fig. 2C, except wherein the gradient of the electric field along the trapping region 14 has been reduced and the magnitude of the electric field along the analytical region 15 has been reduced. The DC travelling potentials 16 are then able to urge the ions further downstream along the ion channel and such that first ions 18 of relatively high ion mobility are urged out of the trapping region and into the analytical region, whereas ions of lower mobility are retained in the trapping region by the DC electric field. The first ions 18 are able to pass through the analytical region and out of the downstream end of the ion channel. At a later time the magnitude of the electric field at each point along the ion channel is reduced further such that second ions having a higher mobility than the first ions are urged out of the trapping region and into the analytical region, whereas ions of lower mobility are retained in the trapping region by the DC electric field. The second ions are able to pass through the analytical region and out of the downstream end of the ion channel. The magnitude of the electric field at each point along the ion channel may be progressively reduced, e.g. by being stepped or continuously scanned, such that ions having progressively lower mobility elute from the downstream end of the ion channel. As an alternative to progressively varying the magnitude of the electric field in order to cause ions to elute from the ion channel, it is preferred that at least one property of the travelling DC potentials (such as amplitude and / or speed) may be progressively varied instead so as to cause ions to elute from the ion channel. For example, ions may be caused to elute from the downstream end of the ion channel by progressively varying a property of the DC potential that is being repeatedly travelled along the ion channel, e.g. such that the DC potentials push ions to elute from the channel in order of decreasing mobility as time progresses. For example, the amplitude of the travelling DC potential may be progressively increased with time. Alternatively, or additionally, the speed of the travelling DC potential in the downstream direction may be progressively decreased with time. Although embodiments have been described in which the DC electric field urges ions in the upstream direction and the DC travelling potentials urge the ions in the downstream direction, it is alternatively contemplated that the DC electric field may urge the ions in the downstream direction and the DC travelling potentials may urge the ions in the upstream direction. In such an embodiment the magnitude of the electric field within the trapping region may decrease as a function of position in the downstream direction (at any given time) to the analytical region, which may have a constant electric field within it. Ions may be caused to elute from the downstream end of the ion channel by progressively increasing the gradient of the electric field along the trapping region with time. Alternatively, ions may be caused to elute from the downstream end of the ion channel by progressively varying a property of the DC potential that is being repeatedly travelled in the upstream direction along the ion channel. For example, the amplitude of the DC potential may be progressively decreased with time. Alternatively, or additionally, the speed of the DC potential in the first direction may be progressively increased with time. It is contemplated that the ion separation device may separate the ions, and cause them to elute, with substantially no gas flow in the upstream or downstream directions. This is useful in arrangements in which it is not practical to maintain a high gas flow rate through the device, e.g. because the total cross-sectional area inside the IMS device is relatively large and this would require the vacuum chamber in which the ion separation device is located to be pumped at a high rate. Alternatively, a gas flow may also be provided through the ion channel in the upstream or downstream direction in order to urge the ions. This gas flow may be used to enhance the mobility resolution of the ion separation device. For example, the gas flow may be provided such that the gas flows through the ion channel in the same direction that the ions are urged by the static DC electric field. Adding the gas flow in this manner increases the axial length over which a given range of mobilities reside and therefore provides a higher mobility resolution. It is alternatively contemplated that the provision of the DC electric field for urging ions along the ion channel in a certain direction may be replaced with a gas flow for urging the ions along the ion channel in that direction. Although five ion channels are illustrated in the embodiment, it will be appreciated that fewer or a greater number of ion channels 13 may be present in the stack of plates 10. For example, preferably at least ten such channels may be provided in the plates so as to increase the charge capacity by a factor of at least ten. Although the electrodes 11 in the illustrated embodiment are continuous (endless) loop electrodes, it is contemplated that other electrodes arrangements may be used. For example, multiple different electrodes may be arranged around the circumference of each aperture 11 so that the different ones of these electrodes may be maintained at different voltages. For instance, the electrodes may be arranged circumferentially around each aperture so that the electrodes along any given ion channel 13 form a multipole ion guide, such as a quadrupole ion guide. Additionally, or alternatively, although circular apertures 11 are shown, the apertures may have other shapes. It is contemplated that different ion channels 13 may have different cross-sectional areas and / or shapes. It is also contemplated that the cross-sectional area and / or shape of any given ion channel 13 may vary along its length. Embodiments have been described in which the plurality of apertures in each plate are discrete, although it is contemplated that the apertures may alternatively be interconnected, e.g. as shown in Fig. 3 Fig. 3 shows an ion separation device according to another embodiment of the first set of embodiments. The ion separation device may have the same construction and methods of operation as the embodiments described in relation to Figs. 2A-2D, except that rather than each plate 10 comprising a plurality of discrete apertures 11 and corresponding electrodes 12 for defining a plurality of respective ion channels 13, each plate instead comprises a single continuous aperture 20 and at least one electrode 12 that are shaped so that the stack of plates 10 defines the plurality of ion channels 13. In the depicted embodiment the single aperture 20 in each plate comprises a plurality of substantially circular aperture portions 22 that are interconnected with each other so as to form the single aperture. The shape of the single aperture corresponds to the shape that would be formed by making a plurality of circular apertures in the plate, where adjacent ones of these circular apertures only partially overlap with each other or touch each other at their sides. The circular aperture portions are arranged in a row across each plate, such that each circular aperture portion is interconnected with at least one of the other circular aperture portions. More specifically, each circular aperture portion at an end of the row is interconnected with the adjacent circular aperture portion in the row, whereas the circular aperture portions between the circular aperture portions that are at the ends of the row are interconnected with two adjacent circular aperture portions in the row. A single continuous electrode 12 may be provided around the circumference of the single aperture 20 in each plate 10, such that the one or more voltage supply may apply one or more voltages to these electrodes so as to radially confine ions within each circular aperture portion 22, thereby defining the plurality of ion channels 13 through the plates 10. The one or more voltage supply may also apply one or more voltages to these electrodes for separating the ions in the ion channels, in a corresponding manner to the techniques described above, i.e. by travelling DC potentials along the ion channels in one direction and maintaining an electric field along the ion channels in the opposing direction. This embodiment has the advantage that the gaps between the ion channels 13 can be reduced, thus allowing more ion channels in an ion separation device of a given volume. As mentioned above, each plate 10 may be a PCB having the aperture 20 therein. The electrode 12 may be a conductive trace on the PCB that is arranged around the aperture on one major surfaces of the PCB, or such a conductive trace may be arranged around the aperture on both major surfaces of the PCB. The conductive trace forming each electrode may additionally, or alternatively, extend around the wall of the aperture (i.e. around the wall that is between the major surfaces of the PCB). The PCBs also have conductive traces for supplying the voltages from the one or more voltage supplies to the electrodes around the apertures in the plates. In use, ions are supplied into the different circular aperture portions 22 in the plate 10 at the upstream end of the ion separation device. The ions are radially confined within the circular aperture portion that they enter and are unable to pass into any of the other circular aperture portions in the same plate due to the RF voltage(s) that is applied to the electrode(s) 12 surrounding the circular aperture portions. As such, even though each plate includes a single aperture 20, the stack of plates 10 still defines a plurality of ion channels 13 therethrough. Less preferably, ions to be moved radially between ion channels, e.g. by applying a potential difference between the electrodes of adjacent ion channels. Although the electrode around each single aperture 20 in each plate 10 is illustrated as a single continuous electrode that extends around the aperture, it is contemplated that other electrode arrangements may be used. For example, a plurality of different electrodes may be spaced around the circumference of the aperture in each plate, e.g. so that the different electrodes may be maintained at different voltages. For instance, such electrodes may be arranged so that the electrodes along any given ion channel form a multipole ion guide, such as a quadrupole ion guide. Additionally, or alternatively, although circular apertures portions are shown, the aperture portions may have other shapes. Although the aperture portions have been described as being arranged in a linear row in each plate, it is contemplated that the aperture portions may be arranged in other configurations. Additionally, or alternatively, although each plate has been described as having only a single aperture formed from interconnected aperture portions, each plate may have multiple such apertures. Although the aperture portions have been described as being substantially circular, the aperture portions may be other shapes. Accordingly, more generally, the aperture 20 may have enlarged portions that define the ion channels 13 and which are separated from each other by narrower aperture portions. In all of the embodiments described above, ions may be separated in each of the ion channels 13 and then exit the downstream ends of the ion channels. It may be desired to transmit ions that exit multiple ion channels, or all ion channels, to the same downstream ion-optical device. As such, it may be necessary to direct the ion beams leaving such ion channels onto the same axis such that the ions from the different channels can then be received by the downstream ion-optical device. This may be achieved in numerous ways, such as by deflecting the ion beams onto the axis, although a preferred method is to provide an ion funnel at the downstream end of the ion channels, as shown in Fig. 4A. Fig. 4A shows the same embodiment as Fig. 2B, except that an ion funnel 24 is provided at the downstream end of the ion channels 13. Ions that exit the downstream end of the ion channels enter the relatively wide entrance end of the ion funnel 24 and are guided, radially inwards, towards the axis that extends through the relatively narrow exit end of the ion funnel as the ions pass downstream through the ion funnel. Ions from all of the ion channels then exit the ion funnel along the axis and pass into a downstream ion-optical device (not shown). It will be appreciated that the embodiment shown in Fig. 3 may also have such an ion funnel arranged at its downstream end. The ion funnel comprises a plurality of plates 26, each of which has an aperture therein. The plates are arranged with their major surfaces parallel to each other and such that the apertures in the plates are aligned with each other to form an ion channel therethrough. The apertures in the plates become progressively smaller as a function of position in the downstream direction. At least one electrode is provided around the circumference of each of the apertures and the ion funnel comprises one or more voltage supply for applying voltages to these electrodes for radially confining ions within the ion funnel and for driving ions downstream through the ion funnel. The plates may be PCBs having the apertures therein. The electrodes may be conductive traces on the PCBs that are arranged around the apertures on one or both major surfaces of the PCB. The conductive trace forming each electrode may additionally, or alternatively, extend around the wall of the aperture (i.e. around the wall that is between the major surfaces of the PCB). The PCBs also have conductive traces for supplying the voltages from the one or more voltage supplies to the electrodes. Fig. 4B shows an example of three plates 26 of the ion funnel 24. The uppermost plate 26a in Fig. 4B has a relatively large aperture therein and is arranged relatively upstream in the ion funnel 24. The central plate 26b in Fig. 4B has an aperture therein with a smaller cross-sectional area and is arranged in the ion funnel downstream of plate 26a. The lowermost plate 26c in Fig. 4B has an aperture therein with an even smaller cross-sectional area and is arranged in the ion funnel downstream of plate 26b. An electrode 28 extends around the circumference of each aperture. In use, ions are supplied by the ion channels 13 into the upstream end of the ion funnel 24. An RF voltage supply supplies RF voltages to the electrodes 28 on the plates 26 of the ion funnel so as to radially confine the ions within the ion funnel. For example, opposite phases of an RF voltage supply may be applied to axially alternate electrodes of the ion funnel in order to produce a pseudo-potential that confines the ions radially within the ion funnel. Different DC voltages may also be applied to different electrodes of the ion funnel so as to provide a potential difference across the ion funnel that urges the ions through the ion funnel in the downstream direction. Alternatively, or additionally, DC voltages could be transiently applied to axially successive electrodes of the ion funnel at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion funnel in the downstream direction in order to urge the ions downstream through the ion funnel. The ion funnel therefore causes the ions from the multiple ion channels to be guided radially inwards towards and along the axis through the exit of the ion funnel. Although the apertures in Fig. 4B are illustrated as being rectangular, it is contemplated that the apertures may have other shapes, such as being oval or circular. It is also contemplated that apertures of different shapes may be provided at different axial positions along the ion guide. For example, the apertures that are arranged over an axial length of the ion funnel that extends from the upstream end of the ion funnel may be rectangular or oval, whereas the apertures that are arranged over an axial length of the ion funnel that extends