Mass Filter

Abstract

a mass filter comprising a plurality of mass filtering electrodes for mass filtering ions passing therethrough, at least a first of the electrodes having a first gap therethrough; a voltage source arranged and configured to apply a voltage to the mass filtering electrodes such that ions having a mass to charge ratio within a mass transmission window are confined by the electrodes and transmit through the mass filter, while at least some ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the first gap to be filtered by the mass filter; a first collector electrode arranged to receive ions transmitting through the first gap, and the voltage source applying a DC potential difference between the first mass filtering electrode and the first collector electrode so as to force ions transmitting through the first gap onto the first collector electrode;

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority to and the benefit of UK Patent Application No. 2301594.4, filed 3 February 2023, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates generally to mass and / or ion mobility spectrometers, and in particular to mass filters that selectively transmit ions within a particular range of mass-to-charge ratios and filter out other ions. The mass filters disclosed herein allow for an extended operational lifetime compared to conventional mass filters. [Background technology]

[0003] It is known to use quadrupole mass filters to selectively transmit ions within a particular range of mass-to-charge ratios. Quadrupole mass filters transmit ions that satisfy a condition of stability within the quadrupole field, the condition being defined by the dimensionless parameters q and a,

[0004]

number

[0005] Ions with values ​​of a and q that result in unstable ion trajectories generally collide with the quadrupole rods and are lost. This property is exploited when a quadrupole rod set is used as a mass filter, so that a large proportion of ions that are not desired to be transmitted by the mass filter collide with the inner surfaces of the rod electrodes. Over time, however, the inner surfaces of the rods become contaminated by ions, and charge accumulates on their surfaces. Eventually, localized charging of the contaminated surfaces leads to degradation of the mass filter's performance. This can result in loss of transmission, loss of resolution, or poor mass peak shape. When this occurs, the mass filter must be removed from the vacuum chamber of the mass spectrometer in which it is located and cleaned. Summary of the Invention

[0006] From a first aspect, the present invention provides a mass filter comprising a plurality of mass filtering electrodes for mass filtering ions passing therethrough, at least a first of the electrodes having a first gap therethrough; a voltage source arranged and configured to apply a voltage to the mass filtering electrodes such that ions having a mass to charge ratio within a mass transmission window are confined by the electrodes and transmit through the mass filter, while at least some ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the first gap to be filtered by the mass filter; and a first collector electrode arranged to receive ions transmitted through the first gap, the voltage source applying a DC potential difference between the first mass filtering electrode and the first collector electrode so as to force ions transmitting through the first gap onto the first collector electrode.

[0007] Providing a gap in the first mass filtering electrode allows ions filtered by the mass filter to pass through the first mass filtering electrode rather than colliding with the inner surface of the electrode and causing contamination and charge buildup. However, it has been found that ions that enter such a gap may still collide with the mass filtering electrode and therefore still be problematic. To eliminate or mitigate this, the present invention provides a collector electrode adjacent the gap to push ions onto the collector electrode. Thus, the majority of the contamination and charge buildup caused by filtered ions occurs in the collector electrode, which may have substantially no or relatively little effect on the electric field that causes mass filtering in the mass filter. Also, it may be easier to clean or replace the collector electrode than the mass filtering electrode, for example because precision in aligning the collector electrode within the mass filter is less critical.

[0008] The mass filter may be configured such that the collector electrodes discussed herein do not have any function in shaping or transporting the ion beam passing through the mass filter, in other words the collector electrodes preferably do not contribute substantially to the electric field defined in the region between the mass filtering electrodes for mass filtering ions.

[0009] The mass filter may be a quadrupole mass filter.

[0010] Thus, the plurality of mass-filtering electrodes may comprise four elongated multipole electrodes.

[0011] The mass filter may be a DC resolving mass filter in which a voltage source applies RF and DC voltages to mass filtering electrodes to define the mass transmission window.

