Patent ID: 12243734

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned above, it is an object of the present invention to provide an ion optical arrangement comprising a collision/reaction cell, which ion optical arrangement is suitable for use in a mass spectrometer, in which the collision/reaction gas can be used only when necessary, while avoiding the relatively complicated dual path ion optics arrangement described in British patent application GB 2 535 754. It is another object of the invention to reduce the noding effect in an ion optical arrangement, such as a collision/reaction cell.

In accordance with the invention, the first object may be achieved by switching the operation modes of the ion optical arrangement between:A. a first operation mode including a pressurized collision cell, andB. a second operation mode along the same optical axis with an evacuated collision cell.
This switching between operation modes can be achieved without exchanging any components, that is, by using the components present in the ion optical arrangement.

The first operation mode uses a radio frequency (RF) electric focusing field while the second operation mode uses a static (DC) electric focusing field. The first mode of operation can be a low energy RF operation mode while the second mode of operation can be a high energy static operation mode.

In GB 2 546 060, which is herewith incorporated by reference in this document, the concept of a pre-mass filtered collision cell interfaced to a multi-collector mass spectrometer is disclosed. An RF quadrupole pre-mass filter is used which also introduces noding effects similar to the RF collision cell. In the collision cell the ion beam trajectories are altered by the collisions and the strong phase correlation to the oscillating RF field of the ions traveling through the quadrupole is disturbed by the collisions and thus leads to less mass dependent transmission effects.

The small dependence of the trajectories depending on the mass is known as “noding”. It is a result of the spatial oscillations of the ions inside a multipole. Depending on the number of oscillations of the ions, they leave the cell with an angle/position vector that is mass dependent. This effect can be amplified by the tuning parameters such as the potentials of the entry and exit lenses which determine the input and exit parameters of the ion beam entering and exiting the collision cell. The DC bias potential of the multipole rods also determines the travel velocity of the ions through the collision cell and has an influence on the noding.

By using higher order multipoles, from quadrupoles (4 poles) to hexapoles (6 poles) or octupoles (8 rods or poles), higher order oscillations are added to the ion trajectories which make the mass dependent differences of the trajectories less pronounced and which at the same time increase the acceptance input aperture of the collision cell. However, this beneficial effect is limited.

The pressurization of the collision cell by introducing a collision gas (e.g. helium) flow in the order of several ml/min results in multiple collisions of the ions with the collision gas, which in turn results in scattering and random movements of the ions. These scattering events further reduce the phase correlation of the ion beam trajectories to the oscillating RF field and thus reduce the noding effect. The more collisions the ions undergo the smaller the noding effect is. Especially for heavier ions multiple collisions result into both a reduction of the kinetic energy and a reduction of the energy spread of the ions, which improves the focusing conditions and which is known as collisional focusing.

The momentum transfer per collision becomes more efficient the more the difference in mass between both collision partners is reduced and might even stop the movement of the ions. For lighter masses approaching the low mass range of helium (He), the overall transmission efficiency through the pressurized collision cell is significantly reduced. This can partly be compensated by adding an axial electrical field gradient to the cell potential that actively drags ions from the entry to the exit aperture and therefore allows for an increased number of collisions as well as for higher transmission for lighter ions.

The noding effect can be reduced by using higher order multipoles with high gas pressures and axial fields, but it cannot be eliminated completely. Accurate and precise isotope ratio measurements using collision cells requires the availability of a calibrated standard and extensive calibration procedures. Tuning parameters need to be controlled carefully.

The invention provides a collision cell that can be switched to a static high energy DC transfer lens to completely avoid noding effects. Collision cells usually require high gas pressure inside the collision cell volume to induce sufficient collisions and chemical reactions (when a reaction gas is used). Therefore, the pumping apertures at the entrance and the exit are usually rather small, in the order of 1-3 mm diameter. For an efficient high energy transport through the collision cell arrangement, an improved pumping mechanism has to be established.