to the downstream end of the ion funnel may be circular. The shapes of the apertures may progressively change along the length of the ion funnel from the shape at the upstream end to the shape at the downstream end. Although each plate 26 is illustrated as having a single electrode 28 that fully encircles the aperture, other electrode arrangements are contemplated, such as is shown in Fig. 4C. Fig. 4C shows an embodiment that is the same as that shown and described in relation to Fig. 4B, except that each plate 26 has a different arrangement of electrodes on it. Rather than having a single electrode that extends around the aperture on each plate, four electrodes are provided around the aperture on each plate. More specifically, first and second electrodes 30,32 are provided at opposing ends of the elongated aperture, and third and fourth electrodes 34,36 are provided on the opposing sides of the aperture that extend between the ends. One or more RF voltage is applied to all of the electrodes 30-36 on a given plate 26 so as to radially confine the ions. For example, the same RF voltage may be applied to all four electrodes on each plate, with the electrodes on different plates being supplied with different phases of the RF voltage. DC voltages are also applied to the electrodes on the plates so as to urge ions downstream through the ion funnel, in the same manner as described in relation to Fig. 4B. On any given plate, the first and second electrodes 30,32 may be maintained at a different DC voltage to the third and fourth electrodes 34,36, so as to assist in urging ions towards the central axis of the ion funnel. As such, the first and second electrodes 30,32have a higher DC potential than the third and fourth electrodes 34,36. The ion funnel could have plates 26 that have the electrode arrangement described in relation to Fig. 4B and also plates 26 that have the electrode arrangement described in relation to Fig. 4C. For example, an axial length of the ion funnel that extends from the upstream end of the ion funnel may have plates having the electrode arrangement shown and described in relation to Fig. 4C, whereas an axial length of the ion funnel that extends to the downstream end of the ion funnel may have plates having the electrode arrangement shown and described in relation to Fig. 4B. Such an arrangement may provide a combination of ion funnelling and transmission to the downstream ion-optical device that is improved over an ion funnel that uses only one of the electrode arrangements. In order to ensure that the ion separation device has a relatively large space-charge capacity, it may be desired to distribute the ion beam substantially evenly between at least some of the ion channels 13. The ion beam may arrive at the ion separation device with a cross-sectional area that is smaller than the area over which the entrances to the ion channels are located. Also, the ion beam may arrive at the ion separation device with a cross-sectional shape that is different to the shape of the area over which the entrances to the ion channels are located. For example, the ion beam may arrive at the ion separation device from an ion guide having a substantially circular cross-sectional shape, whereas the ion channels may be distributed over an area that is larger and substantially rectangular. The ion separation device may therefore comprise an ion distribution region at its upstream end for receiving the ion beam arriving at the ion separation device and distributing it into the ion channels, e.g. as shown in Fig. 5A. Fig. 5A shows an embodiment of an ion separation device having an ion distribution region 40 for receiving an ion beam 42, e.g. from an ion guide 44, and distributing these ions to the multiple ion channels 13. This embodiment is the same as that described in relation to Figs. 4A-4C, except that it additionally includes the ion distribution region 40 for distributing the ions between the ion channels 13. In use an ion beam 42 passes from the ion guide 44 into the ion separation device having a relatively small size in the x-dimension. The ion beam enters the ion distribution region 40 and is allowed or caused to expand in the x-dimension such that the ions are located adjacent to the entrances of all of the ion channels 13. The ion distribution region 40 may also be configured to divide the incoming ion beam into multiple ion beams and supply these ion beams to multiple respective ion channels. For example, the ion distribution region may comprise a plurality of plates, each of which has one or more aperture therein. The plates may be arranged with their major surfaces parallel to each other. At least one electrode is provided around the circumference of each of the apertures and one or more voltage supply applies voltages to these electrodes for radially confining ions and optionally also for driving ions downstream through the ion distribution region. The apertures and electrodes are arranged and configured so as to divide the incoming ion beam into multiple ion beams. The plates may be PCBs having the apertures therein. The electrodes may be conductive traces on the PCBs that are arranged around the apertures on one or both major surfaces of the PCB. The conductive trace forming each electrode may additionally, or alternatively, extend around the wall of the aperture (i.e. around the wall that is between the major surfaces of the PCB). The PCBs also have conductive traces for supplying the voltages from the one or more voltage supplies to the electrodes. Fig. 5C shows an example of three plates 46a-46c of the ion distribution region 40 and a fourth plate 46d that represents the upstream end of the ion channels (i.e. the upstream end of the device shown in Fig. 2A). The first, uppermost plate 46a in Fig. 5C has a rectangular aperture 47 and one or more electrode 48 arranged around it, and is arranged towards the upstream end of the ion distribution region 40. Ions are able to expand in the x-dimension within the aperture. A plurality of such apertured plates may be provided adjacent to each other at the upstream end of the ion distribution region so as to allow the ions received in the ion separation region to disperse in the x-dimension. For example, these plates 46a may be arranged over an ion dispersing portion 41 of the ion distribution region 40, as shown in Fig. 5A. The second plate 46b in Fig. 5C is a plate at a region of the ion distribution region that is downstream of the first plate(s) 46a, i.e. downstream of the ion dispersing portion 41. The aperture 49 in this second plate 46b and the one or more electrodes 50 around it are shaped differently to those on the first plate(s) 46a so as to confine the ions in multiple separate regions of the aperture 49. For example, the edge of the aperture 49 and the one or more electrodes 50 may have a plurality of curved regions for forming the plurality of separate ion confinement regions. The third plate 46c in Fig. 5C is a plate at a region of the ion distribution region that is downstream of the second plate 46b. The aperture 51 in this third plate 46c and the one or more electrodes 52 around it are shaped differently to those on the second plate 46b so as to confine the ions more tightly in multiple separate regions of the aperture 51. For example, the aperture 51 may have substantially circular aperture portions that are interconnected. The second and third plates 46b,46c are aligned such that the ions in any given one of the separate ion confinement regions in the second plate 46b pass downstream into a corresponding separate ion confinement regions in the third plate 46c. The second and third 46b,46c therefore form an ion channelling portion 43 of the ion distribution region 40, as illustrated in Fig. 5A. As mentioned above, the fourth plate 46d in Fig. 5C represents the upstream end of the ion channels 13 (i.e. the upstream end of the device shown in Fig. 2A). Accordingly, this plate 46d has multiple separate apertures 53 that each have one or more electrode 54 around it so as to confine the ions in the separate apertures. For example, the apertures may be substantially circular. The third and fourth plates 46c and 46d are aligned such that the ions in any given one of the separate ion confinement regions in the third plate 46c pass downstream into a corresponding separate aperture 53 in the fourth plate 46d. It will be appreciated that as the ions have already been separated into multiple regions by the second plate 46b, the third plate 46c may be omitted, although it is preferred that it is included so as to provide a more gradual transition of the ion beam from the first plate to the multiple ion beams. In use, ions are supplied into the upstream end of the ion distribution region 40 and enter the ion dispersing portion 41. An RF voltage supply supplies RF voltages to the electrodes on the plates of the ion distribution region so as to radially confine the ions therein. For example, opposite phases of an RF voltage supply may be applied to axially alternate electrodes of the ion distribution region in order to produce a pseudo-potential that confines the ions radially within the ion distribution region. Different DC voltages may also be applied to the electrodes on different plates of the ion distribution region so as to provide a potential difference across the ion distribution region that urges the ions through the ion distribution region in the downstream direction. Alternatively, or additionally, DC voltages could be transiently applied to axially successive electrodes of the ion distribution region at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion distribution region in the downstream direction in order to urge the ions downstream through the ion distribution region. The ions may be trapped in the ion distribution region and not allowed to enter the ion channels for a period during which the ions that have entered the ion distribution region are being distributed to the entrances of the ion channels. For example, the ions may be trapped in this manner within the ion dispersing portion 41.Ions may also be separated axially according to mass to charge ratio or mobility, e.g. whilst they are trapped in the ion dispersing portion 41. For example, DC voltages may be transiently applied to axially successive electrodes of the ion dispersing portion 41 at successive respective times so as to provide DC potentials that repeatedly travel along the length of the ion dispersing portion 41 in the downstream direction. These travelling DC potentials urge the ions in the downstream direction as they pass the ions. DC voltages are also applied to electrodes of the ion dispersing portion 41 so as to provide a DC electric field that urges the ions in the upstream direction. The magnitude of the DC electric field varies as a function of position along the axial length of the ion dispersing portion 41 such that when the ions are driven against this electric field by the travelling DC potentials the ions separate out along the axial length of the ion dispersing portion 41 according to ion mobility or mass to charge ratio, depending on the operating conditions of the ion separation device as will be described further below. Ions having different values of mobility or mass to charge ratio are axially confined at different respective axial equilibrium positions along the ion dispersing portion 41where the force on them due to the electric field is counter-balanced with the time-averaged force on them due to the travelling DC potentials. The ions are allowed to expand in the x-dimension whilst they are trapped within the ion dispersing portion 41, due to diffusion and / or space charge repulsion. Fig. 5B illustrates an example of the electric field profile along the ion separation device during the period in which the ion channels 13 are being operated in the ion trapping phase. As can be seen, the magnitude of the electric field increases as a function of position along the ion dispersing portion 41 of the ion distribution region 40 in the downstream direction. The electric field profile along the trapping region 14 and the analytical region 15 of one of the ion channels 13 is also shown. The electric fields in these regions of the ion channel are operated as has been described above. Fig. 5B also shows the electric field profile along the ion funnel 24. The electric field may be constant along the ion funnel. The electric field in the ion funnel may have the opposite polarity to that in the ion channel. As can be seen from Fig. 5B, the maximum magnitude of the electric field along the axis of the ion dispersing portion 41 of the ion distribution region 40 is lower than or the same as the lowest magnitude of the electric field along the trapping region 14 of the ion channel. For example, the electric field in the ion distribution region may have a maximum value that is less than 0.25 V / mm. This enables the ions to be trapped and separated in the ion dispersing portion 41 using travelling DC potentials that have relatively low amplitudes (and / or high velocities). This minimises the instantaneous force exerted by the travelling potentials on the ions in the radial direction (e.g. y-direction). The magnitude of this force is directly related to the maximum position of the ions in the radial direction (e.g. y-direction) and hence the maximum charge capacity of the device. Once the ions have been distributed across the entrances of the ion channels 13 in the ion dispersing portion 41 of the ion distribution region 40, the ions are urged axially out of the ion dispersing portion 41, through the ion channelling portion 43 and into the ion channels 13. This may be achieved by changing the amplitude or speed of the travelling potentials that travel along the ion dispersing portion 41. For example, the amplitude of the travelling potentials may be increased. The amplitude or speed of the travelling potentials may be changed such that only some of the ions from the ion dispersing portion 41 that have a restricted range of mobilities or mass to charge ratios are passed to the ion channels 13. Alternatively, the amplitude or speed of the travelling potentials may be changed such that all ions from the ion dispersing portion 41 are passed to the ion channels 13. The ions may be urged through the ion channelling portion 43 to the channels 13 using an electric field and / or travelling potentials. For example, the magnitude of the electric field may be constant through the ion channelling portion 43, as is shown in Fig. 5B. Once the ions enter the ion channels 13, they are then separated within the trapping region 14 of the ion channels, as shown in Fig. 5D, and subsequently caused to elute from the channels in the manners described above. Once the ions have been urged out of the ion distribution region 40 and into the ion channels 13, the amplitude and / or speed of the travelling potentials in the ion distribution region 40 may be changed again so accumulate a second population of ions 45 in the ion dispersing portion 41 of the ion distribution region 40, as shown in Fig. 5D. For example, the amplitude of the travelling potentials may be decreased. Once the ions that were urged into the ion channels have eluted, this second population of ions may then be urged into the ion channels, and those ions may be caused to separate and elute in the manner described above. This process of accumulating ions in the ion distribution region whilst ions are being separated and eluting from the ion channels may be repeated, enabling the ion separation device to operate with an overall duty cycle close to 100%. As the ions are separated in the relatively low field, high space-charge capacity ion dispersing portion 41before entering the ion channels 13, this enables a reduction of the time that the ions are required to spend in the higher fields and voltages of the ion channels in order to become separated. This therefore minimises ion losses due to insufficient RF confinement or RF heating and also minimises heating of