Ion separators

Abstract

A method of separating ions comprises providing a force (e.g. DC electric field or gas flow) on ions in an upstream direction through an ion separation device, whilst repeatedly travelling electrical

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2411346.6 filed on 1 August 2024. The entire content of this application is 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 in which both the space-charge capacity and separation resolution are high. SUMMARY From a first aspect the present invention provides a method of separating ions comprising: providing ions to an ion separation device; providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and then increasing the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region. As the maximum amplitude of the travelling electrical potentials that travel along the downstream region is higher than the maximum amplitude of the travelling electrical potentials that travel along the trapping region, the travelling electrical potentials are such that they provide a greater force on the ions in the downstream region than in the trapping region. When the amplitudes of the travelling electrical potentials have been increased, ions having mobilities that are lower than that of the first ions may remain trapped in the trapping region, or ions having mass to charge ratios that are higher than that of the first ions may remain trapped in the trapping region. Within the downstream region, the force on the ions in the upstream direction and the travelling electrical potentials may be such that ions are not trapped in the downstream region. The ion separation device comprises a plurality of electrodes arranged and configured to provide an ion guiding channel therethrough, and the method comprises applying voltages to the electrodes such that said electrical potentials repeatedly travel along the ion channel. The method may comprise transiently applying DC voltages to successive electrodes along the ion separation device in order to travel the electrical potentials downstream through the ion separation device. The DC potentials may be repeatedly travelled along the length of the trapping region and the downstream region. These travelling DC potentials urge the ions downstream as they pass the ions. Said providing the force in the upstream direction through the ion separation device may comprise applying different DC voltages to electrodes at different respective positions along the ion separation device so as to provide a DC electric field that urges ions in the upstream direction. The amplitude of any given travelling electrical potential at any given position along the ion separation device may be relative to the electrical potential due to the DC electric field at that position, wherein the travelling electrical potentials within the trapping region have amplitudes that are smaller than a first value, and the travelling electrical potentials within the downstream region have amplitudes that are equal to or larger than the first value. Optionally, the travelling electrical potentials within the trapping region all have the same amplitude, and the travelling electrical potentials within the downstream region all have the same amplitude. Alternatively, the travelling electrical potentials may have different amplitudes at different positions within the trapping region, all of which are smaller than the first value. Additionally, or alternatively, travelling electrical potentials may have different amplitudes at different positions within the downstream region, all of which are equal to or larger than the first value. The travelling electrical potentials may be travelled along the ion separation device by applying a voltage to each of a plurality of electrodes that are spaced along the ion separation device, where the magnitude of the voltage is varied with time so as to follow a voltage waveform, and wherein the voltages applied to different electrodes at different positions along the ion separation device are at different phases of the voltage waveform. The voltage waveform applied to each electrode may be superimposed on the DC voltage that is applied to that electrode in order to generate the DC electric field, e.g. by capacitive coupling. The voltage waveform has a peak-to-peak amplitude and may be superimposed in a manner so that the waveform is symmetrical about the DC voltage that is applied to that electrode in order to generate the DC electric field, i.e. such that half of the peak-to-peak amplitude is above the DC voltage and half of the peak-to-peak amplitude is below the DC voltage. Alternatively, the voltage waveform may be superimposed in a manner so that the waveform is asymmetrical about the DC voltage that is applied to that electrode in order to generate the DC electric field, e.g. such that a greater proportion of the peak-to-peak amplitude is above the DC voltage than is below the DC voltage. For example, the entire peak-to-peak amplitude may be above the DC voltage that is applied to that electrode in order to generate the DC electric field. The electrical potentials may be travelled along the ion separation device by applying different phases of a voltage waveform having a first peak-to-peak amplitude to different, spaced apart electrodes in the trapping region, and applying different phases of a voltage waveform having a second, larger peak-to-peak amplitude to different, spaced apart electrodes in the downstream region. The DC voltages may be applied such that the magnitude of the electric field increases as a function of position in the downstream direction along the trapping portion. The magnitude of the electric field may progressively increase, without decreasing, as a function of position in the downstream direction along the trapping portion. For example, the magnitude of the electric field may increase substantially linearly along the trapping portion. The magnitude of the electric field may be substantially constant along the downstream region. The voltages may be applied such that the magnitude of the electric field at any point along the downstream region is higher than the maximum magnitude of the electric field in the trapping region. The trapping region and downstream region may be adjacent each other and the magnitude of the electric field may step up at the boundary between the trapping region and downstream region. Said force in the upstream direction may not be varied whilst increasing the amplitudes of the electrical potentials. For example, the magnitudes of the electric field at all locations along the trapping region and / or downstream region may not be varied whilst increasing the amplitudes of the electrical potentials. Said providing the force in the upstream direction through the ion separation device may comprise flowing gas along the ion separation device for urging ions in the upstream direction. The gas flow may be used in combination with the electric field as to enhance the resolution of the ion separation device. Adding the gas flow in this manner increases the axial length of the ion separation device 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 separation device. 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 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. A method has been described in which the amplitudes of the electrical potentials are increased so as to urge first ions out of the trapping region and into, and through, the downstream region of the ion separation device. The amplitudes of the electrical potentials may be increased one or more further times so as to urge one or more groups, respectively, of ions of lower mobility, or higher mass to charge ratio, out of the trapping region and through the downstream region of the ion separation device. The amplitudes of the electrical potentials may be increased substantially continuously and progressively, or may be stepped in a discontinuous manner. Each time that the amplitudes of the electrical potentials are increased, the amplitudes of the electrical potentials passing through the downstream region are maintained higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region. Accordingly, the method may comprise further increasing the amplitudes of the travelling electrical potentials so as to urge second ions out of the trapping region and into, and through, the downstream region of the ion separation device; wherein this increasing of the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region. The second ions have mobilities that are lower than those of the first ions, or the second ions have mass to charge ratios that are higher than those of the first ions. When the amplitudes of the electrical