The present invention additionally provides a solution to the noding problem by providing a collision/reaction cell that varies the number of oscillations the ions undergo in the RF field. That can be done by:1. a variation of the RF frequency, and/or2. a variation of the ions' energy/velocity in axial direction, and/or3. any other lens element that influences the ion velocity.
The variation of the axial energy can be achieved by, for example, superimposing an oscillation on the rod bias voltage (DC potential of the rods that defines the energy the ions have in the multipole) and/or by applying an oscillating voltage to the vanes (which may also be referred to as drag electrodes in some embodiments, seeFIGS.4A &4B).

The amplitude of the applied variation is preferably such that the number of oscillations which the ions undergo changes by at least 1 over the length of the collision cell. As mentioned above, the number of oscillations n is given by the frequency and the velocity of an ion:

n=f·l·m2⁢E
withf=frequency,l=length of multipole,m=ion mass, andE=ion energy.

In an embodiment, the minimum number of oscillations is in the order of 10 (f=1 MHZ, I=100 mm, m=7 amu and E=5 eV). Hence the frequency variation should be at least 10 percent (it is noted that the number of oscillations n is directly proportional to f) or the energy variation should be at least 20 percent.

The solution to the collision cell problem consists of a collision/reaction cell that can be operated in two modes. In a first mode the collision cell is driven with electrical RF fields at low beam energy and high gas pressures in the collision cell mode. In a second mode the collision cell uses DC fields at high beam energy and low gas pressures.

In the RF mode the rods may be driven in two groups. The first group is connected with one of the two RF outputs and the other group of rods is connected with the other RF output (180° phase shifted with respect to the first output, see the rods11and11′ inFIG.2A). The rods of the multipole are aligned to the optical axis of the ion optics. The optical axis can be straight or curved. Due to the alternating potential of the rods the ions are actively refocused into the center of the setup. Without the focusing nature of the RF field the ions would be strayed by the collisions with the gas in the cell.

In the DC mode the setup is driven substantially without gas and at a high beam energy. Therefore, a focusing RF field is not necessary. In accordance with the invention, the RF rods are segmented in at least two or three sections along the optical axis and the setup is switched to DC only mode such that effectively it works as an einzel lens.

Since an einzel lens consists of three cylindrical elements that are placed coaxially on the center axis, the rods may be partitioned into three parts or sections. With such a setup both the RF-field of the multipole as well as the DC potentials of the einzel lens can be applied. Beside the einzel lens concept with three segments a DC-only mode is also possible with just two segments.

In the DC mode the ions can have a much higher energy compared to the RF mode. Ion optics for ion beams with low energy are difficult to focus since the high charge density of the beam leads to a radial space charge expansion of the beam (space charge effects). Ion beam energies of several thousand eV minimize space charge effects and allow beam focusing at high ion beam currents. With the einzel lens arrangement according to the invention the ions stay at energies in the keV (kilo electronvolt) range and thus space charge effects are much reduced compared to beam energies of a few eV.

The described principle of segmenting a multipole lens in order to switch between low energy RF mode and high energy DC mode can be applied not just to a multipole collision cell arrangement but also to a quadrupole mass spectrometer or any other RF multipole arrangement.

This invention allows to switch a low energy RF mode ion optical setup to a high energy DC mode setup along the same optical axis.

Since the two described modes can be altered just by applying different potentials to the lenses, the limiting factor for the switching time is most likely the gas pressure in the housing of the cell. For the RF mode the housing is ideally completely sealed with small (1-3 mm) diameter apertures at the entry and the exit of the collision cell housing. For the operation in the high energy DC einzel lens mode an increased pumping cross section is required to efficiently pump out residual gas as memory from a previous experiment where the arrangement has been operated as a low energy high pressure collision/reaction cell.

The suggested techniques do not require expensive mechanical feed-throughs. In the first case the movement inside the vacuum is induced via a steel capillary. In the second case an electrical feed through is sufficient to induce the movement inside the vacuum. The altered potential of the foil might be combined with the potential of one of the lenses.

