Ion separators

Abstract

A method of separating ions comprising providing ions to an ion separation instrument having an ion attenuation region 22 arranged between first and second ion separator regions 20,24. The first ion separator region 20 is operated in an ion accumulation mode during which opposing forces are applied to the ions such that ions are caused to separate such that ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion attenuation region 22. The ion attenuation region 22 is operated in an attenuation mode, whilst the first ion separator region 20 is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

Description

[0001] 175441 -02v1 (M-4741)

[0002] ION SEPARATORS

[0003] CROSS-REFERENCE TO RELATED APPLICATION

[0004] This application claims priority from and the benefit of United Kingdom patent application No. 2418769.2 filed on 19 December 2024. The entire contents of this application are incorporated herein by reference.

[0005] FIELD OF THE INVENTION

[0006] The present invention relates generally to techniques for separating ions according to a physicochemical property such as ion mobility or mass to charge ratio.

[0007] BACKGROUND

[0008] An ion mobility separator (IMS) is a known device for separating ions according to their mobility through a gas. 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 by providing 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.

[0009] SUMMARY

[0010] From a first aspect the present invention provides a method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions, a second ion separator region for separating ions, and an ion attenuation region arranged between the first and second ion separator regions; operating the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion attenuation region; and operating the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

[0011] As the present invention urges the ions having mobilities above, or mass to charge ratios below, the first threshold value out of the first ion separator region and into the ion attenuation region so as to be attenuated, these ions do not contribute to space-charge effects in the first ion separator region during the ion accumulation mode. As these ions may be relatively abundant, their removal according to the present invention prevents them from building up in the first ion separator region and inhibiting the passage of other ions into and through the first ion separator region.

[0012] As the attenuation region is downstream of the first ion separator region, ions may be attenuated by, for example, radial ejection, at any time and without affecting the passage of ions into and through the first ion separator region. As the attenuation region is upstream of the second ion separator region, ions are able to be attenuated at any time and without having to pass them through the second ion separator region, which may be undesirable in at least some of its modes of operation.

[0013] Said opposing forces are applied to the ions during the ion accumulation mode such that the ions are separated according to mobility or mass to charge ratio, where ions having relatively high mobilities, or relatively low mass to charge ratios, are urged to move further downstream than ions having lower mobilities, or higher mass to charge ratios.

[0014] Said opposing forces may be applied to the ions during the ion accumulation mode such that ions having mobilities below, or mass to charge ratios above, the first threshold value are trapped at different respective equilibrium positions along the first ion separator region.

[0015] Ions are supplied towards and into said first ion separator region during said ion accumulation mode.

[0016] A first potential barrier may be provided at the boundary between the first ion separator region and the ion attenuation region, during the ion accumulation mode; wherein the first potential barrier has a height such that said opposing forces on the ions cause only said ions having mobilities above, or mass to charge ratios below, the first threshold value to be urged over the first potential barrier and into the ion attenuation region.

[0017] The first potential barrier is preferably a DC potential barrier.

[0018] A second potential barrier may be provided at the boundary between the ion attenuation region and the second ion separator region, during the ion accumulation mode; wherein said ions having mobilities above, or mass to charge ratios below, the first threshold value are trapped between the first and second potential barriers during the ion accumulation mode until they are attenuated.

[0019] The second potential barrier is preferably a DC potential barrier.

[0020] A DC potential profile along the attenuation region, during the ion accumulation mode, may be constant other than the first and / or second potential barrier.

[0021] Said opposing forces may be applied to the ions by providing a force on the ions in an upstream direction through the first ion separator region whilst repeatedly travelling electrical potentials downstream through the first ion separator region. For the avoidance of doubt, the downstream direction is the direction from the first ion separator region to the second ion separator region.

[0022] The electrical potentials may be repeatedly travelled through the first ion separator region by applying transient DC voltages to electrodes so that DC potentials travel along the first ion separator. Alternatively, different phases of a periodic or harmonic voltage waveform may be applied to different ones of the electrodes so that potentials travel along the first ion separator region.

[0023] Said providing the force in the upstream direction through the first ion separator region may comprise applying different DC voltages to electrodes at different respective positions along the first ion separator region so as to provide a DC electric field that urges ions in the upstream direction.

[0024] 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 first ion separator region. The magnitude of the electric field may increase monotonically as a function of position in the downstream direction along the first ion separator region.

[0025] Additionally, or alternatively, to providing a DC electric field in the upstream direction, said providing the force in the upstream direction through the first ion separator region may comprise flowing gas along the ion separation instrument for urging ions in the upstream direction.

[0026] Operating the ion attenuation region in the attenuation mode may comprise radially ejecting ions from the attenuation region, or neutralising ions within the attenuation region, so as to attenuate the ions.

[0027] For example, the method may comprise radially confining ions within the ion separation instrument using one or AC or RF voltage, and said operating the ion attenuation region in the attenuation mode may comprise varying the amplitude and / or phase of the one or more AC or RF voltage applied to electrodes in the attenuation region, or stopping applying the one or more AC or RF voltage to the electrodes, such that ions in the attenuation region are not radially confined in the attenuation region.

[0028] Additionally, or alternatively, said operating the ion attenuation region in the attenuation mode may comprise applying one or more voltage to at least one electrode in the attenuation region so as to urge ions radially out of the ion attenuation region. The one or more voltage may be one or more DC voltage or an AC voltage.

[0029] The method may comprise switching said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions from the first ion separator region downstream into the second ion separator region, whilst the ion attenuation region is operated in an ion guiding mode in which it does not attenuate ions or attenuates ions at a lower rate than during the ion accumulation mode; and operating the second ion separator region in an ion accumulation mode whilst the first ion separator region is operating in the ion ejection mode such that ions from the first ion separator region are trapped in the second ion separator region. The ion ejection mode may eject all of the ions that were trapped in the first ion separator region together, e.g. by reducing or removing the force on these ions in the upstream direction. At least some of the separation of the ions according to mobility or mass to charge ratio may be maintained as the ions exit the first ion separator region and enter the second ion separator region.

[0030] Said operating the second ion separator region in its ion accumulation mode may comprise applying opposing forces to the ions in the second ion separator region such that ions having different mobilities or mass to charge ratios are trapped at different respective equilibrium positions along the second ion separator region.

[0031] Said opposing forces may be applied to the ions in the second ion separator region during its ion accumulation mode such that the ions are separated according to mobility or mass to charge ratio, where ions having relatively high mobilities, or relatively low mass to charge ratios, are urged to move further downstream than ions having lower mobilities, or higher mass to charge ratios.

[0032] The range of mobilities or mass to charge ratios trapped in the first ion separator region during its accumulation mode may be the same as, or different to, the range of mobilities or mass to charge ratios trapped in the second ion separator region during its accumulation mode. For example, the range of mobilities or mass to charge ratios trapped in the second ion separator region during its accumulation mode may be smaller than the range of mobilities or mass to charge ratios trapped in the first ion separator region during its accumulation mode.

[0033] Accordingly, said opposing forces in the second ion separator region may be such that ions having mobilities above, or mass to charge ratios below, a second threshold value that is lower than said first threshold value are not trapped and are urged out of the downstream end of the second ion separator region.

[0034] Said opposing forces in the second ion separator region may be applied to the ions by providing a force on the ions in an upstream direction through the second ion separator region whilst repeatedly travelling electrical potentials downstream through the second ion separator region.

[0035] The electrical potentials may be repeatedly travelled through the second ion separator region by applying transient DC voltages to electrodes so that DC potentials travel along the second ion separator. Alternatively, different phases of a periodic or harmonic voltage waveform may be applied to different ones of the electrodes so that potentials travel along the second ion separator region.

[0036] Said providing the force in the upstream direction through the second ion separator region may comprise applying different DC voltages to electrodes at different respective positions along the second ion separator region so as to provide a DC electric field that urges ions in the upstream direction.

[0037] 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 second ion separator region. The magnitude of the electric field may increase monotonically as a function of position in the downstream direction along the second ion separator region. Additionally, or alternatively, to providing a DC electric field in the upstream direction, said providing the force in the upstream direction through the second ion separator region may comprise flowing gas along the ion separation instrument for urging ions in the upstream direction.

[0038] Ions may be trapped in the second ion separator region, in its ion accumulation mode, within a first trapping zone and a second trapping zone that is downstream of the first trapping zone, wherein ions having a first range of mobilities or mass to charge ratios are trapped in the first trapping zone and ions having a second, different range of mobilities or mass to charge ratios are trapped in the second trapping zone. The method may comprise attenuating the ions having the first range of mobilities or mass to charge ratios.

[0039] The ions having the second range of mobilities or mass to charge ratios are eluted from the downstream end of the second ion separator region when it is operating in an elution mode and these ions, or ions derived therefrom are detected by an ion detector. In contrast, the ions having the first range of mobilities or mass to charge ratios are attenuated and so these ions, or ions derived therefrom, are not detected.

[0040] In embodiments in which an electric field is used to separate the ions in the second ion separator region, during its ion accumulation mode, the electric field gradient over the first trapping zone may be greater than the electric field gradient over the second trapping zone.

[0041] The magnitude of the electric field may vary substantially linearly over each of the first and second trapping zones.

[0042] The second trapping zone may be longer than the first trapping zone, in the downstream direction.

[0043] The magnitude of the electric field may be substantially constant over a length of the second ion separator at its downstream end.