potentials have been further increased, ions having mobilities that are lower than that of the second ions may remain trapped in the trapping region or ions having mass to charge ratios that are higher than that of the second ions may remain trapped in the trapping region. The first aspect of 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 first aspect of the present invention also provides a method of mobility and / or mass spectrometry comprising: performing a method as described above, wherein the amplitudes of the electrical potentials are increased one or more times during an elution period such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during the 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. The first aspect of the present invention also provides an ion separation device that is set up and configured to perform the method described above. Accordingly, the first aspect of the present invention also provides an ion separation device comprising: a plurality of electrodes; one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to: control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and then increase the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region. The control circuitry may be configured to control the one or more voltage supplies so as to apply different DC voltages to the electrodes at different respective positions along the ion separation device so as to provide a DC electric field that provides the force on the ions in the upstream direction; and / or the ion separation device may comprise a gas supply and the control circuitry is configured to control the gas supply so as to flow gas along the ion separation device so as to provide the force on the ions in the upstream direction. The first aspect of the present invention also provides a mobility or mass spectrometer comprising the ion separation device 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. The mass analyser may be arranged and configured to detect the separated ions, or ions derived therefrom, so as to determine their mass to charge ratios. The spectrometer may also be configured to 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 provides a mobility or mass spectrometer comprising an ion separation device as described above and a mass filter downstream of the ion separation device. The control circuitry may be configured to increase the amplitudes of the electrical potentials one or more times during an elution period such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during the elution period. The spectrometer may have control circuitry configured to 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 from the ion separation device. The mass filter may be a quadrupole mass filter. lons 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 second aspect the present invention provides a method of separating ions comprising: providing ions to an ion separation device; providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and then decreasing the speed that the electrical potentials travel in the downstream direction so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region. The method according to the second aspect of the present invention may have any of the features described above in relation to the first invention, except that the amplitudes of the travelling electrical potentials need not be varied in the manner described in relation to the first aspect. Rather than varying the amplitudes of the travelling electrical potentials, the speed of the electrical potentials is varied so as to achieve the same effects. For example, when the speed of the electrical potentials has been decreased, ions having mobilities that are lower than that of the first ions remain trapped in the trapping region, or ions having mass to charge ratios that are higher than that of the first ions remain trapped in the trapping region. Ions are not trapped in the downstream region by the force on the ions in the upstream direction and the travelling electrical potentials. The force in the upstream direction may be provided in the same manner as described in relation to the first aspect of the present invention. The ion separation device may comprise a plurality of electrodes that are spaced apart in the downstream direction and the method may comprise applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the spacing between each pair of adjacent electrodes in the trapping region is larger than the spacing between each pair of adjacent electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region. The ion separation device may comprise a plurality of electrodes that are spaced apart in the downstream direction and the method may comprise applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the electrodes in the trapping region have a greater thickness in the downstream direction than the electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region. Said applying voltages to the electrodes may comprise successively applying transient DC voltages to electrodes arranged successively in the downstream direction, optionally wherein the rate at which the voltages are applied to successive electrodes is substantially the same throughout the ion separation device. The ion separation device may comprise a plurality of electrodes that are spaced apart in the downstream direction such that the spacing between each pair of adjacent electrodes is substantially the same throughout the ion separation device, and the method may comprise successively applying transient DC voltages to electrodes arranged successively in the downstream direction so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the rate at which the voltages are applied to successive electrodes is lower in the downstream region than in the trapping region. The method according to the second aspect may comprise further decreasing the speed of the travelling electrical potentials so as to urge second ions out of the trapping region and into, and through, the downstream region of the ion separation device; wherein this decreasing of the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region. The second ions have mobilities that are lower than those of the first ions, or the second ions have mass to charge ratios that are higher than those of the first ions. When the speed of the electrical potentials has been further decreased, ions having mobilities that are lower than that of the second ions remain trapped in the trapping region or ions having mass to charge ratios that are higher than that of the second ions remain trapped in the trapping region. The second aspect of the present invention also provides an ion separation device that is set up and configured to perform the method described above. Accordingly, the present invention provides an ion separation device comprising: a plurality of electrodes; one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to: control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and to then decrease the speed of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region. The control circuitry may be configured to control the one or more voltage supplies so as to apply different DC voltages to the electrodes at different respective positions along the ion separation device so as to provide a DC electric field that provides the force on the ions in the upstream direction. Alternatively, or additionally, the ion separation device may comprise a gas supply and the control circuitry may be configured to control the gas supply so as to flow gas along the ion separation device so as to provide the force on the ions in the upstream direction. The second aspect of the present invention also provides a mobility or mass spectrometer comprising the ion separation device described above (in relation to the second aspect) 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. The mass analyser may be arranged and configured to detect the separated ions, or ions derived therefrom, so as to determine their mass to charge ratios. The spectrometer may also be configured to 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 provides a mobility or mass spectrometer comprising an ion separation device as described above and a mass filter downstream of the ion separation device. The control circuitry may be configured to decrease the speed of the electrical potentials one or more times during an elution period such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during the elution period. The spectrometer may have control circuitry configured to 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 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 a method of separating ions comprising: