Gas collision cell for mass spectrometer

Abstract

A mass / mobility spectrometer comprising: a vacuum chamber, a gas cell 20 arranged in the first vacuum chamber, wherein the gas cell comprises an upstream ion entrance 22 and a downstream ion exit 23;

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2408893.2 filed on 20 June 2024. The entire contents of this application are incorporated herein by reference. FIELD OF THE INVENTION The present invention relates generally to mass and ion mobility spectrometry. Embodiments if the present invention relate to a mass or mobility spectrometer having a collision cell for fragmenting and / or thermalising ions prior to mass analysis. BACKGROUND In mass spectrometry it is known to transmit ions into a collision cell that houses a relatively high pressure gas, in order to collide the ions with the gas molecules so as to fragment and / or thermalise the ions. Such collision cells have a housing for confining the gas and entrance and exit apertures formed in thin walls of the housing for allowing ions to enter and exit the collision cell. These entrance and exit apertures are sized to restrict the gas flow out of the housing, so that the gas inside the housing can be maintained at the relatively high pressure required for fragmentation and / or thermalisation of ions. However, such conventional collision cells can cause excessive scattering of the ions, particularly when the ions exit the exit aperture. SUMMARY From a first aspect the present invention provides a mass or mobility spectrometer comprising: a first vacuum chamber; a gas cell arranged in the first vacuum chamber, wherein the gas cell comprises: an ion entrance at an upstream end thereof; an ion exit at a downstream end thereof; a main body portion between the ion entrance and ion exit; an elongated tube between the main body portion and the ion exit; and an ion guide for radially confining ions within the elongated tube; and control circuity configured to operate the gas cell in a first mode in which first voltages are applied to the ion guide such that ions are axially trapped within the gas cell, and to operate the gas cell in a second mode in which second voltages are applied to the ion guide such that at least some of these ions are ejected or released from the gas cell. Embodiments of the present invention enable ions to be selectively released or ejected from the gas cell in a manner such that the ions undergo relatively little scattering. The spectrometer may be configured to maintain the main body portion of the gas cell at a higher pressure than the first vacuum chamber. The elongated tube may be configured to limit the gas conductance out of the downstream end of the gas cell. The provision of the elongated tube may cause the pressure to drop relatively gradually over a relatively long length of the gas cell towards its exit. As such, the ions experience a relatively low pressure gradient as they travel towards and leave the exit, which minimises the range of energies, radial positions and divergence of ions exiting the gas cell, e.g. by minimising ion scattering caused by gas flow out of the exit. The elongated tube may also reduce the vacuum pumping requirements of the spectrometer, even when there is a relatively large difference between the pressures upstream and downstream of the gas cell, because the elongated tube provides a gradual variation of the pressure, e.g. rather than having to provide multiple separate vacuum chambers to achieve this. Conventionally, the gas conductance limiting apertures of collision cells have been formed in walls of the collision cell having a minimal thickness, in order to reduce the likelihood of ions impacting on the wall as they are transmitted through it. As such, the used of an elongated tube to limit the gas conductance is counter-intuitive, particularly as it is generally desired to minimise the size of the components in the spectrometer. The spectrometer may be configured such that in the first mode the first voltages provide a potential barrier within the elongated tube that axially traps ions within the gas cell; and in the second mode the second voltages: a) urge at least some of the ions over the potential barrier such that they are ejected from the gas cell; and / or b) reduce or remove the potential barrier such that at least some of the ions are ejected or released from the gas cell. Substantially all ions may be trapped in the gas cell during the first mode. The control circuity may be configured to apply voltages to electrodes of said ion guide in said first mode so as to trap ions having a range of different mass to charge ratios within the gas cell, and to vary these voltages in the second mode so as to mass selectively eject or release ions of different mass to charge ratio from the gas cell at different respective times. The spectrometer may comprise a mass analyser arranged to receive ions that exit the gas cell. The mass analyser may be arranged to receive ions that are ejected or released from the gas cell, wherein the mass analyser is configured to intermittently analyse ions, and wherein the control circuitry is configured to control the times at which the ions of different mass to charge ratio are mass selectively ejected from the gas cell, in the second mode, such that these ions of different mass to charge ratio arrive at the mass analyser at substantially the same time, and at a time that the mass analyser is mass analysing ions. This enables the ions to be mass analysed with a relatively high duty cycle. The ions may be ejected or released in reverse order of mass to charge ratio, i.e. starting by ejecting ions of relatively high mass to charge ratio and then ejecting ions of progressively lower mass to charge ratio as time progresses. The mass analyser may be a TOF mass analyser having a pusher for pushing ions into a time of flight region towards an ion detector. The control circuitry may control the times at which the ions of different mass to charge ratio are mass selectively ejected or released from the collision cell, in the second mode, such that these ions of different mass to charge ratio arrive at the pusher at substantially the same time, and at a time that a voltage pulse is applied to the pusher so as to push the ions into the time of flight region towards the detector. The control circuitry may be configured to apply an AC voltage to one or more electrodes of the ion guide in said first mode so as to form a potential barrier that traps the ions of different mass to charge ratio within the gas cell, and to vary the amplitude and / or frequency of the AC voltage with time in the second mode such that ions of different mass to charge ratios are able to pass the potential barrier and be ejected or released from the gas cell at different respective times. In this embodiment the potential barrier is a pseudopotential barrier. The control circuity may be configured to apply one or more voltage to one or more electrodes of said ion guide in said first mode so as to provide a potential barrier for trapping ions having a first range of different mass to charge ratios within the gas cell, and to vary said one or more voltages in the second mode so as to substantially simultaneously eject ions having said first range of mass to charge ratios from the gas cell. The spectrometer may be configured to simultaneously eject ions of all mass to charge ratios in the second mode. The spectrometer may comprise a mass analyser that intermittently analyses ions arranged to receive ions that are ejected from the gas cell, and the control circuitry may be configured to control the time at which the ions are ejected from the gas cell such that ions having a selected second, different range of mass to charge ratios arrive at the mass analyser at substantially the same time, and at a time that the mass analyser is mass analysing ions. The second range of mass to charge ratios is preferably narrower than the first range of mass to charge ratios. The control circuitry may be configured to apply a DC voltage to electrodes of the ion guide in said first mode so as to form a DC potential barrier that traps the ions of different mass to charge ratios within the gas cell, and the control circuitry may be configured to reduce the amplitude of the DC voltage, or remove the DC voltage, in the second mode such that the ions are ejected from the gas cell at substantially the same time. The ion guide extending through the elongated tube radially confines ions. The spectrometer comprises one or more voltage supplies, such as one or more AC voltage supply, for supplying one or more voltages to electrodes of the ion guide for performing the radial confinement of the ions therein. The spectrometer may be configured to apply voltages to the ion guide so as to urge ions downstream through the elongated tube. For example, in embodiments in which the second mode provides a potential barrier in the elongated tube, the ion guide may urge the ions downstream such that they pass over the potential barrier, e.g. in a mass selective manner. The spectrometer may comprise one or more voltage supplies, such as one or more DC voltage supplies, for supplying one or more voltages to electrodes of the ion guide for urging the ions along the ion guide. For example, the ion guide may comprise a plurality of electrodes that are spaced apart along the length of the elongated tube, and the spectrometer may comprise one or more voltage supplies configured to apply different voltages to different ones of the plurality of electrodes that are arranged at different positions along the length of the elongated tube for urging ions in the downstream direction. The one or more voltage supplies may be configured to apply different voltages, such as DC voltages, to different ones of the plurality of electrodes so as to generate a static DC gradient along the length of the elongated tube that urges ions in the downstream direction. Alternatively, or additionally, the one or more voltage supplies may be configured to sequentially apply a voltage, such as a DC voltage, to sequential electrodes along the ion guide such that a potential barrier is travelled along the ions guide so as to urge ions in the downstream direction. The gas cell may also comprise an ion guide extending through the main body portion of the gas cell for radially confining ions and / or urging ions downstream through the main body portion of the gas cell. This ion guide may be the same ion guide that extends through the elongated tube. Alternatively, a first ion guide may extend through the elongated tube and a second different ion guide may extend through the main body portion of the gas cell. As such, the gas cell comprises one or more ion guides. Each of the one or more ion guides may be a multipole rod set ion guide, such as a quadrupole rod set ion guide. Alternatively, each of the one or more ion guides may be an ion tunnel ion guide comprising a plurality of apertured plate electrodes or closed-loop electrodes that are arranged such that the apertures through the electrodes are aligned to form an ion guiding path therethrough. Alternatively, multiple different types of ion guides may be provided in the gas cell. For example, a first multipole ion guide may be arranged in the main body portion and a second multipole ion guide may be arranged in the elongated tube, where the first multipole ion guide is a higher order multipole than the second multipole ion guide. The first multipole ion guide may be an octopole ion guide or hexapole ion guide, and the second multipole ion guide may be a quadrupole ion guide. The spectrometer may comprise one or more ion guide configured to radially converge an ion beam passing through the gas cell such that it is radially compressed as it travels from the main body portion into the elongated tube. For example, the ion guide that extends through the elongated tube may exend at least partially into the main body portion, and this ion guide may be configured to radially converge the ion beam passing through the gas cell such that it is radially compressed as it travels from the main body portion into the elongated tube. Alternatively, a first ion guide may be provided in the main body portion and