Gas collision cell for mass spectrometer

Abstract

A mass or mobility spectrometer comprising: a first vacuum chamber; and a gas / collision cell 20 arranged in the first vacuum chamber, wherein the gas / collision cell comprises: an ion entrance 22 at an

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2407307.4 filed on 22 May 2025. 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 and in particular 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, it has been found that 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; and a collision cell arranged in the first vacuum chamber, wherein the collision 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; and an elongated tube between the main body portion and the ion exit; wherein the spectrometer is configured to maintain the main body portion of the collision cell at a higher pressure than the first vacuum chamber; and wherein the elongated tube is configured to limit the gas conductance out of the downstream end of the collision cell. The inventors have recognised that providing the elongated tube to limit the gas conductance of gas flowing from the main body portion out of the downstream end of the collision cell causes the pressure to drop relatively gradually over a relatively long length of the collision cell towards the ion exit. As such, the ions experience a relatively low pressure gradient as they travel towards and leave the ion exit, which 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 opening. 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 as the conductance limiting aperture is counter-intuitive, particularly as it is generally desired to minimise the size of the components in the mass spectrometer. The elongated tube in the present invention 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 collision 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. The elongated tube has a conduit therethrough that may have any cross-sectional shape, such as being square, an elongated rectangle, circular, oval, or cross-shaped. The elongated tube may be an elongated hollow section. The spectrometer may be configured to transmit ions into the ion entrance of the collision cell and maintain the pressure in the main body portion of the collision cell such that the ions that enter the collision 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. Alternatively, the ions that enter the collision cell may be caused to collide with the gas in the collision cell so that the ions are collisionally activated, e.g. so as to undergo collisional induced unfolding substantially without fragmentation. The spectrometer may be configured to maintain the gas pressure inside the main body portion of the collision cell at >10'3 mbar, e.g. to form CID fragment ions or facilitate collisional cooling of the ions. The spectrometer may be configured to accelerate ions into the collision cell so as to cause the ions to fragment due to collisions with the background gas in the collision 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 collision cell. The spectrometer may comprise a gas supply and gas conduit for supplying gas into the main body portion of the collision cell through a wall of the main body portion. 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 elongated tube may define an elongated gas conduit therethrough, wherein the wall of the elongated tube is sealed such that gas cannot escape through the wall over a length of the tube that is between 4 and 50 mm long; and wherein the mass spectrometer is configured such that the elongated tube has a conductance of <0.4 L / s. The length of the elongated tube that is sealed extends downstream from the downstream end of the main body portion towards the ion exit of the collision cell. Preferably, the collision 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 elongated tube may define an elongated gas conduit therethrough that has a cross-sectional area, in a plane orthogonal to an axis that extends from the ion entrance to the ion exit, that is substantially constant over a length of the tube that is at least 4 mm 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. The main body portion of the collision 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 collision cell may extend from the ion entrance of the collision cell to the upstream end of the elongated tube. The collision cell may comprise one or more ion guide extending through the main body portion and / or the elongated tube for radially confining ions within the one or more ion guide and / or urging ions downstream through the collision cell. The spectrometer may comprise one or more voltage supplies, such as one or more AC voltage supply, for supplying one or more voltages to electrodes of the one or more ion guide for performing the radial confinement of the ions. The spectrometer may comprise one or more voltage supplies, such as one or more DC voltage supply, for supplying one or more voltages to electrodes of the one or more ion guide for urging the ions along the ion guide. For example, the collision cell may have an ion guide comprising a plurality of electrodes that are spaced apart along the length of the elongated tube, and the spectrometer may comprise one or more voltage supply 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 supply 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 supply 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. 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 collision cell. For example, the spectrometer may comprise a first multipole ion guide in the main body portion and a second multipole ion guide in the elongated tube, where the first multipole ion guide is a higher order multipole than the second multipole ion guide. For instance, 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 one or more ion guide may be 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. The elongated tube may be configured such that the ions that have been radially compressed enter a length of the elongated tube having a pressure that is sufficient high to collisionally cool the radially compressed ions, and such that the pressure in the elongated tube decreases from said length in a downstream direction. A gas pumping aperture may be provided through the wall of the elongated tube at, or proximate, the downstream end thereof, and wherein one of the one or more ion guides extends to the axial location at which the gas pumping aperture is located. Additionally, or alternatively, an ion guide may extend through the elongated tube and beyond the downstream end of the elongated tube such that a portion of the ion guide is not enclosed by a wall of the collision cell. The embodiments enable the ions from the collision 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 collision 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 spectrometer may comprise voltage supplies for supplying voltages to electrodes of the