providing ions to an ion separation device that comprises a plurality of electrodes that define an ion guiding channel; providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and then urging ions out of the trapping region and into, and through, a downstream region of the ion separation device using the travelling electrical potentials; wherein the diameter of the ion guiding channel in the downstream region is smaller than the diameter of the ion guiding channel in the trapping region such that the travelling electrical potentials exert a larger downstream force on ions in the downstream region than in the trapping region. The method may comprise urging ions out of the trapping region by continuously varying or stepping the amplitude and / or speed of the travelling potentials, and / or the force in the upstream direction, so that ions having different mobilities or mass to charge ratios are urged out of the trapping region and elute from the ion separation device at different respective times. The method according to the third aspect of the present invention may have any of the features described above in relation to the first aspect of the invention, except that the amplitudes of the travelling electrical potentials need not be varied in the manner described in relation to the first aspect. For example, ions are not trapped in the downstream region by the force on the ions in the upstream direction and the travelling electrical potentials. The force in the upstream direction may be provided in the same manner as described in relation to the first aspect of the present invention. For example, said providing the force in the upstream direction through the ion separation device may comprise applying different DC voltages to electrodes at different respective positions along the ion separation device so as to provide a DC electric field that urges ions in the upstream direction; and the trapping region and downstream region may be adjacent each other and the magnitude of the electric field steps up at the boundary between the trapping region and downstream region. The ion guiding channel along substantially the whole length of the downstream region may have a smaller diameter than the ion guiding channel along the trapping region. The third aspect of the present invention also provides an ion separation device that is set up and configured to perform the method described above. Accordingly, the present invention provides an ion separation device comprising: a plurality of electrodes that define an ion guiding channel; one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to: control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and to then urge ions out of the trapping region and into, and through, a downstream region of the ion separation device using the travelling electrical potentials; wherein the diameter of the ion guiding channel in the downstream region is smaller than the diameter of the ion guiding channel in the trapping region such that the travelling electrical potentials exert a larger downstream force on ions in the downstream region than in the trapping region. The control circuitry may be configured to control the one or more voltage supplies so as to apply different DC voltages to the electrodes at different respective positions along the ion separation device so as to provide a DC electric field that provides the force on the ions in the upstream direction. Alternatively, or additionally, the ion separation device may comprise a gas supply and the control circuitry may be configured to control the gas supply so as to flow gas along the ion separation device so as to provide the force on the ions in the upstream direction. The third aspect of the present invention also provides a mobility or mass spectrometer comprising the ion separation device described above (in relation to the third aspect) 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. The mass analyser may be arranged and configured to detect the separated ions, or ions derived therefrom, so as to determine their mass to charge ratios. The spectrometer may also be configured to 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 provides a mobility or mass spectrometer comprising an ion separation device as described above and a mass filter downstream of the ion separation device. The control circuitry may be configured to vary the amplitude and / or speed of the travelling potentials, and / or the force in the upstream direction, during an elution period so that ions having different mobilities or mass to charge ratios are urged out of the trapping region and elute from the ion separation device at different respective times during the elution period. The spectrometer may have control circuitry configured to 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 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. 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-2B illustrate how the electric field profile along a known ion mobility separator is varied so as to cause ions to elute; Figs. 3A-3B illustrate how the electric field profile along an ion separation device according to an embodiment of the present invention is varied so as to cause ions to elute; Figs. 4A-4B show schematics of the electrode structures of ion separation devices according to embodiments of the invention in which the diameter of the ion channel through the devices vary, Fig. 4C shows a schematic of the electrode structure of an ion separation device according to an embodiment of the invention in which the spacing between the electrodes of the device varies, and Fig. 4D shows a schematic of the electrode structure of an ion separation device according to an embodiment of the invention in which the thickness of the electrodes in the axial direction varies; Figs. 5A-5B illustrate an ion separation device according to an embodiment of the present invention in which multiple apertures are provided in each plate to define a plurality of ion guiding channels; Fig. 6 illustrates an ion separation device according to an embodiment of the present invention in which a single aperture is provided in each plate having a configuration that defines a plurality of ion guiding channels; Fig. 7A shows the same embodiment as Fig. 5B except also having an ion funnel, whereas Figs. 7B and 7C show examples of plates that may be used to form the ion funnel; Figs. 8A-8D show an embodiment of an ion separation device having an ion distribution region at the upstream end, whereas Fig. 8E shows a schematic of an embodiment comprising three ion distribution regions; and Fig. 9 shows another embodiment of an ion separation device, which has a single, relatively wide ion channel therethrough. DETAILED DESCRIPTION Figs. 1A-1C show views relating to 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, as will be described in more detail with reference to Figs. 2A-2B. Fig. 2A illustrates an example of the electric field profile along the IMS device during an ion accumulation phase during which ions are accumulated in the IMS device. As can be seen, the IMS device has a trapping region 14 within which the magnitude of the electric field increases as a function of position in the downstream direction. 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 are driven to elute from the IMS device. Fig. 2A also illustrates the travelling DC potentials 16 that move along the IMS device in the downstream direction, through the trapping region and the analytical region. Fig. 2A also shows that ions 17 of different mobility are trapped at different equilibrium positions along the trapping region, the different mobilities being illustrated by the different sized circles. Once the ions have been separated according to mobility within the IMS 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 will be described with reference to Fig. 2B. Fig. 2B shows the same example as in Fig. 2A, except wherein the magnitude of the electric field at each point along the trapping region 14 and analytical region 15 has been reduced. The travelling DC potentials 16 are then able to urge the ions further downstream along the IMS device 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 IMS device. At a later time the magnitude of the electric field at each point along the IMS device is reduced further such that second ions having a lower mobility than the first ions are urged out of the trapping region and into the analytical region, whereas ions of lower mobility than the second ions 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 IMS device. The magnitude of the electric field at each point along the IMS device is progressively reduced such that ions having progressively