Accordingly, the invention provides at least the following advantages:Switching between high energy DC einzel lens ion optics and low energy RF multipole lens ion optics without intervention to the vacuum system.Switching may be done by switching electronic supplies only.The high pressure collision cell may be operated by a mechanical switch to increase pumping efficiency in case of high energy and high vacuum DC operation mode.Proposed multipole arrangement of hexapole or octupole or even higher order reduces noding effects.There is only one optical axis in the system (no bypass optical axis). This allows a compact geometry and reduced aberrations.Since there is only one ion optical axis, the tuning of the system is much easier compared to a complicated deflection setup where the ion beam has to be steered along a bent bypass axis to circumvent the collision cell or vice versa.The principle of a segmented multipole lens also can be applied to a quadrupole mass filter lens. This allows the ion optical instrument to be switched from a low energy front-end RF multipole lens design to a high energy DC lens design without any noding effects.The high energy DC mode gives higher sensitivity and thus overcomes the limitations of a low energy multipole setup.The switching between the two modes allows to configure a unique instrumental setup which allows to switch to the collision mode for specific applications only and to run the same instrument in the high energy DC mode simply by switching electric power supplies.

FIG.1schematically shows a collision cell according to the prior art. The collision cell1is shown to comprise a housing18in which a multipole arrangement is accommodated. In the example shown, the multipole arrangement is a hexapole arrangement comprising six elongate poles or rods11which constitute electrodes. A radio frequency (RF) voltage may be fed to opposite pairs of poles11to produce an RF electric field. Ions can enter the collision cell through an entrance aperture13and leave the collision cell through an exit aperture15. The RF field produced by the multipole arrangement focuses the ions on the longitudinal axis of the arrangement. This is particularly relevant when a collision gas is present in the collision cell, as collisions may cause the ions to deviate from their path.

FIG.2Ashows a multipole arrangement, in the example shown a hexapole arrangement, without a collision cell. As inFIG.1, RF (radio frequency) voltages may be applied to the alternating rods11and11′ to generate an RF field in the arrangement that focuses ions and guides them through the arrangement. The RF field may be applied when the collision cell is pressurized. It is noted that such a multipole arrangement may not only be used in a collision cell but also in a mass filter, for example.

When the collision cell is not pressurized, or at least has a lower pressure due to which the influence of the gas on the ion trajectories is reduced, the ions can have a higher energy and the RF field is not required to guide the ions. Instead, in accordance with the invention a so-called einzel lens may be used to guide the ions.

FIG.2Bschematically shows an ion optic einzel lens consisting of three consecutive tube sections or rings10A,10B &10C which are electrically isolated relative to each other. A first section10A may have a first voltage V1, the second section10B may have a second voltage V2, while the third section V3may have a third voltage V3. In some embodiments, the first voltage V1and the third voltage V3may be substantially equal, for example both may be equal to −1 kV. The second voltage V2may then, for example, be equal to −2 kV. Depending on the polarity of the ions, positive voltages may be used instead, for example +1 kV, +2 kV and +1 kV respectively. It will be understood that other voltages may also be used, depending on the ions and the dimensions of the multipole arrangement. In some applications, the second voltage V2may be equal to +1 kV, +5 kV, −1 kV or −5 kV, for example. Exemplary trajectories of ions passing through such an arrangement will later be explained in more detail with reference toFIG.6B.

It has been found that it is impractical to combine the rods11and the rings10A-10C in the same collision cell or other multipole arrangement. In accordance with the invention, therefore, the rods11and the rings10A-10C are combined into a single structure, which is schematically shown inFIGS.3A &3B.

