Mass filter having extended operational lifetime

Abstract

A mass filter is disclosed having at least one electrode (42-48) comprising an aperture (43) or recess. Voltages are applied to the electrodes (42-48) of the mass filter such that ions having mass to charge ratios in a desired range are confined by the electrodes and are transmitted along and through the mass filter, whereas ions (47,49) having mass to charge ratios outside of said desired range are unstable and pass into the aperture (43) or recess such that they are filtered out by the mass filter. The aperture (43) or recess reduces or eliminates the number of ions that would otherwise impact the electrode surface facing the ion transmission axis and hence reduces degradation of the ion transmission properties of the mass filter.

Claims

1. A method of mass filtering ions comprising: mass filtering ions using a first mass filter so as to mass selectively transmit only ions having a first range of mass to charge ratios; and mass filtering the ions transmitted by the first mass filter using a second mass filter, wherein the second mass filter only transmits ions having a second range of mass to charge ratios that is a sub-set of the first range of mass to charge ratios; wherein at least one electrode of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that ions that are unstable in the first mass filter pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter; wherein the ions transmitted by the first mass filter are guided into the second mass filter using a RF-only ion guide arranged between the first mass filter and the second mass filter; and wherein the first mass filter, the second mass filter and the RF-only ion guide are located in a single vacuum chamber.

2. The method of claim 1, wherein the first mass filter and/or second mass filter is a multipole mass filter, such as a quadrupole mass filter.

3. The method of claim 1, comprising applying RF and DC voltages to electrodes of the first mass filter and/or to electrodes of the second mass filter so as to confine ions desired to be transmitted between the electrodes and to cause ions that are not desired to be transmitted to be unstable and not confined between the electrodes.

4. The method of claim 1, comprising: guiding the ions into the first mass filter using a second RF-only ion guide arranged directly upstream of the first mass filter.

5. The method of claim 1, wherein at least one of the electrodes of the second mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that ions that are unstable in the second mass filter pass into or through the aperture and/or into the recess such that they are not transmitted by the second mass filter.

6. The method of claim 1, wherein the electrode having the aperture or recess is elongated in a direction along the length of the first mass filter, and wherein the aperture is a slotted aperture or the recess is a slotted recess.

7. The method of claim 1, comprising arranging a conductive grid or mesh over, or in, the aperture or recess so as to support an electric field generated by the electrode.

8. The method of claim 1, wherein ions that pass into or through the aperture or recess are not detected and are neutralised or discarded.

9. The method of claim 1, wherein at least some of the electrodes of the first mass filter are heated.

10. The method of claim 1, further comprising detecting ions transmitted by the mass filter and/or mass analysing ions transmitted by the filter.

11. The method of claim 1, wherein the first mass filter, the second mass filter and the RF-only ion guide are maintained at the same pressure.

12. The method of claim 1, wherein at least one of the electrodes of the first mass filter and/or at least one of the electrodes of the second mass filter is axially segmented so as to comprise separate individual segments that are spaced a distance apart along the longitudinal axis by one or more gaps so as to not be connected such that ions that are unstable in the first mass filter pass into or through the gaps such that they are not transmitted by the first mass filter.

13. The method of claim 1, wherein at least one electrode of the first mass filter comprises a longitudinal recess extending only partially through the thickness of the electrode so as to not form an aperture through the electrode; and wherein the recess is arranged and configured such that ions that are unstable in the first mass filter pass into the recess such that they are not transmitted by the first mass filter.