[0012] The voltage source may apply a first phase of an RF voltage to a first pair of opposing mass filtering electrodes and a second, different phase of the RF voltage to another pair of opposing mass filtering electrodes. The first and second phases may be 180 degrees out of phase with each other. The voltage source may also apply a first DC voltage to the first pair of opposing mass filtering electrodes and a second, different DC voltage to the other pair of opposing mass filtering electrodes.

[0013] The mass filtering electrodes may be elongated and substantially parallel to one another so as to define a central axis along which ions having a mass-to-charge ratio within the mass transmission window are transmitted, and filtered ions may be ejected radially relative to the central axis through a first gap.

[0014] Each elongated mass filtering electrode may be a single continuous electrode. Alternatively, at least one, or at least some, of the elongated electrodes may be axially segmented electrodes. In embodiments with axially segmented electrodes, the same phase and amplitude of RF voltage may be applied to all of the axial segments within any given axially segmented electrode. However, different DC voltages may be applied to different axial segments within any given axially segmented electrode, for example, to force ions downstream through the mass filter.

[0015] A second one of the mass filtering electrodes, positioned opposite the first mass filtering electrode, may have a second gap therethrough, and a voltage source arranged and configured to apply a voltage to the mass filtering electrode such that ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the second gap, and the mass filter comprises a second collector electrode positioned to receive ions transmitted through the second gap.

[0016] A voltage source may apply a DC potential difference between the second mass filtering electrode and the collector electrode so as to urge ions transmitting through the second gap onto the second collector electrode.

[0017] The voltage source may be configured to (i) apply a first DC voltage to the first and second mass filtering electrodes, (ii) apply a second DC voltage of opposite polarity to two other opposing mass filtering electrodes, and (iii) apply a DC voltage of the same polarity as the first DC voltage but of greater magnitude to the first and second collector electrodes.

[0018] The voltage source may be configured to (i) apply a first phase of an RF voltage to the first and second mass filtering electrodes, (ii) apply a second, different phase of an RF voltage to two other opposing mass filtering electrodes, and (iii) apply the first phase of an RF voltage to the first and second collector electrodes.

[0019] An RF voltage may be applied to the first and / or second collector electrodes.

[0020] Each of the first and / or second collector electrodes may include a plurality of conductive regions, and the voltage source may be configured to apply different voltages to different conductive regions so as to force filtered ions onto at least one of the conductive regions.

[0021] The voltage source may be configured to apply different voltages to different conductive regions simultaneously.

[0022] The voltage source may be configured to operate (i) in a first mode in which a first voltage is applied to a first of the conductive regions and one or more different voltages are applied to one or more other conductive regions such that substantially all or a majority of ions striking the collector electrode strike the first conductive region, and (ii) in a second mode in which a voltage is applied to a second of the conductive regions and one or more different voltages are applied to one or more other conductive regions such that substantially all or a majority of ions striking the collector electrode strike the second conductive region.

[0023] The voltage applied to the conductive region is preferably a DC voltage.

[0024] The mass filter may comprise a charge detection circuit configured to detect charge on the first conductive region during the first mode, and to switch the mass filter to the second mode when the detected charge reaches a threshold level.

[0025] The mass filter may be configured to switch from the first mode to the second mode after a maximum preselected amount of time operating in the first mode.

[0026] The mass filter may comprise a charge or current detection circuit configured to detect charge or current on the second conductive region during the second mode, the mass filter being configured to determine the remaining operational life of the collector electrode based on the charge or ion current detected on the second conductive region in the second mode and then switch to the first mode.

[0027] The mass filter may compare the detected ion current or charge to calibration data relating ion current or charge values ​​to the remaining operational life of the collector electrode. A mass and / or mobility spectrometer comprising a mass filter may display the remaining operational life and / or may be configured to provide an alert indicating that the operational life has expired and that, for example, the collector electrode should be replaced or cleaned.

[0028] The first conductive region may not include ion detection circuitry.