FIG.3Ashows a multipole arrangement according to the invention. In the embodiment ofFIG.3A, the rods have been partitioned into sections11A,11B and11C (and similarly into11A′,11B′ and11C′). In the mode of operation illustrated inFIG.3A, the rods function as the rods11in FIG.2A and all three sections of each rod carry the same RF voltage. In this respect, there is no functional difference withFIG.2A. However, in the mode of operation illustrated inFIG.3B, a different DC voltage is applied to consecutive ones of the rod sections11A,11B and11C, so that the rods are used in the same way as the rings10A,10B &10C ofFIG.2B. That is, the rod sections11A-11C can be said to simulate the rings10A-10C. In this way, the rods are used to constitute an einzel lens. It is noted that in the mode of operation ofFIG.3Bthe RF voltage ofFIG.3Ais typically not applied. In the embodiment ofFIGS.3A and3B, the einzel lens structure is constituted by segmented rods. Alternatively, or additionally, it is possible to provide an einzel lens structure by using other segmented electrodes, such as vanes which are often provided in the spacings between the rods of a multipole arrangement. Vanes are typically longitudinal, flat electrodes which may be used to provide axial fields, such as drag fields. By segmenting vanes instead of, or in addition to segmenting rods, an einzel lens structure can also be achieved.

FIGS.4A &4Bschematically show segmented electrodes, constituted by vanes, arranged between the rods of a multipole lens arrangement, whileFIGS.5A &5Bschematically show voltage generation circuits and the resulting voltages.FIGS.4A &5Acorrespond with a first or RF operation mode whileFIGS.4B &5Bcorrespond with a second or DC operation mode.

InFIG.4A, the multipole rods11are supplied with RF voltages. The vanes17(that is,17A,17B &17C together) are in the example shown used for creating an axial electric field gradient which causes a drag force. Depending on the direction of the field gradient, the drag force either accelerates or decelerates the ions passing through the multipole arrangement. Here a positive potential gradient is applied to the electrodes that induces a field gradient in the center of the arrangement. By this gradient the ions are pulled towards the exit (that is, accelerated), even when they almost have come to rest due to complete momentum transfer to the collision gas. The potential gradient can be created by two potentials that are applied on each side of the electrodes in combination with a resistor chain on the electrodes or with a homogeneous resistance of the material itself, as will be explained with reference toFIGS.5A &5B.

FIG.4Bshows the same setup with three potentials applied along the segmented vanes17, as illustrates inFIG.5B. With a potential being applied at the entrance side and at the end of the first segment17A, a constant potential is applied to the first segment. The other two potentials are applied to the second segment17B and the third segment17C respectively. In the second or DC operation mode, the RF power supply for the multipole rods is typically switched off, the rods may be grounded or a DC potential may be supplied to them.

FIGS.5A &5Bschematically show how suitable voltages may be applied to the vanes in the respective operation modes. Each vane17(comprising the vane sections17A,17B &17C) is shown to comprise a resistor network RN which includes electrode elements EE coupled by a series arrangement of resistors R. For each section17A,17B &17C a respective voltage source VSA, VSB & VSC is provided, which can be connected to the resistor network RN via a number of switches SW1to SW5. It is noted that the three vane sections17A,17B &17C are combined here into a single physical vane17which, however, is shown to have three distinct (but electrically connected) electrical sections, each having its own voltage source.

In the first operation mode shown inFIG.5A, the switches SW1to SW5are all open. The first voltage source VS1supplies a voltage to the first resistor and hence to the entrance end (which may typically be arranged near a collision cell entrance13, for example, seeFIG.1) of the vane17A. Switch SW5connects the last resistor and hence the exit end (which may typically be arranged near a collision cell exit15, for example, seeFIG.1) of the vane17C to ground. As a result, there is a voltage or potential gradient U1along the length I of the combined vane17A-17C, as shown inFIG.5A. This voltage gradient U1produces an electric field gradient between the poles11of the multipole arrangement (seeFIG.4A).