lower mobilities elute from the downstream end of the IMS device. As an alternative to progressively reducing the magnitude of the electric field at each point along the IMS device in order to cause ions having progressively lower mobilities to elute, it is known to progressively increase the amplitude of the travelling DC potentials 16 that travel through the trapping and analytical regions in order to cause ions having progressively lower mobilities to elute. Although this can be advantageous over varying the electric field, it has been recognised that increasing the amplitude of the travelling DC potentials not only increases the force on the ions in the downstream direction but it can also result in the force on the ions in the orthogonal, radial direction being increased. Although RF voltages are used to radially confine the ions in the IMS device, the increased force in the radial direction can cause ions to overcome the radial confinement and be lost. It has been recognised that this is a particular problem in the trapping region, as it reduces the charge density of the ions that can be accommodated in this region before ion losses become significant. Also, such higher amplitude travelling DC potentials can cause increased heating of the trapped ions, resulting in a relatively higher probability of dissociation of the ions in the trapping region, which may be undesirable. However, if the amplitude of the travelling DC potentials travelling throughout the IMS device is maintained relatively low so as to avoid the above problems, then the mobility resolution that the IMS device is able to achieve is relatively low. It has been recognised that as the mobility resolution is dominated by the conditions in the analytical region 15 and is less dependent on the conditions in the trapping region 14, whereas the charge capacity of the device is dominated by the conditions and residence time within the trapping region 14, the problems described above can be overcome by travelling the DC potentials through the trapping region 14 whilst having relatively low amplitudes and travelling the DC potentials through the analytical region whilst having higher amplitudes. This enables a relatively high charge capacity and low losses in the trapping region, and a relatively high mobility resolution for ions eluting from the device. Under these conditions the trapping region 14 is operated with a relatively low, maximum DC electric field magnitude, and the amplitudes of the travelling DC potentials in this region are relatively low. The analytical region is operated with a relatively higher maximum DC electric field magnitude and the amplitude of the travelling DC potentials in this region are relatively higher. Figs. 3A-3B illustrate an example of the electric field profile along an ion separation device according to an embodiment of the present invention. Embodiments will now be described in which the ion separation device separates ions according to their mobilities, although the ion separation may be configured and controlled to separate ions according to their mass to charge ratios, as will be discussed further below. The ion separation device may have an electrode configuration as has been described in relation to Figs. 1A-1C, although other configurations are also contemplated. As can be seen from Figs. 3A-3B, the ion separation device has a trapping region 14 within which the magnitude of the electric field increases as a function of position along the ion separation device in the downstream direction. Although the magnitude of the electric field in the trapping region 14 is shown as increasing linearly, the magnitude of the field may increase non-linearly. At the end of the trapping region there is an analytical region 15, along which the magnitude of the electric field is substantially constant. The magnitude of the electric field in the analytical region is higher than at any point in the trapping region. The electric field may step up at the boundary between trapping region 14 and the adjacent analytical region 15. Fig. 3A also illustrates the travelling DC potentials 16 that move along the ion separation device in the downstream direction, through the trapping region and also through the analytical region. The amplitudes of the travelling DC potentials that travel along the trapping region 14 may be lower than the amplitudes of the travelling DC potentials that travel along the analytical region 15 at the same time. Fig. 3A also shows that ions 17 of different mobility are trapped at different equilibrium positions along the ion separation device, the different mobilities being illustrated by the differently 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 increasing the amplitude of the travelling DC potentials, as shown in Fig. 3B. Fig. 3B shows the same example as in Fig. 3A, except wherein the amplitude of the travelling DC potentials that travel along the trapping region 14 has been increased, and the amplitude of the travelling DC potentials that travel along the analytical region 15 has been increased, wherein the amplitude of the travelling DC potentials that travel along the analytical region 15 is higher than the amplitude at which the travelling DC potentials travel along the trapping region 14. The travelling DC potentials 16 are then able to urge the ions in the trapping region further downstream along the ion separation device 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 separation device. At a later time (not shown) the amplitudes of the travelling DC potentials are increased further and in a manner such that the amplitude of the travelling DC potentials that travel along the analytical region 15 is higher than the amplitude at which the travelling DC potentials travel along the trapping region 14. Second ions having a lower mobility than the first ions are urged out of the trapping region and into the analytical region, whereas ions of lower mobility than the second ions 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 separation device. The amplitudes of the travelling DC potentials in both the trapping and analytical regions may be progressively increased such that ions having progressively lower mobility elute from the downstream end of the ion separation device. At any point in time, the maximum amplitudes of the travelling DC potentials that travel along the analytical region 15 may be higher than the maximum amplitudes of the travelling DC potentials that travel along the trapping region 14. In addition, at any point in time, the maximum amplitude of the DC electric field in the analytical region may remain higher than the maximum amplitude of the DC electric field in the trapping region. As described above, the resolution of the device may be dominated by the ion separation conditions in the analytical region 15 and is less dependent on the conditions in the trapping region 14. However, the charge capacity of the device is dominated by the conditions, and ion residence time, within the trapping region 14. As the amplitudes of the travelling DC potentials that travel along the trapping region 14 are relatively low, the ions in the trapping region experience a relatively low force in the radial direction due to the travelling DC potentials, which allows a relatively high amount of charge (i.e. a high number of ions) to be accumulated before ion losses become significant. Additionally, such relatively low amplitude travelling DC potentials cause less heating of the trapped ions, resulting in a relatively low probability of dissociation of the ions in the trapping region 14. On the other hand, as the amplitudes of the travelling DC potentials that travel along the analytical region are relatively high, the ion separation device is provided with a relatively high resolution. Embodiments of the present invention may therefore operate the trapping region 14 with a relatively low maximum DC electric field magnitude and relatively low amplitude travelling DC potentials, and operate the analytical region 15 with a higher maximum electric field magnitude and higher amplitude travelling DC potentials. The magnitude of the maximum amplitude of any given travelling DC potential described herein at any given location may be the maximum amplitude relative to the background electrical potential at that position, e.g. relative to the electrical potential due to the DC electric field in this embodiment. In each of Figs. 3A-3B, the magnitude of the DC electric field is shown as a function of position along the ion separation device, whereas the maximum amplitudes of the travelling DC potentials are illustrated (rather than the electric field due to the traveling DC potentials). The