the ions from the DC travelling potentials in the ion channels. Although ions have been described as being separated in the ion dispersing portion 41 of the ion distribution region 40 by travelling potentials downstream and arranging an electric field in the upstream direction, it is contemplated that the travelling potentials may instead be caused to travel in the upstream and the electric field may be provided in the downstream direction. In both of these embodiments, the electric field may be replaced with a gas flow. However, it is preferred to use the combination of travelling potentials and an opposing electric field to cause the separation of the ions described herein, as it avoids the relatively large gas pumping requirement that would be needed to separate ions in multiple ion channels. Also, this technique enables the ion separation device to switch from separating ions predominantly by mass to charge ratio to separating ions predominantly by mobility, or vice versa, which cannot be achieved using a separation technique that employs as gas flow and static electric field. It is also contemplated that ions may be separated according to mobility or mass to charge ratio in the x-dimension within the ion distribution region, and then ions having different ranges of mobility or mass to charge ratio may be caused to enter different respective ion channels. For example, ions may be separated according to mass to charge ratio in the x-dimension within the ion distribution region and then ions may be separated in the ion channels according to mobility (or mass to charge ratio as described later herein). Alternatively, ions may be separated according to mobility in the x-dimension within the ion distribution region and then ions may be separated in the ion channels according to mobility (or mass to charge ratio). In these embodiments ions may be caused to separate in the x-dimension within the ion distribution region in a corresponding manner to how the ions are separated in the ion channels, e.g. by providing an array of electrodes in the ion distribution region, applying voltages to these electrodes so as to travel DC potentials in one direction in the x-dimension, and applying voltages to these electrodes so as to maintain an electric field in the opposing direction in the x-dimension. In the embodiment shown in Figs. 5A-5D, the plates 10 provide a single array of ion channels 13 that are arranged in a row, although the ion channels in the array may be arranged in other configurations. However, multiple such arrays (e.g. rows) of ion channels may be provided in parallel in order to increase the charge capacity of the device, either by providing additional apertures 11 in the plates 10, or by providing additional plates 10 to provide the additional array(s) of ion channels 13. The ion distribution region 40 may be configured to distribute ions to the entrances of all of the ion channels 13. Alternatively, each array of ion channels may have its own ion distribution region 40, e.g. as shown in Fig. 5E. Fig. 5E shows a schematic of an embodiment comprising three ion distribution regions 40a-c, three stacks of plates 10a-c for forming three respective arrays of ion channels 13, and a single ion funnel 24. The ion beam 42 may be switched to being diverted to different ion distribution regions 40a-c at different times, or it may alternatively be split or diverged such that ions enter all of the ion distribution regions simultaneously. Ions enter each of the ion distribution regions and are processed as described above. Ions that are ejected from any given ion distribution region pass into a respective array of ion channels 13, where they are separated according to mass to charge ratio or mobility, as has been described above. Ions that elute from the ion channels 13 then pass into the ion funnel 24 and are funnelled towards an exit. Although embodiments have been described above in which the ion separation device has a plurality of ion channels in order to provide a high space-charge capacity separation device, it is alternatively contemplated that a single relatively wide ion channel may be provided instead of multiple ion channels, e.g. as described below in relation to Fig. 6. Fig. 6 shows an embodiment of an ion separation device that is the same as those described above, such as in relation to Fig. 2A, except that the plates 10 having the plurality of ion channels 13 therethrough are replaced by plates 10 having a single, relatively wide ion channel therethrough. Each of the plates 10 may take the form described in relation to Fig. 4C. That is, four electrodes are provided around the aperture 60 on each plate. First and second electrodes 61,62 are provided at opposing ends of the elongated aperture 60, and third and fourth electrodes 63,64 are provided on the opposing sides of the aperture 60 that extend between the ends. One or more RF voltage is applied to all of the electrodes 61-64 on a given plate so as to radially confine the ions. For example, the same RF voltage may be applied to all four electrodes on each plate, with the electrodes on different plates being supplied with different phases of the RF voltage. DC voltages are also applied to the electrodes on the plates so as to perform the ion separation and elution processes in the ion channel in a corresponding manner to the embodiments described above. On any given plate, the first and second electrodes 61,62 may be maintained at a different DC voltage to the third and fourth electrodes 63,64, so as to assist in urging ions towards the central axis of the ion channel. As such, the first and second electrodes 61,62 have a higher DC potential than the third and fourth electrodes 62,63 on any given plate 10. The ion separation device of this embodiment may have an ion funnel 24 downstream of the ion channel for funnelling the ions down. The ion funnel may have any of the configurations described above, such as in relation to Figs. 4B and 4C. The ion separation device may also have an ion distribution region at its upstream end that traps ions from an ion beam received at the ion separation device and allows or causes the ions to diverge in the radial direction so as to extend over the entrance of the relatively wide ion channel. The ion separation device may comprise a series of plates corresponding to the plates shown in Fig. 6, or it may have a configuration corresponding to one of the ion funnels 24 described herein, except where the funnel allows the ion beam to expand as it travels downstream rather than causing it to converge. Accordingly, such an ion funnel is arranged with its narrow end upstream and its wider end downstream. The ion separation device may be operated in the same manner as in the other embodiments described above in order to separate ions and transmit them into the ion channel. The embodiments described above provide a first plurality of ion channels 13, or a first single relatively wide ion channel, in order for the ion separation device to have a relatively high space-charge capacity. Embodiments are contemplated in which one or more additional aperture is provided in each of the plates such that the ion separation device has a second plurality of ion channels, or a second relatively wide ion channel. In such embodiments the first plurality of ion channels or first relatively wide ion channel may be operated so as to elute ions according to ion mobility or mass to charge ratio, whilst the second plurality of ion channels or second relatively wide ion channel is being filled with ions. After the first plurality of ion channels or first relatively wide ion channel has eluted all of the ions, the ion separation device may be switched so as to operate in a mode in which the first plurality of ion channels or first relatively wide ion channel is filled with ions, whilst the second plurality of ion channels or second relatively wide ion channel is being operated so as to elute ions according to ion mobility or mass to charge ratio. Then, after the second plurality of ion channels or second relatively wide ion channel has eluted all of the ions, the ion separation device may be switched again so as to operate in a mode in which the second plurality of ion channels or second relatively wide ion channel is filled with ions, whilst the first plurality of ion channels or first relatively wide ion channel is being operated so as to elute ions according to ion mobility or mass to charge ratio. The ion separation device may be repeatedly switched between the modes whilst it substantially continuously receives ion, thus allowing it to operate with a high duty cycle. The first plurality of ion channels or first relatively wide ion channel may be controlled so as to separate and elute ions according to mobility. The second plurality of ion channels or second relatively wide ion channel may also be controlled so as to separate and elute ions according to mobility, or alternatively to separate and elute ions according to mass to charge ratio. In order to perform these techniques, the ion channels in each plate of the embodiments described above may be duplicated, e.g. as shown in Fig. 7. Fig. 7 shows one of the plates 10 of an embodiment that corresponds to Fig. 6, except wherein the relatively wide aperture 60 and electrodes 61-64 on each plate have been duplicated so as to form a second ion channel. The two ion channels may be used to implement the high duty cycle mode switching technique described above. As shown in Fig. 7, each plate 10 may also include a bypass aperture 66 and electrode 68 such that the plates 10 form a bypass ion channel through which ions may be directed without being separated according to mass to charge ratio or mobility. One or more RF voltage is applied to the bypass electrodes so as to radially confine the ions in the bypass ion channel. DC voltages may also be applied to these electrodes 68 so as to urge ions through the bypass ion channel, but these are operated such that the ions are substantially not separated according to mass to charge ratio or mobility as they pass through that ion channel. Although Fig. 7 shows an embodiment corresponding to that in Fig. 6 except wherein the ion channel is duplicated, it will be appreciated that each plate 10 may instead correspond to a plate in one of the other embodiments, such as Figs. 2A or 3, but wherein the ion channels 13 are duplicated. Alternatively, rather than duplicating the ion channel(s) so as to provide two sets of ion channels having the same form, the two sets of ion channel may be of different forms. Various changes may be made to the first set of embodiments above without departing from the scope of the invention as set forth in the accompanying claims. For example, although the plates of the ion separation device have been described as being PCBs, the plates may not be PCBs and may instead each comprise another form of apertured substrate having the electrodes thereon. For example, the plates themselves could be sheet electrodes having apertures therethrough for forming the ion channel(s). The plurality of ion channels, or relatively wide ion channel, in the ion separation devices described above have been described as separating and eluting ions according to mobility. However, it is alternatively contemplated that ions may be separated in and eluted from the plurality of ion channels, or relatively wide ion channel, according to mass to charge ratio. The operating parameters of the ion separation device may be switched between a first mode in which ions are separated in and eluted from the plurality of ion channels, or relatively wide ion channel, according to mobility and a second mode in which ions are separated in and eluted from the plurality of ion channels, or relatively wide ion channel, according to mass to charge ratio. The ion separation device may be switched between the modes during a single experimental run, e.g. whilst ions are being substantially continually supplied into the mass or mobility spectrometer having the ion separation device. Alternatively, ions may be separated in and eluted from at least one or at least some of the plurality of ion channels according to mobility, and ions may be separated in and eluted from at least one or at least some of the plurality of ion channels according to mass to charge ratio. Although the ion distribution region of the ion separation devices described above has been described as separating and eluting ions according to mobility, it is contemplated that ions may alternatively be separated in and eluted from this region according to mass to charge ratio. The operating parameters of the ion separation device may be switched between a first mode in which ions are separated in and eluted from the ion distribution region according to mobility and a second mode in which ions are separated in and eluted from the ion distribution region according to mass to charge ratio. In respect of selecting whether to separate ions according to mobility or mass to charge ratio, it is known that in devices in which a DC potential is repeatedly travelled along the device in order to separate ions by mobility, there is a mass to charge ratio dependence in the ion separation, e.g. as described in K. Richardson, D. Langridge, K. Giles, Fundamentals of travelling wave ion mobility revisited: I. Smoothly moving waves, International Journal of Mass Spectrometry, Volume 428, 2018, Pages 71-80. Operational parameters of the separator, such as pressure and / or speed of the travelling DC potentials, may be selected such that it predominantly separates ions by mobility, such that it predominantly separates ions by mass to charge ratio, or such that it operates in a mode where the ion separation is significantly dependent on both mobility and mass to charge ratio. The ion separation devices of the embodiments described herein may separate ions predominantly according to mass to charge ratio by providing ion separation conditions that are such that the ions lose only a relatively small portion of their kinetic energy between being accelerated by subsequent travelling DC potentials. Under such conditions the ions do not reach a mobility-related terminal velocity. Such ion separation conditions include relatively low gas pressures, relatively high speed DC travelling potentials, or the gas in the separation region having relatively low mass molecules (such as helium or hydrogen). Conversely, ions may be separated predominantly according to mobility by providing ion separation conditions that are such that the travelling DC potentials cause the ions to reach a mobility-related terminal velocity due to collisions with the gas inside the device. Such ion separation conditions include relatively high gas pressures, relatively low speed DC travelling potentials, or the gas in the separation region having relatively high mass molecules (such as nitrogen). The ion separation device may be operated to select either mass to charge ratio separation or mobility separation by varying these operating conditions. For example, the device may switch from the mobility separation mode to the mass to charge ratio separation mode by performing at least one of the following: increasing the speed of the travelling DC potentials; increasing the amplitude of the travelling DC potentials; reducing the pressure of the gas in the device; and changing the gas in the device to have a lower collisional cross-sectional area. Conversely, the device may switch from the mass to charge ratio separation mode to the mobility separation mode by performing at least one of the following: decreasing the speed of the travelling DC potentials; decreasing the amplitude of the travelling DC potentials; increasing the pressure of the gas in the device; and changing the gas in the device to have a higher collisional cross-sectional area. As has been described above, the ion separation device described herein may separate ions according to mobility using the force from a DC static electric field and the opposing force due