14. The method of claim 1, wherein the aperture and/or recess extend a full length of said at least one electrode.

15. The method of claim 1, wherein pressure in the vacuum chamber is 0.1 mbar.

16. A mass and/or ion mobility spectrometer comprising: a first mass filter comprising a plurality of electrodes; a second mass filter comprising a plurality of electrodes arranged downstream of the first mass filter so as to receive ions transmitted by the first mass filter; a RF-only ion guide arranged between the first mass filter and the second mass filter so as to guide the ions transmitted by the first mass filter into the second mass filter, wherein the first mass filter, the second mass filter and the RF-only ion guide are located in a single vacuum chamber of the spectrometer; one or more voltage supplies; and a controller set up and configured to: control said one or more voltage supplies so as to apply voltages to the first mass filter so that it mass selectively transmits only ions having a first range of mass to charge ratios, wherein at least one of the electrodes of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that when said voltages are applied to the first mass filter ions become unstable in the first mass filter and pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter to the second mass filter; and control said one or more voltage supplies so as to apply voltages to the second mass filter so that it mass filters the ions transmitted by the first mass filter, and such that the second mass filter only transmits ions having a second range of mass to charge ratios that is a sub-set of the first range of mass to charge ratios.

17. A mass and/or ion mobility spectrometer comprising: a first mass filter comprising a plurality of electrodes; a second mass filter comprising a plurality of electrodes arranged downstream of the first mass filter so as to receive ions transmitted by the first mass filter; a RF-only ion guide arranged between the first mass filter and the second mass filter so as to guide the ions transmitted by the first mass filter into the second mass filter, wherein the spectrometer is configured to maintain the first mass filter, the second mass filter and the RF-only ion guide at the same pressure; one or more voltage supplies; and a controller set up and configured to: control said one or more voltage supplies so as to apply voltages to the first mass filter so that it mass selectively transmits only ions having a first range of mass to charge ratios, wherein at least one of the electrodes of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that when said voltages are applied to the first mass filter ions become unstable in the first mass filter and pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter to the second mass filter, and control said one or more voltage supplies so as to apply voltages to the second mass filter so that it mass filters the ions transmitted by the first mass filter, and such that the second mass filter only transmits ions having a second range of mass to charge ratios that is a sub-set of the first range of mass to charge ratios.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic of a prior art instrument comprising a pre-filter positioned upstream of a main analytical quadrupole;

(3) FIG. 2 shows a schematic of an instrument according to a first embodiment the present invention, which corresponds to the arrangement shown in FIG. 1 except that it comprises a low resolution analytical quadrupole between the pre-filter and the main analytical quadrupole;

(4) FIG. 3 shows a schematic of an instrument according to another embodiment the present invention, which corresponds to the embodiment shown in FIG. 2 except that it comprises a pre-filter between the low resolution analytical quadrupole and the main analytical quadrupole;

(5) FIG. 4 shows a cross-sectional view of a quadrupole rod set having slotted apertured electrodes, according to an embodiment of the present invention;

(6) FIG. 5 shows a cross-sectional view of a quadrupole rod set having grooved recessed electrodes, according to an embodiment of the present invention;

(7) FIG. 6 shows a perspective view of a of a quadrupole rod set that is axially segmented, according to an embodiment of the present invention

(8) FIG. 7 shows the relative transmissions of ions through different instruments;

(9) FIG. 8 shows the locations in an instrument at which the ions impact on the rod electrodes;

(10) FIG. 9 shows the positions at which ions impact on the rod electrodes in an analytical quadrupole;

(11) FIG. 10 shows the positions at which ions impact on the rod electrodes in a pre-filter quadrupole;

(12) FIG. 11 shows an embodiment of a band pass filter; and

(13) FIG. 12 shows an embodiment of low mass cut-off filter.

DETAILED DESCRIPTION

(14) FIG. 1 shows a cross-sectional view (in the y-z plane) of a schematic of a prior art instrument comprising a short RF-only pre-filter or Brubaker lens 2 positioned directly upstream of a main analytical quadrupole 4. This RF-only pre-filter 2 is supplied with an RF voltage having approximately 50-90% of the amplitude of the RF voltage that is applied to the main analytical quadrupole mass filter 4. The purpose of the pre-filter is to control fringing fields at the entrance to the main resolving quadrupole so as to allow ions to enter the RF-confined environment without becoming unstable and without initially experiencing the effects of the resolving DC applied to the main analytical quadrupole mass filter 4. An RF voltage and a DC resolving voltage is applied to the main analytical quadrupole mass filter 4 in order to mass filter the ions. An RF-only post-filter 6 is also provided at the exit of the analytical quadrupole mass filter 4 for conditioning ions for acceptance into a downstream device (not shown).