one or more ion guide, wherein the electrodes of the one or more ion guide are mounted to the edges of printed circuit boards (PCBs) that have conductive traces configured to supply voltages from voltage sources to the electrodes. The PCBs may extend through the wall of the elongated tube and into the conduit therein, such that the electrodes are mounted to the edges of the PCBs at locations that are spaced from the wall of the conduit. The PCBs may be stacked with layers of insulating material so as to define said conduit. The one or more ion guide may have gaps between its electrodes, and one or more conductive member may be located radially outside of the one or more ion guide such that ions that pass through the gaps impact on the conductive member and are neutralised. For example, the ion guide may be a multipole rod set ion guide and the gaps may be between pairs of circumferentially adjacent poles of the multipole. Additionally, or alternatively, the ion guide may be axially segmented and the gaps may be between axial segments. The one or more conductive member may be grounded. The wall(s) of the elongated tube may be arranged radially outward of the electrodes of the ion guide therein. For example, the electrodes of the ion guide may be arranged on the interior the wall(s) of the elongated tube. 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 collision cell may be arranged such that ions exit the collision cell into the first vacuum chamber; or the spectrometer may further comprise 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 collision cell is a differential pumping aperture between the first and second vacuum chambers. The spectrometer may further comprise a mass analyser arranged to receive ions from the collision cell, wherein the mass analyser is located in the first vacuum chamber or in a second vacuum that is downstream of the first vacuum chamber, and where the spectrometer is configured to maintain the second vacuum chamber at a lower pressure than the first vacuum chamber. The spectrometer may be configured to maintain the mass analyser at a pressure of <10 4, <10 5, <10 6, <10 7, <10 8, or< 10'9. The mass analyser may be a Time of Flight (TOF) mass analyser having a pusher for pushing packets of ions towards an ion detector. The 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. The spectrometer may comprise a mass filter upstream of the collision cell that is configured to only transmit ions having a restricted range of mass to charge ratios to the collision cell. Although embodiments have been described in which the collision cell is used to collide ions with a gas therein in order to fragment, cool, or activate the ions, it is contemplated that the ions may be caused to undergo alternative processes in the cell. For example, ions may 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. Alternatively, ions may be separated by mobility in the cell by urging the ions through gas in the cell using one or more electric field. Accordingly, more generally, from a second aspect the present invention provides a mass or mobility spectrometer comprising: a first vacuum chamber; and 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; and an elongated tube between the main body portion and the ion exit; wherein the spectrometer is configured to maintain the main body portion of the gas cell at a higher pressure than the first vacuum chamber; and wherein the elongated tube is configured to limit the gas conductance out of the downstream end of the gas cell. 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 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. The spectrometer according to the second aspect of the present invention may have any of the features described in relation to the first aspect of the invention. For example, the gas cell may comprise one or more ion guide extending through the main body portion, and / or through the elongated tube, for radially confining ions within the one or more ion guide. The one or more ion guide may be configured to urge ions downstream through the gas cell. The one or more ion guide may be 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. Although embodiments have been described in which an elongated tube is provided at the downstream end of the cell, it is contemplated that a corresponding elongated tube may alternatively, or 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. The gas cell itself is novel it its own right and therefore 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; and (i) a first elongated tube between the main body portion and the ion exit, wherein the elongated tube is configured to limit the gas conductance out of the downstream end of the gas cell; and / or (ii) a second elongated tube between the main body portion and the ion entrance, wherein the elongated tube is configured to limit the gas conductance out of the upstream end of the gas cell. The gas cell may have any of the features described above in relation to the first or second aspects of the present invention, without being limited to having the other features of the spectrometer. For example, the gas cell may comprise one or more ion guide extending through the main body portion, and / or through the first elongated tube, for radially confining ions within the one or more ion guide. The one or more ion guide may also, or alternatively, extend through the second elongated tube. The one or more ion guide may be configured to urge ions downstream through the gas cell. The one or more ion guide may be 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. The elongated tube between the main body portion and the ion entrance may have any of the features described herein in relation to the elongated tube between the main body portion and the ion exit. The present invention also provides a method of mass spectrometry comprising performing the techniques described herein. Accordingly, the present invention provides a method of mass or mobility spectrometry comprising: providing a spectrometer or gas cell as described above; and introducing ions into the collision or gas cell to thereby collide the ions with a gas therein, or fragment or react the ions inside the cell. The present invention also provides a method of manufacturing or designing a gas cell as described above, comprising: a) selecting a gas conductance required to be achieved through the first or second elongated tube; b) selecting an internal surface profile shape for that elongated tube or an ion guide to be arranged therein; c) obtaining a relationship that relates the gas conductance through the selected internal profile shape to the dimensions that define the volume enclosed by the internal profile shape; and d) determining, from said relationship, the dimensions that provides said gas conductance required. The method may then comprise providing or constructing a gas cell in which the elongated tube or ion guide therein has