magnitude of the DC electric field increases along the trapping region, but the maximum amplitudes of the travelling DC potentials (relative to the potentials of the underlying DC electric field) are the same at all points along the trapping region. Similarly, the magnitude of the electric field is constant along the analytical region, and the maximum amplitudes of the travelling DC potentials (relative to the potentials of the underlying DC electric field) are the same at all points along the analytical region. The travelling DC potentials may be travelled along the ion separation device by applying a voltage to each of a plurality of electrodes that are spaced along the ion separation device, where the magnitude of the voltage is varied with time so as to follow a voltage waveform (e.g. such as the waveform shown in Fig. 1B), and wherein the voltages applied to different electrodes at different positions along the ion separation device are at different phases of the voltage waveform. The voltage waveform applied to each electrode may be superimposed on the DC voltage that is applied to that electrode in order to generate the DC electric field, e.g. by capacitive coupling. The voltage waveform has a peak-to-peak amplitude and may be superimposed in a manner so that the waveform is symmetrical about the DC voltage that is applied to that electrode in order to generate the DC electric field, i.e. such that half of the peak-to-peak amplitude is above the DC voltage and half of the peak-to-peak amplitude is below the DC voltage. Alternatively, the voltage waveform may be superimposed in a manner so that the waveform is asymmetrical about the DC voltage that is applied to that electrode in order to generate the DC electric field, e.g. such that a greater proportion of the peak-to-peak amplitude is above the DC voltage than is below the DC voltage. For example, the entire peak-to-peak amplitude may be above the DC voltage that is applied to that electrode in order to generate the DC electric field. As described above, at any given time, the maximum amplitude of the travelling DC potentials that travel along the analytical region is higher than the maximum amplitude of the travelling DC potentials that travel along the trapping region, such that the travelling DC potentials in the analytical region provide a greater force on the ions in the downstream direction than the travelling DC potentials in the trapping region do. This may be achieved by applying different phases of a voltage waveform having a first peak-to-peak amplitude to different, spaced apart electrodes in the trapping region, and applying different phases of a voltage waveform having a second, larger peak-to-peak amplitude to different, spaced apart electrodes in the analytical region. Preferably, the frequency of the voltage waveform applied to the electrodes in the trapping region is the same as the frequency of the voltage waveform applied to the electrodes in the downstream region. These voltages may be applied to the different electrodes with different phases such that the travelling DC potentials travel continuously along both the trapping and analytical regions, differing between the trapping region and analytical region only in amplitude. Embodiments have been described in which the amplitudes of the travelling DC potentials travelling through the trapping and analytical regions are increased in order to elute ions. Additionally, or alternatively, the speed at which the travelling DC potentials travel through the trapping and analytical regions may be decreased in order to elute ions. In such embodiments, ions are trapped in the trapping region as has been discussed above, i.e. using the opposing forces due to the travelling DC potentials and the electric field. The speed at which the travelling DC potentials travel along the trapping region 14 is then decreased, and the speed at which the travelling DC potentials that travel along the analytical region 15 is decreased, wherein the speed of the travelling DC potentials that travel along the analytical region 15 is lower than the speed at which the travelling DC potentials travel along the trapping region 14. The travelling DC potentials 16 are then able to urge the ions in the trapping region further downstream along the ion separation device 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 separation device. At a later time the speed at which the travelling DC potentials travel along the trapping region 14 is decreased further and the speed at which the travelling DC potentials travel along the analytical region 15 is decreased further, wherein the speed of the travelling DC potentials that travel along the analytical region 15 is lower than the speed at which the travelling DC potentials travel along the trapping region 14. This causes second ions having a lower mobility than the first ions to be urged out of the trapping region and into the analytical region, whereas ions of lower mobility than the second ions 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 separation device. The speed of the travelling DC potentials in both the trapping and analytical regions may be progressively decreased such that ions having progressively lower mobility elute from the downstream end of the ion separation device. At any point in time, the speed of the travelling DC potentials that travel along the analytical region 15 may be lower than the speed at which the travelling DC potentials travel along the trapping region 14. As such, this method may provide the advantages described above. The above embodiments enable the travelling DC potentials to provide a higher driving force downstream through the analytical region than through the trapping region by using higher amplitude and / or lower speed travelling DC potentials in the analytical region 15 than in the trapping region 14. However, alternatively, or additionally, the driving force downstream through the analytical region may be made higher than through the trapping region by providing these regions with different electrode structures, as will be described with reference to Figs. 4A-4D. Fig. 4A shows a schematic of the electrode structure of an ion separation device according to an embodiment of the invention in which the diameter of the ion channel through the ion separation device varies. The ion separation device may be formed from a stack of plate electrodes that are stacked together in the axial direction. Each plate electrode may have an aperture therethrough, and the plates may be arranged such that the apertures are aligned so as to form the ion channel through the ion separation device. The apertures that are in the electrodes located in the trapping region 14 may be larger than the apertures that are in the electrodes located in the analytical region 15. Travelling DC potentials and a DC electric field are used to separate ions, as has been described above. However, as the apertures in the trapping region are larger than the apertures in the analytical region, the force on the ions at the central axis of the ion separation device due to a travelling DC potential generated by transiently applying the same amplitude voltage to successive electrodes will be smaller in the trapping region than in the analytical region. As such, this geometry may provide the advantages described above. Although the ion separation device has been described as being formed from a stack of apertured electrodes, it is contemplated that one or more different ion guide structures may be used instead, such as a segmented multipole (e.g. quadrupole) ion guide. In such embodiments the inscribed radius of the multipole in the trapping region is larger than the inscribed radius of the multipole in the analytical region. Fig. 4B shows an embodiment that is the same as that shown and described in relation to Fig. 4A, except that rather than the diameter of the apertures (or inscribed radius) stepping abruptly at the boundary between the trapping region and the analytical region, the diameter of the apertures (or inscribed radius) changes progressively along the ion separation device. For example, the diameter (or inscribed radius) may step down multiple times along the ion separation device. Although Fig. 4B illustrates this stepping down occurring only in the analytical region, it may instead occur only in the trapping region, or in both the trapping and analytical regions. Fig. 4C shows a schematic of the electrode structure of an ion separation device according to an embodiment of the invention in which the spacing between the electrodes of the ion separation device varies. As described above, the ion separation device may be formed from a stack of plate electrodes