a second ion guide may be provided in the elongated tube, where the first and second ion guides are configured such that the second ion guide radially confines the ion beam to a smaller diameter than the first ion guide does. The control circuity may be configured to apply the first voltages to the ion guide in the first mode such that ions are axially trapped within the gas cell by a potential barrier, where the potential barrier is spaced apart from the downstream end of the ion guide. For example, the potential barrier may be spaced from the downstream end of the ion guide by a distance of >2 mm, >4 mm, >6 mm, >8 mm, >10 mm, >15 mm, >20 mm, >25 mm, >30 mm, >35 mm, >40 mm, >45 mm, or >50 mm. The spectrometer may be configured to maintain the pressure at the location of said potential barrier at >10'3 mbar. The region of the elongated tube that extends upstream from the potential barrier may also be at a pressure of >10'3 mbar. This relatively high pressure is useful for collisionally cooling the ions. At least a portion of the elongated tube that is downstream of the potential barrier maybe as a pressure of less than 10'3 mbar. The spectrometer may comprise one or more voltage supply for applying different voltages to different electrodes of the ion guide so as to provide a driving force on the ions in the downstream direction that increases with increasing proximity to the downstream end of the ion guide. This increases the urging force on the ions as the pressure within the elongated tube drops, enabling ions to be removed from the gas cell quickly and without compromising the phase space. The different voltages that provide the driving force that increases with increasing proximity to the downstream end of the ion guide may be provided to the length of the ion guide that is downstream of the potential barrier. In embodiments in which a static DC gradient is used to urge ions downstream through the ion guide, the magnitude of the static DC gradient may increase as a function of proximity to the exit of the ion guide. In embodiments in which travelling DC potentials are used to urge ions downstream through the ion guide, the amplitude of the DC potentials may increase as it travels downstream or the speed of the DC potentials may vary (e.g. decrease) as they travel downstream. Optionally, voltages for urging ions downstream through the ion guide are not applied to the downstream end portion of the ion guide. This may be unnecessary because the pressure in this region may be sufficiently low that minimal gas collisions occur. However, the RF voltage for radially confining the ions may be applied to the downstream end portion of the ion guide. The gas cell may comprise an ion guide configured to radially compress an ion beam as it passes from the main body portion to the elongated tube; wherein the gas cell is configured such that ions in the ion beam that have been radially compressed enter a length of the elongated tube having a pressure that is sufficiently high to collisionally cool these ions, and such that the pressure in the elongated tube decreases from said length in a downstream direction. For example, the gas cell may be configured such that ions in the ion beam that have been radially compressed enter a length of the elongated tube having a pressure of >10'3 mbar. Ions may be trapped in said length in the first mode and at least some of these ions are released or ejected from said length in said second mode. A gas pumping aperture may be provided through the wall of the elongated tube at, or proximate, the downstream end thereof, and the ion guide may extend to the axial location at which the gas pumping aperture is located. Additionally, or alternatively, the ion guide may extend beyond the downstream end of the elongated tube such that a portion of the ion guide is not enclosed by a wall of the gas cell. These embodiments enable the ions from the gas cell to be transitioned to the relatively low pressure at which the first vacuum chamber is maintained whilst still being confined in the ion guide. As such, the ions may be transmitted downstream of the gas cell with relatively minimal scattering caused by the background gas. For example, according to option (i) above, the portion of the ion guide that is at the axial location at which the gas pumping aperture is located may be maintained at the same pressure as the first vacuum chamber. The wall of the elongated tube may be arranged radially outward of the electrodes of the ion guide therein. Alternatively, the elongated tube may be formed by filling gaps between the electrodes of the ion guide, e.g. with electrically insulating material, such that gas cannot pass between the electrodes of the ion guide in the radial direction. However, this latter option is less preferable, e.g. as ions may impact the filling material and therefore charge may build up on it and this may affect the electric fields within the ion guide. For the avoidance of doubt, the radial direction referred to herein is any direction orthogonal to the longitudinal axis along which the one or more ion guide guides the ions. The spectrometer may be configured to transmit ions into the ion entrance of the gas cell and maintain the pressure in the main body portion of the gas cell such that the ions that enter the gas cell are: (i) fragmented by collision induced dissociation in the main body portion; or (ii) collisionally cooled by colliding with gas molecules in the main body portion. The spectrometer may be configured to accelerate ions into the gas cell so as to cause the ions to fragment due to collisions with the background gas in the gas cell, i.e. via Collision Induced Dissociation (CID). The spectrometer may comprise voltage supplies that supply voltages to electrodes within the spectrometer so as to generate a potential difference that causes this acceleration of the ions into the gas cell. Alternatively, the ions that enter the gas cell may be caused to collide with the gas in the gas cell so that the ions are collisionally activated, e.g. so as to undergo collisional induced unfolding substantially without fragmentation. Alternatively, it is contemplated that the ions may be caused to undergo alternative processes in the gas cell, such as being fragmented or reacted by other techniques. For example, the spectrometer may be configured to fragment ions in the gas cell by using an AC electric field to oscillate the ions through gas within the cell (CID), or by causing the ions to undergo Surface Induced Dissociation (SID), Electron Transfer Dissociation or Electron Capture Dissociation (ECD). Alternatively, the spectrometer may be configured to cause the ions to be reacted with other ions or neutral molecules in the gas cell that cause them to dissociate into fragment ions, or to be reacted with other ions or neutral molecules that cause them to form product ions other than fragment ions, such as adduct ions. Alternatively, ions may be separated by mobility in the gas cell by urging the ions through gas in the gas cell using one or more electric field. The spectrometer may be configured to maintain the gas pressure inside the main body portion of the gas cell at >10'3 mbar. The spectrometer may comprise a gas supply and gas conduit for supplying gas into the main body portion of the gas cell through a wall of the main body portion. Preferably, the gas cell is configured such that, in use, gas cannot escape from it other than through the ion entrance and through the downstream end of the elongated tube. The wall of the elongated tube may be sealed such that gas cannot escape through the wall over a length of the tube that is at least 4mm long, at least 5 mm long, at least 6 mm long, at least 7 mm long, at least 8 mm long, at least 9 mm long, or at least 10 mm long. Preferably, the elongated tube has a length of less than or equal to 50 mm. For example, it may be: less than or equal to 40 mm, less than or equal to 30 mm, less than or equal to 20 mm, or less than or equal to 10 mm. The main body portion of the gas cell may define a gas conduit therethrough that has a cross-sectional area that is larger than the cross-sectional area of the interior of the elongated tube, wherein cross-sectional area is in a plane orthogonal to an axis that extends from the ion entrance to the ion exit. The main body portion of the gas cell may extend from the ion entrance of the gas cell to the upstream end of the elongated tube. The gas cell may be arranged such that ions exit the gas cell into the first vacuum chamber. Alternatively, the spectrometer may further comprises a second vacuum chamber adjacent to the first vacuum chamber and that is maintained at a lower pressure than the first vacuum chamber, wherein the ion exit of the gas cell is a differential pumping aperture between the first and second vacuum chambers. As mentioned above, the spectrometer may comprise a mass analyser arranged to receive ions from the gas cell. The mass analyser may be a Time of Flight (TOF) mass analyser. The TOF mass analyser may have a pusher that receives ions from the gas cell and pushed packets of ions towards an ion detector. The TOF mass analyser may be a Multi-Reflecting TOF (MRTOF) mass analyser that reflects the ions multiple times using multiple ion mirrors as the ions drift from the pusher towards the ion detector. The spectrometer may be configured to maintain the mass analyser at a pressure of <10 4, <10 5, <10 6, <10 7, <10 s, or< 10'9. The mass analyser may be located in the first vacuum chamber or in a second vacuum that is downstream of the first vacuum chamber, where the spectrometer is configured to maintain the second vacuum chamber at a lower pressure than the first vacuum chamber. The spectrometer may comprise one or more vacuum pumps configured to pump down the first and second vacuum chambers such that the pressure at the downstream exit of the elongated tube is lower than the pressure at the upstream entrance to the elongated tube. From a second aspect the present invention provides a gas cell for a mass or mobility spectrometer comprising: an ion entrance at a first, upstream end thereof; an ion exit at a second, downstream end thereof; a main body portion between the ion entrance and ion exit; an elongated tube between the main body portion and the ion exit; an ion guide for radially confining ions within the elongated tube; and control circuity configured to operate the gas cell in a first mode in which first voltages are applied to the ion guide such that ions are axially trapped within the gas cell, and to operate the gas cell in a second mode in which second voltages are applied to the ion guide such that at least some of these ions are ejected or released from the gas cell. The gas cell may have any of the features described above in relation to the first aspect of the present invention, without being limited to having the other features of the spectrometer such as the first vacuum chamber. The present invention also provides a method of mass and / or mobility spectrometry comprising performing the techniques described herein. Accordingly, the present invention provides a method of mass and / or mobility spectrometry comprising: providing a spectrometer or gas cell as described herein; introducing ions into the gas cell; and detecting ions that have exited the gas cell, or ions derived therefrom, so as to determine their mass to charge ratio and / or mobility. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments together with other arrangements given for illustrative purposes only will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a mass spectrometer according to a known arrangement; Figs. 2A and 2B show schematics of collision cells according to embodiments of the present invention; Fig. 3 shows an example of an ion guide that may be used inside the collision; Fig. 4A shows how ions exit the collision cell according to an embodiment of the present invention having an elongated tube as shown in Fig. 4B; Fig. 5 shows an example of how the probability of transmission of gas through a tube having a circular cross-sectional shape varies; Fig. 6 represents the cross-sectional area defined by the perimeter along the inside surfaces of a quadrupole ion guide; and Figs. 7-9 show examples of how the probability of transmission of gas through tubes having different cross-sectional shapes vary. DETAILED DESCRIPTION Fig. 1 shows a mass spectrometer according to a known arrangement. The spectrometer comprises an ion source 2 for generating ions, a first ion guide 3 arranged in a first vacuum chamber 4, a second ion guide 5 arranged in a second vacuum chamber 6, a quadrupole mass filter 7 and a collision cell 8 comprising a quadrupole rod set ion guide 9 arranged in a third vacuum chamber 10. The spectrometer also comprises ion optics 11 arranged in a fourth vacuum chamber 12 for focussing ions into a TOF mass analyser 13 that is arranged in a fifth vacuum chamber 14. Gas is pumped out of the vacuum chambers by one or more vacuum pump so as to reduce the pressure inside them. In use, ions are generated by the ion source 2 and pass into the first vacuum chamber 4. The ions are guided through the first and second vacuum chambers 4,6, by the first and second ion guides 3,5 respectively, and into the third vacuum chamber 10. The ions then pass into the mass filter 7, which is set to be capable of only transmitting ions having a mass to charge ratio in a restricted range of mass to charge ratios. The precursor ions that are transmitted by the mass filter are accelerated into the collision cell 8 and caused to fragment into fragment ions due to collisions with the gas molecules in the collision cell 8, i.e. via Collision Induced Dissociation (CID). Voltages are applied to the ion guide 9 in the collision cell 8 such that the fragment ions are trapped therein and guided out of the exit of the collision cell. The fragment ions exit from the collision cell 8, as will be described in more detail below, pass through the ion optics 11 in the fourth vacuum chamber 12 and into the fifth vacuum chamber 14. The ions then arrive at the pusher 15 of the TOF mass analyser 13, and a voltage pulse is applied to the pusher 15 so that the pusher pushes a packet of the ions into the time of flight region 16 towards an ion mirror 17. The ions are reflected back through the time of flight region 16 by the ion mirror 17 and onto the ion detector 18. The ions can therefore be mass analysed by the TOF mass analyser 13 in the known manner, i.e. by determining the mass to charge ratios of the ions based on their flight time from the pusher 15 to the detector 18. In order to fragment the precursor ions in the collision cell 8, and also to collisionally cool the resulting fragment ions prior to mass analysis in the TOF mass analyser 13, the collision cell 8 is maintained at a relatively high pressure, such as between 10'3 and 5x1 O'2 mbar. In contrast, the pressure in the other vacuum chambers, such as vacuum chambers 10, 12 and 14, is desired to be relatively low such that the ions have minimal collisions with the background gas therein as the ions pass through these vacuum chambers. This is particularly important for the vacuum chamber 14 that houses the TOF mass analyser 13. For example, it is typically desired for this vacuum chamber 14 to be at a pressure between 10'4 and 10'7 mbar, or even lower. The vacuum chambers are evacuated by vacuum pumps in order to achieve the desired pressures therein. Generally the further downstream that a vacuum chamber is arranged, the lower the pressure that vacuum chamber is maintained at. However, the collision cell 8 is maintained at a relatively high pressure compared to the regions upstream and downstream of it, so as to enable the fragmentation and collisional cooling of the ions to take place. This relatively high pressure in the collision cell 8 may be achieved by introducing gas into the collision cell via a conduit 19. The collision cell 8 has entrance and exit apertures for allowing ions to enter and exit the collision cell, respectively. The ion entrance and exit apertures are provided in thin plates. The cross-sectional area of each of the entrance and exit apertures is significantly smaller than the average cross-sectional area of the volume inside the collision cell 8 that contains gas. As such, the entrance and exit apertures control the rate at which gas exits the relatively high pressure collision cell 8 into the surrounding area. These apertures are therefore known as (gas) conductance limiting apertures. It has been recognised that such thin plate conductance limiting apertures can be problematic, particularly at the ion exit aperture of the collision cell 8. More specifically, the thin plate results in a relatively large pressure gradient between the upstream and downstream sides of the exit aperture, which causes a relatively fast gas flow out of the collision cell 8 that can cause scattering of the ions. This can affect how downstream ion-optical elements process the ions. For example, such ion scattering may broaden the range of energies, radial positions and divergence of ions exiting the collision cell, which may affect the accuracy and the mass resolution with which the TOF mass analyser 13 is able to mass analyse the ions. The inventors have recognised that it is desirable to configure the collision cell such that the ions do not encounter a large pressure gradient as they travel towards and pass through the exit aperture, but rather that the pressure drops gradually over a relatively long length of the collision cell towards the exit. As such, the ions experience a relatively low pressure gradient as they travel towards the exit and pass out of the collision cell. This minimises the range of energies, radial positions and divergence of ions exiting the collision cell, e.g. by minimising ion scattering caused by gas flow out of the exit. This is particularly beneficial in arrangements where the ions exiting the collision cell are directed into a mass analyser, since it is typically desired for the ion beam to meet certain conditions prior to mass analysis, such as having a relatively low range of energies and / or spatial positions. The inventors have realised that this can be achieved by providing the collision cell with an elongated tube that limits the gas conductance out of the collision cell, rather than only using an aperture in a thin plate to limit the gas conductance out of the collision cell, as has been conventionally used. Ions may be radially confined within the elongated tube by applying voltages, such as RF voltages, to electrodes of an ion guide arranged within the elongated tube. The elongated tube is desirably configured such that the pressure inside the downstream end of the ion guide therein is the same as the pressure of the vacuum chamber that the ions enter when they leave the collision cell. As such, the ions remain radially confined by the ion guide of the collision cell until they are at a relatively low pressure at which there will be a relatively low rate of collisions with gas molecules. Fig. 2A shows a schematic of a collision cell 20 according to an embodiment of the present invention. The collision cell comprises a housing 21 for retaining gas therein, although it will be appreciated that gas will escape from the housing through the ion entrance 22 into the housing and the ion exit 23 from the housing. The collision cell has a main body portion 24 at the upstream end that receives ions through the ion entrance. The main body portion is configured such that it defines an elongated gas conduit therethrough that has a relatively large internal cross-sectional area, in the plane orthogonal to the longitudinal axis from the entrance 22 to the exit 23. The collision cell also has a downstream portion 25 that is an elongated tube having a gas conduit therethrough that has a smaller internal cross-sectional area (in the plane orthogonal to the longitudinal axis from the entrance 22 to the exit 23) than the main body portion 24, such that the elongated tube 25 defines the gas conductance limit for gas leaving the collision cell 20 at the downstream end. The collision cell may also have a gas conduit 26 for supplying gas into the collision cell through the wall 21 of the collision cell, for reasons that will be described below. Optionally, a gas pumping port 27 is provided through the wall of the elongated tube 25 proximate or at its downstream end, for reasons that will be described below. The gas pumping port 27 may be a recess, such as a slot, through the wall of the elongated tube that extends a distance from the downstream end of the tube part way upstream along the tube. In other words, the circumference of the gas pumping port 27 may be only partially bounded by the wall of the tube and may be open at the downstream end of the tube. Less preferably, the gas pumping port 27 may be an aperture that has a circumference that is entirely bounded by the wall of the tube. The collision cell 20 also comprises one or more ion guide 28 for radially confining ions therein and for guiding the ions in the downstream direction from the ion entrance 22 towards the ion exit 23. Each of the one or more ion guides 28 may be of any form. For example, each ion guide may be an ion tunnel ion guide such as a stacked ring ion guide, or may be a multipole rod set ion guide such as an axially segmented multipole rod set ion guide. It is also contemplated that multiple different forms of ion guide may be provided in the collision cell in order to guide the ions from the ion entrance to the ion exit. In the embodiment depicted in Fig. 2A, an axially segmented multipole rod set ion guide 28 extends through the collision cell 20, where the ion guide has a relatively large inscribed radius over a first length 28a that is arranged in the main body 24 of the collision cell and that extends downstream from the ion entrance 22, a relatively smaller inscribed radius over a second length 28b that extends through the elongated tube 25 to the ion exit 23, and an ion funnel portion 28c between the first and second lengths and that has an inscribed radius that decreases in the downstream direction so as to funnel ions from the first length of the ion guide into the second length of ion guide. In this embodiment the ion funnel portion is formed from a plurality of axial segments of the multipole rod set ion guide, where these different axial segments have different inscribed radii and are arranged such that the radii decrease in the downstream direction. One or more AC voltage supply is connected to the ion guide 28 so as to apply one or more AC voltages to electrodes of the ion guide so as to radially confine ions inside the ion guide. The amplitude and / or phase of the one or more AC voltage that is applied to the ion guide may differ for different portions of the ion guide. For example, a first AC voltage having a first amplitude and phase may be applied to the first length 28a that is optimised for radially confining the precursor ions that enter the collision cell and the fragment ions that have relatively high energy when they are produced. A second AC voltage having a different amplitude and / or phase may be applied to the second length 28b that is optimised for radially confining fragment ions after they have been collisionally cooled in the collision cell. The second AC voltage may also be applied to the ion funnel portion 28c, or a third AC voltage having an amplitude and / or phase that is different to that of the first and / or second AC voltage may be applied to the ion funnel portion. The third AC voltage may be selected to be optimised for funneling ions through the ion funnel portion. One or more voltage supply may also be provided that applies voltages to the electrodes of the ion guide so as to urge ions in the downstream direction. For example, different DC voltages may be applied to different axial segments of the ion guide 28 so as to form a DC potential difference along the ion guide that urges the ions in the downstream direction. The DC potential differences along the first length 28a, second length 28b and ion funnel portion 28c of the ion guide may be the same or may be different from each other. For example, the DC potential difference along the first length may be relatively high so as to urge ions through the relatively high pressure gas in that region, whereas the DC potential difference along the second length may be lower as that region is at a