to travelling DC potentials. However, the device may be operated in a further mode in which it need not drive the ions in opposing directions in order to separate them according to mobility. Rather, the ions may only be driven in the downstream direction, through a substantially static background gas, such that the ions separate according to mobility. For example, the ions may only be driven in the downstream direction using a static DC electric field, or may only be driven in the downstream direction using travelling DC potentials. Accordingly, ions need not be confined axially during the mobility separation of this further mode. As such, the device may separate ions relatively quickly in the further mode, and separate ions more slowly and / or with higher mobility resolution in the other modes described herein. Embodiments of the invention have been described that cause ions to elute from the ion separation device in an order according to their mobility or mass to charge ratio. The ions may be caused to elute from the device in order of increasing or decreasing mobility, or in order of increasing or decreasing mass to charge ratio, during an elution period. A mass filter, such as a quadrupole mass filter, may be provided downstream of the ion separation device so as to receive ions that elute from the ion separation device. The mass filter is controlled such that it is only capable of transmitting ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time. The mass filter may be operated in a bandpass mode in which said range is wide enough that multiple precursor ions can be transmitted simultaneously, or in a resolving mode in which the range is narrow such that only a single precursor ion species is transmitted. The range may be scanned or stepped over a pre-selected range of mass to charge ratios and in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities from the ion separation device. Ions that are transmitted by the mass filter, or ions derived therefrom, are detected by an ion detector or are mass analysed in a mass analyser, such as a Time of Flight mass analyser. For example, the ions that are transmitted by the mass filter may be fragmented or reacted to produce fragment or product ions that are then detected or mass analysed. The mass to charge ratio and / or mobility of the detected or mass analysed ions may be determined by the spectrometer. If the ion separation device elutes ions in an order according to their mobility, then the spectrometer may also associate a mobility related parameter, such as collisional cross-section, with each detected mass to charge ratio. The above-described acquisition process may be repeated, e.g. whilst the ion separation device substantially continuously receives ions. Between consecutive instances of the above-described acquisition process, the mass spectrometer may be operated in a different manner, such as in a survey mode. In the survey mode the fragmentation or reaction cell may be deactivated such that precursor ions are mass analysed. The mass filter may also be operated in a wideband mode or as an RF ion guide such that ions are substantially not mass filtered. The ion separation device may or may not separate ions according to mobility or mass to charge ratio in this survey mode. The mass spectral data obtained by the mass analyser in the survey mode may be associated with the mass spectral data obtained in the other acquisition modes and / or may be used to control how the spectrometer is operated in a subsequent acquisition mode. The ion separation device disclosed herein may be used in any one of the techniques described in WO 2017 / 178835 A1 or US 10727036, which are incorporated herein by reference, in order to perform the mobility or mass to charge ratio separation described therein. For example, in the above described embodiment of the present invention in which the mass spectrometer comprises the ion separation device, a downstream mass filter, a fragmentation or reaction cell, and a mass analyser, the mass spectrometer may be operated to perform a first, survey acquisition on a sample being analysed. In the survey acquisition the analyte ions from the sample are not separated in the ion separation device and the mass filter is operated in a wide band mode or as an RF ion guide so as to substantially not mass filter ions passing therethrough. The mass spectrometer is operated such that the ions transmitted by the mass filter are subjected to a relatively low level of fragmentation or reaction in the fragmentation or reaction cell, such as substantially no fragmentation or reaction. The ions then pass to the mass analyser and are mass analysed so as to provide mass spectral data. This data is then interrogated so as to identify ion species or mass to charge ratio ranges that are of interest. A second acquisition is then performed on the sample, in which ions are separated in the ion separation device disclosed herein such that they elute in order, or reverse order, of mass to charge ratio or mobility during an elution period. The eluting ions pass to the mass filter, which is operated in a bandpass or resolving mode, and which is scanned or stepped over a pre-selected range of mass to charge ratios and in synchronism with said elution period. The operational parameters of the mass filter, such as the speed it is scanned or stepped with and / or its mass resolution and / or width of mass transmission window, may be controlled based on information in the mass spectral data from the survey scan. For example, the detection of ions may be relatively sparse in a mass to charge ratio region of the survey scan as compared to one or more other regions, and so in the second acquisition the mass filter may be scanned or stepped at a faster rate over this region than the other one or more regions (or may jump this sparse region). The mass spectrometer is operated such that the ions transmitted by the mass filter are subjected to a relatively high level of fragmentation or reaction in the fragmentation or reaction cell so as to produce fragment or other product ions. The operational parameters of the fragmentation or reaction cell, such as collision energy or reaction time, may be controlled based on information in the mass spectral data from the survey scan. The fragment or product ions from the fragmentation or reaction cell, and any remaining precursor ions, are then mass analysed in the mass analyser. According to another embodiment, the first, survey acquisition described above is performed, except wherein the ion separation device separates and elutes ions in order, or reverse order, of ion mobility. The mass analyser obtains mass spectral data in which the mass to charge ratios of the ions are associated with their mobilities. This data is then interrogated so as to identify ion species of interest or regions of the mobility-m / z space that are of interest. The second acquisition described above is then performed. For example, the mass filter may be controlled so as to transmit ions with different charge states and mass to charge ratio values at different times during the elution period of the ion separation device. The mass to charge ratio at which the mass transmission window of the mass filter is centred on may change non-monotonically during the elution period. Additionally, or alternatively, to transmitting different charge states at different times and / or varying the mass transmission window non-monotonically, during the second acquisition the width of the mass transmission window and / or speed at which the mass filter is scanned or stepped may be varied during the elution period based on the data from the survey scan. Other parameters, such as the collision energy used to fragment ions, may also change non-monotonically during the elution period of the second acquisition based on information determined in the survey scan. For example, such other parameters may be varied during the elution period based on a mass to charge ratio and / or mobility of ions of interest detected in the survey scan. For instance, ions may be determined to have different charge states from the survey scan data (e.g. based on their mobilities and mass to charge ratios, and / or based on their isotope patterns) and the fragmentation cell may be controlled such that the ions of different charge states are subjected to different fragmentation energies or fragmentation techniques. In the embodiments described above that include the second acquisition, it is contemplated that the mobility or mass to charge ratio separation conditions in the ion separation device may be varied during the second acquisition, based on information obtained in the preceding survey scan. In the embodiments described above that include the second acquisition, it is contemplated that the mass analyser operating conditions may be varied during the second acquisition, based on information obtained in the preceding survey scan. For example, the mass analyser may be a time of flight mass analyser that operates in an enhanced duty cycle mode in which an ion trap pulses ions towards the mass analyser at a time so that ions of interest arrive at the pusher of the mass analyser at the time the pusher is activated, so as to pulse these ions toward the detector of the mass analyser. The timing at which ions are released from the ion trap, relative to the time that the pusher is pulsed, may be varied based on information from the mass spectral data obtained in the survey scan, e.g. so as to optimise the duty cycle for an ion of interest identified in the survey scan. A chromatography device, such as a liquid chromatography device, may be provided for separating the sample supplied to the mass spectrometer. In the embodiments described above that include the second acquisition, the ion separation device and / or mass filter may be disabled during the second acquisition if the mass spectrometer determines that relatively few ion species are being detected by the mass analyser during the survey scan. In the embodiments described above that include the second acquisition, the mass filter may be controlled so as not to transmit, or to partially attenuate, an undesired ion species, such as an interference ion, detected in the survey scan. According to another embodiment, the ion separation device described herein may be operated in a mobility separation mode and a downstream mass filter may be scanned or stepped in synchronism with the elution period of the ion separation device so as to follow a trendline in the mobility-m / z space for ion species of interest, such as ions that have a particular charge state or a range of charge states. This improves the specificity and / or duty cycle of the spectrometer for the ions of interest. When the ion separation device disclosed herein is operated, different ion species may take different durations to elute from the ion separation device. For example, if a species of ion is present in multiple different conformations, then that species may have a relatively broad range of mobilities and so may elute from the ion separation device over a longer duration than an ion species that is present with fewer conformations, especially if the ion separation device is operated in a mobility separation mode that does not resolve those conformations. It may therefore be desired to vary the rate at which the downstream mass filter is stepped based on the duration that the eluting ion species is expected to elute for. For example, in embodiments in which the mass filter is stepped between transmitting different ranges of mass to charge ratio at different times, the mass filter is controlled to transmit each range of mass to charge ratio for a dwell period. The dwell period may be varied based on the ion species that are expected to elute from the ion separation device. If an ion species having a first mass to charge ratio and a relatively wide mobility peak, e.g. due to having a relatively high number of conformers, is expected to elute from the ion separation device over a first duration then the mass filter may be controlled to transmit ions having the first mass to charge ratio for a first, relatively long dwell time whilst it receives and transmits these ions. In contrast, when an ion species having a second mass to charge ratio and a narrower mobility peak, e.g. due to having fewer conformers, is expected to elute from the ion separation device over a second duration then the mass filter may be controlled to transmit ions having the second mass to charge ratio for a second, shorter dwell time whilst it receives and transmits these ions. This may help to optimise the transmission of the ions of interest through the mass filter. Fig. 8A shows an ion separation device 80 according to a second set of embodiments that act as ion filters. The ion separation device has a first ion guide 81, a second ion guide 82, an ion entrance region 83 at a first end of the first ion guide, an ion exit region 84 at a first end of the second ion guide, and an ion transfer region 85 arranged at a second end of the first and second ion guides. The ion entrance region is configured to receive ions travelling in the x-dimension and transfer them orthogonally such that they pass, in the z-dimension, into the first end of the first ion guide. Ions are caused to pass along the first ion guide to its second end at which the transfer region is located. As such, the electrodes of the ion entrance region 83 first ion guide 81 define a first ion channel. Ions reaching the transfer region 85 are transferred into the adjacent second end of the second ion guide 82. Ions are then caused to travel to the first end of the second ion guide and into the ion exit region. As such, the electrodes of the second ion guide 82 and the ion exit region 84 define a second ion channel. Ions reaching the ion exit region 84 are transferred orthogonally, in the x-dimension, out of the ion separation device. Each of the ion entrance region, first ion guide, second ion guide and ion exit region comprises two planar arrays of electrodes arranged such that the ions are caused to travel between the arrays. Each array is formed from electrodes that are arranged in rows that extend in one dimension (e.g. the z-dimension) and columns that extend in an orthogonal dimension (e.g. the x-dimension). Each row and each column comprises a plurality of electrodes. Fig. 8B shows an example of a portion of the first ion guide 82, showing a single row of electrodes from each of the two planar arrays of electrodes that form the ion guide. In use, one or more RF voltage supply is connected to the electrodes of the ion separation device so as to confine the ions between the arrays, i.e. in the y-dimension. DC voltage supplies are also connected to at least some of the electrodes for confining and separating ions in the other dimensions, i.e. the x- and z- dimensions. Ions 86 are supplied into the ion entrance region 83 in the x-dimension and are confined between its two arrays of electrodes by the RF voltages applied to these electrodes. Ions may be received between the central columns of electrodes of the two arrays, and may be prevented from moving laterally, in the z-dimension, over the initial portion of the ion entrance region by applying DC voltages to the columns of electrodes on either side of the central columns. The ions may be urged through the ion entrance region, in the x-dimension, by applying voltages to the electrodes thereof. For example, different voltages may be applied to these different electrodes so as to arrange a potential difference in the x-dimension that urges ions through it, or transient DC voltages may be successively applied to successive electrodes in the x-dimension so as to travel DC potentials in the x-dimension that urge ion along. DC potentials 87 are travelled along the ion entrance region and the first ion guide in a first direction in the z-dimension by successively applying transient DC voltages to successive electrodes in z-dimension. These voltages may