that are stacked together in the axial direction. Axially adjacent plate electrodes in the trapping region 14 may be spaced apart by a greater amount than the axially adjacent plate electrodes in the analytical region 15. All pairs of axially adjacent plate electrodes in the trapping region may be spaced apart by the same amount and / or all pairs of axially adjacent plate electrodes in the analytical region may be spaced apart by the same amount. Alternatively, the spacing between adjacent pairs of electrodes may decrease progressively as a function of position in the downstream direction along the ion separation device. Travelling DC potentials and a DC electric field are used to separate ions, as has been described above. The DC voltage that is successively applied to axially successive electrodes so as to form the travelling DC potentials may be applied to the electrodes in both the trapping and analytical regions at a constant rate. However, as the spacing between electrodes is lower in the analytical region than in the trapping region, the speed at which the travelling DC potentials travel along the analytical region is slower than the speed at which the travelling DC potentials travel along the trapping region. As such, this geometry may provide the advantages described above. Although the ion separation device has been described as being formed from a stack of apertured electrodes, it may instead be a segmented multipole (e.g. quadrupole) ion guide. In such embodiments the gaps between the electrodes are gaps between axial segments of the ion guide. Fig. 4D shows a schematic of the electrode structure of an ion separation device according to an embodiment of the invention in which the thickness of the electrodes in the axial direction varies. All of the plate electrodes in the trapping region 14 may have a first thickness, whereas all of the plate electrodes in the analytical region 15 may have a second, smaller thickness. Alternatively, the thickness of the electrodes may decrease progressively as a function of position in the downstream direction along the ion separation device. Travelling DC potentials and a DC electric field are used to separate ions, as has been described above. The DC voltage that is successively applied to axially successive electrodes so as to form the travelling DC potentials may be applied to the electrodes in both the trapping and analytical regions at a constant rate. However, as the thickness of the electrodes is lower in the analytical region than in the trapping region, the speed at which the travelling DC potentials travel along the analytical region is slower than the speed at which the travelling DC potentials travel along the trapping region. As such, this geometry may provide the advantages described above. Again, although the ion separation device has been described as being formed from a stack of apertured electrodes, it may instead be a segmented multipole (e.g. quadrupole) ion guide. In such embodiments the thicknesses of the electrodes correspond to the lengths of the axial segments of the ion guide. It will be appreciated that embodiments are envisaged in which thicknesses of the electrodes may be different in the trapping and analytical regions as well as the diameters of the apertures being different in these different regions and / or the gaps between electrodes being different in these different regions. Although the above embodiments have been described as separating the ions by mobility, they may instead be controlled so as to separate the ions by mass to charge ratio, as will be described in more detail further below. Although embodiments that separate ions in a single ion channel have been described, the ion separation device may have multiple ion channels that separate ions. Figs. 5A-5B 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. 5B shows a view of the ion separation device of Fig. 5A 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. 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 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 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. 6. Fig. 6 shows an ion separation device according to another embodiment. The ion separation device may have the same construction and methods of operation as the embodiments described in relation to Figs. 5A-5B, 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. 7A. Fig. 7A shows the same embodiment as Fig. 5B, 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. 6 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. 7B shows an example of three plates 26 of the ion funnel 24. The uppermost plate 26a in Fig. 7B has a relatively large aperture therein and is arranged relatively upstream in the ion funnel 24. The central plate 26b in Fig. 7B 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. 7B 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. 7B 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. 7. Fig. 7C shows an embodiment that is the same as that shown and described in relation to Fig. 7B, 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. 7B. 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 30,32 have 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. 7B and also plates 26 that have the electrode arrangement described in relation to Fig. 7C. 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. 7C, 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. 7B. 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. 8A. Fig. 8A 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. 7A-7C, 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. 8C 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. 5A). The first, uppermost plate 46a in Fig. 8C 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. 8A. The second plate 46b in Fig. 8C 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. 8C 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. 8A. As mentioned above, the fourth plate 46d in Fig. 8C represents the upstream end of the ion channels 13 (i.e. the upstream end of the device shown in Fig. 5A). 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 41 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. 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. 8B 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. 8B 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. 8B, 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. 8B. 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. 8D, 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. 8D. 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 41 before 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 travelling DC potentials in the ion channels. 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. 8A-8D, 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. 8E. Fig. 8E 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. 9. Fig. 9 shows an embodiment of an ion separation device that is the same as those described above, such as in relation to Fig. 5A, 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. 7C. 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. 7B and 7C. 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. 9, 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. 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, in the ion separation devices described above, ions have been described as being separated and eluting from the device according to mobility. However, it is alternatively contemplated that ions may be separated in and eluted from the ion separation device 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 device according to mobility and a second mode in which ions are separated in and eluted from the device 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, in embodiments that include a plurality of ion channels, 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 embodiments have been described above in which the ion distribution region of the ion separation device separates and elutes 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 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 travelling DC 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 travelling DC 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. 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, 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 according to 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 according to 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 scanned or 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. Embodiments have been described 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 so as 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.