(15) FIG. 2 shows a cross-sectional view (in the y-z plane) of a schematic of an instrument according to an embodiment the present invention. The instrument is similar to that shown in FIG. 1, except that it further comprises a relatively short, low performance analytical quadrupole mass filter 8 positioned directly upstream the main analytical quadrupole mass filter 4. A short RF-only pre-filter or Brubaker lens 2 may be positioned directly upstream of the short analytical quadrupole mass filter 8. One or more RF-only post filter 6 may be positioned downstream of the main analytical quadrupole mass filter 4.

(16) In operation, an RF voltage supply 12 applies an RF voltage to the electrodes of the pre-filter or Brubaker lens 2. The pre-filter or Brubaker lens 2 may comprise a quadrupole rod set. A DC voltage may not be applied to the pre-filter or lens 2. An RF voltage supply 14 and a DC voltage supply 16 apply RF and DC voltages, respectively, to the electrodes of the low performance analytical quadrupole mass filter 8 such that the low performance analytical quadrupole mass filter 8 is only capable of transmitting ions having a first range of mass to charge ratios. An RF voltage supply 18 and a DC voltage supply 20 apply RF and DC voltages, respectively, to the electrodes of the main analytical quadrupole mass filter 4 such that the main analytical quadrupole mass filter 4 is only capable of transmitting ions having a second range of mass to charge ratios, which is narrower than the first range of mass to charge ratios transmitted by the low performance analytical quadrupole mass filter 8. An RF voltage supply 22 applies an RF voltage to the electrodes of the post-filter 6, which may comprise a quadrupole rod set. A DC voltage may not be applied to the post-filter 6. A controller 24 is provided so as to control the above described voltage supplies.

(17) In use, ions are transmitted into the pre-filter or lens 2 and guided through the pre-filter or lens 2 and into the low performance analytical quadrupole mass filter 8. The RF voltage applied to the pre-filter or lens 2 may be of lower amplitude than the RF voltage applied to the low performance analytical quadrupole mass filter 8 and/or to the main analytical quadrupole mass filter 4 so as to reduce transmission losses on entry to the low performance analytical quadrupole mass filter 8 due to fringe fields. The RF-only pre-filter or lens 2 may also act as a low mass cut-off filter since the RF voltage supply 13 may be controlled so as to apply RF voltages that radially confine only ions above a particular cut-off mass to charge ratio.

(18) The ions are then transmitted into the low performance analytical quadrupole mass filter 8. The RF and DC voltages applied to mass filter 8 cause only ions in the first range of mass to charge ratios to be radially confined and hence transmitted to the exit of the mass filter 8. Ions having mass to charge ratios outside of this range are filtered out by the mass filter 8, e.g. by being radially excited into the electrodes of the mass filter 8. These ions are not transmitted to the exit of the mass filter 8.

(19) Ions in the first range of mass to charge ratios are then transmitted into the main analytical mass filter 4. The RF and DC voltages applied to main analytical mass filter 4 cause only ions in the second, narrower range of mass to charge ratios to be radially confined and hence transmitted to the exit of the main analytical mass filter 4. Ions having mass to charge ratios outside of this second range are filtered out by the main analytical mass filter 4, e.g. by being radially excited into the electrodes of the mass filter 4. These ions are not transmitted to the exit of the main analytical mass filter 4. The provision of the low performance analytical quadrupole mass filter 8 enables many ions outside of the second range of mass to charge ratios to be filtered out upstream of the main analytical filter 4. As such, these ions are not required to be filtered out by the main analytical filter 4 and hence do not impact on the electrodes of the main analytical filter 4. This helps avoid contamination of the main analytical filter 4 and reduces surface charging of the main analytical filter 4, which would degrade its ion transmission properties.