be applied to one or both of the arrays in each of the ion entrance region and the first ion guide. Different DC voltages are applied to different electrodes in the ion entrance region and the first ion guide so as to maintain a DC electric field 88 that urges ions in a second direction in the z-dimension that is opposite to the first dimension. The travelling potentials and the DC electric field cause ions to be separated by mass to charge ratio as will be described in relation to Fig. 8C (and / or by mobility as described elsewhere herein). Fig. 8C illustrates the separation of the ions in the ion separation device of Fig. 8A shortly after ions 86 enter the ion separation device, in a mode in which the device separates ions by mass to charge ratio. As can be seen, the DC travelling potentials 87 are able to urge ions 89 having mass to charge ratios below a first threshold value to travel in the first direction against the opposing force due to the DC electric field 88. In contrast, ions 90 having mass to charge ratios above the first threshold value are caused to travel in the second direction by the force due to the electric field acting against the time-averaged force due to the travelling potentials. As such, ions having mass to charge ratio ions below the first threshold are urged out of the ion entrance region 83 and caused to travel through the first ion guide 81 (i.e. first ion channel) in the first direction, whereas ions having mass to charge ratio ions above the first threshold are caused to pass out of the ion entrance region in the second direction and may be discarded. Ions that are received into the first ion guide are confined between its two arrays of electrodes by the RF voltages applied to these electrodes. Ions may be received between the central rows of electrodes of the two arrays, and may be prevented from moving laterally, in the x-dimension, by applying DC voltages to the rows of electrodes on either side of the central row. The ions 89 continue to travel along the first ion guide 81 until they approach the second end of the first ion guide, at which point these ions are transferred into the adjacent portion of the second ion guide 82 (into the second ion channel), i.e. the second end of the second ion guide. The ions may be transferred between the ion guides by applying different voltages to different electrodes in the columns of electrodes at the second ends of the ion guides. Ions that are received into the second ion guide are confined between its two arrays of electrodes by the RF voltages applied to these electrodes. DC potentials 91 are also travelled along the second ion guide in the first direction in the z-dimension by successively applying transient DC voltages to successive electrodes in z-dimension. These voltages may be applied to one or both arrays in the second ion guide. Different DC voltages are applied to different electrodes in the second ion guide (and optionally the ion exit region) so as to maintain a DC electric field 92 that urges ions in the second direction in the z-dimension that is opposite to the first dimension. The electric field and / or travelling potentials in the second ion guide are different to those in the first ion guide so as to enable further mass to charge ratio separation, as will be described in relation to Fig. 8D. For example, the electric field in the different ion guides may have different magnitudes. Alternatively, or additionally, the speed and / or amplitude of the travelling potentials in the first ion guide may be different to in the second ion guide. As can be seen from Fig. 8D, the electric field 92 in the second ion guide that acts in the second direction is able to urge ions 93 having mass to charge ratios above a second threshold value to travel in the second direction against the time-averaged force due to the DC travelling potentials 91. In contrast, ions 94 having mass to charge ratios below the second threshold value are caused to travel in the first direction by the DC travelling potentials against the electric field. As such, the ions having mass to charge ratios above the second threshold value are caused to travel through the second ion guide in the second direction, whereas the ions having mass to charge ratios below the second threshold value are caused to pass out of the second ion guide in the first direction and may be discarded. Ions may be prevented from moving laterally, in the x-dimension, out of the second ion guide by applying DC voltages to the rows of electrodes on either side of the central row in each array of electrodes. The ions 93 continue to travel along the second ion guide and enter the ion exit region 84. Ions may pass to the region between the central columns of electrode of the two arrays in the ion exit region 84, and may be prevented from moving further in the second direction by applying DC voltages to the columns of electrodes adjacent to the central columns. The ions may be urged through the ion exit region, in the x-dimension, such that the ions exit the ion separation device by applying voltages to the electrodes of the ion exit region. For example, different voltages may be applied to these different electrodes so as to arrange a potential difference in the x-dimension that urges ions through it, or transient DC voltages may be successively applied to successive electrodes in the x-dimension so as to travel DC potentials 95 in the x-dimension that urge ions along. The range of mass to charge ratios that are transmitted from the ion entrance region 83 to the ion exit region 84 can be selected / adjusted by selecting / altering the magnitude of the electric field and / or an operating parameter of the travelling potentials (such as amplitude or speed) in the ion entrance region and first ion guide (i.e. in the first ion channel), and / or by selecting / altering the magnitude of the electric field and / or an operating parameter of the travelling potentials (such as amplitude or speed) in the second ion guide (i.e. in the second ion channel). In the embodiment described above the ion separation device is operated as a band-pass mass to charge ratio filter having a high mass cut-off (i.e. the first threshold) set by the operating conditions in the ion entrance region and the low mass cut-off (i.e. the second threshold) set by the operating conditions in the second ion guide. However, it will be appreciated that the low mass cut-off (i.e. the first threshold) may set by the operating conditions in the ion entrance region and the high mass cut-off (i.e. the second threshold) may be set by the operating conditions in the second ion guide, e.g. by reversing the directions of the travelling potentials and electric fields, as shown in Fig. 8E. Instead of being operated as a band-pass mass to charge ratio filter having high and low mass cut-offs, the ion separation device may instead be operated as a low pass mass to charge ratio filter having only a high mass cut-off or a high pass mass to charge ratio filter having only a low mass cut-off. For example, referring to Fig. 8D, the ions may exit the ion separation device at the second end of the first ion guide instead of being transmitted into a second ion guide. Alternatively, if a longer separation path is desired the ions may be transferred into the second ion guide, but ions may be separated in the second ion guide in the same manner as in the first ion guide. For example, in order to do this in the embodiment shown in Fig. 8D, the travelling potentials may travel along the second ion guide in the second direction towards the ion exit region and the DC electric field may urge ions in the first, opposite direction. Fig. 8F shows the device of Fig. 8A when being operated in a by-pass mode such that it substantially does not separate ions by mass to charge ratio. In this mode, voltages are applied to the ion entrance region 83 and the ion exit region 84 such that ions 86 pass in the x-dimension from the ion entrance to the ion exit region without passing through the first and second ion guides 81,82. For example, ions may maintained between the central columns of electrodes of the two arrays in each of the ion entrance region and ion exit region, and prevented from moving laterally (in the z-dimension) by applying DC voltages to the columns of electrodes on either side of the central columns. The ions may be urged through the ion entrance region and ion exit region, in the x-dimension, by applying voltages to the electrodes thereof. For example, transient DC voltages may be successively applied to successive electrodes in the x-dimension so as to travel DC potentials 96 in the x-dimension that urge ion along. Alternatively, different voltages may be applied to these different electrodes so as to arrange a potential difference in the x-dimension that urges ions along. During this by-pass mode, ions are preferably not urged in the z-dimension by the travelling potentials or electric field described above. Although embodiments have been described in which ions are caused to continually flow through the ion guides 81,82, other embodiments are contemplated in which each ion guide is operated in one of the manners described above in relation to the ion channels in the first set of embodiments. For example, in each of the first and second ion guides 81,82, ion may be accumulated and then eluted in the manner described above, e.g. as will be described below in relation to Fig. 9. Fig. 9 shows an embodiment that may be implemented using the same electrode structure as has been described above in relation to Fig. 8A, except wherein different voltages are applied to the electrodes so that ions are accumulated in and then eluted from the first ion guide 81, accumulated in the second ion guide 82, and then eluted from the second ion guide. According to this embodiment, ions enter the ion entrance region 83 and substantially all ions are caused to enter the first ion guide 81, e.g. by being driven into the first ion guide by travelling potentials 87. The ions may enter and exit the ion entrance region 83 in orthogonal directions as has been described above or, because substantially all ions are transmitted into the first ion guide, ions may pass into, through and out of the ion entrance region in the z-dimension. Different DC voltages are applied to different electrodes along the first ion guide so as generate a DC electric field 88 that urges ion in the second direction. Fig. 9 illustrates an example of the potential profile 97 along the first ion guide during an ion trapping phase. As can be seen, the magnitude of the electric field is constant along the portion of the first ion guide at its first end. The first ion guide has a downstream trapping region within which the magnitude of the electric field increases as a function of position along the ion guide towards the second end. Optionally, at the end of the trapping region there is an analytical region along which the magnitude of the electric field is substantially constant. In use, the travelling potentials 87 move along the first ion guide in the first, downstream direction, through the trapping region and also through the analytical region. Fig. 9 shows an example in which ions of different mass to charge ratio 98 are trapped at different equilibrium positions along the first ion guide, the different mass to charge ratios being illustrated by the different sized circles. Once the ions have been separated according to mass to charge ratio within the first ion guide, the ions are caused to elute from said analytical region in order of mass to charge ratio as described in relation to the first set of embodiments, e.g. by lowering the DC electric field gradient along the first ion guide and / or by changing an operational parameter of the travelling potentials such as speed or amplitude. The eluting ions reach the transfer region 85, and one or more voltages are selectively applied to the electrodes in the transfer region such that only a restricted range of mass to charge ratios that elute from the analytical region are transferred into the second ion guide 82. The timing at which these voltages are applied may be synchronised relative to the elution cycle such that only ions having a pre-selected range of mass to charge ratios are transferred to the second ion guide. The voltages that selectively transfer the ions between the ion guides 81,82 may be applied to the electrodes shown in Fig. 8A, and / or one or more additional gate electrode 99 to which these voltages are applied may be provided as shown in Fig. 9. Different DC voltages are applied to different electrodes along the second ion guide so as generate a DC electric field that urges ion in the first direction. Fig. 9 illustrates an example of the potential profile 100 along the second ion guide during an ion trapping phase. As can be seen, the magnitude of the electric field is constant along the portion of the second ion guide at its second end. The second ion guide has a trapping region within which the magnitude of the electric field increases as a function of position along the second ion guide towards the first end. At the end of the trapping region there is an analytical region along which the magnitude of the electric field is substantially constant. In use, travelling potentials 101 move along the second ion guide from the second end towards the first end, through the trapping region and through the analytical region. Fig. 9 shows an example in which ions of different mass to charge ratio 102 are trapped at different equilibrium positions along the second ion guide, the different mass to charge ratios being illustrated by the different sized circles. Once the ions have been separated according to mass to charge ratio within the second ion guide, the ions are caused to elute from said analytical region in order of mass to charge ratio as described above in relation to the first ion guide (and the first set of embodiments), e.g. by lowering the DC electric field gradient the second ion guide and / or by changing an operational parameter of the travelling potentials such as speed or amplitude. The eluting ions reach the ion exit region 84 and are caused to exit the ion separation device as described above. Therefore, the ions may exit the ion separation device in the x-dimension. However, the ions could instead leave the ion separation device simply by continuing in the second direction. Once the ions have been urged out of the first ion guide 81, the amplitude or speed of the travelling potentials in the first ion guide may be changed so accumulate a second population of ions in the trapping region of the first ion guide. For example, the amplitude of the travelling potentials may be decreased. Once the ions that were transferred into the second ion guide have eluted and left the ion exit, the second population of ions may then be caused to elute from the analytical region of the first ion guide and some of these ions that have a restricted range of mass to charge ratios may be transferred from the first ion guide to the second ion guide. This restricted range may have the same or different mass to charge ratios to the range that was previously transferred. The transferred ions may then be caused to separate and elute from the second ion guide in the manner described above. This process of accumulating ions in the first ion guide whilst ions are being separated in and eluted from the second ion guide may be repeated, enabling the ion separation device to operate with an overall duty cycle close to 100%. The ion separation in the first ion guide may be controlled independently of the ion separation in the second ion guide, e.g. such that the DC electric field profile in their respective downstream directions differ, and / or such that their travelling potentials have different speeds or amplitudes. It is contemplated that the directions in which the DC potentials travel and the directions of the electric fields may be reversed such that the ions elute from each analytical region in