Claims

1. A method of separating ions comprising: providing ions to an ion separation device; providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thenincreasing the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

2. The method of claim 1, wherein when the amplitudes of the travelling electrical potentials have been increased, ions having mobilities that are lower than that of the first ions remain trapped in the trapping region, or ions having mass to charge ratios that are higher than that of the first ions remain trapped in the trapping region.

3. The method of claim 1 or 2, wherein, within the downstream region, the force on the ions in the upstream direction and the travelling electrical potentials are such that ions are not trapped in the downstream region.

4. The method of claim 1,2 or 3, wherein said providing the force in the upstream direction through the ion separation device comprises applying different DC voltages to electrodes at different respective positions along the ion separation device so as to provide a DC electric field that urges ions in the upstream direction.

5. The method of claim 4, wherein the amplitude of any given travelling electrical potential at any given position along the ion separation device is relative to the electrical potential due to the DC electric field at that position, wherein the travelling electrical potentials within the trapping region have amplitudes that are smaller than a first value, and the travelling electrical potentials within the downstream region have amplitudes that are equal to or larger than the first value;optionally wherein the travelling electrical potentials within the trapping region all have the same amplitude, and wherein the travelling electrical potentials within the downstream region all have the same amplitude.

6. The method of claim 4, wherein the electrical potentials are travelled along the ion separation device by applying different phases of a voltage waveform having a first peak-to-peak amplitude to different, spaced apart electrodes in the trapping region, and applying different phases of a voltage waveform having a second, larger peak-to-peak amplitude to different, spaced apart electrodes in the downstream region.

7. The method of claim 4, 5 or 6, wherein the DC voltages are applied such that the magnitude of the electric field increases as a function of position in the downstream direction along the trapping portion.

8. The method of any one of claims 4-7, wherein the magnitude of the electric field is substantially constant along the downstream region.

9. The method of any one of claims 4-8, wherein the trapping region and downstream region are adjacent each other and the magnitude of the electric field steps up at the boundary between the trapping region and downstream region.

10. The method of any preceding claim, wherein said providing the force in the upstream direction through the ion separation device comprises flowing gas along the ion separation device for urging ions in the upstream direction.

11. The method of any preceding claim, comprising further increasing the amplitudes of the travelling electrical potentials so as to urge second ions out of the trapping region and into, and through, the downstream region of the ion separation device; wherein this increasing of the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

12. The method of claim 11, wherein when the amplitudes of the electrical potentials have been further increased, ions having mobilities that are lower than that of the second ions remain trapped in the trapping region or ions having mass to charge ratios that are higher than that of the second ions remain trapped in the trapping region.

13. A method of mobility and / or mass spectrometry comprising:performing a method as claimed in any preceding claim, wherein the amplitudes of the electrical potentials are increased one or more times during an elution period such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during the elution period; andcontrolling 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 filterreceives ions having different mass to charge ratios or mobilities from the ion separation device.

14. An ion separation device comprising:a plurality of electrodes;one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to:control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thenincrease the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

15. The ion separation device of claim 14, wherein the control circuitry is configured to control the one or more voltage supplies so as to apply different DC voltages to the electrodes at different respective positions along the ion separation device so as to provide a DC electric field that provides the force on the ions in the upstream direction; and / or wherein the ion separation device comprises a gas supply and the control circuitry is configured to control the gas supply so as to flow gas along the ion separation device so as to provide the force on the ions in the upstream direction.

16. A method of separating ions comprising:providing ions to an ion separation device;providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thendecreasing the speed that the electrical potentials travel in the downstream direction so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region.

17. The method of claim 16, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction and the method comprises applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the spacing between each pair of adjacent electrodes in the trapping region is larger than the spacing between each pair of adjacent electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region.

18. The method of claim 16 or 17, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction and the method comprises applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the electrodes in the trapping region have a greater thickness in the downstream direction than the electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region.

19. The method of any one of claims 17 or 18, wherein said applying voltages to the electrodes comprises successively applying transient DC voltages to electrodes arranged successively in the downstream direction, and optionally wherein the rate at which the voltages are applied to successive electrodes is substantially the same throughout the ion separation device.

20. The method of claim 16, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction such that the spacing between each pair of adjacent electrodes is substantially the same throughout the ion separation device, wherein the method comprises successively applying transient DC voltages to electrodes arranged successively in the downstream direction so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the rate at which the voltages are applied to successive electrodes is lower in the downstream region than in the trapping region.

21. An ion separation device comprising:a plurality of electrodes;one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to:control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions ofdifferent mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and to thendecrease the speed of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region.

22. A method of separating ions comprising:providing ions to an ion separation device that comprises a plurality of electrodes that define an ion guiding channel;providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thenurging ions out of the trapping region and into, and through, a downstream region of the ion separation device using the travelling electrical potentials;wherein the diameter of the ion guiding channel in the downstream region is smaller than the diameter of the ion guiding channel in the trapping region such that the travelling electrical potentials exert a larger downstream force on ions in the downstream region than in the trapping region.

23. The method of claim 22, wherein said providing the force in the upstream direction through the ion separation device comprises applying different DC voltages to electrodes at different respective positions along the ion separation device so as to provide a DC electric field that urges ions in the upstream direction; and wherein the trapping region and downstream region are adjacent each other and the magnitude of the electric field steps up at the boundary between the trapping region and downstream region.

24. The method of claim 22 or 23, wherein the ion guiding channel along substantially the whole length of the downstream region has a smaller diameter than the ion guiding channel along the trapping region.

25. An ion separation device comprising:a plurality of electrodes that define an ion guiding channel;one or more voltage supplies for applying voltages to the electrodes; andcontrol circuitry configured to:control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions ofAmendments to the claims have been made as follows:Claims:5 1. A method of separating ions comprising:providing ions to an ion separation device;providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios10 become trapped at different respective axial positions along a trapping region of the ion separation device; and thenincreasing the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of15 the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

2. The method of claim 1, wherein when the amplitudes of the travelling electrical potentials have been increased, ions having mobilities that are lower than that of the first 20 ions remain trapped in the trapping region, or ions having mass to charge ratios that are higher than that of the first ions remain trapped in the trapping region.