lower pressure and so it requires less force to urge the ions through it. The DC potential difference along the ion funnel portion is selected so as to urge ions through the converging funnel, and may be different to the DC potential gradients arranged along the first and second lengths. Alternatively or additionally to using DC potential differences to urge ions downstream, a DC voltage may be successively applied to successive electrodes of the ion guide such that a DC potential repeatedly travels along the ion guide so as to urge ions from the entrance to the exit of the ion guide. The DC potential may have different amplitudes and / or speeds in the first length 28a, second length 28b and ion funnel portion. For example, the DC potential may have a first amplitude and speed along the first length, whereas it may have a different amplitude and / or speed along the second length. The amplitude and / or speed of the DC potential in the ion funnel portion is selected so as to urge ions through the converging funnel, and may be different to that of the DC potentials that travel along the first and second lengths. Although the DC voltages for urging ions along the ion guide 28 have been described as being applied to the AC electrodes of the ion guide that radially confine the ions therein, it is contemplated that the DC voltages may instead be applied to other electrodes. For example, DC electrodes may be arranged radially outwards of the AC electrodes or circumferentially between them. For instance, the electrodes may be vane, rod or plate electrodes that are shaped or positioned relative to the central axis of the ion guide such that when one or more DC voltage is applied to them the ions are urged axially along the ion guide. AC and / or DC voltages are also applied to electrodes of the ion guide 28b for preventing at least some of the ions from passing downstream and out of the collision cell, for reasons that will be discussed further below. For example, AC and / or DC voltages may be applied to electrodes of the ion guide 28b in the elongated tube 25 so as to axially confine ions within the collision cell until it is desired to eject them towards a downstream mass analyser, such as a Time of Flight (TOF) mass analyser. For instance, the ions may be axially trapped within the elongated tube 25 for a time period sufficient to allow them to be collisionally cooled and thermalised before the ions are then caused to be ejected from the collision cell towards the mass analyser. Alternatively, or additionally, ions may be axially trapped within the elongated tube 25 until it is desired to eject them to the mass analyser, as will be described further below. Although the first length, second length and ion funnel portion of the ion guide have been described as being an axially segmented multipole rod set ion guide, such as a quadrupole ion guide, it is contemplated that a different type of ion guide may be used. For example, the ion guide may be formed from a plurality of apertured plates, such as a ring electrodes, that are stacked in the axial direction with the apertures aligned so as to form an ion guiding channel therethrough. Additionally, or alternatively, the different portions of the ion guide may be formed from different types of ion guide. For example, a first relatively high order multipole ion guide such as an octopole ion guide may be arranged at the upstream end of the collision cell and a second lower order multipole ion guide such as a quadrupole ion guide may be arranged at the downstream end of the collision cell. This arrangement provides an ion guide having a relatively high ion acceptance at the upstream end of the collision cell and an ion guide having relatively high ion focusing at the downstream end of the collision cell. The downstream end of the first ion guide, or the upstream end of the second ion guide, may have an inscribed diameter that decreases as a function of position in the downstream direction so that ions are funnelled down as they move towards the exit of the collision cell. Alternatively, a third ion guide may be arranged between the first and second ion guides for funnelling the ions from the first ion guide into the second ion guide. The third ion guide may be of a different form to the first and second ion guides, such as being formed from a plurality of plates electrodes that are stacked in the axial direction, e.g. a stacked of apertured plates such as a ring ion guide. Fig. 2B shows another embodiment that is the same as that shown in Fig. 2A, except that there is no ion funnel portion that gradually funnels the ions. As described above, the ion guide 28 in the elongated tube 25 may take any form. However, multipole ion guides, such as quadrupole ion guides, have been found to be advantageous. Fig. 3 shows a cross-sectional view through the elongated tube 25 of an embodiment of the invention. In this embodiment the elongated tube comprises an axially segmented quadrupole ion guide 28. The electrodes of the quadrupole ion guide are located on the edges of printed circuit boards (PCBs) 29 that are configured to supply the required voltages to the electrodes. More specifically, the PCBs 29 are connected to one or more AC voltage supply and include conductive traces that supply one or more AC voltage to the electrodes of the ion guide 28 so as to radially confine ions therein. The PCBs may also be connected to one or more DC voltage supply and include conductive traces that supply one or more DC voltage to the electrodes of the ion guide so as to urge ions axially along the ions guide, e.g. in one or the manners described above. The PCBs are connected to one or more AC and / or DC voltage supply and include conductive traces that supply one or more AC and / or DC voltage to electrodes of the ion guide so as to prevent at least some of the ions from passing downstream and out of the collision cell, for reasons that will be discussed further below. The electrodes that form any given pole of the quadrupole ion guide 28 may be located on the edge of the same PCB 29, such that the conductive traces on the PCB supply voltages from the voltage supplies to those electrodes. As such, four PCBs 29 are provided for the four respective poles of the ion guide 28. A wall 30 is provided circumferentially around the ion guide for substantially preventing gas from flowing radially out of the ion guide through the gaps between the electrodes of the ion guide. In the depicted embodiment this wall 30 around the ion guide 28 may be provided by arranging the PCBs 29 with their major surfaces parallel to each other and stacking them together in a manner such that each PCB 29 in the stack is sandwiched between other layers. For example, each PCB 29 may be sandwiched between layers of electrically insulating material such as PEEK. The layers are stacked so as to define a conduit through the stack in which the ion guide 28 is located, where the wall 30 of the conduit is arranged so as to substantially prevent gas from flowing radially out of the ion guide 28 through the gaps between the electrodes of the ion guide. In the depicted embodiment, two of the PCBs 29 for two respective poles of the multipole are arranged on a first layer of insulating material 31 such that these PCBs are in the same plane with their edges that face each other being spaced apart so as to provide space for the electrodes of the ion guide 28 that are attached to these edges. Second layers of insulating material 32 are arranged on the other side of these PCBs to the first layer of insulating material 31. The remaining two PCBs for the other poles of the multipole are arranged on these second layers of insulating material 32 such that said remaining two PCBs are in the same plane with their edges that face each other being spaced apart so as to provide space for the electrodes of the ion guide 28 that are attached to these edges. A third layer of insulating material 33 is then arranged on the other side of said two remaining PCBs to the second layers of insulating material 32 such that the layers of insulating material and PCBs form the conduit in which the ion guide 28 is located. The edges of the PCBs 29 to which the electrodes are connected may extend beyond the edges of the layers of insulating material 31,32,33 that they are sandwiched between such that the PCBs 29 extend into the conduit. This assists in mounting the electrodes at their desired locations in the conduit, and enables electrodes having a wider variety of shapes and sizes to be employed as the location at which each electrode is mounted is spaced apart from the wall of the conduit. During use, some stray ions may pass through the gap between each adjacent pair of poles of the multipole ion guide 28 and impact on the wall 30 of the conduit. In order to avoid this causing electrical charge to build up on the wall, which could affect the electric field in the ion guide, an electrically conductive strip 34 may be provided on the portion of the wall 30 adjacent to each of the gaps for conducting the electrical charge from the stray ions away from the ion guide 28. These conductive strips 34 may be grounded by electrical connections 35 that extend through the insulating layers 31,32,33. The electrically conductive strips 34 may therefore be arranged to extend over at least a portion of the length of the ion guide 28. The electrically conductive strips may be embedded in the electrically insulating layers such that their surfaces that face the ion guide are flush with the surfaces of the insulating layers that face the ion guide. This helps minimise the distance between the wall 30 of the conduit and the electrodes of the ion guide, which helps minimise gas flow out of the ion guide in the radial direction. However, it is contemplated that the electrically conductive strips may simply be arranged on top of the insulating layers rather than being embedded in them. Although the embodiment in Fig. 3 illustrates an axially segmented quadrupole rod set ion guide 28, it is contemplated that other forms of ion guide may be confined in a stack of layers in a similar manner, such as higher order multipole rod-set ion guides. In use, the collision cell 20 is arranged in a mass spectrometer such that the pressure inside the main body 24 of the collision cell is higher than the pressure outside the exit 23 of the collision cell, and optionally also higher than the pressure outside the ion entrance 22 of the collision cell. For example, the collision cell described in relation to Fig. 1 may be replaced with a collision cell 20 according to an embodiment of the present invention. As such, an embodiment of the present invention provides a mass spectrometer comprising an ion source 2 for generating ions, a first ion guide 3 arranged in a first vacuum chamber 4, a second ion guide 5 arranged in a second vacuum chamber 6, and a quadrupole mass filter 7 and collision cell 20 arranged in a third vacuum chamber 10. The spectrometer also comprises ion optics 11 that may be arranged in a fourth vacuum chamber 12 for focusing ions into a mass analyser, such as a Time of Flight mass analyser that is arranged in a fifth vacuum chamber 14. The vacuum chambers are pumped by vacuum pumps such that the vacuum chambers that are further downstream have lower pressures. For example, the ion source chamber may be substantially at atmospheric pressure and the vacuum chamber 14 in which the mass analyser 13 is located may be at a pressure of between 10'4 and 10'9 mbar. As described above, the collision cell 20 may be supplied with a gas conduit 26 such that the pressure inside the main body 24 of the collision cell is higher than the pressure of the vacuum chamber 10 in which the collision cell is located. For example, the pressure inside the main body 24 of the collision cell may be between 10'3 and 5x1 O'2 mbar. As such, gas escapes through the ion entrance 22 and ion exit 23 of the collision cell. In the embodiments that include the gas pumping port 27 in the wall of the elongated tube 25, gas also leaves the collision cell through this port such that the exit end of the elongated tube may be pumped down to the pressure of the vacuum chamber 10 that the collision cell is arranged within. Ions are generated