order from high to low mass to charge ratio instead of from low to high mass to charge ratio. Other modes of operation are contemplated, such as operating one of the ion guides as a non-selective ion trap, i.e. one which accumulates ions and ejects all ions at substantially the same time. Alternatively, ions may be driven through one of the ion guides without being separated by mass to charge ratio (or mobility). The embodiment described in relation to Fig. 9 may be operated in a bypass mode, as has been described in relation to Fig. 8E. Although embodiments have been described in relation to Figs. 8-9 in which the electrodes are arranged in planar arrays, between which the ions travel, other electrode geometries may be used, e.g. as shown in Figs. 10A-10B. Figs. 10A-10B show cross-sections (in the x-y plane) through an embodiment of an ion separation device as has been described in relation to Figs. 8-9, except wherein at least the electrodes of the ion guides have different geometries. Fig. 10A shows a crosssection through the ion separation device at a location towards the first end of the ion guides 81,82, whereas Fig. 10B shows a cross-section through the ion separation device at a location towards the second end of the ion guides where the ion transfer region 85 is located. As can be seen, each of the ion guides may be a stack of apertured electrodes such as ring electrodes. As shown in Fig. 10B, the electrodes of the ion guides within the ion transfer region may only partially encircle the ion guiding axes of the ion guides such that ions can be transferred radially from the first ion guide to the second ion guide. More specifically, in the ion transfer region the sides of the ions guides that face each other are open so that ions can be transferred from the first ion guide to the second ion guide. The ions may be moved from the first ion guide to the second ion guide in this region by applying different DC voltages to the electrodes of the different ion guides. Although the ion separation device has been described as separating ions according to mass to charge ratio, its operating conditions may be such that it operates as a mobility separator instead. For example, the ion separation device may separate ions predominantly according to mass to charge ratio by providing ion separation conditions that are such that the ions lose only a relatively small portion of their kinetic energy between being accelerated by subsequent travelling DC potentials. Under such conditions the ions do not reach a mobility-related terminal velocity. Such ion separation conditions include relatively low gas pressures, relatively high speed DC travelling potentials, or the gas in the separation region having relatively low mass molecules (such as helium or hydrogen). Conversely, ions may be separated predominantly according to mobility by providing ion separation conditions that are such that the travelling DC potentials cause the ions to reach a mobility-related terminal velocity due to collisions with the gas inside the device. Such ion separation conditions include relatively high gas pressures, relatively low speed DC travelling potentials, or the gas in the separation region having relatively high mass molecules (such as nitrogen). The ion separation device may be operated to select either mass to charge ratio separation or mobility separation by varying these operating conditions. For example, the device may switch from the mobility separation mode to the mass to charge ratio separation mode by performing at least one of the following: increasing the speed of the travelling DC potentials; increasing the amplitude of the travelling DC potentials; reducing the pressure of the gas in the device; and changing the gas in the device to have a lower collisional cross-sectional area. Conversely, the device may switch from the mass to charge ratio separation mode to the mobility separation mode by performing at least one of the following: decreasing the speed of the travelling DC potentials; decreasing the amplitude of the travelling DC potentials; increasing the pressure of the gas in the device; and changing the gas in the device to have a higher collisional cross-sectional area. It is contemplated that one of the ion guides may be operated as a mass to charge ratio filter and the other ion guide may be operated as an ion mobility filter. For example, an ion guide having a high mass cut-off (i.e. that only transmit ions below a threshold mass to charge ratio towards the device exit) may be followed by an ion guide having a high mobility cut-off (i.e. that only transmit ions below a threshold mobility towards the device exit). Alternatively, the ion separation device may comprise an ion guide having a high mass cut-off followed by an ion guide having a low mass cut-off, such that these ion guides provide a m / z bandpass filter. The device may also comprise an ion guide having a high mobility cut-off followed by an ion guide having a low mobility cut-off, such that these ion guides provide a mobility bandpass filter. Alternatively, the ion guides forming the mobility bandpass filter may be provided upstream of the ion guides forming the m / z bandpass filter. The ion separation device may have a static buffer gas therein, i.e. substantially no gas flow through it. This significantly reduces the burden on the vacuum pumps that maintain the vacuum chamber in which the ion separation device is located at the desired pressure. Also, as a gas flow is not used to separate the ions there are fewer constraints on the form of electrodes that are used in the ion separation device. For example, gas flow separator devices require a laminar gas flow therein and this restricts the electrode arrangements that may be used. It will therefore be appreciated that the ion separation device according to the embodiment described above is not limited to the electrode configuration shown. However, embodiments are contemplated in which a gas flow is used to urge ions instead of, or as well as, the electric fields described. Although the present invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims. For example, embodiments have been described in relation to both the first and second sets of embodiments in which voltages are applied to electrodes of the ion separation device so that DC potentials travel along the ion separation device and urge ions along it, for example, to separate ions according to mobility or mass to charge ratio. However, there are several ways to provide potentials that travel along the ion separation device so as to urge ions along it. For instance, a periodic or harmonic voltage waveform having an amplitude that oscillates with time may be applied to the electrodes of the ion separation device, where different phases of the voltage waveform are applied to electrodes that are located at different axial locations along the ion separation device so as to cause ions to be urged along the device. The voltage waveform may be any periodic or harmonic wave including, but not limited to, a sine or cosine wave, a square wave, a trapezoidal wave, a triangular wave or a sawtooth wave. The voltage waveform may be a waveform that has a continuously varying amplitude. For example, a first electrode (or multiple axially adjacent electrodes) may be supplied with a harmonically oscillating voltage waveform with an initial phase shift of 0 degrees. A second electrode (or multiple axially adjacent electrodes) may be supplied with the voltage waveform, but with the initial phase shifted by 90 degrees. A third electrode (or multiple axially adjacent electrodes) may be supplied with the voltage waveform, but with the initial phase shifted by 180 degrees. A fourth electrode (or multiple axially adjacent electrodes) may be supplied with the voltage waveform, but with the initial phase shifted by 270 degrees. This pattern of applying the voltage waveform to electrodes of the ion separation device may then be repeated along the device. The voltage waveform may be applied to the electrodes of the device so as to effectively form a substantially smoothly varying potential that travels along the ion separation device and acts to urge ions axially along the device. The frequency of this four-phase voltage supply and the distance between the electrodes (i.e. pitch of the electrodes) may be selected so as to dictate the velocity of the travelling potential. Such a system is described, for example, in A.W. Colburn et al., Physics Procedia 1 (2008) 51-60, Colburn et al. It will be appreciated that a waveform having more than four phases, or even three phases, may be applied to respective electrodes so as to urge the ions along the device. It is contemplated that the amplitude and / or frequency of the voltage waveform may be altered with time. Additionally, or alternatively, the phases of the voltage waveform that are applied to the different electrodes may be altered with time, e.g. so that the potential travels along the ion separation device in the opposite direction. In addition to the phase shifted voltage waveform, axially adjacent electrodes may be supplied with opposite phases of a separate RF waveform for radially confining ions within the device. This enables the radially confining potential and the axial travelling potential to be adjusted independently of each other. The confining RF waveform and the phase shifted travelling potential waveforms may be superimposed. These oscillating waveforms may be capacitively coupled to the electrodes, optionally allowing complex DC potentials to be simultaneously applied to the electrodes, for example, using DC resistive divider circuits between the electrodes in the axial direction. Aspects and embodiments on the present invention are listed in the clauses below: 1. An ion separation device comprising: a plurality of plates, wherein each of the plates has one or more apertures therethrough and at least one electrode around each of the one or more apertures, and wherein the plates are arranged so that the one or more apertures and electrodes are aligned to form a plurality of ion channels that extend through each of the plates; and one or more voltage supplies for applying voltages to the electrodes for causing ions to be separated in the plurality of channels according to mass to charge ratio or mobility. 2. The device of clause 1, wherein the plates are arranged with their major surfaces parallel to each other such that the ion channels extend along the ion separation device, through the plates, along axes that are substantially orthogonal to the major surfaces of the plates. 3. The device of clause 1 or 2, wherein said one or more apertures in each plate is a plurality of discrete apertures, and wherein the plates are arranged so that the apertures in the plates are aligned to form said plurality of ion channels that extend through each of the plates. 4. The device of clause 1 or 2, wherein said one or more apertures in each plate comprises an aperture having said at least one electrode around, wherein the aperture and at least one electrode are shaped so as to define the plurality of ion channels through the plates. 5. The device of clause 4, wherein the aperture in each plate comprises a plurality of enlarged aperture portions that are separated from each other by smaller aperture portions, and the at least one electrode is arranged around the aperture so as to form said plurality of ion channels which extend through the plurality of respective enlarged aperture portions in each of the plates. 6. The device of clause 5, wherein the shape of the aperture in each plate corresponds to the shape that would be formed by making a plurality of substantially circular apertures in the plate, where adjacent ones of these circular apertures only partially overlap with each other or touch each other at their sides. 7. The device of any preceding clause, wherein each plate has a further aperture, or further aperture portion, so that the plates define a further ion channel; and wherein the ion separation device is configured to transmit ions through said further ion channel substantially without separating them according to mobility or mass to charge ratio. 8. The device of any preceding clause, wherein each plate is a printed circuit board (PCB) having said one or more apertures therethrough, and wherein the one or more electrodes are conductive traces on the PCBs. 9. The device of any preceding clause, wherein the one or more voltage supplies comprises: a first voltage supply configured to apply voltages to the electrodes on the plates such that an electric potential repeatedly travels along each ion channel for urging ions in one direction along that ion channel; and a second voltage supply configured to provide different voltages to electrodes at different respective positions along each ion channel so as to provide an electric field for urging ions in an opposite direction to said one direction, and / or a gas supply for flowing gas along each ion channel for urging ions in an opposite direction to said one direction, for causing ions to be separated according to mass to charge ratio or mobility. 10. The device of clause 9, wherein the ion separation device is configured to cause ions to elute from each ion channel in an order according to mobility or mass to charge ratio by: (i) controlling said first voltage supply so as to progressively vary the speed and / or amplitude of the electric potential that repeatedly travels along that ion channel; and / or (ii) controlling said second voltage supply so as to progressively vary the electric field maintained along that ion channel. 11. The device of any preceding clause, comprising an ion funnel arranged at the downstream end of the ion channels so as to receive and funnel ions exiting the ion channels. 12. The device of clause 11, wherein the ion funnel comprises: a plurality of plates arranged with their major surfaces parallel to each other, wherein each of the plates has an aperture therein that is surrounded by one or more electrode, wherein the apertures become progressively smaller in the downstream direction and are aligned with each other to form an ion channel therethrough; and at least one voltage supply for applying voltages to these electrodes for radially confining ions within the ion funnel and for driving ions downstream through the ion funnel. 13. The device of clause 11 or 12, wherein the apertures that are arranged over an axial length of the ion funnel that extends from the upstream end of the ion funnel are elongated, such as being rectangular or oval, and the apertures that are arranged over an axial length of the ion funnel that extends to the downstream end of the ion funnel are circular. 14. The device of clause 11, 12 or 13, wherein the aperture in each of at least some of the plates in the ion funnel is an elongated aperture, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodes are provided on opposing sides of the aperture that extend between said opposing ends, and wherein the at least one voltage supply is configured to apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes for urging ions towards the centre of the elongated aperture. 15. The device of clause 14, wherein the ion funnel comprises plates having said elongated apertures and said first to fourth electrodes at its upstream end, and plates at its downstream end that each have a single continuous electrode surrounding an aperture. 16. The device of any preceding clause, comprising an ion distribution region at the upstream end of the ion separation device for receiving a beam of ions and expanding or deflecting the beam of ions such that ions are distributed over the entrances of said plurality of ion channels. 