[0044] The ions having the first range of mobilities or mass to charge ratios may be attenuated by: i) being urged upstream into the attenuation region and operating the attenuation region in the attenuation mode; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

[0045] For example, the method may comprise radially confining ions within the ion separation instrument using one or AC or RF voltage, and the step of attenuating the ions may comprise varying the amplitude and / or phase of the one or more AC or RF voltage applied to electrodes in the first trapping zone, or stopping applying the one or more AC or RF voltage to the electrodes, such that ions in the first trapping zone are not radially confined. Additionally, or alternatively, the step of attenuating the ions may comprise applying one or more voltage to at least one electrode in the first trapping zone so as to urge ions radially out of the first trapping zone. The one or more voltage may be one or more DC voltage or an AC voltage.

[0046] Ions may be trapped in the first ion separator region, in its ion accumulation mode, within a third trapping zone and a fourth trapping zone that is downstream of the third trapping zone. Ions having a third range of mobilities or mass to charge ratios may be trapped in the third trapping zone and ions having a fourth, different range of mobilities or mass to charge ratios are trapped in the fourth trapping zone.

[0047] In embodiments in which an electric field is used to separate the ions in the first ion separator region, during its ion accumulation mode, the electric field gradient over the third trapping zone may be greater than the electric field gradient over the fourth trapping zone.

[0048] The magnitude of the electric field may vary substantially linearly over each of the third and fourth trapping zones.

[0049] The fourth trapping zone may be longer than the third trapping zone, in the downstream direction.

[0050] In embodiments in which an electric field is used to separate the ions in the first and second ion separator regions, during their ion accumulation modes, the magnitude of the electric field at the upstream end of the second trapping zone of the second ion separator region may be the same as, or different to, the magnitude of the electric field at the upstream end of the fourth trapping zone of the first ion separator region. Preferably, the magnitude of the electric field at the upstream end of the second trapping zone of the second ion separator region is higher than the magnitude of the electric field at the upstream end of the fourth trapping zone of the first ion separator region. This narrows the range of mobilities or mass to charge ratios that are trapped in the second trapping region of the second ion separator, i.e. those ions that are subjected to the elution mode.

[0051] The method may comprise switching said second ion separator region from its ion accumulation mode to an elution mode in which ions are caused to elute from the downstream end of the second ion separator region in an order according to mobility or mass to charge ratio.

[0052] Ions may be caused to elute in this manner by progressively varying one or both of said opposing forces. For example, in the embodiments in which providing said opposing forces comprises repeatedly travelling electrical potentials downstream through the ion separation instrument, the amplitude of the electrical potentials may be progressively increased with time, or the speed at which the electrical potentials travel along the second ion separator region may progressively decrease over time, so as to cause causes the ions to progressively elute from the downstream end of the second ion separator region in an order according to mobility or mass to charge ratio.

[0053] The method may comprise switching said first ion separator region from said ion ejection mode back to said ion accumulation mode when the second ion separator region is switched to operate in said elution mode. Optionally, the method may comprise switching said ion attenuation region from said ion guiding mode back to said attenuation mode when the first ion separator region is switched back to said ion accumulation mode. As such, the first ion separator region receives ions from an upstream source of ions and traps them therein during the period that the second ion separator region elutes ions therefrom a downstream end thereof.

[0054] The method may comprise providing individual voltage supplies for independently supplying different voltages to different electrodes along the first and / or second ion separator region. This allows the desired electric field profile to be established along the first and / or second ion separator region such that the mobility or mass to charge ratio range of interest is distributed over the desired, e.g. maximum, length of the region. This results in the instrument having the optimum space-charge capacity and best performance.

[0055] The ion separation instrument may comprise electrodes arranged to form a plurality of ion channels and an ion distribution region at the upstream end of the ion separation instrument, and the method may comprise receiving a beam of ions at the ion distribution region and expanding or deflecting the beam of ions such that ions are distributed over the entrances of said plurality of ion channels.

[0056] The first ion separator region, and optionally the ion attenuation region, maybe located in the ion distribution region; and said second ion separator region may be provided in each of the plurality of ion channels.

[0057] The present invention also provides a method of mobility and / or mass spectrometry comprising: performing a method as described herein; and controlling a mass filter downstream of the ion separation instrument 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 the period over which the elution mode is performed such that said range is different when the mass filter receives ions having different mass to charge ratios or mobilities from the ion separation instrument.

[0058] Although a first aspect of the present invention has been described in which ions are separated such that ions having higher mobilities, or lower mass to charge ratios, are urged downstream before ions having lower mobilities, or higher mass to charge ratios, this need not be the case. Alternatively, the ions may be separated such that ions having lower mobilities, or higher mass to charge ratios, are urged downstream before ions having higher mobilities, or lower mass to charge ratios.

[0059] Accordingly, from a second aspect the present invention also provides a method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions, a second ion separator region for separating ions, and an ion attenuation region arranged between the first and second ion separator regions; operating the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that ions having mobilities below, or mass to charge ratios above, a first threshold value are urged into the ion attenuation region; and operating the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

[0060] The method may have any of the features described above in relation to the first aspect of the present invention, except that the ions are urged downstream in the opposite order of mobility, or mass to charge ratio.

[0061] For example, a first potential barrier may be provided at the boundary between the first ion separator region and the ion attenuation region, during the ion accumulation mode; wherein the first potential barrier has a height such that said opposing forces on the ions cause only said ions having mobilities below, or mass to charge ratios above, the first threshold value to be urged over the first potential barrier and into the ion attenuation region. The other ions remain trapped in the first ion separator region.

[0062] Similarly, when the second ion separator region is operating in its ion accumulation mode the opposing forces in the second ion separator region may be such that ions having mobilities below, or mass to charge ratios above, a second threshold value that is lower than said first threshold value are not trapped and are urged out of the downstream end of the second ion separator region.

[0063] From a third aspect the present invention provides a method of separating ions comprising: providing ions to an ion separation instrument having a first region and a second, ion separator region; operating the first region in an ion accumulation mode in which it accumulates ions; switching said first region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second, ion separator region whilst the second, ion separator region is operating in an ion accumulation mode such that ions having a first range of mobilities or mass to charge ratios are trapped in a first trapping zone of the second, ion separator region and ions having a second, different range of mobilities or mass to charge ratios are trapped in a second trapping zone of the second, ion separator region; and removing ions having the first range of mobilities or mass to charge ratios from the second, ion separator region by: i) urging these ions out of the second, ion separator region in the upstream direction; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

[0064] This aspect of the invention removes the ions without having to pass them downstream through the second, ion separator region. As such, time is not taken up operating the second, ion separator region in such a mode.

[0065] The method according to this aspect of the invention may have any of the features described above in relation to the first aspect of the present invention.

[0066] For example, the first region may be a first ion separator region which, in its ion accumulation mode, applies opposing forces to the ions therein such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region.

[0067] An ion attenuation region may be arranged between the first region and the second, ion separator region. The ion attenuation region may be operated as described in relation to the first aspect.

[0068] Where the ions are urged upstream they may be urged into the attenuation region. The attenuation region may be located between the first and second ion separator regions, as described above, or it may be located upstream of the first ion separator region.

[0069] Alternatively, the ions may be urged upstream without being attenuated. For example, they may be stored for being analysed later.

[0070] Although a first aspect of the present invention has been described in which an ion attenuation region is provided between the first and second ion separator regions, it is contemplated that the ion attenuation region need not be provided and that the ions that would have been attenuated in the ion attenuation region are simply not trapped in the second ion separator region during its ion accumulation mode, when ions are transferred from the first ion separator region to the second ion separator region.

[0071] Accordingly, from an fourth aspect the present invention provides a method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions and a second ion separator region for separating ions; operating the first ion separator region in an ion accumulation mode in which it accumulates ions; switching said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second ion separator region whilst the second ion separator region is operating in an ion accumulation mode such that ions from the first ion separator region having a first range of mobilities or mass to charge ratios are trapped in the second ion separator region, whereas ions from the first ion separator region having mobilities above said first range, or mass to charge ratios below said first range, are not trapped in the second ion separator region and are instead urged out of the downstream end of the second ion separator region.

[0072] This method may have any of the features described above in relation to the first aspect of the present invention, except that it may or may not have the ion attenuation region between the first and second ion separator regions.

[0073] For example, said operating the first ion separator region in its ion accumulation mode, and / or said operating the second ion separator region in its ion accumulation mode, may comprise applying opposing forces to the ions such that ions having different mobilities or mass to charge ratios are caused to separate within the first and / or second ion separator region.

[0074] Said opposing forces may be applied to the ions by providing a force on the ions in an upstream direction through the first ion separator region whilst repeatedly travelling electrical potentials downstream through the first ion separator region.

[0075] Said providing the force in the upstream direction through the first and / or second ion separator region may comprise applying different DC voltages to electrodes at different respective positions along the first and / or second ion separator region so as to provide a DC electric field that urges ions in the upstream direction.

[0076] The first ion separator region may comprise an ion holding region at its downstream end. When the first ion separator region is operated in its ion accumulation mode, opposing forces may be applied to the ions such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion holding region. Preferably, ions are not attenuated in the ion holding region.

[0077] The first threshold value may be the same as, greater than or less than, the highest mobility value, or the lowest mass to charge ratio value, in said first range. For example, the first threshold value may be the same as the highest mobility value, or the lowest mass to charge ratio value, in said first range such that when the ions are transferred from the first ion separator region to the second ion separator region, the ions from the ion holding region do not become trapped in the second ion separator region during its ion accumulation mode and are instead urged out of the downstream end of the second ion separator region.