[0029] The mass filter may be operated primarily in the first mode and may be switched to the second mode for a smaller proportion of time during an experimental run.

[0030] The surface of the first collector electrode facing the first gap may have a ridge towards the first gap, and / or the surface of the second collector electrode facing the second gap may have a ridge towards the second gap.

[0031] Such a raised profile of the collector electrode may enhance the ion focusing and attracting properties of the collector electrode.

[0032] The ridge may extend along the length of the collector electrode, for example, substantially parallel to the gap.

[0033] The peak of the ridge may be aligned with the center of the gap in the width direction of the gap (ie, perpendicular to the length of the gap).

[0034] The ridge may have a constant height along the length of the collector electrode, or its height may vary as a function of length along the collector electrode, for example so that the electric field between the collector electrode and the first mass-filtering electrode varies as a function of length.

[0035] At least a portion of the surface of the first and / or second collector electrodes facing the first and / or second gaps, respectively, may have a modified surface, making it easier for an operator to see contamination on that surface due to ion impingement than would be the case in the absence of the modified surface.

[0036] At least the portion of the surface that is modified can be a discrete portion of the surface that is disposed adjacent to the gap.

[0037] At least a portion of the surface may be modified relative to other portions of the collector electrode to have a different topology and / or to be less reflective to visible light, such as by being etched or coated.

[0038] The first and / or second collector electrodes may be or comprise conductive plates arranged to face the first and / or second gaps, respectively.

[0039] The plate can be removed from the mass filter to analyze contamination on the plate resulting from mass filtered ions impinging on the plate.

[0040] Each of the first and / or second collector electrodes may comprise a stack of conductive stripping strips positioned to receive ions passing through the first and / or second gap, respectively.

[0041] This allows the outermost peel strips of the stack to be removed as contamination builds up, exposing clean, uncontaminated peel strips.

[0042] The peel strip that has been removed can be analyzed in the same manner as described herein for the plate.

[0043] The stack can be disposed on the body of the collector electrode and configured such that applying a voltage to the body of the collector electrode causes the voltage to penetrate all of the peel strips.

[0044] The release strips in each stack may be adhered to each other, and optionally to the body of the collector electrode, using a conductive adhesive.

[0045] The mass filter may comprise one or more structural supports configured to mount the first and / or second collector electrodes to the mass filtering electrodes such that the first and / or second collector electrodes are located adjacent the first and / or second gaps, respectively.

[0046] The one or more structural supports may be, for example, an electrically insulating material such as PEEK.

[0047] The collector electrode is electrically conductive, such as made from steel.

[0048] Each of the collector electrodes may be releasably mounted to one or more structural supports such that it may be removed and replaced, for example, each collector electrode may be releasably mounted to one or more structural supports using screws.

[0049] The first and / or second collector electrodes may be located relatively close to the first and / or second mass filtering electrodes, respectively, for example the distance between the first and / or second collector electrodes and the first and / or second mass filtering electrodes, respectively, may be less than 10 mm or less than 5 mm at their nearest points.

[0050] One or more structural supports may be configured to releasably mount the first and / or second collector electrodes in multiple or different orientations or positions relative to the mass-filtering electrode so that each collector electrode is (i) rotatable so that different surfaces of the collector electrode receive ions from the gap at different respective times, and / or (ii) movable so that different portions of the same surface on the collector electrode receive ions from the gap at different respective times.

[0051] The first gap may be a slot through the first mass filtering electrode and / or the second gap may be a slot through the second mass filtering electrode.

[0052] Thus, the periphery of each slot can be completely surrounded by electrode material.

[0053] Alternatively, the first electrode may have two separate, spaced apart electrode portions defining a first gap therebetween, and similarly, the second electrode may have two separate, spaced apart electrode portions defining a second gap therebetween.

[0054] The mass filter is preferably configured so as not to trap ions in three dimensions whilst mass filtering is taking place, and therefore the mass filter and the voltages applied to it allow ions to enter the mass filter at the entrance end and ions within the mass transmission window to continue through the mass filter and exit at the downstream end without being trapped axially.