(20) The low performance analytical quadrupole mass filter 8 may be provided with the same amplitude and frequency RF voltage as the main analytical filter 4. It will therefore be appreciated that they may have the same RF voltage supply. However, the low performance analytical quadrupole mass filter 8 may be provided with the a lower amplitude DC voltage than the main analytical filter 4 such that the resolution for the low performance analytical quadrupole mass filter 8 is lower than that of the main analytical mass filter 4, but the set mass transmission window of both mass filters 8,4 may be centered on substantially the same mass to charge ratio value.

(21) Ions in the second range of mass to charge ratios that are transmitted by the main mass filter 4 are transmitted downstream, e.g. into the post-filter 6. The RF voltage applied to the post-filter radially confines these ions so that they are guided downstream.

(22) It has been recognised that fringing fields between the low resolution mass filter 8 and the main analytical mass filter 4 may cause a reduction in the performance of the main analytical mass filter 4. More specifically, the transmission of the main analytical mass filter at operational mass resolution may be reduced by these fringing field. FIG. 3 shows a schematic of an embodiment for overcoming this.

(23) FIG. 3 shows a schematic of an instrument according to another embodiment of the present invention. This instrument corresponds to that shown in FIG. 2, except that a further RF-only pre-filter 30 is positioned directly between the low performance quadrupole mass filter 8 and the main analytical mass filter 4. The pre-filter 30 may comprise a quadrupole rod set. An RF voltage supply 32 is controlled by the controller 24 so as to apply an RF voltage to the electrodes of the pre-filter 30 for radially confining ions within the pre-filter 30 and guiding them between the low resolution mass filter 8 and main analytical mass filter 4. The RF-only pre-filter 30 effectively shields the main analytical mass filter 4 from the low resolution mass filter 8. In this instrument the performance of the main analytical mass filter 8 is therefore not compromised.

(24) In operation the amplitude of the RF voltage applied to pre-filters 2 and 30 may be the same. As such voltage supplies 12 and 32 may be the same supply. The RF voltage applied to pre-filters 2 and 30 may be, for example, approximately 67% of the amplitude of the RF voltage that is applied to the low performance mass filter 8 and/or main analytical quadrupole 4.

(25) An example of operation using typical operating parameters will now be described. The amplitude of the RF voltage, V, applied to the electrodes of the main analytical mass filter 4, at a given frequency may be set such that ions of interest having a mass to charge ratio M have a value of q=0.706. This may be the point directly below the apex of the Mathieu stability diagram for the main analytical mass filter 4.

(26) The RF only pre-filter 2 acts as a low-mass cut-off such that ions having mass to charge ratio values such that q>0.908 become unstable and will be lost to the electrodes of the pre-filter 2.

(27) If the amplitude of the RF voltage applied to the pre-filters 2,30 is 67% of that applied to the electrode rods of the main analytical mass filter 4 then the low-mass cut-off value M.sub.L of the pre-filters 2,30 is given by:

(28) $\begin{array}{cc}{M}_L=\frac{M0.7060.67}{0.908}=0.52M& \left(3\right)\end{array}$
Therefore, all ions having a mass to charge ratio below M.sub.L will be lost to the electrodes of the pre-filter 2.

(29) The low resolution mass filter 8 may typically be operated with a mass to charge ratio transmission window of 20 Da. Under these conditions only mass to charge ratio values of M+/10 Da will be transmitted to the main analytical mass filter 4, assuming the mass transmission window is centered on the mass to charge ratio of interest M. The main analytical mass filter 4 is typically operated with a mass to charge ratio transmission window of 0.5 to 1 Da, which may also be centered on the mass to charge ratio of interest M.

(30) As described previously, the presence of the low resolution mass filter 4 ensures that the majority of unwanted ions do not impact upon the electrodes of the main analytical mass filter 4, thus minimising contamination and subsequent charging of the electrodes of the main analytical mass filter 4.