In the second operation mode shown inFIG.5B, the switches SW1to SW5are all closed. The first voltage source VS1supplies a first voltage to the first vane section17A, the second voltage source VS2supplies a second, different voltage to the second vane section17B while the third voltage source VS3supplies a third voltage to the third vane section17C. In the example shown inFIG.5B, the first voltage and the third voltage are substantially equal, while the second voltage is higher than both the first and the third voltage. This voltage distribution U2causes the vane sections to act as an einzel lens.

FIG.6Aschematically shows a multipole arrangement of a collision cell1according to the invention in a first operation mode, in which the rods are used as an RF multipole. The collision cell1is shown to have a housing18, in which the multipole arrangement is accommodated. The collision cell1is further shown to comprise an entrance electrode12and an exit electrode14, which comprises an exit opening15. All three segments11A,11B &11C of each rod have the same DC voltage in this first or RF operation mode, as inFIG.3A. This DC voltage may or may not be equal to zero (ground).

In the partially expandedFIG.6Cit can be seen that the ions do not follow straight lines but have oscillating trajectories. It can also be seen, as shown inFIG.6A, that the ions fan out evenly at the exit opening15. This is the suppressed noding effect that may occur in the RF operation mode and which will later be discussed in more detail. The electrical field lines EFL are also schematically shown inFIG.6C.

FIG.6Bschematically shows the same ion guide as inFIG.6A, but where different DC voltages are applied to each of the sections of the rods, as inFIG.3B, so as to provide an einzel lens. An RF voltage is not applied inFIG.6B. The trajectories of the ions (three different trajectories T are shown) depend on the entrance angles but no longer on the substantially random parameters as in the RF operation mode shown inFIG.6A. The DC voltages that may be used are, for example, between −1 kV and −2 kV at a beam energy of 2 keV (high energy). It can be seen that the einzel lens causes ions having different trajectories to pass through the exit opening15. The einzel lens can therefore be said to focus the ions in the second or DC operation mode, in which the ions may have a high energy.

It is noted that according to another aspect of the invention, the collision cell may be heated to reduce so-called memory effects. That is, by heating the collision cell to a temperature of, for example, 50° C., stray ions are less likely to remain on the electrodes (rods and/or vanes) and on the inner walls of the collision cell. It will be understood that stray ions which remain behind in an experiment may detrimentally influence any further experiment. A suitable temperature range is 40° C. to 70° C., preferably 45° C. to 55° C. Heating a collision cell is preferably achieved using electric heating.

As mentioned above, a problem that may arise in a multipole arrangement is noding. This effect is illustrated inFIG.7. A multipole arrangement, which may be part of a collision cell1or of a mass filter, comprises rods or poles11, to which an RF voltage may be applied. An entrance electrode (front plate)12is provided with an entrance opening13for letting an ion beam IB enter the multipole arrangement. An exit electrode (back plate)14is provided with an exit opening15for letting the (modified) ion beam IB′ exit the multipole arrangement.

As can be seen, some ions follow slightly different trajectories, resulting in the modified ion beam IB′. While the original ion beam IB was substantially uniform, the ion beam IB′ exiting the multipole arrangement is no longer uniform, different ions exiting at slightly different angles. The trajectories shown inFIG.7are of ions having the same energy but different masses. Since different masses follow different trajectories, the probability that ions pass through the exit opening15(instead of hitting the end plate14) is also mass dependent. In addition, the focusing of the ions emerging from the multipole arrangement in a subsequent ion optical device (such as a mass analyzer) may also become mass dependent. It will be clear that this is undesirable. In embodiments of the invention, therefore, the RF frequency of the voltage supplied to the rods is varied. That is, the RF frequency is not kept constant but is changed over time. Frequency changes of at least 10% are preferred, although smaller frequency changes such as 5% may in some embodiment also be used, also depending on the length of the multipole arrangement. Frequency changes of 15% or 20% may, however, be more effective in some multipole arrangements. That is, at an RF frequency of 1 MHz, for example, the frequency is preferably made to vary at least from 0.90 MHz to 1.10 MHz (−10% and +10%). The resulting RF frequency may vary over time in various ways: sawtooth, square or sinusoidal, for example.