by the ion source 2 and pass into the first vacuum chamber 4. The ions are guided through the first and second vacuum chambers 4,6, by the first and second ion guides 3,5 respectively, and into the third vacuum chamber 10. The ions then pass into the mass filter 7, which may be set to be only capable of transmitting ions having a mass to charge ratio in a restricted range of mass to charge ratios, such as only being able to transmit a single precursor ion species. Ions that are capable of being transmitted by the mass filter 7 are radially confined by the mass filter and guided along it in the downstream direction and then pass into the collision cell 20. In contrast, ions that are not capable of being transmitted by the mass filter are filtered out by the mass filter and are not transmitted into the collision cell. The precursor ions that are transmitted by the mass filter are accelerated into the ion entrance 22 of the collision cell 20 and into the main body 24 that is maintained at a relatively high pressure. The ions may be accelerated in this manner by applying a potential difference between components of the mass spectrometer. As such, the ions are caused to fragment into fragment ions due to collisions with the gas molecules in the main body of the collision cell, i.e. via Collision Induced Dissociation (CID). The resulting fragment ions are then collisionally cooled, i.e. thermalised, inside the collision cell by being collided with the gas molecules. As described above, voltages are applied to the ion guide(s) 28 in the collision cell so as to radially confine the precursor ions that enter the ion guide and also the fragment ions that are produced. Voltages may also be applied to the ion guide(s) 28 so as to guide the ions downstream from the main body portion 24 into the elongated tube 25 and towards the ion exit 23. The interior of the elongated tube 25 has a smaller cross-sectional area than that of the main body 24 of the collision cell 20 so as to limit the gas conductance out of the downstream end of the collision cell. As such, the beam of ions passing axially through the collision cell may be caused to radially converge by the ion guide(s) 28 inside the collision cell, e.g. by an ion funnel, such that they are able to pass from the main body 24 of the collision cell into the elongated tube 25. As gas is unable to escape the collision cell other than via the ion entrance 22 and ion exit 23, the gas pressure within the elongated tube 25 decreases as a function of distance towards its downstream end. As the elongated tube limits the gas conductance out of the collision cell and is relatively long, the pressure gradient along the elongated tube, and at its downstream end, is relatively low. This minimises the range of energies, radial positions and divergence of ions exiting the collision cell, when they are caused to do so. In embodiments in which the wall of the elongated tube 25 includes the gas pumping aperture 27, gas is evacuated from the collision cell through the gas pumping aperture. In these embodiments the axial portion of the elongated tube 25 at which the gas pumping aperture is located may be maintained at substantially the same pressure as the vacuum chamber that the collision cell is arranged within. In such embodiments the portion of the elongated tube 25 between the main body 24 of the collision cell and the gas pumping aperture 27 limits the gas conductance out of the downstream end of the collision cell. As this portion is relatively long, the pressure gradient across it is relatively low. In these embodiments the ions passing through the ion guide 28 in the elongated tube 25 will enter a region of the elongated tube which is at the same pressure at the vacuum chamber that the collision cell is arranged within, whilst still being confined within the ion guide inside the elongated tube. As such, the ions will be subjected to a relatively low rate of collisions with gas molecules as they exit the collision cell, which helps minimise ion scattering and improves transmission of the ions in the downstream direction. By way of example only, the portion of the ion guide in the main body 24 of the collision cell that receives the precursor ions may have an inscribed diameter of 5mm, whereas the portion of the ion guide in the elongated tube 25 may have an inscribed diameter of 3mm. However, these ion guide portions may have other diameters. As the ion beam is compressed as it travels from the main body to the elongated tube of the collision cell, it is desirable to collisionally cool the compressed ion beam within the elongated tube. As such, the collision cell may be configured such that a length of the elongated tube that is downstream of where the ion beam is radially compressed is maintained at a sufficiently high pressure to thermalise the ions in the compressed ion beam. The pressure inside the elongated tube decreases from this thermalisation region to the downstream end of the elongated tube. The fragment ions exit from the collision cell, pass through the ion optics 11 in the fourth vacuum chamber 12 and into the fifth vacuum chamber 14. The ions then arrive at the pusher 18 of the TOF mass analyser 13, and a voltage pulse is applied to the pusher so that the pusher pushes a packet of the ions into the time of flight region 16 (e.g. a field-free region) towards an ion detector 18. The ions separate out according to their mass to charge ratios as they pass through the time of flight region, and then strike the ion detector. As such, the separated ions arrive at the ion detector at different times, wherein the time at which an ion arrives at the detector is related to its mass to charge ratio. The mass to charge ratio of any given ion can be determined from the duration of time between the time at which it was pushed into the time of flight region and the time at which it was detected by the ion detector. The TOF mass analyser 26 is therefore able to obtain data from the signal detected at the detector and determine the mass to charge ratios of the ions pushed into the mass analyser, and their intensities, and form a mass spectrum. The ions may be reflected by one or more ion mirror 17 between the pusher 15 and the ion detector 18 in order to provide a relatively long flight path through the time of flight region 16. This enables ions of different mass to charge ratios to separate out to a greater degree as they travel though the time of flight region and hence provides the TOF mass analyser with a higher mass resolution. The TOF mass analyser may be a Multi-Reflecting TOF mass analyser that reflects the ions multiple times using multiple ion mirrors as the ions drift from the pusher towards the ion detector. Such mass analysers are required to be maintained at a particularly low pressure, and as such the collision cell of the present invention is particularly beneficial in mass spectrometers having these mass analysers since there may be a relatively large pressure difference between the pressure inside the main body 24 of the collision cell and the pressure of the vacuum chamber that it is arranged within. The gas pressure and path length between the exit 23 of the collision cell and the pusher of the TOF mass analyser are such that the ions will encounter relatively few collisions with gas molecules between these two devices. Therefore, ions that have different mass to charge ratios will have different flight times from the collision cell to the pusher of the TOF mass analyser. More specifically, ions having a relatively low mass to charge ratio will have a shorter flight time than ions having a higher mass to charge ratio. However, it may be desired that all ions of interest arrive at the pusher at substantially the same time, i.e. at the time at which a voltage pulse is applied to the pusher so as to push ions into the time of flight region of the TOF mass analyser. If this does not occur then some ions will pass beyond the pusher before the voltage pulse is applied to it (i.e. before it is activated), or will arrive at the pusher after the voltage pulse has been applied to it (i.e. after it has been deactivated). These ions will not be mass analysed by the TOF mass analyser, resulting in the TOF mass analyser having a relatively low duty cycle. In order to improve the duty cycle of the TOF mass analyser, embodiments of the present invention release the ions from the collision cell to the TOF mass analyser in reverse order of mass to charge ratio such that all of the ions of interest are at the pusher at the time it is activated, i.e. at the time that the voltage pulse is applied to the pusher. In other words, ions may be mass selectively ejected from the collision cell such that ions having a relatively high mass to charge ratio are ejected first and then ions of progressively lower mass to charge ratios are ejected as time progresses. As the ions of relatively high mass to charge ratio travel to the pusher relatively slowly and ions of lower mass to charge ratio travel to the pusher faster, this process may be controlled such that all ions of interest arrive at the pusher at the time it is activated and hence all of these ions are mass analysed. In order to do this, a pseudo-potential barrier may be generated within the elongated tube 25 by applying one or more AC voltage to at least one axial segment of the ion guide 28b therein, for preventing ions passing the pseudo-potential barrier in the axial direction. Ions may be urged along the ion guide 28b in the downstream direction to the pseudo-potential barrier, e.g. by maintaining a potential difference along that ion guide and / or by successively applying an electrical potential to successive axial segments of the ion guide such that a potential barrier is repeatedly travelled in the downstream direction along the ion guide. The AC voltage(s) that provides the pseudo-potential barrier may initially be set so as to prevent all ions, or at least all ions of interest, from passing the pseudo-potential barrier in the downstream direction such that these ions cannot exit the collision cell. When it is desired to eject ions from the collision cell to the TOF mass analyser, the ions are ejected in reverse order of mass to charge ratio by the following process. The AC voltage(s) applied to electrodes of the ion guide so as to provide the pseudo-potential barrier is varied, e.g. by varying the amplitude and / or frequency of the AC voltage, such that the highest mass to charge ratio ions are able to be urged downstream beyond the pseudo-potential barrier and to travel to the exit of the collision cell, whereas the pseudopotential barrier prevents ions of lower mass to charge ratio from being urged downstream beyond the pseudo-potential barrier and they remain trapped within the collision cell. The amplitude and / or frequency of the AC voltage may then be progressively varied with time such that ions of progressively lower mass to charge ratios are able to be urged downstream beyond the pseudo-potential barrier as time progresses. For example, the amplitude and / or frequency of the AC voltage may be progressively and continuously scanned with time so as to achieve the above described effect. Less preferably, the amplitude and / or frequency of the AC voltage may be stepped between different spaced apart values with time so as to achieve the above described effect. The rate at which the AC voltage is varied with time is selected such that ions having the whole range of mass to charge ratios that were trapped in the collision cell 24, or at least ions having a range of mass to charge ratios of interest, arrive at the pusher of the TOF mass analyser 26 at substantially the same time. Accordingly, the AC voltage(s) used to form the pseudo-potential barrier are varied in synchronism with the time at which the pusher of the TOF mass analyser is activated such that at least the ions having the mass to charge ratios of interest arrive at the pusher at substantially the same time such that they are mass analysed together. In order to fragment the ions in the collision cell, and also to collisionally cool and thermalise the fragment ions prior to mass analysis in the TOF mass analyser, at least part of the collision cell is maintained at a relatively