[0078] A first potential barrier may be provided in the first ion separator region so as to define the upstream end of the ion holding region, during the ion accumulation mode. The first potential barrier may have a height such that said opposing forces on the ions cause only said ions having mobilities above, or mass to charge ratios below, the first threshold value to be urged over the first potential barrier and into the ion holding region. The first potential barrier is preferably a DC potential barrier.

[0079] A second potential barrier may be provided at the downstream end of the ion holding region, during the ion accumulation mode. Said ions having mobilities above, or mass to charge ratios below, the first threshold value become trapped between the first and second potential barriers during the ion accumulation mode. The second potential barrier is preferably a DC potential barrier.

[0080] A DC potential profile along the ion holding region, during the ion accumulation mode, may be constant other than the first and / or second potential barrier.

[0081] The present invention also provides a method of mass and / or ion mobility spectrometry comprising: providing any of the methods described above; and mass and / or ion mobility analysing ions eluting from the ion separation instrument, or ions derived therefrom.

[0082] The present invention also provides an ion separation instrument comprising: a plurality of electrodes; one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to perform any of the methods described above.

[0083] The first and second aspects of the present invention provide an ion separation instrument comprising: a first ion separator region for separating ions; a second ion separator region for separating ions; an ion attenuation region arranged between the first and second ion separator regions; and control circuitry configured to: operate the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that either: (a) ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion attenuation region; or (b) ions having mobilities below, or mass to charge ratios above, a first threshold value are urged into the ion attenuation region; and operate the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

[0084] The third aspect of the present invention provides an ion separation instrument comprising: a first region; a second, ion separator region for separating ions; and control circuitry configured to: operate the first region in an ion accumulation mode in which it accumulates ions; switch said first region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second, ion separator region whilst the second, ion separator region is operating in an ion accumulation mode such that ions having a first range of mobilities or mass to charge ratios are trapped in a first trapping zone of the second, ion separator region and ions having a second, different range of mobilities or mass to charge ratios are trapped in a second trapping zone of the second, ion separator region; and remove ions having the first range of mobilities or mass to charge ratios from the second, ion separator region by: i) urging these ions out of the second, ion separator region in the upstream direction; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

[0085] The fourth aspect the present invention provides an ion separation instrument comprising: a first ion separator region; a second ion separator region; and control circuitry configured to: operate the first ion separator region in an ion accumulation mode in which it accumulates ions; switch said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second ion separator region whilst the second ion separator region is operating in an ion accumulation mode such that ions from the first ion separator region having a first range of mobilities or mass to charge ratios are trapped in the second ion separator region, whereas ions from the first ion separator region having mobilities above said first range, or mass to charge ratios below said first range, are not trapped in the second ion separator region and are instead urged out of the downstream end of the second ion separator region.

[0086] The present invention also provides a mass and / or mobility spectrometer comprising any one of the ion separation instruments described herein. The spectrometer may comprise a mass analyser and / or detector for mass analysing and / or detecting ions eluting from the ion separation device, or ions derived therefrom.

[0087] BRIEF DESCRIPTION OF THE DRAWINGS

[0088] Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0089] Figs. 1A-1C show views of a known ion mobility separator;

[0090] Figs. 2A-2B illustrate one technique of how the electric field profile along a known ion mobility separator may be varied so as to cause ions to elute;

[0091] Figs. 3A-3B illustrate another technique of how a known ion mobility separator may be varied so as to cause ions to elute;

[0092] Fig. 4 shows a schematic of an arrangement in which two IMS devices are provided in series;

[0093] Figs. 5A-5D show electrical DC potential profiles that are maintained along an ion separation instrument according to an embodiment of the present invention at four different times during its operation;

[0094] Fig. 6 shows the magnitude of the electric field along the ion separation instrument in an embodiment corresponding to that shown in Fig. 5D, except that slightly different DC potential profiles are provided along the instrument;

[0095] Figs. 7A-7B illustrate an instrument according to an embodiment of the present invention that has multiple ion channels for separating and eluting ions;

[0096] Fig. 8 illustrates an ion separation instrument 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;

[0097] Fig. 9A shows the same embodiment as Fig. 7B except also having an ion funnel, whereas Figs. 9B and 9C show examples of plates that may be used to form the ion funnel;

[0098] Figs. 10A-10B show an embodiment of an ion separation instrument having an ion distribution region at the upstream end;

[0099] Figs. 11A-11 D show electrical DC potential profiles that are maintained along the ion separation instrument of Fig. 10A at four different times during its operation; and

[0100] Fig.12 shows the magnitude of the electric field along the ion separation instrument in an embodiment corresponding to that shown in Fig. 11 D, except that slightly different DC potential profiles are provided along the instrument.

[0101] DETAILED DESCRIPTION

[0102] 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. 1 B, 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.

[0103] 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, or by progressively increasing the amplitude of the travelling DC potentials, as will be described in more detail with reference to Figs. 3A- 3B. Fig. 2A illustrates an example of the electric field profile along the IMS device during an ion accumulation mode during which ions are accumulated in the IMS device. As can be seen, the IMS device has a trapping region 10 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 11 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 12 that move along the IMS device in the downstream direction, through the trapping region and the analytical region. Fig. 2A also shows that ions 13 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.

[0104] 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 10 and analytical region 11 has been reduced. The travelling DC potentials 12 are then able to urge the ions further downstream along the IMS device and such that first ions 14 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 14 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.

[0105] 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 12 that travel through the trapping and analytical regions in order to cause ions having progressively lower mobilities to elute, e.g. as will be described in relation to Figs. 3A-3B.

[0106] Figs. 3A-3B illustrate an example of the electric field profile along an IMS device that causes ions having progressively lower mobilities to elute by progressively increasing the amplitude of the travelling DC potentials 12 that travel through the trapping and analytical regions. As can be seen from Figs. 3A-3B, the IMS device has a trapping region 10 within which the magnitude of the electric field increases as a function of position along the IMS device in the downstream direction. At the end of the trapping region there is an analytical region 11 , along which the magnitude of the electric field is substantially constant. Fig. 3A also illustrates the travelling DC potentials 12 that move along the IMS device in the downstream direction, through the trapping region and also through the analytical region. Fig. 3A also shows that ions 13 of different mobility are trapped at different equilibrium positions along the trapping region, the different mobilities being illustrated by the differently 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 during an elution mode by progressively increasing the amplitude of the travelling DC potentials, as shown in Fig. 3B.

[0107] Fig. 3B shows the same example as in Fig. 3A, except wherein the amplitude of the travelling DC potentials that travel along the IMS device has been increased. The travelling DC potentials 12 are then able to urge the ions in the trapping region 10 further downstream along the IMS device and such that first ions 14 of relatively high ion mobility are urged out of the trapping region 10 and into the analytical region 11 , whereas ions of lower mobility are retained in the trapping region by the DC electric field. The first ions 14 are able to pass through the analytical region and out of the downstream end of the IMS device. At a later time (not shown) the amplitudes of the travelling DC potentials are increased further. 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 amplitudes of the travelling DC potentials may be progressively increased such that ions having progressively lower mobility elute from the downstream end of the IMS device.

[0108] In each of Figs. 3A-3B, the magnitude of the DC electric field is shown as a function of position along the device, whereas the maximum amplitudes of the travelling DC potentials 12 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 10, but the maximum amplitudes of the travelling DC potentials (relative to the potentials of the underlying DC electric field) may be the same at all points along the trapping region. Similarly, the magnitude of the electric field is constant along the analytical region 11 , and the maximum amplitudes of the travelling DC potentials (relative to the potentials of the underlying DC electric field) may be the same at all points along the analytical region.

[0109] It is possible to provide two IMS devices in series, such that an upstream IMS device can accumulate ions from a source of ions, whilst a downstream IMS device elutes ions according to mobility, as will be described in relation to Fig. 4.

[0110] Fig. 4 shows a schematic of an arrangement in which first and second IMS devices 1a, 1b are provided in series. Ions are received into an entrance end of the first IMS device 1a during an ion accumulation mode of the first device. The ions are axially confined within the first IMS device by the opposing forces on them, as discussed above. For example, as described in relation to Figs. 3A-3B, transient DC potentials may be successively applied to successive electrodes arranged along the first IMS device such that travelling DC potentials repeatedly travel in the downstream direction, whilst different DC potentials are applied to different electrodes along the IMS device so as to form a DC electric field that urges ions in the upstream direction. An example of the voltage profile along the first IMS device is shown in Fig. 4 below the first IMS device 1a. The ions are accumulated in the first IMS device in a manner such that they are axially confined at different axial location depending on their mobility.

[0111] When sufficient ions have been accumulated in the first IMS device, they are allowed to exit the downstream end of the first IMS device and pass into the upstream end of the second IMS device 1b by switching the first IMS device to an ion ejection mode. All of the ions are urged out of the first IMS device at substantially the same time, e.g. by removing the DC electric field acting in the upstream direction. Once the ions have exited the downstream end of the first IMS device, the first IMS device may switch back to the ion accumulation mode and ions are allowed to enter the upstream end of the first IMS device.

[0112] During the period that the second IMS device receives ions from the first IMS device it is operated in an ion accumulation mode during which the ions are axially confined within the second IMS device by opposing forces on them, e.g. as described above in relation to Figs. 3A. Transient DC potentials are successively applied to successive electrodes arranged along the second IMS device such that travelling DC potentials repeatedly travel in the downstream direction, whilst different DC potentials are applied to different electrodes along the second IMS device so as to form a DC electric field that urges ions in the upstream direction. An example of the voltage profile along the second IMS device is shown in Fig. 4 below the second IMS device 1b. The ions are accumulated in the second IMS device in a manner such that they are axially confined at different axial location depending on their mobility.