Instead of, or in addition to changing the RF frequency to reduce the noding effect, it is also possible to superimpose a (preferably RF) frequency upon any DC bias voltage that is supplied to the multipole arrangement, even when the DC bias voltage is zero.

As mentioned above, an aspect of the invention is operating a collision cell in a pressurized mode and in an evacuated (that is, non-pressurized) mode. This requires that the collision cell can be pressurized and depressurized rapidly. In particular, a pressure release mechanism is desired that is fast and effective.

According to an aspect of the invention, therefore, valve mechanisms are provided which are particularly suitable for use in a collision cell having a pressurized and an evacuated operation mode, such as, but not limited to, the collision cell of the present invention.

FIGS.8A &8Bshow a mechanism20for adjusting the pumping cross section of a collision cell housing18having rods11. The mechanism20is shown to comprise a door or flap21which is connected via a hinge22to the housing18of the collision cell1. The flap21can be operated by an actuator23of which one end is connected to the flap21and the other end is connected to a support element24attached to the housing18.

The actuator23shown inFIGS.8A &8Bis a Bourdon tube. A Bourdon tube comprises a bent tube. The bending radius of the bent tube can be decreased if the pressure difference between the inner part and the outer part of tube increases. To this end, a gas tube25, which is also connected to the support element24, is connected with the actuator23. In the embodiment shown, the gas flows from the gas tube25through a channel in the support element24into the actuator23when the gas pressure in the gas tube25is higher than the gas pressure surrounding the actuator23. By letting gas flow into the actuator, its bending radius decreases (the actuator straightens) and the flap is opened. Conversely, the gas flows from the actuator23through the support element24into the gas tube25when the gas pressure in the gas tube25is lower than in the actuator23. By letting gas flow out of the actuator, its bending radius increases (the actuator curves) and the flap is closed.

Thus, by providing a pressure difference between the gas tube25and the air (or other gas) outside the actuator23, the flap can be quickly opened or closed, thus allowing the gas pressure in the interior of the collision cell1to quickly assume the gas pressure on its outside.

It is noted that the collision cell1may be accommodated in a near-vacuum environment, while the gas tube may be connected with an environment under atmospheric pressure. The gas used for inflating the inflatable actuator may be air. As the interior volume of the actuator23and the gas tube25may be small, only a small amount of air or other gas is needed to inflate the actuator. This air or other gas may be provided by a gas reservoir or by a pump. Thus, a small pump or valve can be sufficient to indirectly operate the relatively large flap.

By using a Bourdon tube or similar actuator, a fast and effective pressure regulation of a collision cell can be achieved. However, a Bourdon tube is not the only type of actuator that may be used in a collision cell or similar pressurized chamber, as will be further explained with reference toFIG.9.

FIG.9schematically shows an electrostatic opening mechanism used in a collision cell. The collision cell1is shown to comprise a housing18in which rods11are accommodated. An ion beam IB can pass through the collision cell1, through openings in the front plate12and back plate14respectively. In the embodiment shown, part of the wall of the housing18is provided with through holes16which can be closed off by a movable foil. This foil is located in a spacing between the housing18and a plate19. Both the housing18and the plate19contain electrically conductive material and may both be made of metal, or at least contain a metal layer or other conductive layer. The plate19, which extends substantially parallel to the housing18, may be flat but may alternatively be curved to accommodate any curvature of the housing18.

In the embodiment shown, the foil comprises two layers: a conductive layer30and an electrically insulating layer31. A further electrically insulating layer32is attached to the plate19. In an alternative embodiment, the foil consists of three layers: the conductive layer30and both insulating layers31&32. Further layers may be added, as long as the foil remains sufficiently flexible. A suitable material for the insulating layers31&32is Kapton, but other materials, for example other polyimides, may also be used. The conductive layer may be made of copper foil, for example.