the dimensions determined in step d). The present invention also provides a method of manufacturing or designing a gas cell as described above, comprising: a) selecting a first gas conductance that is required to be achieved through the first or second elongated tube; b) selecting a cross-sectional profile for that elongated tube or an ion guide to be arranged therein, and calculating a second gas conductance based on the area enclosed by the cross-sectional profile; c) obtaining a relationship that relates the probability of transmission of gas molecules through the elongated tube or ion guide therein as a function of a parameter that is based on physical dimensions that define the volume enclosed by the elongated tube or ion guide; d) dividing the first gas conductance by the second gas conductance so as to determine a value of said probability of transmission that is required to achieve said first gas conductance; f) determining, from said relationship, the value of said parameter that provides said value of the probability of transmission; and g) selecting the physical dimensions of the elongated tube or ion guide therein based on the value of the parameter. The second gas conductance may be the theoretical gas conductance through an aperture having said area but no length. The second gas conductance may therefore be proportional to said area, e.g. approximately 11.6 times said area (in cm2). In step c) of each of the methods described above, said relationship may relate the probability of transmission of gas molecules through the elongated tube or ion guide therein as a function of a parameter that is based on the length of the elongated tube or ion guide therein and the effective diameter of the cross-sectional profile, wherein the effective diameter is given by 4A / LP, where A is said area and Lp is the internal perimeter of said cross-sectional profile. The gas conductance required to be achieved through the first or second elongated tube maybe <0.4 L / s; and / or the length of the elongated tube or ion guide therein may be <50 mm. For example, said relationship may relate the probability of transmission of gas molecules through the elongated tube or ion guide therein as a function of the length of the elongated tube divided by the effective diameter. The length of the elongated tube or ion guide therein may be selected and the other dimensions of the elongated tube or ion guide may be calculated from the parameter determined in step f), such as the effective diameter. However, method are contemplated in which different physical parameters that define the volume enclosed by the elongated tube or ion guide are used. The method of designing the gas cell may be performed a computer implemented method that performs the above-described steps. The method may extend to a method of manufacturing or designing a spectrometer as described herein. The present invention also provides an ion guide comprising: a tube having one or more internal walls that define a conduit therethrough; and a plurality of printed circuit boards that extend through the one or more internal walls of the tube and into the conduit therein, wherein a plurality of electrodes are mounted to the edges of the printed circuit boards at locations that are spaced from the one or more walls of the conduit. The electrodes may therefore be spaced away from the one or more walls of the conduit. The plurality of printed circuit boards may be stacked with layers of other material so as to define said conduit. Said other material is preferably electrically insulating material. The ion guide may be a multipole ion guide, wherein one or more of said electrodes form each pole of the multipole ion guide, and wherein the electrodes forming different poles are provided on the edges of different respective printed circuit boards. The ion guide may comprise one or more voltage supplies for supplying voltages to the electrodes, wherein the printed circuit boards have conductive traces configured to supply voltages from voltage sources to the electrodes. 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; and Fig. 3 shows an example of an ion guide that may be used inside the collision cell of the present invention; Fig. 4 shows an example of how the probability of transmission of gas through a tube having a circular cross-sectional shape varies; Fig. 5 represents the cross-sectional area defined by the perimeter along the inside surfaces of a quadrupole ion guide; and Figs. 6-8 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 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 small 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 through the ion guide 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 downstream through the ion guide. 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 along the ion guide, 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. 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 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 focussing 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 are also 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 then out of 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. 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. 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 20 may be mounted. It will be appreciated that the mass spectrometer may have fewer or additional 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 20 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. 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 alternatively, or 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. 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. The cell may therefore be a collision, fragmentation or reaction cell. 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. 4 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. 4 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 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. 4, 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. 5 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. 5, 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. 6 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. 4, and also a plot for a tube defined by a quadrupole ion guide and having a cross-sectional area as is shown in Fig. 5. Fig. 7 shows the same plots as in Fig. 6, 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. 4, 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. 8 shows Fig. 7 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. 8 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. 5, 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. 8 it can be seen that the tube provides this probability of 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. 5, 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 <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.

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