[0113] The second IMS device is then operated in an elution mode, e.g. as has been described in relation to Fig. 2B or 3B. In this mode the magnitude of the DC electric field or the amplitude of the travelling DC potentials is progressively varied with time so as to cause ions to elute from the downstream end of the second IMS device in an order according to their mobility. After the ions have eluted from the second IMS device, the second IMS device may be switched back to the ion accumulation mode and the first IMS device may be switched back to the ion ejection mode so as to again fill the second IMS device with ions from the first IMS device. The above described cycle may be repeated as many times as desired, with the first IMS device accumulating ions whilst the second IMS device elutes ions according to mobility. This provides the instrument with a relatively high duty cycle since, although ions must be prevented from entering the second IMS device whilst it is operating in the elution mode, ions are not lost during this period because they are accumulated in the first IMS device.

[0114] However, although the arrangement shown in Fig. 4 provides the instrument with a high duty cycle, accumulating the ions in the IMS devices can result in the ions experiencing detrimental space-charge effects if excess charge is accumulated. This is problematic, for example, in that if excess charge builds up in the first IMS device then space-charge effects may cause ions to overflow into the second IMS device during its elution mode, thus causing an undesirable ion signal during the elution mode. It is recognised that only ions having a limited range of mobilities may be of interest, e.g. ions having mobilities above and / or below a certain mobility value may not be of interest and so could be discarded without further analysis. However, although ions are separated by mobility in the first IMS device, it is difficult to remove the ions having mobilities that are not of interest from the first IMS device. For example, in the arrangement described above ions having undesirably high mobilities will be trapped near the downstream end of the first IMS device, but it is undesirable to eject them in the downstream direction when the second IMS device is performing its elution mode because these undesired ions would then give rise to an ion signal during the elution mode. Furthermore, ions having mobilities above the range that is capable of being trapped in the first IMS device will pass into the second IMS device and may then give rise to an undesirable ion signal during the elution mode.

[0115] An ion separation instrument according to an embodiment of the present invention will now be described that has two ion separator regions arranged in series, such as has been shown and described in relation to Fig. 4. It also has an ion attenuation region arranged between the two ion separator regions, as will be described below. The ion separation instrument may have an electrode configuration such as that shown in relation to Figs. 1A-1C and 4. Voltages are applied to these electrodes so as to perform the functions described below.

[0116] Figs. 5A-5D show electrical DC potential profiles that are maintained along the ion separation instrument at four different times during its operation. As shown in Fig. 5A, the ion separation instrument has a first ion separator region 20, a second ion separator region 24 and an attenuation region 22 therebetween. Different DC voltages are applied to different electrodes along the first ion separator region such that the DC potential monotonically increases as a function of distance in the downstream direction along the first ion separator region so as to generate an electric field for urging ions in the upstream direction. The electrodes in the attenuation region 22 may be maintained at the same, constant DC voltage, except that an electrode at the upstream end of the attenuation region may be maintained at a higher DC voltage so as to generate a first DC potential barrier 26 at the boundary between the first ion separator region 20 and the attenuation region 22. Different DC voltages are applied to different electrodes along the second ion separator region 24 such that the DC potential monotonically increases as a function of distance in the downstream direction along the second ion separator region so as to generate an electric field for urging ions in the upstream direction. The DC voltage at the upstream end of the second ion separator region may be substantially the same as the constant DC voltage applied in the attenuation region 22. Optionally, an electrode at the downstream end of the attenuation region 22 may be maintained at a relatively high DC voltage, e.g. higher than the constant DC voltage in the attenuation region, so as to generate a second DC potential barrier 28 at the boundary between the attenuation region 22 and the second ion separator region 24. Fig. 5A shows the instrument when the first ion separator region 20 is operating in an accumulation mode. In this mode, ions are supplied into an upstream end of the first ion separator region, e.g. as a continuous ion beam, whilst travelling DC potentials (not shown) are travelled along the instrument in the downstream direction so as to urge the ions against the electric field that is acting in the upstream direction. The opposing forces due to the travelling DC potentials and the electric field cause ions 30 of different mobility to be urged to different axial positions along the instrument. More specifically, the higher the mobility of the ion, the further downstream the travelling DC potentials are able to urge the ion. As such, ions having relatively high mobilities are urged over the first DC potential barrier 26 located at the downstream end of the first ion separator region 20 and pass into the attenuation region 22. The combination of the DC potential profile, the travelling DC potentials and the height of the first DC potential barrier are selected such that only ions having mobilities above a first pre-selected threshold value are urged into the attenuation region, e.g. ions having relatively high mobilities that are not of interest and that are not desired to be analysed further.

[0117] As mentioned above, a second DC potential barrier 28 may be provided at the downstream end of the attenuation region 22. This second DC potential barrier traps the ions having mobilities above the first pre-selected threshold value in the attenuation region. The second DC potential barrier may or may not have a greater amplitude than the first DC potential barrier. It is contemplated that the second DC potential barrier need not be provided and instead the DC potential profile at the upstream end of the second ion separator region 24 may be such that the ions having mobilities above the first pre-selected threshold value are unable to leave the attenuation region 22 in the downstream direction.

[0118] At least some of the ions that are trapped in the attenuation region 22 may then be attenuated, such as by being neutralised in the instrument or by being radially ejected from the instrument. For example, RF voltages may be applied to the electrodes along the instrument for radially confining the ions therein, and when it is desired to attenuate the ions the amplitude and / or frequency of the RF voltages applied to the electrodes in the attenuation region may be varied such that at least some of these ions are no longer radially confined and either leave the instrument radially or impact on the electrodes and become neutralised. This may be performed, for instance, by reducing the amplitude of the RF voltages or removing the RF voltages in the attenuation region. Alternatively, or additionally, one or more voltages may be applied to electrodes in the attenuation region so as to excite the ions therein so as to cause them to be radially ejected from the instrument or to move radially so as to impact on electrodes and be neutralised. For instance, this may be performed by applying an AC voltage that increases the amplitude of oscillation of the ions in the radial direction. The above-described attenuation process is indicated in Fig. 5A by the arrows in the attenuation region 22 and it is performed during at least part of the duration in which the first ion separator region 20 is operated in the accumulation mode.

[0119] The ions of interest, which have mobilities below the first pre-selected threshold value remain in the first ion separator region 20 during the accumulation mode. The opposing forces on these ions due to the travelling DC potentials and the electric field cause ions of different mobilities to be axially trapped at different respective equilibrium positions along the first ion separator region, at which the opposing forces on each ion are balanced. The ions are illustrated in Fig. 5A by circles, where larger circles represent lower mobilities. The DC potential profile in the first ion separator region 20 may be such that the ions of interest are distributed over substantially the entire axial length of the first ion separator region.

[0120] Fig. 5B shows the instrument when the first ion separator region 20 is at the end of the accumulation mode, and when the unwanted ions of higher mobility have been attenuated in the attenuation region 22. After the ions in the attenuation region have been attenuated, the attenuation region is restored to its non-attenuating mode. The first ion separator region is then switched to an ion ejection mode whilst the second ion separator region is operating in an ion accumulation mode. As such, ions in the first ion separator region are transferred downstream, through the attenuation region and into the second ion separator region 24. This may be performed by removing the first and second DC potential barriers 26,28 at the ends of the attenuation region 22 and controlling the travelling DC potentials so as to urge the ions from the first ion separator region 20 to the second ion separator region 24. For example, the amplitudes of the travelling DC potentials that travel along the instrument may be increased, and / or their speed of travel may be decreased, so that the travelling DC potentials are able to urge the ions from the first ion separator region to the second ion separator region. The DC potential profile that generates the electric field directed in the upstream direction may remain the same as previously described, except for the removal of the DC potential barriers.

[0121] Fig. 5C shows the instrument after the ions 30 have been urged from the first ion separator region 20 into the second ion separator region 24, whilst the second ion separator region is operating the ion accumulation mode. The opposing forces on the ions in the second ion separator region due to the travelling DC potentials and the electric field cause ions of different mobilities to be axially trapped at different respective equilibrium positions along the second ion separator region, at which the opposing forces on each ion are balanced. Ions may continue to be supplied into the upstream end of the first ion separator region 20 throughout the above described process. Alternatively, ions may be prevented from entering the upstream end of the first ion separator region after the attenuation step has finished and whilst ions are being transferred from the first ion separator region to the second ion separator region. After the ions have been transferred from the first ion separator region to the second ion separator region the first DC potential barrier 26, and optionally the second DC potential barrier 28, is restored at the ends of the attenuation region 22. The instrument then switches the first ion separator region back to operating in the accumulation mode and switches the attenuation region to performing the attenuation again, as described in relation to Figs. 5A-5B. Simultaneously, the second ion separator region is switched to an elution mode so as to cause the ions trapped therein to elute in an order according to their mobility, as shown in Fig. 5D. Fig. 5D shows the instrument at a time when the first ion separator region has been switched back to operating in the accumulation mode and whilst ions are being attenuated in the attenuation region again. As described above, whilst this is occurring the second ion separator region is controlled so as to cause the ions trapped therein to elute in an order according to their mobility. This may be performed by progressively varying a property of the travelling DC potentials that are travelling along the second ion separator region such that ions are urged out of the downstream end of the second ion separator region in an order from high mobility to low mobility. For example, the amplitude of the travelling DC potentials may be progressively increased, and / or the speed of the travelling DC potentials may be progressively decreased, such that ions are urged out of the downstream end of the second ion separator region in an order from high mobility to low mobility. As an alternative to varying a property of the travelling DC potentials to cause the ions to elute, or additionally, the gradient of the DC electric field may be progressively reduced such that ions are urged out of the downstream end of the second ion separator region by the travelling DC potentials in an order from high mobility to low mobility. Fig. 5D illustrates the ions in the second ion separator region part way through the elution process, after the highest mobility ions that were trapped during its accumulation mode have already been eluted.