[0055] The mass filter or mass spectrometer disclosed herein preferably does not have circuitry to determine the mass-to-charge ratio of ions striking the first and / or second collector electrodes.

[0056] It is contemplated that the mass filter need not necessarily apply a DC potential difference between a first mass filtering electrode and a first collector electrode to force ions transmitting a first gap onto the first collector electrode, and similarly, the mass filter need not necessarily apply a DC potential difference between a second mass filtering electrode and a second collector electrode to force ions transmitting a second gap onto a second collector electrode.

[0057] Thus from a second aspect the present invention provides a mass filter comprising a plurality of mass filtering electrodes for mass filtering ions passing therethrough, at least a first of the electrodes having a first gap therethrough; a voltage source arranged and configured to apply a voltage to the mass filtering electrodes such that ions having a mass to charge ratio within a mass transmission window are confined by the electrodes and transmit through the mass filter, while at least some ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the first gap to be filtered by the mass filter; and a first collector electrode arranged to receive ions transmitted through the first gap.

[0058] The second aspect of the invention may have any of the optional features described in relation to the first aspect of the invention, except that a DC potential difference does not necessarily have to be placed between the first and / or second mass filtering electrodes and the first and / or second collector electrodes, respectively, to force ions onto the collector electrodes.

[0059] The present invention also provides a mass and / or ion mobility spectrometer comprising a mass filter as described herein, further comprising a detector or analyser for detecting or analysing ions within a mass transmission window that are transmitted by the mass filter, or ions derived therefrom.

[0060] The mass filter may be a bandpass mass filter that filters ions that would otherwise impinge on components of the spectrometer downstream of the mass filter, and / or the mass filter may be a bandpass mass filter that filters ions that would otherwise impinge on components of the spectrometer downstream of the mass filter. -4 The pressure may be maintained at or above 1000 mbar.

[0061] The downstream component may be an aperture wall or electrode through which ions pass, or an ion optical component such as an ion lens, a further mass filter, or an ion guide. For example, a bandpass mass filter may transmit ions having a mass-to-charge ratio within a first mass transmission window and filter ions having a mass-to-charge ratio outside this window; the transmitted ions may then pass into a further mass filter having a second mass transmission window smaller than and located within the first mass transmission window. For example, the width of the second mass transmission window may be such that it is only capable of transmitting a single ion species. The bandpass mass filter protects the further mass filter by filtering a portion of the ions that would otherwise impinge on the electrodes of the further mass filter. Ions that pass through the bandpass mass filter and are within the second mass transmission window will also pass through the further mass filter.

[0062] A bandpass mass filter may have a mass transmission window that is capable of transmitting multiple ion species simultaneously. For example, the mass transmission window may have a width of ≥ 50 Da, ≥ 100 Da, ≥ 150 Da, or ≥ 200 Da, although mass transmission windows of other widths are also contemplated.

[0063] The mass filters described herein may be located in relatively high pressure regions of the spectrometer, e.g., relatively upstream regions, so as to perform pre-filtering of ions to protect downstream components. For example, the mass filter may be located in a 10 -2 mbar~10 -3 The mass filter may be used at pressures of up to 1000 mbar, however, it is contemplated that the mass filter may be used at lower pressures.

[0064] The present invention also provides a method of mass filtering ions, comprising providing a mass filter as described herein and applying a voltage to the mass filter such that the mass filter transmits only ions having a mass to charge ratio within the mass transmission window and filters ions having a mass to charge ratio outside the mass transmission window, and at least some of the filtered ions pass through the first and / or second gaps onto the first and / or second collecting electrodes, respectively.