(31) Many unwanted ions will impinge on the surfaces of the electrodes of the pre-filter 2 and low resolution mass filter 8. Although the performance of both of these devices is more robust to surface contamination and charging (e.g. since they are operated at relatively low resolutions), these devices may eventually become sufficiently contaminated that ion transmission through them is affected. In order to reduce surface contamination of these components, elongated slotted apertures or grooved recesses may be provided in the rod electrodes such that all or some of the ions which have unstable trajectories within these devices either pass through the rod electrodes or impinge on surfaces which are remote from, or are shielded from, the surfaces closest to the central ion transmission axis.

(32) FIG. 4 shows a cross-sectional view (in the x-y plane) of an embodiment of the low performance mass filter 8 described above. The mass filter 8 comprises four elongated rod electrodes 42-48 having longitudinal axes that extend in the z-direction. The RF voltage supply 14 is provided for delivering RF confinement voltages of opposite phases to different rod electrodes, as is known in the art. The DC power supply 16 is provided for delivering DC resolving voltages of opposite polarities to different rod electrodes, as is known in the art. Each of the rod electrodes 42-48 comprises a tapered slotted aperture 43 that extends all of the way through the electrode, from an ion entrance opening facing the ion optical axis through the mass filter to an ion exit opening facing radially outward from the mass filter. The slot 43 tapers outwardly in a direction from the ion entrance opening to the ion exit opening, i.e. the slot 43 has a cross sectional area in the x-z plane that increases in a direction from the ion entrance opening to the ion exit opening. A grid or mesh electrode 45 may be provided over the ion entrance opening of each slot 43 for substantially maintaining the electric field profile of a conventional quadrupole rod electrode, i.e. a rod electrode not having a slot 43.

(33) FIG. 4 shows the trajectories 47 of positive ions that have mass to charge ratios that are higher than the mass to charge ratio which the mass filter 8 is set to transmit, i.e. for ions outside of the first range of mass to charge ratios. These ions exit the mass filter 8 in the y-direction through the slots 43. FIG. 4 also shows the trajectories 49 of negative ions that have mass to charge ratios that are lower than the mass to charge ratio which the mass filter 8 is set to transmit, i.e. for ions outside of the first range of mass to charge ratios. These ions exit the mass filter 8 in the x-direction through the slots 43. It will therefore be appreciated that the mass filter 8 is able to filter out ions without these filtered ions impacting on the electrodes 42-48 and hence without the filtered ions causing surface contamination and charging of the electrodes 42-48. Some of the filtered ions may impact on the electrodes 42-48, on the side walls of the slotted apertures 43 between the ion entrance openings and ion exit openings. However, even if this causes surface contamination and charging, this occurs away from the ion optical axis through the mass filter 8 and hence is less problematic.

(34) FIG. 5 shows a cross-sectional view (in the x-y plane) of another embodiment of the low performance mass filter 8. This embodiment is the same as that shown and described in relation to FIG. 4, except that each of the rod electrodes 42-48 comprises a grooved recess 50 in the inner surface of the electrode, rather than an aperture 43 extending entirely through the electrode. Each recess 50 extends part way through its respective electrode 42-47, from an ion entrance opening facing the ion optical axis through the mass filter 8 to an ion exit opening facing radially outward from the mass filter 8. The recess 50 may taper outwardly in a direction from the ion entrance opening to the ion exit opening (not shown), i.e. the recess 50 may has a cross-sectional area in the x-z plane that increases in a direction from the ion entrance opening to the ion exit opening. A grid or mesh electrode 45 may be provided over the ion entrance opening of each recess 50 for substantially maintaining the electric field profile of a conventional quadrupole rod electrode, i.e. a rod electrode not having a recess 50.

(35) FIG. 5 shows the trajectories 52 of positive ions that have mass to charge ratios that are higher than the mass to charge ratio which the mass filter 8 is set to transmit, i.e. for ions outside of the first range of mass to charge ratios. These ions travel in the y-direction and enter the recesses 50 in the electrodes 42,46 of the mass filter 8. FIG. 5 also shows the trajectories 54 of negative ions that have mass to charge ratios that are lower than the mass to charge ratio which the mass filter 8 is set to transmit, i.e. for ions outside of the first range of mass to charge ratios. These ions travel in the x-direction and enter the recesses 50 in the electrodes 44,48 of the mass filter 8. It will therefore be appreciated that the mass filter 8 is able to filter out ions without these filtered ions impacting on the inner surfaces of the electrodes 42-48 that face the ion transmission axis, and hence without the filtered ions causing surface contamination and charging of the electrodes 42-48 at these surfaces. As such, ions with stable trajectories through the mass filter 8 are shielded from surface charging on contaminated areas.