[0122] Once all of the ions have been caused to elute from the second ion separator region, the first ion separator region may be switched back to the ejection mode and the second ion separator region may be switched back to operating in the accumulation mode, so that ions are transferred from the first ion separator region into the second ion separator region in the same manner as described above in relation to Figs. 5B-5C. The first ion separator region may then be switched back to operating in the accumulation mode and the second ion separator region switched back to operating in the elution mode, so as to cause ions to elute from the second ion separator region according to mobility whilst ions are again simultaneously accumulated in the first ion separator region and attenuated in the attenuation zone.

[0123] It will be appreciated that the process described above in relation to Figs. 5A-5D may be cycled through repeatedly as many times as desired during an experiment.

[0124] The instrument may be configured to operate such that the range of mobilities trapped in the first ion separator region during its accumulation mode is the same as, or different to, the range of mobilities trapped in the second ion separator region during its accumulation mode. For example, the range of mobilities trapped in the second ion separator region during its accumulation mode may be smaller than the range of mobilities trapped in the first ion separator region during its accumulation mode. As described above, ions having mobilities up to the first threshold mobility value may be trapped in the first ion separator region during its accumulation mode, whereas only ions having mobilities up to a second, lower threshold mobility value may be trapped in the second ion separator region during its accumulation mode. As described above, ions above the first threshold mobility value pass into the attenuation region and are attenuated therein. When the first ion separator region is operated in the ejection mode and the second ion separator region is operated in its accumulation mode, the remaining ions having mobilities below the first threshold mobility value are transferred from the first ion separator region to the second ion separator region. Any ions having mobilities above the second threshold mobility value will pass out of the downstream end of the second ion separator region, even though it is operating in the accumulation mode, and so will not be accumulated or subjected to the subsequent elution mode. These ions may therefore not be analysed further and may be discarded.

[0125] Fig. 6 shows the magnitude of the electric field along the ion separation instrument in an embodiment corresponding to that shown in Fig. 5D, except that slightly different DC potential profiles are provided along the instrument. It is to be noted that Fig. 5D shows the DC potential profile along the instrument, whereas Fig. 6 shows the electric field profile along the instrument. In the embodiment of Fig. 6 the DC potential profile along the first ion separator region 20, during its accumulation mode, is set such that there is a first trapping zone 20a at its upstream end and a second trapping zone 20b at its downstream end, where the first trapping zone has a greater electric field gradient than the second trapping zone. The gradient of the electric field may be substantially linear along each of the first and second trapping zones. The first trapping zone traps ions of relatively low mobility within it, such as ions that are not of interest for further analysis, whereas the second trapping zone traps ions having higher mobilities, apart from ions having mobilities above the first threshold mobility value which pass into the attenuation zone and are attenuated as discussed above. The second trapping zone 20b may be longer than the first trapping zone 20a, which allows the field gradient in the second trapping zone to be relatively low such that the ions of interest are trapped in a manner such that they are spatially distributed over a relatively long length of the first ion separator region 20, thus providing relatively low space-charge effects in this zone.

[0126] The DC potential profile along the second ion separator region 24, during its ion accumulation mode, is set such that there is a first trapping zone 24a at its upstream end and a second, downstream trapping zone 24b, where the first trapping zone has a greater electric field gradient than the second trapping zone. The gradient of the electric field may be substantially linear along each of the first and second trapping zones 24a, 24b. The second ion separator region 24 may also have an analytical region 24c having a constant electric field and that is arranged downstream of the second trapping zone 24b. The first trapping zone 24a traps ions of relatively low mobility within it, such as ions that are not of interest for further analysis, whereas the second trapping zone 24b traps the ions having higher mobilities, optionally apart from ions having mobilities above a second threshold mobility value which, as has been described above, may pass downstream through the analytical region 24c and out of the instrument so as to be discarded. The second trapping zone 24b may be longer than the first trapping zone 24a, which allows the electric field gradient in this zone to be relatively low such that the ions of interest are trapped in a manner such that they are spatially distributed over a relatively long length of the second ion separator region 24, thus providing relatively low space-charge effects in this zone. The gradient of the electric field in the second trapping zone 24b of the second ion separator region 24 may be lower than the gradient of the electric field in the second trapping zone 20b of the first ion separator region 20. This allows ions trapped in the second trapping zone 24b of the second ion separator region to be dispersed over a relatively large volume. The magnitude of the electric field at the downstream end of the second trapping zone 24b of the second ion separator region may be lower than the magnitude of the first DC potential barrier 26 at the upstream end of the attenuation region 22, such that the highest mobility ion that is trapped in the second ion separator region 24 during its accumulation mode has a lower mobility than the highest mobility ion trapped in the first ion separator region 20 during its accumulation mode.

[0127] The first trapping zone 24a of the second ion separator region may be used to trap low mobility ions that are not of interest, which may be attenuated as described further below. The magnitude of the electric field at the upstream end of the second trapping zone 24b of the second ion separator region may be the same as, or different to, the magnitude of the electric field at the upstream end of the second trapping zone 20b of the first mobility separation region. For example, as shown, the magnitude of the electric field at the upstream end of the second trapping zone 24b of the second ion separator region may be higher than the magnitude of the electric field at the upstream end of the second trapping zone 20b of the first mobility separation region.

[0128] As mentioned above, the first trapping zone 24a of the second ion separator region may be configured to trap low mobility ions, in the accumulation mode, that are not of interest for further analysis. It may be desired to remove these ions from the instrument before the accumulation mode is performed again. These ions may be removed before, during or after the elution mode has been performed to elute the ions of interest from the second trapping zone 24b of the second ion separator region. This removal of ions may be accomplished in several ways. For example, after the ions in the second trapping zone 24b have been eluted, the DC electric field in the second ion separator region 24 may be lowered such that the unwanted ions in the first trapping zone 24a are removed from the downstream end of the instrument and may be discarded. Additionally, or alternatively, the amplitude and / or speed of the travelling DC potentials that travel through the second ion separator region 24 may be varied, e.g. discontinuously or rapidly, such that the unwanted ions in the first trapping zone 24a are urged out of the downstream end of the instrument and may be discarded. This may involve increasing the amplitude, and / or decreasing the speed, of the travelling DC potentials. Alternatively, the voltages that are applied to the electrodes in the first trapping zone 24a may be varied in a corresponding manner to that described above in relation to the attenuation region 22, in order to attenuate the ions therein. For example, the amplitude and / or frequency of the RF voltages applied to electrodes in the first trapping zone 24a for radially confining the ions may be changed such that ions are not radially confined and are allowed to travel radially outward and exit the instrument or impact on an electrode. Alternatively, the direction of the travelling DC potentials passing through the first trapping zone 24a may be reversed so that they travel in the upstream direction and drive the unwanted ions from the first trapping zone 24a into the attenuation region 22. These ions may then be attenuated in the attenuation region according to any one of the techniques previously described.

[0129] Although embodiments have been described in which ions are attenuated in an attenuation region 22 located between the first and second ion separator regions, it is contemplated that the attenuation region may be omitted. Optionally, the downstream end of the first ion separator region may include an ion holding region that is the same as the attenuation region described except that it does not attenuate the ions. In these embodiments, when ions are transferred from the first ion separator region 20, and from the holding region if present, into the second ion separator region 24 that is being operated in its accumulation mode, the second ion separator region 24 may be operated such that it does not trap ions having mobilities above a second mobility threshold value and instead these ions pass downstream and out of the instrument. For example, the maximum electric field magnitude in the second ion separator region, during its accumulation mode, may be lower than the maximum electric field magnitude in the first ion separator region during its accumulation mode (and optionally also lower than the maximum electric field magnitude in the holding region, if it is present). Additionally, or alternatively, the amplitude of the travelling DC potentials in the second ion separator region, during its accumulation mode, may be higher than the amplitude of the travelling DC potentials in the first ion separator region, during its accumulation mode (and optionally also higher than in the holding region, if it is present). Additionally, or alternatively, the speed of the travelling DC potentials in the second ion separator region, during its accumulation mode, may be lower than the speed of the travelling DC potentials in the first ion separator region, during its accumulation mode (and optionally also lower than in the holding region, if it is present). Ions having mobilities above the second threshold mobility value are therefore not trapped in the accumulation mode and so do not contribute to the total charge trapped in the second ion separator region during its accumulation mode.

[0130] Alternatively, or additionally, to attenuating ions that have mobilities outside of the range of interest in the manner described above, these ions may be prevented from entering the instrument by filtering ions according to mobility upstream of the instrument.