3. The method of claim 1 or 2, wherein, within the downstream region, the force on the ions in the upstream direction and the travelling electrical potentials are such that ions are 25 not trapped in the downstream region.

4. The method of claim 1,2 or 3, wherein said providing the force in the upstream direction through the ion separation device comprises applying different DC voltages to electrodes at different respective positions along the ion separation device so as to provide 30 a DC electric field that urges ions in the upstream direction.

5. The method of claim 4, wherein the amplitude of any given travelling electrical potential at any given position along the ion separation device is relative to the electrical potential due to the DC electric field at that position, wherein the travelling electrical 35 potentials within the trapping region have amplitudes that are smaller than a first value, and the travelling electrical potentials within the downstream region have amplitudes that are equal to or larger than the first value;optionally wherein the travelling electrical potentials within the trapping region all have the same amplitude, and wherein the travelling electrical potentials within the 40 downstream region all have the same amplitude.16 02 266. The method of claim 4, wherein the electrical potentials are travelled along the ion separation device by applying different phases of a voltage waveform having a first peak-to-peak amplitude to different, spaced apart electrodes in the trapping region, and applying different phases of a voltage waveform having a second, larger peak-to-peak amplitude to different, spaced apart electrodes in the downstream region.

7. The method of claim 4, 5 or 6, wherein the DC voltages are applied such that the magnitude of the electric field increases as a function of position in the downstream direction along the trapping portion.

8. The method of any one of claims 4-7, wherein the magnitude of the electric field is substantially constant along the downstream region.

9. The method of any one of claims 4-8, wherein the trapping region and downstream region are adjacent each other and the magnitude of the electric field steps up at the boundary between the trapping region and downstream region.

10. The method of any preceding claim, wherein said providing the force in the upstream direction through the ion separation device comprises flowing gas along the ion separation device for urging ions in the upstream direction.

11. The method of any preceding claim, comprising further increasing the amplitudes of the travelling electrical potentials so as to urge second ions out of the trapping region and into, and through, the downstream region of the ion separation device; wherein this increasing of the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

12. The method of claim 11, wherein when the amplitudes of the electrical potentials have been further increased, ions having mobilities that are lower than that of the second ions remain trapped in the trapping region or ions having mass to charge ratios that are higher than that of the second ions remain trapped in the trapping region.

13. A method of mobility and / or mass spectrometry comprising:performing a method as claimed in any preceding claim, wherein the amplitudes of the electrical potentials are increased one or more times during an elution period such that ions having different mass to charge ratios or mobilities elute from the ion separation device at different respective times during the elution period; andcontrolling 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 filterreceives ions having different mass to charge ratios or mobilities from the ion separation device.

14. An ion separation device comprising:a plurality of electrodes;one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to:control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thenincrease the amplitudes of the electrical potentials so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said increasing the amplitudes is performed such that the amplitudes of the electrical potentials passing through the downstream region are higher than the amplitudes of the electrical potentials simultaneously passing through the trapping region.

15. The ion separation device of claim 14, wherein the control circuitry is configured to control the one or more voltage supplies so as to apply different DC voltages to the electrodes at different respective positions along the ion separation device so as to provide a DC electric field that provides the force on the ions in the upstream direction; and / or wherein the ion separation device comprises a gas supply and the control circuitry is configured to control the gas supply so as to flow gas along the ion separation device so as to provide the force on the ions in the upstream direction.

16. A method of separating ions comprising:providing ions to an ion separation device;providing a force on the ions in an upstream direction through the ion separation device whilst repeatedly travelling electrical potentials downstream through the ion separation device, such that ions of different mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and thendecreasing the speed that the electrical potentials travel in the downstream direction so as to urge first ions out of the trapping region and into, and through, a downstream region of the ion separation device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region.

17. The method of claim 16, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction and the method comprises applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the spacing between each pair of adjacent electrodes in the trapping region is larger than the spacing between each pair of adjacent electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region.

18. The method of claim 16 or 17, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction and the method comprises applying voltages to the electrodes so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the electrodes in the trapping region have a greater thickness in the downstream direction than the electrodes in the downstream region such that when the voltages are applied to the electrodes the travelling electrical potentials travel slower along the downstream region than along the trapping region.

19. The method of any one of claims 17 or 18, wherein said applying voltages to the electrodes comprises successively applying transient DC voltages to electrodes arranged successively in the downstream direction, and optionally wherein the rate at which the voltages are applied to successive electrodes is substantially the same throughout the ion separation device.

20. The method of claim 16, wherein the ion separation device comprises a plurality of electrodes that are spaced apart in the downstream direction such that the spacing between each pair of adjacent electrodes is substantially the same throughout the ion separation device, wherein the method comprises successively applying transient DC voltages to electrodes arranged successively in the downstream direction so as to repeatedly travel the electrical potentials downstream through the ion separation device, wherein the rate at which the voltages are applied to successive electrodes is lower in the downstream region than in the trapping region.

21. An ion separation device comprising:a plurality of electrodes;one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to:control the ion separation device so as to provide a force on the ions in an upstream direction through the ion separation device, whilst controlling the one or more voltage supplies so as to apply voltages to the electrodes so as to repeatedly travel electrical potentials downstream through the ion separation device, such that ions ofdifferent mobilities or different mass to charge ratios become trapped at different respective axial positions along a trapping region of the ion separation device; and to thendecrease the speed of the electrical potentials so as to urge first ions out ofthe trapping region and into, and through, a downstream region of the ion separation5 device; wherein said decreasing the speed is performed such that the speed of the electrical potentials passing through the downstream region is lower than the speed of the electrical potentials simultaneously passing through the trapping region.CXICXIA

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