(36) FIG. 6 shows a perspective view of another embodiment of the low performance mass filter 8. The mass filter 8 is configured and operates in the same manner as the mass filters described above, except that the electrodes 42-48 of the mass filter 8 need not comprise apertures 43 or recesses 52. Each of the rod electrodes 42-48 of the mass filter 8 is segmented in the longitudinal direction (z-direction), with gaps 60 between the axial segments of the rod set. The mass filter 8 is operated in the same way as described above, such that ions having mass to charge ratios outside of the first range are not stably confined and are radially excited to the extent that they are not transmitted by the mass filter 8. The gaps 60 reduce the surface area of the electrodes 42-48 on which the unstable ions may impact, thus reducing surface charging and contamination of these electrodes 42-48. The axial spacing between the electrode segments in the longitudinal direction (z-direction) may be chosen to be as large as possible and/or the thickness of the electrode segments in the longitudinal direction (z-direction) may be chosen to be as small as possible, provided that the required resolution of the mass filter 8 is maintained in order to minimise the surface area that can be contaminated by filtered ions.

(37) Although the electrodes 48-48 have circular cross-sections (in the x-y plane), other shapes may be used. For example, the electrodes may be substantially hyperbolic (in the x-y plane), or they may have a substantially circular inner bore (e.g. may be annular).

(38) It is also contemplated that the configurations shown in FIGS. 4 and 5 may be axially segmented in the manner shown and described in relation to FIG. 6 in order to further reduce the contamination close to the ion optical axis of the mass filter 8.

(39) The provision of slotted apertures and/or grooved recesses in the electrodes of the mass filter 8 may have an impact on the analytical performance of a quadrupole mass filter, as it may reduce the transmission of ions of interest as the mass resolution is increased. However, at lower resolutions the transmission of the quadrupole is not significantly affected and hence this arrangement is suitable at least for use as the low resolution band-pass mass to charge ratio filter 8 used to protect the higher resolution analytical quadrupole mass filter 4.

(40) The instability of low mass to charge ratio ions within the RF-only pre-filter device 2 may not be as directional as in the case of a resolving quadrupole mass filter. However, slotted apertures and/or grooved recesses may be provided in such a pre-filter 2, or the pre-filter 2 may be segmented, so as to reduce the extent of surface contamination and decrease the effects of surface charging.

(41) An ion optical model (SIMION 8) was constructed in order to demonstrate the principal of operation of the instrument shown in FIG. 3. The RF-only quadrupole filters 2 and 30, and the low resolution analytical quadrupole mass filter 8 were each 16 mm in length. The analytical quadrupole mass filter 4 was 130 mm in length. All of the rod electrodes had a radius of 6 mm and were arranged to form an inscribed circle of radius 5.33 mm. The frequency of the RF voltage applied to all of the rods was set to 1.185 MHz. The main analytical mass filter 4 was set to transmit a mass to charge ratio of 556. This corresponds to an RF amplitude of 1601.8 V (0-peak). The same amplitude of RF voltage was applied to the low resolution mass filter 8. The low resolution mass filter 8 was modeled non-tapered slotted apertures. Each of the slots either had a width in the x-direction or y-direction of 1 mm. The amplitude of the RF voltage applied to the RF-only filters 2 and 30 was set to 67% of the amplitude of the main analytical mass filter 4, i.e. 1073.2 V (0-pk). The kinetic energy of the ions entering the quadrupole assembly was modeled as 1 eV. A resolving DC voltage of 268.7 V was applied to the main analytical mass filter 4, resulting in a mass to charge ratio transmission window of approximately 0.5 Da. Different DC resolving voltages were modeled as being applied to the low resolution mass filter 8 corresponding to theoretical mass to charge ratio transmission windows of 60, 40, 20 and 10 Da, so as to examine the effect on the transmission of ions having a mass to charge ratio of 556 through the entire instrument.