high pressure, such as >10'3 mbar or between 10'3 and 5x1 O'2 mbar. For example, the portion of the collision cell at, and upstream of, the pseudo-potential barrier may be at such a relatively high pressure. As the pressure downstream of the collision cell, and hence at the exit of the collision cell, will be lower than this, it is desired that the pseudo-potential barrier is provided at a location within the collision cell that is spaced upstream from the exit of the elongated tube 25. In embodiments in which the wall of the elongated tube 25 includes the gas pumping aperture 27, the pseudo-potential barrier is provided at a location within the collision cell that is spaced upstream from the gas pumping aperture 27. Fig. 4A shows a SIMION model illustrating how ions exit a collision cell according to an embodiment of the present invention having an elongated tube as shown in Fig. 4B. In the model, the elongated tube 25 of the collision cell is an axially segmented quadrupole rod set of the type described in relation to Fig. 3. The electrodes of the segmented quadrupole rod set were modelled to have an inscribed radius ro of 2.5 mm. Each axial segment was modelled as having an axial length of 4 mm and a spacing of 1 mm between axially adjacent segments, with two phases of an RF voltage of 300 V (peak-to-peak) and 2 MHz being applied thereto so as to form a quadrupolar radial ion confinement field. However, the axial segments 40 to which the AC voltage for forming the pseudo-potential barrier is applied (i.e. the four circumferentially spaced segments at the same axial location) was modelled as being 0.5 mm long, with only a single phase of a 2 MHz AC voltage being applied thereto. As described above, the pseudo-potential barrier axially traps a population of ions in the collision cell upstream of the pseudo-potential barrier. The voltages that are applied to the collision cell are then changed so as to allow mass selective ejection of the ions from the collision cell and onward transmission of these ions in the downstream direction. In the trapping mode, the barrier electrodes 40 were modelled as having a DC voltage of 4 V. The set of four axial electrodes 42 adjacent the upstream side of the barrier electrodes 40 were modelled as having a DC voltage of 0 V. The set of four axial electrodes 44 upstream of and adjacent to these electrodes were modelled as having a DC voltage of -0.5 V. The segmented electrodes between these electrodes 44 and the upstream end of the elongated tube 25 were modelled to have DC voltages so as to provide a DC axial gradient of 0.005 V / mm for urging ions downstream along the collision cell to the pseudo-potential barrier. The amplitude of the AC voltage for forming the pseudo-potential barrier was modelled as being 1 kV (peak-to-peak) in the trapping mode, with a trapping time of 500 ps. In the ion ejection mode, the barrier electrodes 40 were modelled as having a DC voltage of -1 V. The set of four axial electrodes 42 adjacent the upstream side of the barrier electrodes 40 were modelled as having a DC voltage of 0.3 V. The set of four axial electrodes 44 upstream of and adjacent to these electrodes 42 were modelled as having a DC voltage of -0.1 V. The segmented electrodes between these electrodes 44 and the upstream end of the elongated tube 25 were modelled to have DC voltages so as to provide a DC axial gradient of 0.005 V / mm for urging ions downstream along the collision cell to the pseudo-potential barrier. The segmented electrodes between the barrier electrodes 40 and the exit opening 23 of the collision cell were modelled to each have a DC voltage of -2V. The amplitude of the AC voltage for forming the pseudo-potential barrier was modelled as being linearly scanned down from an amplitude of 1 kV (peak-to-peak) to an amplitude of 500 V (peak-to-peak) over a time period of 500 ps. The model did not include a pressure gradient along the elongated tube 25, but rather serves to illustrate that ions can be trapped within the elongated tube 25 and mass selectively ejected. The gas pressure in the collision cell was modelled as being 6 x10-3 Torr of Nitrogen. Fig. 4A shows the intensity of ions having just passed the pseudo-potential barrier, in the downstream direction, as a function of time since the pseudo-potential barrier started to be scanned down. Fig. 4A shows five plots for ions of five different mass to charge ratios. More specifically, Fig. 4A shows a first plot 50 for ions of m / z = 1000, a second plot 51 for ions of m / z = 800, a third plot 52 for ions of m / z = 600, a fourth plot 53 for ions of m / z = 400, and a fifth plot 54 for ions of m / z = 200. As can be seen, the ions of highest mass to charge ratio are driven over the pseudo-potential barrier first, followed by progressively lower mass to charge ratios as the pseudo-potential barrier is progressively scanned down. As the ions traverse the portion of the collision cell between the pseudo-potential barrier and the exit opening 23 they may experience further collisional cooling and a gradual drop in pressure before they exit the collision cell. The ions may be urged from the downstream side of the pseudo-potential barrier to the exit opening 23 of the collision cell by maintaining a static DC gradient along the portion of the ion guide 28b in this region, or by repeatedly travelling DC potentials along this portion in the downstream direction. This may serve to ensure that ions continue to pass through the collision cell and exit it in reverse order of mass to charge ratio, such that the ions of different mass to charge ratio arrive at the pusher of the TOF mass analyser at the same time, and at the time that a voltage pulse is applied to the pusher so as to push the ions into the time of flight region to be mass analysed. In the embodiments in which a static DC gradient or travelling DC potentials are provided along the downstream end portion of the ion guide 28b, the static DC gradient or travelling DC potentials may be configured such that the drive force that they apply to the ions increases as a function of proximity to the exit opening 23 of the collision cell. For example, in embodiments in which a static DC gradient is used, the magnitude of the static DC gradient may increase as a function of proximity to the exit opening 23 of the collision cell. In embodiments in which travelling DC potentials are used, the amplitude of the DC potentials may increase as it travels downstream and / or the speed of the DC potentials may vary (e.g. decrease) as they travel downstream. This increases the urging force on the ions as the pressure within the collision cell drops, enabling ions to be removed from the collision cell quickly and without compromising the phase space. The DC gradient or travelling DC potentials may not be applied along a length at the very end of the collision cell, e.g. along a length where the pressure is sufficiently low that minimal collisional cooling occurs. Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims. For example, the above described mass spectrometer is merely an example of a mass spectrometer in which the collision cell may be mounted. It will be appreciated that the mass spectrometer may have fewer, additional or different ion-optical components and / or vacuum chambers to those described above. For example, the ion source 2 may be located in a vacuum chamber instead of being an atmospheric pressure ion source. Additionally, or alternatively, the first vacuum chamber 4 and / or second vacuum chamber 6 (and the ion guide(s) therein) may be omitted. Additionally, or alternatively, the mass filter 7 may be omitted or replaced with an ion guide. It is also contemplated that a different type of mass analyser to the TOF mass analyser 13 may be used, such as an electrostatic ion trap mass analyser, a Fourier Transform mass analyser or a quadrupole mass analyser. Furthermore, the collision cell may be arranged in a vacuum chamber such that the ion exit 23 of the collision cell is spaced apart from the differential pumping aperture in the downstream wall of that vacuum chamber, or such that the ion exit of the collision cell forms the differential pumping aperture. Although embodiments have been described in which the collision cell housing 21 is separate from the ion guide(s) 28, it is contemplated that the collision cell housing (i.e. the walls of the collision cell) may be formed by blocking the gaps between the electrodes of the ions guide(s) inside the collision cell. For example, for ion guides having electrodes that are spaced apart from each other in the circumferential direction by gaps, such as multipole ion guides, these gaps may be filled with a filler material, such an electrical insulator, in order to prevent gas escaping the collision cell by passing through the gaps. For ion guides having electrodes that are spaced apart from each other in the axial direction by gaps, such as an axially segmented multipole ion guide or an ion guide formed from a stack of apertured plate electrodes, these gaps may be filled with a filler material, such an electrical insulator, in order to prevent gas escaping the collision cell by passing through the gaps. Embodiments are contemplated in which an apertured disc is provided at the exit end of the elongated tube 25, where the aperture has a smaller cross-sectional area than the internal cross-sectional area of the elongated tube. This disc may assist in defining the gas conductance out of the collision cell. Although embodiments have been described in which the collision cell 20 has a single elongated tube 25 at the downstream end of the main body portion 24, it is contemplated that multiple such elongated tubes may be provided in series, where the internal cross-sectional areas of these tubes decreases in a direction towards the exit of the collision cell. Embodiments have been described in which an elongated tube 25 is provided at the downstream end of the collision cell, but it is contemplated that a corresponding elongated tube may additionally be provided at the upstream end of the collision cell so as to provide a relatively low pressure gradient at the entrance to the collision cell. Such an elongated tube need not have the mass-selective pseudo-potential barrier that has been described above therein. Although the precursor ions have been described as being fragmented by CID in the collision cell, they may alternatively be fragmented or reacted by other techniques in a cell that is at a higher pressure than the region at the exit of the cell. For example, the ions may be fragmented in the cell by using an AC electric field to oscillate the ions through gas within the cell (CID), or by Surface Induced Dissociation (SID), or by Electron Transfer Dissociation, or by Electron Capture Dissociation (ECD). Alternatively, the ions may be reacted with other ions or neutral molecules in the cell that cause them to dissociate into fragment ions, or they may be reacted with other ions or neutral molecules that cause them to form product ions other than fragment ions, such as adduct ions. Alternatively, the ions may be subjected to collisional cooling or collision induced activation / unfolding by colliding the ions within the cell and substantially without fragmenting the ions or reacting them to form other ions. For example, the ions may be fragmented upstream of the collision cell and the collision cell may not be used for fragmentation but may instead be used to collisionally cool the ions. The cell may therefore be a collision, fragmentation or reaction cell. Although the mass selective ejection has been described as being performed by driving ions over a pseudo-potential barrier that is varied with time, it is contemplated that it may be performed in other ways. For example, the pseudo-potential barrier may not be varied with time, whereas the potential difference that urges ions downstream along the ion guide 28b or a property of the potentials that are repeatedly travelled downstream along the ion guide 28b (such as amplitude or speed) may be varied with time so as to cause ions having different mass to charge ratios to pass the pseudo-potential barrier and exit the collision cell at different respective times. It is contemplated that