[0131] During accumulation of the ions in the first and second ion separator regions, or a second trapping zone thereof, it may be desirable to set the DC potential profile along each of these regions / zones such that the ratio of the highest magnitude of the electric field to the lowest magnitude of the electric field is such that ions having the mobility range of interest are spread substantially over the entire length of this region / zone. In order to achieve this for different mobility ranges of interest, different ratios are required. For example, peptide precursor and product ions that are of interest in one experiment may span a larger range of mobilities than metabolite molecules of interest in another experiment. It is therefore desirable to be able to change the ratio of the highest to lowest DC electric field magnitude in order to optimise the trapping of ions for different applications or operational modes. It is common to supply different voltages to different electrodes in an ion guide by using a resistive divider formed from a chain of resistors arranged in series, where one electrical potential is applied to one end of the resistor chain and another electrical potential is applied at the other end of the resistor chain. Although the absolute magnitude of the electric field generated by this technique can be changed by altering the applied electrical potentials, the ratio of the highest to lowest electric field magnitude cannot be changed by altering these potentials alone, as this ratio is fixed by the resistor values chosen for the resistive divider chain.

[0132] In order to allow the ratio of the highest to lowest electric field magnitudes to be changed, embodiments of the present invention provide individual voltage supplies for independently supplying different voltages to different electrodes along the first and / or second ion separator region, and / or the second trapping zones thereof. This allows the desired electric field profile to be established along the first and / or second ion separator region, and / or the second trapping zones thereof, such that the mobility range of interest is distributed over the desired, e.g. maximum, length of the region / zone. This results in the instrument having the optimum space-charge capacity and best performance. This also allows the electric field profile to be changed rapidly in this region. The electrodes in the other regions can also be driven in this way, rather than using resistor chains.

[0133] Approaches where individual electrodes are driven by separately controllable voltage supplies have been made practical by the availability of miniaturised operational amplifiers. These may be used to independently drive the voltages applied to the electrodes. Analog devices may be used, such as Analog Devices Inc. ADHV4702 which is capable of providing + / -110V output. These devices may be mounted inside or outside of the vacuum envelope of the spectrometer in order to drive the electrodes. Multiple such amplifiers can be controlled by multiple output Digital to Analog convertors (DAC), such as the Analog Devices Inc. AD5370, 40 channel 16 bit ADC device.

[0134] The DC field in different regions of the instrument may be supported by resistive divider chains, multiple individual voltage supplies, or combinations of both these approaches so as to provide the optimum field for best performance for each application.

[0135] Although embodiments that separate ions in a single ion channel have been described, the instrument may have multiple ion channels that separate ions.

[0136] Figs. 7A-7B illustrate an instrument according to an embodiment of the present invention that has multiple ion channels for separating and eluting ions. The instrument comprises a plurality of plates 100, each of which has a plurality of apertures 110 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.

[0137] An electrode 120 is provided around the circumference of each of the apertures 110 and the instrument 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 100 may be printed circuit boards (PCBs) having the apertures 110 therein. The electrodes 120 may be conductive traces on the PCBs that are arranged around the apertures 110 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.

[0138] Fig. 7B shows a view of the instrument of Fig. 7A in the x-z plane, at a location that extends through the apertures 110. As can be seen, the apertures 110 form a plurality of elongated ion channels 130 that extend axially in the z-direction. In use, ions are supplied into the ion channels 130 through the apertures 110 in the upstream end of instrument. Each of the ion channels may be operated in the manner described above. Accordingly, an RF voltage supply supplies RF voltages to the electrodes 120 on the plates 100 so as to radially confine the ions within each ion channel 130. 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 120 of each ion channel 130 at successive respective times so as to provide the travelling DC potentials that repeatedly travel along the length of the ion channel in the downstream direction. As described above, these travelling DC potentials urge the ions in the downstream direction as they pass the ions. DC voltages are also applied to electrodes 120 of each ion channel so as to provide a DC electric field that urges the ions in the upstream direction.

[0139] Although five ion channels are illustrated in the embodiment, it will be appreciated that fewer or a greater number of ion channels 130 may be present in the stack of plates 100. 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.

[0140] Although the electrodes 110 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 110 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 130 form a multipole ion guide, such as a quadrupole ion guide. Additionally, or alternatively, although circular apertures 110 are shown, the apertures may have other shapes. It is contemplated that different ion channels 130 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 130 may vary along its length.

[0141] 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. 8.

[0142] Fig. 8 shows an instrument according to another embodiment. The instrument may have the same construction and methods of operation as the embodiments described in relation to Figs. 7A-7B, except that rather than each plate 100 comprising a plurality of discrete apertures 110 and corresponding electrodes 120 for defining a plurality of respective ion channels 130, each plate instead comprises a single continuous aperture 200 and at least one electrode 120 that are shaped so that the stack of plates 100 defines the plurality of ion channels 130. In the depicted embodiment the single aperture 200 in each plate comprises a plurality of substantially circular aperture portions 220 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.

[0143] A single continuous electrode 120 may be provided around the circumference of the single aperture 200 in each plate 100, 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 220, thereby defining the plurality of ion channels 130 through the plates 100. 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 130 can be reduced, thus allowing more ion channels in a given volume.

[0144] In use, ions are supplied into the different circular aperture portions 220 in the plate 100 at the upstream end of the instrument. 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) 120 surrounding the circular aperture portions. As such, even though each plate includes a single aperture 200, the stack of plates 100 still defines a plurality of ion channels 130 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.

[0145] Although the electrode around each single aperture 200 in each plate 100 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.

[0146] Although the aperture portions have been described as being substantially circular, the aperture portions may be other shapes. Accordingly, more generally, the aperture 200 may have enlarged portions that define the ion channels 130 and which are separated from each other by narrower aperture portions.

[0147] In all of the embodiments described above that have multiple ion channels, ions may be separated in each of the ion channels 130 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. 9A.

[0148] Fig. 9A shows the same embodiment as Fig. 7B, except that an ion funnel 240 is provided at the downstream end of the ion channels 130. Ions that exit the downstream end of the ion channels enter the relatively wide entrance end of the ion funnel 240 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. 8 may also have such an ion funnel arranged at its downstream end.

[0149] The ion funnel comprises a plurality of plates 260, 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.

[0150] Fig. 9B shows an example of three plates 260 of the ion funnel 240. The uppermost plate 260a in Fig. 9B has a relatively large aperture therein and is arranged relatively upstream in the ion funnel 240. The central plate 260b in Fig. 9B has an aperture therein with a smaller cross-sectional area and is arranged in the ion funnel downstream of plate 260a. The lowermost plate 260c in Fig. 9B has an aperture therein with an even smaller cross-sectional area and is arranged in the ion funnel downstream of plate 260b. An electrode 280 extends around the circumference of each aperture.

[0151] In use, ions are supplied by the ion channels 130 into the upstream end of the ion funnel 240. An RF voltage supply supplies RF voltages to the electrodes 280 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.

[0152] Although the apertures in Fig. 9B 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.

[0153] Although each plate 260 is illustrated as having a single electrode 280 that fully encircles the aperture, other electrode arrangements are contemplated, such as is shown in Fig. 9C.

[0154] Fig. 9C shows an embodiment that is the same as that shown and described in relation to Fig. 9B, except that each plate 260 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 300,320 are provided at opposing ends of the elongated aperture, and third and fourth electrodes 340,360 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 on a given plate 260 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. 9B. On any given plate, the first and second electrodes 300,320 may be maintained at a different DC voltage to the third and fourth electrodes 340,360, so as to assist in urging ions towards the central axis of the ion funnel. As such, the first and second electrodes 300,320 have a higher DC potential than the third and fourth electrodes 340,360.

[0155] The ion funnel could have plates 260 that have the electrode arrangement described in relation to Fig. 9B and also plates 260 that have the electrode arrangement described in relation to Fig. 9C. 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. 9C, 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. 9B. 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.

[0156] In order to ensure that the instrument 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 130. The ion beam may arrive at the instrument 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 instrument 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 instrument 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 instrument may therefore comprise an ion distribution region at its upstream end for receiving the ion beam arriving at the instrument and distributing it into the ion channels, e.g. as shown in Fig. 10A.

[0157] Fig. 10A shows an embodiment of an instrument having an ion distribution region 400 for receiving an ion beam 420, e.g. from an ion guide 440, and distributing these ions to the multiple ion channels 130. This embodiment may be the same as that described in relation to Figs. 9A-9C, except that it additionally includes the ion distribution region 400 for distributing the ions between the ion channels 130. In use an ion beam 420 passes from the ion guide 440 into the ion separation instrument having a relatively small size in the x- dimension. The ion beam enters the ion distribution region 400 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 130.

[0158] The ion distribution region 400 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.

[0159] Fig. 10C shows an example of three plates 400a-400c of the ion distribution region 400 and a fourth plate 400d that represents the upstream end of the ion channels 130 (i.e. the upstream end of the device shown in Fig. 7 A). The first, uppermost plate 400a in Fig. 10C has a rectangular aperture 470 and one or more electrode 480 arranged around it, and is arranged towards the upstream end of the ion distribution region 400. 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 distribution region to disperse in the x-dimension. For example, these plates 400a may be arranged over an ion dispersing portion 410 of the ion distribution region 400, as shown in Fig. 10A.