[0065] The present invention also provides methods of mass and / or ion mobility spectrometry, including methods of mass filtering ions as described herein, which involve detecting and / or mass analysing and / or ion mobility analysing ions transmitted by or derived from a mass filter. [Brief explanation of the drawings]

[0066] Various embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] 1 illustrates a mass spectrometer according to one embodiment of the present invention. [Figure 2] 1 shows a conventional quadrupole mass filter. [Figure 3] 1 shows a quadrupole mass filter with slotted electrodes. [Figure 4A] 1A-1D show different views of a mass filter according to an embodiment of the present invention; [Figure 4B] 1A-1D show different views of a mass filter according to an embodiment of the present invention; [Figure 4C] 1A-1D show different views of a mass filter according to an embodiment of the present invention; [Figure 4D] 1A-1D show different views of a mass filter according to an embodiment of the present invention; [Figure 5A] 4A-4D show simulations of ion trajectories in the mass filter of FIGS. 4A-4D. [Figure 5B]4A-4D show simulations of ion trajectories in the mass filter of FIGS. 4A-4D. [Figure 6] 1 illustrates a collector electrode according to one embodiment having multiple discrete conductive regions. [Figure 7] 1 illustrates a collector electrode according to one embodiment having a raised surface. [Figure 8] 1 shows a collector electrode according to one embodiment having a modified area to aid in visualization of contamination thereon. [Figure 9] 1 illustrates a collector electrode according to one embodiment having a stack of conductive release strips. [Figure 10] 10 shows a portion of a mass filter according to an embodiment in which the collector electrode is rotatable; DETAILED DESCRIPTION OF THE INVENTION

[0067] FIG. 1 shows a block diagram of a mass spectrometer according to one embodiment of the present invention, comprising an ion source 2 , a quadrupole mass filter 4 , a fragmentation or reaction cell 6 and an orthogonal acceleration time-of-flight mass analyzer 8 .

[0068] In operation, ions are generated by the ion source 2 and passed through the quadrupole mass filter 4. A voltage is applied to the quadrupole mass filter 4 to allow only ions within a specific range of mass-to-charge ratios, known as the mass transmission window, to be transmitted. Ions with mass-to-charge ratios outside this window are filtered out and not transmitted by the mass filter 4. Ions transmitted by the mass filter pass to the fragmentation or reaction cell 6, which may fragment the ions or react with other ions or molecules to produce fragment or other product ions. These fragment or product ions are transmitted onward to the time-of-flight mass analyzer 8, where they are mass analyzed along with unfragmented precursor ions. The mass-to-charge ratio transmission window of the mass filter 4 may be scanned over time during an analytical run, as known in the art, so that different ranges of mass-to-charge ratios are transmitted by the mass filter 4 at different times. The different precursor ions transmitted at different times may then be associated with their respective product ions in a known manner.

[0069] Although particular equipment has been described, it will be understood that the invention is not limited to mass spectrometers having the above features. For example, fragmentation or reaction devices may be switched on and off in different modes, or may not even be present. Additionally or alternatively, the time-of-flight mass analyzer may be replaced by a different type of mass analyzer. Additionally or alternatively, additional ion optical devices may be provided between the ion source and the mass filter, and / or between the mass filter and the mass analyzer.

[0070] As explained above, the mass filter 14 may be required at any given moment to transmit only ions within a particular mass transmission window, filtering a relatively large proportion of the ions it receives. Traditionally, this has led to the electrodes of the mass filter becoming fouled relatively quickly, as explained below with reference to Figure 2.

[0071] FIG. 2 shows a schematic diagram of a conventional quadrupole mass filter 10. The mass filter comprises four parallel rod electrodes 12a, 12b. An RF voltage source 14 is connected to these electrodes to supply a first phase of RF voltage to a first pair of opposing electrodes 12a and a second phase of RF voltage to the other pair of electrodes 12b, the first and second phases being 180 degrees out of phase with each other. In other words, opposite phases of RF voltage are applied to circumferentially adjacent rods. A DC voltage source 16 is also connected to these electrodes to supply a first DC voltage to the first pair of opposing electrodes 12a and a second, different DC voltage to the other pair of electrodes 12b. In other words, the mass filter is a DC-resolving mass filter.