a pseudo-potential barrier need not be used to mass selectively eject ions from the collision cell. For example, ions may be urged downstream along the ion guide 28b by repeatedly travelling potentials along the ion guide in the downstream direction, whereas a DC electric field may urge ions in the upstream direction. A property of the travelling potentials, such as amplitude or speed, or the magnitude of the electric field may be varied with time so as to cause ions having different mass to charge ratios to exit the collision cell at different respective times. Alternatively, ions may be urged downstream along the ion guide 28b by a DC electric field, whereas ions may be urged in the upstream direction by repeatedly travelling potentials in the upstream direction along the ion guide. A property of the travelling potentials, such as amplitude or speed, or the magnitude of the electric field may be varied with time so as to cause ions having different mass to charge ratios to exit the collision cell at different respective times. Alternatively, the mass selective ejection of ions from the collision cell may be performed by other means. For example, ions could be mass selectively ejected from the collision cell by using an AC field to excite (e.g. resonantly excite) ions of a specific mass to charge such that the overcome a DC or AC potential barrier. The AC field used to excite the ions may be varied with time, e.g. by varying its frequency, such that ions are ejected from the collision cell in reverse order of mass to charge ratio. It is also contemplated that the ions may be non-mass selectively ejected from the collision cell 24, rather than being mass selectively ejected. For example, a DC barrier may be provided in the elongated tube 25 so as to axially trap ions within the collision cell by applying one or more DC voltage to one or more electrodes of the ion guide 28b. The voltages applied to the collision cell may then be changed so as to lower the DC barrier and eject ions from the collision cell to the mass analyser. For example, ions of all mass to charge ratios may be simultaneously ejected from the collision cell to the TOF mass analyser. The time at which the pusher of the TOF mass analyser is pulsed may be synchronised with the time that the ions are simultaneously ejected from the collision cell such that at least ions having a preselected range of mass to charge ratios arrive at the pusher at the time that it is pulsed. As described above in relation to Fig. 1, it is common practice to maintain the collision cell 8 at a higher pressure than the regions outside the entrance and exit apertures of the collision cell. The entrance and exit apertures control the rate at which gas exits the relatively high pressure collision cell 8 into the surrounding area and these apertures are therefore known as gas conductance limiting apertures. Conventionally, these apertures are circular apertures in walls of the collision cell. The gas conductance of a circular aperture though a wall that is infinitely thin is given by: C® 11.6 x A where C is the gas conductance in litres per second and A is the area of the aperture in cm2. In reality the wall that the aperture is formed in also has a thickness, and this thickness affects the gas conductance through the aperture. More specifically, the gas conductance C described above is scaled by the probability of transmission of a gas molecule through the aperture. Fig. 5 shows an example of how the probability of transmission of gas through a tube having a circular cross-section varies as a function of L / D, where L is the length of the aperture and D is the diameter of the aperture. The transmission curve shown in Fig. 5 can be approximated well by two different mathematical functions. For a range of L / D from 0 to 2 the transmission curve is well approximated by y = 0.0546x4 - 0.3125x3 + 0.718x2 -0.9435x + 0.9981 (R2=1). In contrast, for the range of L / D >2 the transmission curve is well approximated by y = 0.6054x'°733 (R2=0.9976). For example, according to the equation for gas conductance above, a 2 mm diameter aperture in an infinitely thin wall would have a gas conductance of 0.3644 L / s, whereas if the aperture has a length such that the value of L / D gives a probability of transmission of 0.65, then the aperture would have a gas conductance of 0.2369 L / s (i.e. 0.3644 L / s x 0.65). A collision cell is typically required to provide an ion path through the gas therein such that the ions are fragmented, or conditioned for further analysis such as being collisionally cooled. In order to achieve these functions, the ions must undergo an adequate number of collisions with the gas molecules in the collision cell, which is dependent on both the gas pressure in the collision cell and the ion path length through the cell. In order to ensure that the mass spectrometer is a reasonable size, gas cells are typically restricted to having a length of less than 200 mm. This results in the gas cell being required to be at a pressure of at least 10'3 mbar. The actual pressure required depends on what the gas cell is required to do, e.g. whether large molecules such as proteins are being analysed or whether relatively small molecules are being analysed. The pressure in a gas cell having entrance and exit apertures is given by: P = Tp / C where P is the pressure in the gas cell in mbar, Tp is the gas throughput in mbar-L / s, and C is the gas conductance in L / s. By way of example, if gas is introduced into the gas port of the collision cell at a rate of 0.35 mL / min from a gas source at atmospheric pressure then this results in a throughput Tp of approximately 5.8 x 10'3 mbar L / s. If each of the entrance and exit apertures of the collision cell is a circular aperture of 2 mm diameter and 0.5 mm length then it can be seen from Fig. 4 that each of these apertures would have a probably of transmission of around 0.8. Accordingly, the total conductance of the two apertures is approximately 0.585 L / s. From the above equation for gas pressure it can then be derived that the gas pressure in the collision cell would be approximately 1 x 10'2 mbar. Although the convention approach of providing the gas conductance apertures in thin walls of the collision cell has its advantages, particularly around the thickness of the pumping aperture leading to a relatively short collision cell, it is not without its drawbacks. For example, in a TOF mass spectrometer is it often desired to couple of the RF ion guide in the collision cell to ion transfer optics that do not have RF ion confinement. At this interface, there is a small region in which the ions are not confined by RF voltages and where the pressure is relatively high. The ions can therefore undergo collisions with the background gas as they are accelerated out of the collision cell, causing scattering of the ions. Also, these ions are accelerated by the relatively large pressure gradient across the exit aperture of the collision cell. These ions therefore have a relatively large range of velocities and positional spread as they enter the TOF mass analyser, which limits the quality of the mass spectral data that the mass analyser is able to obtain. The embodiments of the present invention described herein overcome these problems, by providing a relatively short collision cell across which the pressure drops gradually and which does not introduce excessive energy, velocity or positional spreads to the ion beam passing downstream of the collision cell. As described above, the embodiments of the present invention provide a collision cell having an elongated tube that defines the gas conductance at its downstream end and which may radially confine the ions using RF voltages. As such, ions are able to pass out of the exit of the collision cell whilst retaining the energy, velocity and positional spreads that they had in the collision cell. This approach exploits a similar principle to that shown in Fig. 5, which recognises that increasing the length of the aperture reduces the gas conductance through it. As has been described above, the gas conductance limiting tube in the embodiments of the present invention need not have a circular cross-section and may instead have another cross-sectional shape. As such, the elongated tube was modelled as having an effective diameter, DEff. For example, although the internal surface of the elongated tube could be substantially cylindrical, e.g. if it contained a stacked ring ions guide (SRIG), it could be rectilinear, or it could have a different geometry such as being defined by the internal wall of a multipole ion guide such as a quadrupole ion guide. The effective diameter EffD is given by: EffD = 4A / LP where EffD is the effective diameter in cm, A is the cross-sectional area in cm2 and Lp is the length of the internal perimeter of the elongated tube in cm. Fig. 6 represents the cross-sectional area defined by the perimeter along the inside surfaces of a quadrupole ion guide. In this example the area is approximately 0.1702 cm2 and the perimeter Lp of this area is approximately 2.51 cm, leading to an effective diameter of approximately 0.2712 cm. When calculating the gas conductance for a cross-sectional area as shown in Fig. 6, the above-described equation for the gas conductance of a zero-thickness aperture gives a gas conductance of approximately 1.97 L / s (i.e. 11.6 x 0.1702). As mentioned above, this value must be scaled by the probability of transmission in order to determine the gas conductance of a tube having this cross-sectional area. The probability of transmission can be modelled, e.g. using Molflow+ version 2.8.11. Fig. 7 shows the probability of transmission of gas through two tubes having different shaped cross-sections, as a function of L / EffD, where L is the length of the tube and EffD is the effective diameter of the aperture, as has been described above. The graph shows a plot for a cylindrical tube, which is the same as that in Fig. 5, and also a plot for a tube defined by a quadrupole ion guide and having a cross-sectional area as is shown in Fig. 6. Fig. 8 shows the same plots as in Fig. 7, but also shows additional plots for a tube having a square cross-sectional area, a tube having a triangular cross-sectional area, and a tube having a rectangular cross-sectional area (in which each of the long sides of the rectangle is four times longer than each of the short sides). As can be seen, the plots for the tubes having different cross-sectional areas follow similar trajectories. As mentioned above in relation to Fig. 5, the plots may not be accurately described by a single analytical function, but they can be easily bounded into multiple sections that are represented by multiple respective analytical functions. Fig. 9 shows Fig. 8 replotted on a log-log scale and introduces an addition plot that represents the probability of transmission equal to 0.7 x (L / EffD)'0 45 - 0.16. From Fig. 9 it can be seen that the probability of transmission is above 0.7 x (L / EffD)'0 45 - 0.16 irrespective of the cross-sectional shape of the tube over a range of L / EffD ratios of >1.5. This allows one to bound the calculations related to gas conductance. For example, if the gas conductance is required to be <0.395 L / s using the quadrupole shaped tube shown in Fig. 6, then the maximum probability of transmission of that tube that is required can be calculated by dividing the required maximum gas conductance by the gas conductance for the quadrupole shaped tube having zero thickness, i.e. 0.395 / 1.97 — 0.2. From the relationship shown in Fig. 9 it can be seen that the tube provides this probability of 5 transmission when it has a value of L / EffD of 4.5, which means that the length of the tube must be 4.5 times that of the effective diameter in order to provide the required gas conductance. In the example of Fig. 6, where the effective diameter is 0.2712 cm, this means that the tube must have a length of 1.22 cm to achieve the required conductance. Practically, for best gas cell performance, typical conductance requirements will be 10 <0.4 L / s and typical length requirements will be <50 mm. The required effective diameter can be determined from these values using the relationships described above.