17. The device of clause 16, wherein the ion distribution region comprises: a plurality of plates, each of which has one or more aperture therein; wherein the plates are arranged with their major surfaces parallel to each other and at least one electrode is provided around the circumference of each of the apertures; and a voltage supply for applying voltages to these electrodes for radially confining ions. 18. The device of clause 16 or 17, wherein the apertures and electrodes in the plates of the ion distribution region are arranged and configured so as to divide the incoming ion beam into multiple ion beams. 19. The device of clause 18, wherein the ion distribution region comprises: an ion dispersing portion at an upstream end thereof that includes one or more first plates, each of which has a first, elongated aperture therethrough and one or more electrodes arranged around the elongated aperture so as to form a single first ion channel through the first plates; and an ion channelling portion arranged downstream of the ion dispersing portion and that includes one or more second plates, wherein each of the one or more second plates has a second, elongated aperture therethrough and one or more electrodes arranged around the one or more apertures that are configured so as to receive ions from the first ion channel and confine them in multiple separate ion channels; wherein each of the plurality of ion channels for separating ions is arranged to receive ions from a respective one of the multiple separate ion channels of the ion distribution region. 20. The device of any one of clauses 16-19, configured to vary voltages that are applied to the electrodes in the ion distribution region so as to alternate between an ion accumulation mode in which ions are trapped in the ion distribution region and an ion ejection mode in which ions are urged from the ion distribution region into the plurality of ion channels. 21. The device of clause 20, configured to separate ions according to mass to charge ratio or mobility in the ion distribution region during the ion accumulation mode and / or the ion ejection mode. 22. The device of clause 20 or 21, wherein the ion separation device is configured to separate ion in, and elute ions from, the plurality of ion channels whilst the ion distribution region is being operated in the ion accumulation mode. 23. The device of any one of clauses 16-22, wherein the magnitude of the electric field in the ion distribution region has a maximum value that is less than or equal to the minimum value of the magnitude of the electric field in the plurality of ion channels. 24. An ion separation device comprising: a plurality of plates, wherein each of the plates has an elongated aperture therethrough, wherein the plates are arranged so that the apertures are aligned to form an ion channel that extends through each of the plates, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodes are provided on opposing sides of the aperture that extend between the opposing ends; and at least one voltage supply configured to: apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes; and apply voltages to the electrodes on the plurality of plates so as to separate ions according to mass to charge ratio or mobility along the ion channel. 25. A mobility and / or mass spectrometer comprising: an ion separation device as described in any preceding clause; and an ion detector for detecting the separated ions, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. 26. A method of separating ions according to mobility or mass to charge ratio comprising: providing an ion separation device as described in any one of clauses 1-24; and applying voltages to electrodes in the ion separation device so as to separate ions in the ion channel(s) according to mobility or mass to charge ratio. 27. A method of mobility and / or mass spectrometry comprising: separating ions according to mobility and / or mass to charge ratio using a method as described in clause 26; and detecting the separated ions, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. 28. A method as described in clause 26 or 27, wherein the ions are trapped in the ion separation device as they are separated along the ion separation device according to mobility or mass to charge ratio; and the method comprises: controlling the ion separation device such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during an elution period; and controlling a mass filter downstream of the ion separation device such that the mass filter only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities from the ion separation device. 29. An ion filter for filtering ions according to mass to charge ratio or mobility, comprising: a first plurality of electrodes arranged for guiding ions along a first ion channel having a longitudinal axis therethrough; an ion entrance for introducing ions into said first ion channel in a direction orthogonal to the longitudinal axis; a first voltage supply configured to apply voltages to the first plurality of electrodes such that an electric potential repeatedly travels along the first ion channel for urging ions in a first direction along the first ion channel; and a second voltage supply configured to provide different voltages to said electrodes at different respective positions along the first ion channel so as to provide an electric field for urging ions in a second direction along the ion channel that is opposite to said first direction, and / or a first gas supply for providing a gas flow that urges ions in the second direction along the first ion channel; wherein the voltages applied by the first voltage supply, and the voltages applied by the second voltage supply and / or the gas flow, are such that ions in the first ion channel will be separated according to mass to charge ratio or mobility. 30. The ion filter of clause 29, further comprising a first ion exit for transmitting ions out of the ion filter; wherein the voltages applied by the first voltage supply, and the voltages applied by the second voltage supply and / or the gas flow, are such that either: (a) ions having a mass to charge ratio or mobility that is above a first threshold value are urged to move along the first ion channel from its first end towards its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio or mobility that is below the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter; or (b) ions having a mass to charge ratio or mobility that is below a first threshold value are urged to move along the ion channel from its first end to its second end and downstream towards the ion exit, whereas ions having a mass to charge ratio or mobility that is above the first threshold value are urged to move along the ion channel in the opposite direction such that they are filtered out by the ion filter. 31. The ion filter of clause 30, comprising a second ion exit arranged such that ions that are urged to move along the ion channel towards its first end pass out of the second ion exit, such that they are filtered out by the ion filter. 32. The ion filter of clause 29, 30 or 31, wherein said separation of the ions causes some of the ions to pass to a second end of the first ion channel; and wherein the ion filter comprises: a second plurality of electrodes arranged for guiding ions along a second ion channel having a longitudinal axis therethrough, wherein the second ion channel has an ion entrance adjacent the second end of the first ion channel for receiving ions therefrom; a third voltage supply configured to apply voltages to the second plurality of electrodes such that an electric potential repeatedly travels along the second ion channel for urging ions in a third direction along the second ion channel; and a fourth voltage supply configured to provide different voltages to said electrodes at different respective positions along the second ion channel so as to provide an electric field for urging ions in a fourth direction along the second ion channel that is opposite to said third direction, and / or a second gas supply for providing a second gas flow that urges ions along the second ion channel in said fourth direction; wherein the voltages applied by the third voltage supply, and the voltages applied by the fourth voltage supply and / or the second gas flow, are such that ions in the second ion channel will be separated according to mass to charge ratio or mobility. 33. The ion filter of clause 32, wherein the ion entrance of the second ion channel is in a side of the second ion channel that is adjacent to the second end of the first ion channel for receiving ions that exit the side of the first ion channel. 34. The ion filter of clause 33, wherein the ion filter is configured to eject ions from the second end of the first ion channel in a direction orthogonal to its longitudinal axis, and receive ions into the second ion channel in a direction orthogonal to its longitudinal axis. 35. The ion filter of clause 32, 33 or 34, wherein the voltages applied by the third voltage supply, and the voltages applied by the fourth voltage supply and / or the second gas flow, are such that either: (c) ions having a mass to charge ratio or mobility that is above a second threshold value that is different to said first threshold value are urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio or mobility that is below the second threshold value are urged to move along the second ion channel in the opposite direction such that they are filtered out by the ion filter; or (d) ions having a mass to charge ratio or mobility that is below a second threshold value that is different to said first threshold value are urged to move along the second ion channel towards a first of its ends and hence downstream towards the ion exit of the ion filter, whereas ions having a mass to charge ratio or mobility that is above the second threshold value are urged to move along the ion second channel in the opposite direction such that they are filtered out by the ion filter. 36. The ion filter of clause 35, wherein the mass filter comprises a third ion exit arranged such that ions that are urged to move along the second ion channel in said opposite direction pass out of the third exit, such that they are filtered out by the ion filter. 37. The ion filter of any one of clauses 32-36, wherein the first voltage supply, and the second voltage supply and / or first gas supply, are configured to be operated in: (i) an ion accumulation mode so as to cause ions having different mass to charge ratios or mobilities to be confined at different respective locations along a trapping region of the first ion channel; and then (ii) an elution mode in which ions are caused to elute from the trapping region towards the ion entrance to the second ion channel in an order according to mass to charge ratio or mobility. 38. The ion filter of clause 37, comprising an ion gate between the first and second ion channels, wherein the ion filter is configured to synchronise the opening and closing of the ion gate relative to the start of the elution mode of the first ion channel such that only ions having a restricted range of mass to charge ratios or mobilities are transmitted by the ion gate from the first ion channel to the second ion channel. 39. The ion filter of any one of clauses 29-38, wherein the first plurality of electrodes forming the first ion channel are arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the first ion channel orthogonally to its longitudinal axis; and / or wherein the second plurality of electrodes forming the second ion channel are arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the second ion channel orthogonally to its longitudinal axis. 40. The ion filter of any one of clauses 29-39, wherein the ion filter comprises arrays of electrodes between which the ions travel, wherein each array comprises a plurality of rows of electrodes and a plurality of orthogonal columns of electrodes, and wherein each row and each column comprises a plurality of electrodes; and wherein the first and / or second ion channels are formed by these arrays of electrodes. 41. An ion filter for filtering ions according to mass to charge ratio or mobility, comprising: a first plurality of electrodes arranged for guiding ions in a first direction along a first ion channel having an ion entrance at a first end for receiving ions and an ion exit in a side thereof at its second end; a second plurality of electrodes arranged for guiding ions in a second direction opposite to the first direction along a second ion channel having an ion entrance in a side thereof that is adjacent to the ion exit of the first ion channel for receiving ions from the first ion channel; one or more first voltage supply configured to apply voltages to the electrodes of each of the first and second ion channels such that an electric potential repeatedly travels along each of the first and second ion channel for urging ions in one direction along each of those ion channels; and one or more second voltage supply configured to provide different voltages to the electrodes of each of the first and second ion channels at different respective positions along each of the first and second ion channels so as to provide an electric field for urging ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel, and / or one or more gas supply for providing one or more gas flow that urges ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel; wherein the voltages applied by the one or more first voltage supply, and the voltages applied by the one or more second voltage supply and / or the one or more gas flows, are such that ions in the first and second ion channels will be separated according to mass to charge ratio or mobility. 42. The ion filter of clause 41, wherein the first plurality of electrodes forming the first ion channel are arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the first ion channel orthogonally to its longitudinal axis; and wherein the second plurality of electrodes forming the second ion channel are arranged and configured such that, when the ion filter is operated in a bypass mode, ions pass into, straight through and out of the second ion channel orthogonally to its longitudinal axis. 43. A mobility and / or mass spectrometer comprising: an ion filter as described in any one of clauses 29-42; and an ion detector for detecting ions transmitted by the ion filter, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. 44. A mass spectrometer comprising: an ion filter as described in any one of clauses 29-42; a mass filter arranged downstream of a first ion exit of the ion filter; and control circuitry configured to: control the ion filter such that ions having different mass to charge ratios or mobilities elute from the first exit of the ion filter at different respective times during an elution period; and control the mass filter such that it only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities. 45. A method of filtering ions according to mobility or mass to charge ratio comprising: providing an ion filter as described in any one of clauses 29-42; and applying voltages to the electrodes in the ion filter so as to separate ions in the ion channel(s) according to mobility or mass to charge ratio and thus transmit ions having a restricted range of mobility or mass to charge ratio and filter out other ions. 46. A method of mobility and / or mass spectrometry comprising: filtering ions according to mobility and / or mass to charge ratio using a method as described in clause 45; and detecting the ions transmitted by the ion filter, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio. 47. A method of mass spectrometry comprising: providing an ion filter as described in any one of clauses 29-42; providing a mass filter downstream of a first ion exit of the ion filter; controlling the ion filter such that ions having different mass to charge ratios or mobilities elute from the first exit of the ion filter at different respective times during an elution period; and controlling the mass filter such that it only transmits ions having mass to charge ratios within a restricted range of mass to charge ratios at any given time, wherein said range is scanned or stepped in synchronism with said elution period such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities.