(42) FIG. 7 shows the results of the model described above. It shows the relative transmission of an ensemble of ions having a mass to charge ratio of 556, for a narrow quadrupole scan from m/z=554.4 to m/z=555.6 under different conditions of the low resolution mass filter 8. Plot 70 shows the relative transmission of ions having a mass to charge ratio of 556 for the arrangement of the prior art instrument shown in FIG. 1. The three closely spaced plots 72,74,76 show the relative transmission for the embodiment of the invention shown in FIG. 3. These transmission plots 72,74,76 were generated with the DC resolving voltage set for the low resolution mass filter 8 such that the theoretical resolution of this device was 80 Da, 40 Da and 20 Da respectively. No overall drop in ion transmission was observed for these settings. However, plot 78 shows the results for a theoretical transmission window of 10 amu on low resolution mass filter 8 and results in a reduction of 40-50% in transmission.

(43) FIG. 8 shows the position in z- and y-directions at which ions having a mass to charge ratio of 586 exit the radius of the inscribed circle bounded by quadrupoles 4,8 and 30 in FIG. 3. The slotted, low-resolution mass filter 8 was set to transmit a mass to charge ratio range of 20 Da, centered at a mass to charge ratio of 556. It can be seen from FIG. 8 that 97% of all ions having a mass to charge ratio of 586 (30 amu higher than the central mass to charge ratio set to be transmitted) reach the inner surfaces of the rods within a radial region of +/0.5 mm in the y-direction, corresponding to the position of the 1 mm slots in the rods, and within 16 mm in the z-direction, corresponding to the length of the low resolution mass filter 8. It can be seen that ions having a mass to charge ratio of 586 are not incident on the RF-only pre-filter 30. Only 3% of the ions having a mass to charge ratio of 586 are incident on the electrodes of the main analytical mass filter 4 within the first 16 mm of its length. It is therefore evident that the low resolution mass filter 8 protects the main analytical mass filter 4 from being contaminated by undesired ions having a mass to charge ratio of 586.

(44) FIG. 9 shows a histogram of the number of ions that travel in the y-direction and reach the surfaces of the rods of the low resolution mass filter 8, verses their position in the x-direction relative to the centres of the slots 43 in the rods. The data was modeled for ions having a mass to charge ratio of 1080 and under the same conditions as described in relation to FIG. 8. It can be seen that the majority of the ions pass through the 1 mm wide slots in the rods and so will not contribute significantly to surface contamination on the rods. As described above in relation to equation 3, mass to charge ratios below 289 (=5560.52) will become unstable in the RF-only pre-filter 2 and will be lost to the rod electrodes of the pre-filter 2.

(45) FIG. 10 shows a histogram 100 of the number of ions that travel in the y-direction and reach the surfaces of the rods of the pre-filter 2, verses their position in the x-direction relative to the centres of the rods; and shows a histogram of the number of ions 102 that travel in the x-direction and reach the surfaces of the rods of the pre-filter 2, verses their position in the y-direction relative to the centres of the rods. The data was modeled for ions having a mass to charge ratio of 184 and under the same conditions as described in relation to FIG. 8. It can be seen from FIG. 10 that, although ejection is less directional than that for the resolving quadrupole shown in FIG. 9, ions of this mass to charge ratio are ejected towards the rods in both the x- and y-dimensions. It can be seen that in this case a 2 mm wide slot in each of the pre-filter rods 2 would result in approximately 50% of the low mass ions passing through the slots, and hence not significantly contributing to surface contamination in the pre-filter 2. Even larger slots may be provided in this RF pre-filter 2 without significantly affecting the performance of the device, resulting in a further reduction in surface contamination.