[0072] As is well known in the art, only ions with mass-to-charge ratios that fall within a particular mass transmission window have stable trajectories through the mass filter. Therefore, only these ions are transmitted forward by the mass filter, while all other ions are filtered by the mass filter. Ions that are filtered typically collide with the rod electrodes 12a, 12b, resulting in contamination buildup on the inner surfaces of the electrodes. Because contamination acts as an electrical insulator, when additional ions that are to be filtered collide with the contaminated surface, this causes a charge to build up on the contaminated surface, which disrupts the electric field of the mass filter and therefore alters the performance of the mass filter.

[0073] For example, as described below in connection with Figure 3, it is known to alleviate the problems described above by providing slots through the electrodes of the mass filter so that fewer ions impinge on the inner surfaces of the electrodes 12a, 12b.

[0074] Figure 3 shows a cross-sectional view (in the xy plane) of a known mass filter which is the same as that described with respect to Figure 2, except that each of the rod electrodes 12a, 12b is provided with a slotted aperture 18 which extends entirely through the electrode, from an ion entrance aperture facing the ion optical axis through the mass filter to an ion exit aperture facing radially outwards from the mass filter. A grid or mesh electrode 20 may be provided over the ion entrance aperture of each slot 18 to substantially maintain the electric field profile.

[0075] Figure 3 shows trajectories 22, 24 of analyte ions having a mass to charge ratio that lies outside the mass transmission window. Positive ions having a mass to charge ratio that lies outside the mass transmission window that the mass filter is designed to transmit are unstable and exit the mass filter in the y direction through slot 18, as shown by trajectory 22. Negative ions having a mass to charge ratio that lies outside the mass transmission window that the mass filter is designed to transmit are also unstable and exit the mass filter in the x direction through slot 18, as shown by trajectory 24. It will therefore be appreciated that the mass filter is able to filter ions without a substantial proportion of these filtered ions impinging on the inner radial surfaces of electrodes 12a, 12b and therefore without the filtered ions causing significant contamination and charging of the inner radial surfaces of the electrodes.

[0076] Figures 4A-4D show different views of a mass filter in accordance with one embodiment of the present invention. The mass filter comprises four elongated mass filtering electrodes 26a, 26b which act to mass filter ions, although one of the electrodes 26b has been omitted from Figures 4B-4C for illustrative purposes only. As best shown in Figure 4D, an RF voltage source 28 is connected to a first pair of opposing electrodes 26a to supply a first phase of RF voltage to these electrodes, and a second phase of RF voltage to a second pair of electrodes 26b. In other words, different RF phases may be applied to adjacent mass filtering electrodes 26a, 26b in a circumferential direction about a central axis through the mass filter. The first and second phases are preferably 180 degrees out of phase with each other. Preferably, RF voltages of the same amplitude are applied to all of the electrodes 26a, 26b. A DC voltage source 30 is also connected to these electrodes 26a, 26b so as to supply a first DC voltage to a first pair of opposing electrodes 26a and a second, different DC voltage to the other pair of electrodes 26b, so that the mass filter acts as a DC resolving mass filter.

[0077] RF and DC voltages are applied to the mass filtering electrodes 26a, 26b so that ions having a mass to charge ratio within the mass transmission window are transmitted by the mass filter, while other ions are filtered by the mass filter in the same manner as described above in relation to Figures 2-3. As shown in Figure 4A, an aperture plate or wall 32 may be positioned at the downstream end of the mass filter having an aperture through which ions transmitted by the mass filter pass.

[0078] Each electrode of the first pair of electrodes 26a has a gap 34 in the form of a slotted aperture extending radially therethrough to allow ions filtered by the mass filter to pass through the electrode 26a. Each slot 34 may have a length extending along most of the length of the electrode within it. Alternatively, the slot may have a relatively short length and be located at an axial location where most of the ion ejection is expected to occur, for example, at the entrance end of the mass filter. Each slot 34 may have a width of less than 2 mm, such as about 1 mm. Such a width has been found to allow the ejection of filtered ions without significantly disrupting the electric field within the mass filter. However, other widths are contemplated.