[0160] The second plate 400b in Fig. 10C is a plate at a region of the ion distribution region that is downstream of the first plate(s) 400a, i.e. downstream of the ion dispersing portion 410. The aperture 490 in this second plate 400b and the one or more electrodes 500 around it are shaped differently to those on the first plate(s) 400a so as to confine the ions in multiple separate regions of the aperture 490. For example, the edge of the aperture 490 and the one or more electrodes 500 may have a plurality of curved regions for forming the plurality of separate ion confinement regions. The third plate 400c in Fig. 10C is a plate at a region of the ion distribution region that is downstream of the second plate 400b. The aperture 510 in this third plate 400c and the one or more electrodes 520 around it are shaped differently to those on the second plate 400b so as to confine the ions more tightly in multiple separate regions of the aperture 510. For example, the aperture 510 may have substantially circular aperture portions that are interconnected. The second and third plates 400b, 400c are aligned such that the ions in any given one of the separate ion confinement regions in the second plate 400b pass downstream into a corresponding separate ion confinement regions in the third plate 400c. The second and third plates 400b, 400c therefore form an ion channelling portion 430 of the ion distribution region 400, as illustrated in Fig. 10A.

[0161] As mentioned above, the fourth plate 400d in Fig. 10C represents the upstream end of the ion channels 130 (i.e. the upstream end of the device shown in Fig. 7A). Accordingly, this plate 400d has multiple separate apertures 530 that each have one or more electrode 540 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 400c and 400d are aligned such that the ions in any given one of the separate ion confinement regions in the third plate 400c pass downstream into a corresponding separate aperture 530 in the fourth plate 400d. It will be appreciated that as the ions have already been separated into multiple regions by the second plate 400b, the third plate 400c 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.

[0162] In use, ions are supplied into the upstream end of the ion distribution region 400 and enter the ion dispersing portion 410. 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 and into the ion channels 130. 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 and into the ion channels 130.

[0163] Although embodiments have been described in which each of the channels 130 performs the separation techniques described above, e.g. in relation to Figs. 5A-5D, it is contemplated that the ion distribution region 400 in Fig. 10A may be used to perform part of the separation technique. For example, the first ion separator region 20 and the attenuation region 22 may be provided in the ion distribution region 400, as will be described below in relation to Figs. 11A-11 D.

[0164] Figs. 11 A-11 D show electrical potential profiles that are maintained along the ion separation instrument of Fig. 10A at four different times during operation of an embodiment in which the first ion separator region 20 and the attenuation region 22 are provided in the ion dispersing portion 410 of the ion distribution region 400 of Fig. 10A. The second ion separator region 24 is provided along each of the ion channels 130. The DC profile along the ion funnel 240 at the downstream end of the ion channels 130 is also illustrated.

[0165] This embodiment operates in the same manner as has been described in relation to Figs. 5A-5D, except that the ion channelling portion 430 of the ion distribution region 400 is located between the attenuation region 22 and the second ion separator region 24. The electrodes in the ion channelling portion 430 may be maintained at the same, constant DC voltage, as is shown.

[0166] Fig. 12 shows the magnitude of the electric field along the ion separation instrument in an embodiment corresponding to that shown in Fig. 11 D, except that slightly different DC potential profiles are provided along the instrument. This embodiment operates in the same manner as has been described in relation to Fig. 6, except that the ion channelling portion 430 of the ion distribution region 400 is located between the attenuation region 22 and the second ion separator region 24. That is, the first ion separator region 20, which is located in the ion dispersing portion 410 of the ion distribution region 400, has the first trapping zone 20a and the second trapping zone 20b. Similarly, the second ion separator region 20, which is located in each ion channel 130, has the first trapping zone 24a, the second trapping zone 24b, and the analytical region 24c. The electric field profile along the ion funnel 240 at the downstream end of the ion channels 130 is also illustrated. As mentioned above, the first trapping zone 24a of the second ion separator region 24 may be configured to trap low mobility ions, in the accumulation mode, that are not of interest for further analysis. These ions may be removed from the instrument before another cycle of the accumulation mode is performed, as has been described above. Low mobility ions having mobilities that are outside of the mobility range of interest may also be trapped in the ion channelling portion 430 of the ion distribution region 400. These ions may also be removed from the instrument before another cycle of the accumulation mode is performed, in a manner corresponding to that described above in relation to removing ions that are not of interest from the first trapping zone 24a of the second ion separator region.

[0167] 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.

[0168] For example, although embodiments have been described in which ions are separated in the ion separator regions and eluted from the instrument by travelling DC potentials in the downstream direction against an electric field that urges ions in the upstream direction, it is contemplated that these functions may alternatively be performed by providing the DC electric field in the downstream direction and travelling the DC potentials in the upstream direction. In contrast to the embodiments described above, this will cause ions to separate along each ion separator region such that the ions have a decreasing mobility as a function of distance in the downstream direction. Ions will therefore elute from the instrument, in the elution mode, in an order from low mobility to higher mobility. In such embodiments, ions may be transferred from the first ion separator region to the second ion separator region by switching off or otherwise varying the travelling DC potentials in the first ion separator region. Ions may be eluted from the second ion separator region by progressively increasing the magnitude of the electric field and / or by decreasing the amplitude (and / or increasing the speed) of the travelling DC potentials in the second ion separator region. It will be appreciated that in these embodiments it is ions having a mobility below a first threshold mobility value, which are not of interest, that are transmitted from the first ion separator region to the attenuation zone for attenuation when the first ion separator region is operating in its accumulation mode (rather than ions having mobilities above the threshold as in the previously described embodiments). Similarly, the second ion separator region may be configured such that ions having a mobility below a second threshold mobility value, which are not of interest, are not trapped in its accumulation mode and pass out of the instrument in the downstream direction (rather than ions having mobilities above the second threshold as in the previously described embodiments). Also, it is the high mobility ions that are not of interest that are trapped at the upstream end of the second ion separator region when it is operating in its accumulation mode, and it is these ions that may be passed to the attenuation region for attenuation or attenuated in another way (rather than ions having low mobilities as in the previously described embodiments).

[0169] Although embodiments have been described in which the ions are separated and eluted according to mobility by being urged in opposing directions by travelling DC potentials and an electric field, it is contemplated that the ions may be urged in opposing directions by other means, such as by replacing the DC electric field or travelling DC potentials that urge the ions on one of the directions with a gas flow that urges the ions in that same direction. The gas flow rate may be varied in order to cause the ions to elute from the instrument.

[0170] In the ion separation instruments described above, ions have been described as being separated in the ion separator regions according to their mobility and eluting from the instrument according to mobility. However, it is alternatively contemplated that ions may be separated in the ion separator regions according to mass to charge ratio and eluted from the instrument according to mass to charge ratio.

[0171] The embodiments that separate and elute ions according to mass to charge ratio operate in the same manners as the mobility separating embodiments that have been described above. However, as mass to charge ratio and mobility are generally inversely proportional, where the above-described embodiments cause ions to separate along each ion separator region such that the ions have an increasing mobility as a function of distance in the downstream direction, they will instead cause ions to separate along each ion separator region such that the ions have an decreasing mass to charge ratio as a function of distance in the downstream direction. Ions will therefore elute from the instrument, in the elution mode, in an order from low mass to charge ratio to higher mass to charge ratio. It will be appreciated that in these embodiments it is ions having a mass to charge ratio below a first threshold mass to charge ratio value, which are not of interest, that are transmitted from the first ion separator region to the attenuation zone for attenuation when the first ion separator region is operating in its accumulation mode (rather than ions having mobilities above a first threshold as in the previously described embodiments). Similarly, the second ion separator region may be configured such that ions having a mass to charge ratio below a second threshold mass to charge ratio value, which are not of interest, are not trapped in its accumulation mode and pass out of the instrument in the downstream direction (rather than ions having mobilities above the second threshold as in the previously described embodiments). Also, it is the high mass to charge ratio ions that are trapped at the upstream end of the second ion separator region when it is operating in its accumulation mode, and it is these ions that may be passed to the attenuation region for attenuation or attenuated in another way (rather than ions having low mobilities as in the previously described embodiments).

[0172] Similarly, where the above-described embodiments cause ions to separate along each ion separator region such that the ions have a decreasing mobility as a function of distance in the downstream direction, they will instead cause ions to separate along each ion separator region such that the ions have an increasing mass to charge ratio as a function of distance in the downstream direction. Ions will therefore elute from the instrument, in the elution mode, in an order from high mass to charge ratio to lower mass to charge ratio. It will be appreciated that in these embodiments it is ions having a mass to charge ratio above a first threshold mass to charge ratio value, which are not of interest, that are transmitted from the first ion separator region to the attenuation zone for attenuation when the first ion separator region is operating in its accumulation mode. Similarly, the second ion separator region may be configured such that ions having a mass to charge ratio above a second threshold mass to charge ratio value, which are not of interest, are not trapped in its accumulation mode and pass out of the instrument in the downstream direction. Also, it is the low mass to charge ratio ions that are trapped at the upstream end of the second ion separator region when it is operating in its accumulation mode, and it is these ions that may be passed to the attenuation region for attenuation or attenuated in another way.

[0173] The operating parameters of the ion separation instrument may be switched between a first mode in which ions are separated in and eluted from the instrument according to mobility and a second mode in which ions are separated in and eluted from the instrument according to mass to charge ratio. The ion separation instrument may be switched between the modes during a single experimental run, e.g. whilst ions are being substantially continually supplied into the spectrometer having the ion separation instrument.

[0174] 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.

[0175] 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.

[0176] The ion separation instruments 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 instrument may be operated to select either mass to charge ratio separation or mobility separation by varying these operating conditions. For example, the instrument 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 instrument; and changing the gas in the instrument to have a lower collisional cross-sectional area. Conversely, the instrument 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 instrument; and changing the gas in the instrument to have a higher collisional cross-sectional area.