(46) The RF-only pre-filter 30 may not have slotted apertures or grooved recesses in order that the entrance conditions to the main analytical mass filter 4 are maintained at ideal conditions for transmission and resolution of the main analytical mass filter 4. This pre-filter 30 may be maintained at the same RF amplitude as pre-filter 2. As such, there will be substantially no ions incident on the surfaces of the rod electrodes in pre-filter 30.

(47) It will be appreciated that under the conditions described it would be expected that a significant number of ions with mass to charge ratios greater than 586 Da and less than 526 will pass into or through the slots in the low resolution analytical mass filter 8 or in the pre-filter 2. Therefore, these ions would not contribute significantly to any performance losses due to contamination and surface charging.

(48) Although the low performance mass filter 8 has been described as being used to protect and extend the operational lifetime of the higher performance analytical mass filter 4, the apparatus described may be used for many other applications where a low mass cut-off or mass to charge ratio band pass is required.

(49) For example, FIG. 11 illustrates a low resolution band pass mass filter having reduced surface contamination characteristics. The instrument comprises a first RF-only filter 110 having longitudinal slotted apertures or grooved recesses of the type described above, followed by a low performance analytical mass filter 112 having slotted apertures or grooved recesses of the type described above, followed by a second RF-only mass filter 114 of the type describe above but having no slotted apertures or grooved recesses. This instrument may be used as a robust band pass filter, e.g. prior to another downstream analytical device other than, or in addition to, the main analytical mass filter 4 as previously described. For example, the downstream analytical device may be an ion trap or time of flight mass analyser.

(50) Alternatively, the instrument of FIG. 11 may be used as a low performance robust band pass filter arranged downstream of a separate analytical device. For example, the band pass filter may be arranged downstream of an ion mobility separator (IMS). The range of mass to charge ratios passed by the band pass filter may be fixed or may be scanned in synchronism with the delivery of ions from the upstream device. For example, the band pass filter may be used to select ions eluting from an upstream IMS device corresponding to particular charge states. This may be achieved because ions of a given charge state tend to follow a relationship between ion mobility and mass to charge ratio, and the IMS device and band pass filter may be used in combination so as to only transmit ions following such a relationship.

(51) FIG. 12 shows a simple, robust low mass cut-off filter 120. The filter comprises a set of RF-only quadrupole rods having longitudinal slots or grooves of the type described above for minimising surface contamination. This device may be used downstream of an IMS device, e.g. to prevent ions with certain ion mobility drift times and (e.g. maximum) mass to charge ratio values reaching a downstream mass analyser. This may be used to discriminate against ions with different charge states, since ions having the same charge state but different mass to charge ratios may be received at the filter, but only ions of one of the mass to charge ratio values may be transmitted.

(52) In all of the arrangements described the presence of slotted apertures or grooved recesses in the electrodes, or axially segmented electrodes, reduces surface contamination of the electrodes and hence extends the operational lifetime of the various mass filters and/or of a downstream mass or ion mobility analyser.

(53) 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.

(54) For example, the slotted apertures and/or grooved recesses in the rods may be present over only part of the length of the rods, or may be present over the entire length of the rods.

(55) The presence of an RF-only pre-filter 2 upstream of the low resolution mass filter 8 may not be required for operation. This is because the mass filter 8 is operated at relatively low resolution and therefore the entrance conditions may not have a significant effect on transmission for ions at the centre of the mass to charge ratio transmission window. In this case, low mass to charge ratios may be ejected through one set of slotted apertures and high mass to charge ratios may be ejected through the other set of slotted apertures in the mass filter 8.

(56) It is contemplated that the inscribed radii of the different rod sets may be different.

(57) Different DC voltages may be applied to the different rod sets so as to control the energy of the ions through each rod set.

(58) A dipole excitation voltage may be applied to the low resolution mass filter 8 and/or the RF-only filter 2 in order to help move ions in a direction towards the slotted apertures or recesses as the ions become unstable.

(59) It is also contemplated that the main analytical mass filter 4 may comprise apertures or recesses, or be axially segmented, as described in relation to the low resolution mass filter 8 in order to reduce surface contamination.