Claims

:5 1. A mass or mobility spectrometer comprising:a first vacuum chamber;a gas cell arranged in the first vacuum chamber, wherein the gas cell comprises: an ion entrance at an upstream end thereof; an ion exit at a downstream end thereof; a main body portion between the ion entrance and ion exit; an elongated tube between the main10 body portion and the ion exit; and an ion guide for radially confining ions within the elongated tube; wherein the gas cell comprises one or more ion guide configured to radially converge an ion beam passing through the collision cell such that it is radially compressed as it travels from the main body portion into the elongated tube; andcontrol circuity configured to operate the gas cell in a first mode in which first15 voltages are applied to the ion guide for radially confining ions within the elongated tube such that ions are axially trapped within the gas cell, and to operate the gas cell in a second mode in which second voltages are applied to the ion guide for radially confining ions within the elongated tube such that at least some of these ions are ejected or released from the gas cell.

202. The spectrometer of claim 1, wherein the spectrometer is configured to maintain the main body portion of the gas cell at a higher pressure than the first vacuum chamber.

3. The spectrometer of claim 1 or 2, wherein the elongated tube is configured to limit 25 the gas conductance out of the downstream end of the gas cell.

4. The spectrometer of claim 1,2, or 3, configured such that in the first mode the first voltages provide a potential barrier within the elongated tube that axially traps ions within the gas cell; and in the second mode the second voltages:30 a) urge at least some of the ions over the potential barrier such that they areejected from the gas cell; and / orb) reduce or remove the potential barrier such that at least some of the ions are ejected or released from the gas cell.35 5. The spectrometer of any preceding claim, wherein the control circuity is configuredto apply voltages to electrodes of said ion guide for radially confining ions within the elongated tube in said first mode so as to trap ions having a range of different mass to charge ratios within the gas cell, and to vary these voltages in the second mode so as to mass selectively eject or release ions of different mass to charge ratio from the gas cell at40 different respective times.

6. The spectrometer of claim 5, comprising a mass analyser arranged to receive ions that are ejected or released from the gas cell, wherein the mass analyser is configured to02 02 26nalyse ions, and wherein the control circuitry is configured to control the times at which the ions of different mass to charge ratio are mass selectively ejected from the gas cell, in the second mode, such that these ions of different mass to charge ratio arrive at the mass analyser at substantially the same time, and at a time that the mass 5 analyser is mass analysing ions.

7. The spectrometer of claim 5 or 6, wherein the control circuitry is configured to apply an AC voltage to one or more electrodes of the ion guide for radially confining ions within the elongated tube in said first mode so as to form a potential barrier that traps the ions of 10 different mass to charge ratio within the gas cell, and wherein the control circuitry is configured to vary the amplitude and / or frequency of the AC voltage with time in the second mode such that ions of different mass to charge ratios are able to pass the potential barrier and be ejected or released from the gas cell at different respective times.15 8. The spectrometer of any one of claims 1-4, wherein the control circuity is configuredto apply one or more voltage to one or more electrodes of said ion guide for radially confining ions within the elongated tube in said first mode so as to provide a potential barrier for trapping ions having a first range of different mass to charge ratios within the gas cell, and to vary said one or more voltages in the second mode so as to substantially20 simultaneously eject ions having said first range of mass to charge ratios from the gas cell.

9. The spectrometer of claim 8, further comprising a mass analyser that intermittently analyses ions arranged to receive ions that are ejected from the gas cell, and wherein the control circuitry is configured to control the time at which the ions are ejected from the gas 25 cell such that ions having a selected second, different range of mass to charge ratios arrive at the mass analyser at substantially the same time, and at a time that the mass analyser is mass analysing ions.

10. The spectrometer of any preceding claim, configured to apply voltages to the ion 30 guide for radially confining ions within the elongated tube so as to urge ions downstream through the elongated tube.

11. The spectrometer of any preceding claim, wherein the control circuity is configured to apply the first voltages to the ion guide for radially confining ions within the elongated 35 tube in the first mode such that ions are axially trapped within the gas cell by a potential barrier, where the potential barrier is spaced apart from the downstream end of the ion guide for radially confining ions within the elongated tube.

12. The spectrometer of claim 11, configured to maintain the pressure at the location of 40 said potential barrier at >10'3 mbar.

13. The spectrometer of claim 11 or 12, comprising one or more voltage supply for applying different voltages to different electrodes of the ion guide for radially confining ions02 02 26gated tube so as to provide a driving force on the ions in the downstream direction that increases with increasing proximity to the downstream end of the ion guide for radially confining ions within the elongated tube.5 14. The spectrometer of any preceding claim, wherein voltages for urging ionsdownstream through the ion guide for radially confining ions within the elongated tube are not applied to the downstream end portion of the ion guide for radially confining ions within the elongated tube.10 15. The spectrometer of any preceding claim, wherein the gas cell comprises an ionguide configured to radially compress an ion beam as it passes from the main body portion to the elongated tube; wherein the gas cell is configured such that ions in the ion beam that have been radially compressed enter a length of the elongated tube having a pressure that is sufficiently high to collisionally cool these ions, and such that the pressure in the15 elongated tube decreases from said length in a downstream direction.

16. The spectrometer of claim 15, wherein ions are trapped in said length in the first mode and at least some of these ions are released or ejected from said length in said second mode.2017. The spectrometer of any preceding claim, wherein:(i) a gas pumping aperture is provided through the wall of the elongated tube at, or proximate, the downstream end thereof, and wherein the ion guide for radially confining ions within the elongated tube extends to the axial location at which the gas pumping25 aperture is located; and / or(ii) wherein the ion guide for radially confining ions within the elongated tube extends beyond the downstream end of the elongated tube such that a portion of the ion guide is not enclosed by a wall of the gas cell.30 18. The spectrometer of any preceding claim, wherein the spectrometer is configured totransmit ions into the ion entrance of the gas cell and maintain the pressure in the main body portion of the gas cell such that the ions that enter the gas cell are: (i) fragmented by collision induced dissociation in the main body portion; or (ii) collisionally cooled by colliding with gas molecules in the main body portion.3519. The spectrometer of any preceding claim, wherein the main body portion of the gas cell defines a gas conduit therethrough that has a cross-sectional area that is larger than the cross-sectional area of the interior of the elongated tube, wherein cross-sectional area is in a plane orthogonal to an axis that extends from the ion entrance to the ion exit.4020. The spectrometer of any preceding claim, comprising a Time of Flight (TOF) mass analyser arranged to receive ions from the gas cell.02 26jell for a mass or mobility spectrometer comprising:an ion entrance at a first, upstream end thereof;an ion exit at a second, downstream end thereof;a main body portion between the ion entrance and ion exit;5 an elongated tube between the main body portion and the ion exit;an ion guide for radially confining ions within the elongated tube; andcontrol circuity configured to operate the gas cell in a first mode in which first voltages are applied to the ion guide such that ions are axially trapped within the gas cell, and to operate the gas cell in a second mode in which second voltages are applied to the 10 ion guide such that at least some of these ions are ejected or released from the gas cell;wherein the gas cell comprises one or more ion guide configured to radially converge an ion beam passing through the collision cell such that it is radially compressed as it travels from the main body portion into the elongated tube.15 22. A method of mass and / or mobility spectrometry comprising:providing a spectrometer or gas cell as claimed in any preceding claim;introducing ions into the gas cell; anddetecting ions that have exited the gas cell, or ions derived therefrom, so as to determine their mass to charge ratio and / or mobility.20CXIA

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