Claims

1. An ion separation device comprising:a plurality of plates, wherein each of the plates has one or more apertures therethrough and at least one electrode around each of the one or more apertures, and wherein the plates are arranged so that the one or more apertures and electrodes are aligned to form a plurality of ion channels that extend through each of the plates; andone or more voltage supplies for applying voltages to the electrodes for causing ions to be separated in the plurality of channels according to mass to charge ratio or mobility, wherein the one or more voltage supplies comprises:a first voltage supply configured to apply voltages to the electrodes on the plates such that an electric potential repeatedly travels along each ion channel for urging ions in one direction along that ion channel; anda second voltage supply configured to provide different voltages to electrodes at different respective positions along each ion channel so as to provide an electric field for urging ions in an opposite direction to said one direction, and / or a gas supply for flowing gas along each ion channel for urging ions in an opposite direction to said one direction, for causing ions to be separated according to mass to charge ratio or mobility.

2. The device of claim 1, wherein the plates are arranged with their major surfaces parallel to each other such that the ion channels extend along the ion separation device, through the plates, along axes that are substantially orthogonal to the major surfaces of the plates.

3. The device of claim 1 or 2, wherein said one or more apertures in each plate is a plurality of discrete apertures, and wherein the plates are arranged so that the apertures in the plates are aligned to form said plurality of ion channels that extend through each of the plates.

4. The device of claim 1 or 2, wherein said one or more apertures in each plate comprises an aperture having said at least one electrode around, wherein the aperture and at least one electrode are shaped so as to define the plurality of ion channels through the plates.

5. The device of claim 4, wherein the aperture in each plate comprises a plurality of enlarged aperture portions that are separated from each other by smaller aperture portions, and the at least one electrode is arranged around the aperture so as to form said plurality of ion channels which extend through the plurality of respective enlarged aperture portions in each of the plates.

6. The device of claim 5, wherein the shape of the aperture in each plate corresponds to the shape that would be formed by making a plurality of substantially circular apertures in the plate, where adjacent ones of these circular apertures only partially overlap with each other or touch each other at their sides.

7. The device of any preceding claim, wherein each plate has a further aperture, or further aperture portion, so that the plates define a further ion channel; and wherein the ion separation device is configured to transmit ions through said further ion channel substantially without separating them according to mobility or mass to charge ratio.

8. The device of any preceding claim, wherein each plate is a printed circuit board (PCB) having said one or more apertures therethrough, and wherein the one or more electrodes are conductive traces on the PCBs.

9. The device of any preceding claim, wherein the ion separation device is configured to cause ions to elute from each ion channel in an order according to mobility or mass to charge ratio by: (i) controlling said first voltage supply so as to progressively vary the speed and / or amplitude of the electric potential that repeatedly travels along that ion channel; and / or (ii) controlling said second voltage supply so as to progressively vary the electric field maintained along that ion channel.

10. The device of any preceding claim, comprising an ion funnel arranged at the downstream end of the ion channels so as to receive and funnel ions exiting the ion channels.

11. The device of claim 10, wherein the ion funnel comprises: a plurality of plates arranged with their major surfaces parallel to each other, wherein each of the plates has an aperture therein that is surrounded by one or more electrode, wherein the apertures become progressively smaller in the downstream direction and are aligned with each other to form an ion channel therethrough; and at least one voltage supply for applying voltages to these electrodes for radially confining ions within the ion funnel and for driving ions downstream through the ion funnel.

12. The device of claim 10 or 11, wherein the apertures that are arranged over an axial length of the ion funnel that extends from the upstream end of the ion funnel are elongated, such as being rectangular or oval, and the apertures that are arranged over an axial length of the ion funnel that extends to the downstream end of the ion funnel are circular.

13. The device of claim 10, 11 or 12, wherein the aperture in each of at least some of the plates in the ion funnel is an elongated aperture, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodesare provided on opposing sides of the aperture that extend between said opposing ends, and wherein the at least one voltage supply is configured to apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes for urging ions towards the centre of the elongated aperture.

14. The device of claim 13, wherein the ion funnel comprises plates having said elongated apertures and said first to fourth electrodes at its upstream end, and plates at its downstream end that each have a single continuous electrode surrounding an aperture.

15. The device of any preceding claim, comprising an ion distribution region at the upstream end of the ion separation device for receiving a beam of ions and expanding or deflecting the beam of ions such that ions are distributed over the entrances of said plurality of ion channels.

16. The device of claim 15, wherein the ion distribution region comprises: a plurality of plates, each of which has one or more aperture therein; wherein the plates are arranged with their major surfaces parallel to each other and at least one electrode is provided around the circumference of each of the apertures; and a voltage supply for applying voltages to these electrodes for radially confining ions.

17. The device of claim 15 or 16, wherein the apertures and electrodes in the plates of the ion distribution region are arranged and configured so as to divide the incoming ion beam into multiple ion beams.

18. The device of claim 17, wherein the ion distribution region comprises: an ion dispersing portion at an upstream end thereof that includes one or more first plates, each of which has a first, elongated aperture therethrough and one or more electrodes arranged around the elongated aperture so as to form a single first ion channel through the first plates; andan ion channelling portion arranged downstream of the ion dispersing portion and that includes one or more second plates, wherein each of the one or more second plates has a second, elongated aperture therethrough and one or more electrodes arranged around the one or more apertures that are configured so as to receive ions from the first ion channel and confine them in multiple separate ion channels;wherein each of the plurality of ion channels for separating ions is arranged to receive ions from a respective one of the multiple separate ion channels of the ion distribution region.

19. The device of any one of claims 15-18, configured to vary voltages that are applied to the electrodes in the ion distribution region so as to alternate between an ion accumulation mode in which ions are trapped in the ion distribution region and an ionejection mode in which ions are urged from the ion distribution region into the plurality of ion channels.

20. The device of claim 19, configured to separate ions according to mass to charge ratio or mobility in the ion distribution region during the ion accumulation mode and / or the ion ejection mode;optionally wherein the ion separation device is configured to separate ions in, and elute ions from, the plurality of ion channels whilst the ion distribution region is being operated in the ion accumulation mode.

21. The device of any one of claims 15-20, wherein the magnitude of the electric field in the ion distribution region has a maximum value that is less than or equal to the minimum value of the magnitude of the electric field in the plurality of ion channels.

22. An ion separation device comprising:a plurality of plates, wherein each of the plates has an elongated aperture therethrough, wherein the plates are arranged so that the apertures are aligned to form an ion channel that extends through each of the plates, wherein first and second electrodes are provided at opposing ends of the elongated aperture, and third and fourth electrodes are provided on opposing sides of the aperture that extend between the opposing ends; andat least one voltage supply configured to:apply voltages to the first and second electrodes such that they are maintained at higher DC potentials than the third and fourth electrodes; andapply voltages to the electrodes on the plurality of plates so as to separate ions according to mass to charge ratio or mobility along the ion channel.

23. A mobility and / or mass spectrometer comprising:an ion separation device as claimed in any preceding claim; andan ion detector for detecting the separated ions, or ions derived therefrom, so as to determine their mobility and / or mass to charge ratio.

24. An ion filter for filtering ions according to mass to charge ratio or mobility, comprising:a first plurality of electrodes arranged for guiding ions along a first ion channel having a longitudinal axis therethrough;an ion entrance for introducing ions into said first ion channel in a direction orthogonal to the longitudinal axis;a first voltage supply configured to apply voltages to the first plurality of electrodes such that an electric potential repeatedly travels along the first ion channel for urging ions in a first direction along the first ion channel; anda second voltage supply configured to provide different voltages to said electrodes at different respective positions along the first ion channel so as to provide an electric field for urging ions in a second direction along the ion channel that is opposite to said first direction, and / or a first gas supply for providing a gas flow that urges ions in the second direction along the first ion channel;wherein the voltages applied by the first voltage supply, and the voltages applied by the second voltage supply and / or the gas flow, are such that ions in the first ion channel will be separated according to mass to charge ratio or mobility.

25. An ion filter for filtering ions according to mass to charge ratio or mobility, comprising:a first plurality of electrodes arranged for guiding ions in a first direction along a first ion channel having an ion entrance at a first end for receiving ions and an ion exit in a side thereof at its second end;a second plurality of electrodes arranged for guiding ions in a second direction opposite to the first direction along a second ion channel having an ion entrance in a side thereof that is adjacent to the ion exit of the first ion channel for receiving ions from the first ion channel;one or more first voltage supply configured to apply voltages to the electrodes of each of the first and second ion channels such that an electric potential repeatedly travels along each of the first and second ion channel for urging ions in one direction along each of those ion channels; andone or more second voltage supply configured to provide different voltages to the electrodes of each of the first and second ion channels at different respective positions along each of the first and second ion channels so as to provide an electric field for urging ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel, and / or one or more gas supply for providing one or more gas flow that urges ions in a direction that is opposite to the direction that the electric potential repeatedly travels along that ion channel;wherein the voltages applied by the one or more first voltage supply, and the voltages applied by the one or more second voltage supply and / or the one or more gas flows, are such that ions in the first and second ion channels will be separated according to mass to charge ratio or mobility.A

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