[0177] Embodiments of the invention have been described that cause ions to elute from the ion separation instrument 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 instrument so as to receive ions that elute from the ion separation instrument. 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 instrument.

[0178] 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 instrument 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.

[0179] The above-described acquisition process may be repeated, e.g. whilst the ion separation instrument 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 instrument 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.

[0180] Embodiments have been described in which voltages are applied to electrodes of the ion separation instrument so that DC potentials travel along the ion separation instrument 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 instrument 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 instrument, where different phases of the voltage waveform are applied to electrodes that are located at different axial locations along the ion separation instrument so as to cause ions to be urged along the instrument. 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.

[0181] 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 instrument may then be repeated along the instrument. The voltage waveform may be applied to the electrodes of the instrument so as to effectively form a substantially smoothly varying potential that travels along the ion separation instrument and acts to urge ions axially along the instrument. 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.

[0182] 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 instrument.

[0183] 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 instrument in the opposite direction.

[0184] 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 instrument. 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

175441 -02v1Claims:

1. A method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions, a second ion separator region for separating ions, and an ion attenuation region arranged between the first and second ion separator regions; operating the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion attenuation region; and operating the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

2. The method of claim 1, wherein said opposing forces are applied to the ions during the ion accumulation mode such that ions having mobilities below, or mass to charge ratios above, the first threshold value are trapped at different respective equilibrium positions along the first ion separator region.

3. The method of claim 1 or 2, wherein a first potential barrier is provided at the boundary between the first ion separator region and the ion attenuation region, during the ion accumulation mode; wherein the first potential barrier has a height such that said opposing forces on the ions cause only said ions having mobilities above, or mass to charge ratios below, the first threshold value to be urged over the first potential barrier and into the ion attenuation region.

4. The method of claim 3, wherein a second potential barrier is provided at the boundary between the ion attenuation region and the second ion separator region, during the ion accumulation mode; wherein said ions having mobilities above, or mass to charge ratios below, the first threshold value are trapped between the first and second potential barriers during the ion accumulation mode until they are attenuated.

5. The method of claim 3 or 4, wherein a DC potential profile along the attenuation region, during the ion accumulation mode, is constant other than the first and / or second potential barrier.

6. The method of any preceding claim, wherein said opposing forces are applied to the ions by providing a force on the ions in an upstream direction through the first ionseparator region whilst repeatedly travelling electrical potentials downstream through the first ion separator region.

7. The method of claim 6, wherein said providing the force in the upstream direction through the first ion separator region comprises applying different DC voltages to electrodes at different respective positions along the first ion separator region so as to provide a DC electric field that urges ions in the upstream direction.

8. The method of any preceding claim, wherein operating the ion attenuation region in the attenuation mode comprises radially ejecting ions from the attenuation region, or neutralising ions within the attenuation region, so as to attenuate the ions.

9. The method of any preceding claim, comprising switching said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions from the first ion separator region downstream into the second ion separator region, whilst the ion attenuation region is operated in an ion guiding mode in which it does not attenuate ions or attenuates ions at a lower rate than during the ion accumulation mode; and operating the second ion separator region in an ion accumulation mode whilst the first ion separator region is operating in the ion ejection mode such that ions from the first ion separator region are trapped in the second ion separator region.

10. The method of claim 9, wherein said operating the second ion separator region in its ion accumulation mode comprises applying opposing forces to the ions in the second ion separator region such that ions having different mobilities or mass to charge ratios are trapped at different respective equilibrium positions along the second ion separator region.

11. The method of claim 9 or 10, wherein said opposing forces in the second ion separator region are such that ions having mobilities above, or mass to charge ratios below, a second threshold value that is lower than said first threshold value are not trapped and are urged out of the downstream end of the second ion separator region.

12. The method of 9, 10 or 11, wherein ions are trapped in the second ion separator region, in its ion accumulation mode, within a first trapping zone and a second trapping zone that is downstream of the first trapping zone, wherein ions having a first range of mobilities or mass to charge ratios are trapped in the first trapping zone and ions having a second, different range of mobilities or mass to charge ratios are trapped in the second trapping zone; and wherein the method comprises attenuating the ions having the first range of mobilities or mass to charge ratios.

13. The method of claim 12, wherein the ions having the first range of mobilities or mass to charge ratios are attenuated by: i) being urged upstream into the attenuationregion and operating the attenuation region in the attenuation mode; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

14. The method of any one of claims 10-13, comprising switching said second ion separator region from its ion accumulation mode to an elution mode in which ions are caused to elute from the downstream end of the second ion separator region in an order according to mobility or mass to charge ratio.

15. The method of claim 14, comprising switching said first ion separator region from said ion ejection mode back to said ion accumulation mode when the second ion separator region is switched to operate in said elution mode; and optionally comprising switching said ion attenuation region from said ion guiding mode back to said attenuation mode when the first ion separator region is switched back to said ion accumulation mode.

16. The method of any preceding claim, comprising providing individual voltage supplies for independently supplying different voltages to different electrodes along the first and / or second ion separator region.

17. The method of any preceding claim, wherein the ion separation instrument comprises electrodes arranged to form a plurality of ion channels and an ion distribution region at the upstream end of the ion separation instrument, wherein the method comprises receiving a beam of ions at the ion distribution region and expanding or deflecting the beam of ions such that ions are distributed over the entrances of said plurality of ion channels.

18. The method of claim 17, wherein the first ion separator region, and optionally the ion attenuation region, is located in the ion distribution region; and wherein said second ion separator region is provided in each of the plurality of ion channels.

19. A method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions, a second ion separator region for separating ions, and an ion attenuation region arranged between the first and second ion separator regions; operating the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that ions having mobilities below, or mass to charge ratios above, a first threshold value are urged into the ion attenuation region; and operating the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

20. A method of separating ions comprising: providing ions to an ion separation instrument having a first region and a second, ion separator region; operating the first region in an ion accumulation mode in which it accumulates ions; switching said first region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second, ion separator region whilst the second, ion separator region is operating in an ion accumulation mode such that ions having a first range of mobilities or mass to charge ratios are trapped in a first trapping zone of the second, ion separator region and ions having a second, different range of mobilities or mass to charge ratios are trapped in a second trapping zone of the second, ion separator region; and removing ions having the first range of mobilities or mass to charge ratios from the second, ion separator region by: i) urging these ions out of the second, ion separator region in the upstream direction; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

21. A method of separating ions comprising: providing ions to an ion separation instrument having a first ion separator region for separating ions and a second ion separator region for separating ions; operating the first ion separator region in an ion accumulation mode in which it accumulates ions; switching said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second ion separator region whilst the second ion separator region is operating in an ion accumulation mode such that ions from the first ion separator region having a first range of mobilities or mass to charge ratios are trapped in the second ion separator region, whereas ions from the first ion separator region having mobilities above said first range, or mass to charge ratios below said first range, are not trapped in the second ion separator region and are instead urged out of the downstream end of the second ion separator region.

22. A method of mass and / or ion mobility spectrometry comprising: providing a method as claimed in any preceding claim; and mass and / or ion mobility analysing ions eluting from the ion separation instrument, or ions derived therefrom.

23. An ion separation instrument comprising: a plurality of electrodes; one or more voltage supplies for applying voltages to the electrodes; and control circuitry configured to perform the method of any preceding claim.

24. An ion separation instrument comprising:a first ion separator region for separating ions; a second ion separator region for separating ions; an ion attenuation region arranged between the first and second ion separator regions; and control circuitry configured to: operate the first ion separator region in an ion accumulation mode during which opposing forces are applied to the ions in the ion separation instrument such that ions having different mobilities or mass to charge ratios are caused to separate within the first ion separator region such that either: (a) ions having mobilities above, or mass to charge ratios below, a first threshold value are urged into the ion attenuation region; or (b) ions having mobilities below, or mass to charge ratios above, a first threshold value are urged into the ion attenuation region; and operate the ion attenuation region in an attenuation mode, whilst the first ion separator region is operating in the ion accumulation mode, so as to attenuate at least some of the ions that have been urged into the ion attenuation region.

25. An ion separation instrument comprising: a first region; a second, ion separator region for separating ions; and control circuitry configured to: operate the first region in an ion accumulation mode in which it accumulates ions; switch said first region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second, ion separator region whilst the second, ion separator region is operating in an ion accumulation mode such that ions having a first range of mobilities or mass to charge ratios are trapped in a first trapping zone of the second, ion separator region and ions having a second, different range of mobilities or mass to charge ratios are trapped in a second trapping zone of the second, ion separator region; and remove ions having the first range of mobilities or mass to charge ratios from the second, ion separator region by: i) urging these ions out of the second, ion separator region in the upstream direction; or ii) radially ejecting ions from the first trapping zone; or iii) neutralising ions within the first trapping zone.

26. An ion separation instrument comprising: a first ion separator region; a second ion separator region; and control circuitry configured to: operate the first ion separator region in an ion accumulation mode in which it accumulates ions; switch said first ion separator region from said ion accumulation mode to an ion ejection mode in which it ejects ions downstream into the second ion separator regionwhilst the second ion separator region is operating in an ion accumulation mode such that ions from the first ion separator region having a first range of mobilities or mass to charge ratios are trapped in the second ion separator region, whereas ions from the first ion separator region having mobilities above said first range, or mass to charge ratios below said first range, are not trapped in the second ion separator region and are instead urged out of the downstream end of the second ion separator region.

Images