As mentioned above, the flexible foil is located in the spacing between the housing18and the plate19. One edge of the foil may be attached to the housing18while the opposite edge may be attached to the plate19, such that the foil bridges the spacing. By applying DC voltages to the conductive layer, the position of the foils can be changed, as shown inFIG.9Aby the arrows which indicate the possible movement of the substantially S-shaped spacing-bridging portion of the foil.

Referring toFIG.9B, the housing18will typically be connected to ground (GND). The conductive plate19can be connected to a high voltage, indicated by HV inFIG.9B, thus creating a voltage difference over the spacing between the housing18and the plate19. If the conductive layer30is connected to a high voltage, then the foil will be repelled by the plate19and attracted by the housing18. As a consequence, the foil will tend to move towards the housing and the S-shaped spacing bridging part will move to the right (see alsoFIG.9A). In other words, electrical forces Felpulling the foil towards the housing cause a mechanical force Fmto the right inFIG.9B. The foil will cover the through holes16and the interior of the collision cell will be closed off.

Referring toFIG.9C, the through holes16can be opened by connecting the conductive layer30to ground instead of to the high voltage (HV). This will cause the foil to be repelled by the housing18and to be attracted by the plate19, which in turn cause the S-shaped spacing bridging part to move to the left (see alsoFIG.9A). In other words, electrical forces Felpulling the foil towards the plate19cause a mechanical force Fmto the left inFIG.9C. The foil will no longer cover the through holes16and the interior of the collision cell will be open to the surrounding atmosphere.

As the movement of the foil is controlled by voltages, which can be switched extremely quickly, and as the foil can have a very low mass, the movement of the foil can be very quick. Accordingly, the pressure inside the collision cell1can be adjusted very rapidly and switching between a pressurized state and an evacuated state can be carried out almost instantly.

The exemplary mass spectrometer10schematically shown inFIG.10comprises a collision cell1, which can be a collision cell as described above, but may be replaced by another type of ion guide. The mass spectrometer10may further comprise a plasma source1, such as an ICP (inductively coupled plasma) source, for generating an ion beam IB1. The mass spectrometer may further comprise a mass filter3, such as a magnetic sector mass filter. In the magnetic sector mass filter, the ion beam IB1is separated into partial beams IB2having different m/z (mass versus charge) ratios, which partial beams can be detected by the detector assembly4, which may be a multiple detector assembly. The mass spectrometer10may further comprise a pump for lowering the gas pressure in the collision cell1, a valve associated with the pump, a voltage source5for supplying DC and AC (RF) voltages to the collision cell1, and a controller for controlling the various components of the mass spectrometer10. The valve may comprise a foil-based valve and/or a Bourdon tube-based valve as described above.

Aspects of the invention comprise:a) A multipole collision cell with variation of the number of oscillations in RF mode in order to average mass dependent trajectories (noding effect).b) A multipole collision cell that is able to transmit an ion beam without RF potentials (no noding effect).c) The ability to transmit high energy ions (kilovolt range).d) Segmented multipoles (two, three or more segments per rod).e) A multipole collision cell where not the rods but drag electrodes (such as vanes) are segmented in order to transmit ions in a DC-only mode.f) A collision cell that is switchable between collision mode (filled with gas) and transmission mode (no gas) where the pumping cross section can be switched according to the cell mode.g) No additional cross section in gas mode and additional cross section for transmission mode. These aspects of the invention may be used in isolation or in combination.

Although the invention has been described above mainly with reference to a collision gas, a reaction gas may additionally, or alternatively, be used. That is, the present invention also provides a reaction cell, as well as a collision/reaction cell. In some embodiments, the cell may have not two but three modes of operation: a collision mode, a reaction mode and a vacuum mode. It will be understood that in the vacuum mode, the pressure inside the cell may be greater than zero, but very small, such that any gas present in the cell has a negligible influence on the ions entering the cell.

It will be understood by those skilled in the art that the invention is not limited to the embodiments shown and that many additions and/or modifications can be made without departing from the scope of the invention as defined in the appending claims.