[0079] Behind each slot 34 is a collector electrode 36, so that filtered ions passing through the slot impinge on the collector electrode 36. In Figure 4A, one of the collector electrodes 36 is illustrated as being purely transparent, in order to allow viewing of the slot 34 behind it.

[0080] The electrodes 26, 36 described above may be held in a fixed position relative to one another by one or more structural supports, as shown, for example, in FIG. 4D . For example, a first structural support 38 may be secured to one of the collector electrodes 36, one of the first pair of electrodes 26 a, and one of the second pair of electrodes 26 b. The first structural support may be secured to these electrodes using, for example, fastening members that engage holes 40 in the electrodes (best seen in FIG. 4B ). For example, the fastening members may be threaded into the holes in the electrodes. A second structural support may be provided that is secured to the other of the collector electrode 26, the other of the first pair of electrodes 26 a, and the other of the second pair of electrodes 26 b. Again, fastening members may be provided that secure these electrodes to the second structural support in a manner corresponding to that described above with respect to the first structural support. The first and second structural supports may then be secured to one another so that neither of the electrodes 26, 36 can move relative to one another. The securing member may releasably secure the electrode to the structural support so that at least the collector electrode may be removed, for example to be cleaned.

[0081] Although specific electrodes have been described as being fixed to each structural support, it is contemplated that different combinations of electrodes may be fixed to each structural support, and that more than two structural supports may be used to fix the positions of the electrodes relative to one another.

[0082] The DC voltages applied to the mass filtering electrodes 26a, 26b are selected to urge unstable analyte ions within the mass filter towards and through the slot 34. For example, if positive ions are being mass filtered, the DC voltage applied to the first electrode 26a may be more negative than the DC voltage applied to the second electrode 26b. Alternatively, if negative ions are being mass filtered, the DC voltage applied to the first electrode 26a may be more positive than the DC voltage applied to the second electrode 26b.

[0083] As shown in FIG. 4D , a DC voltage source 42 can supply a DC voltage to the collector electrode 36. This DC voltage is preferably different from the DC voltage applied to the first pair of electrodes 26a, such that a DC potential difference is maintained between each electrode of the first pair of electrodes 26a and its adjacent collector electrode 36. The voltage can be selected to push ions exiting the slot 34 onto the collector electrode 36. For example, if positive ions are being mass filtered, the collector electrode 36 can be maintained at a more negative DC voltage than the first pair of electrodes 26a. Conversely, if negative ions are being mass filtered, the collector electrode 36 can be maintained at a more positive DC voltage than the first pair of electrodes 26a. This DC potential difference helps prevent ions being ejected through the slot 34 from circling and landing on the first pair of electrodes 26a and the second pair of electrodes 26b. In addition to or as an alternative to applying a DC voltage to the collector electrode, RF voltage source 28 may apply an RF voltage to collector electrode 36. This RF voltage may have the same phase and / or amplitude as the RF voltage applied to first electrode 26a.

[0084] Figures 5A-5B show simulations of ion trajectories within a mass filter. Figure 5A shows trajectories 44 of ions having a mass-to-charge ratio that falls within the mass transmission window. These ions are radially confined by the mass filtering electrodes 26a, 26b and have stable trajectories as they pass along the central axis of the mass filter from the upstream end to the downstream end. Figure 5B shows trajectories 46 of ions having a mass-to-charge ratio that falls outside the mass transmission window. These ions are not radially confined by the mass filtering electrodes 26a, 26b; instead, they are unstable and therefore pass radially through the slots 34 in the first pair of electrodes 26a and impact the collector electrode 36.

[0085] As explained above, the slot 34 may have a relatively small width, which helps to prevent, for example, penetration of a field due to the voltage applied to the collector electrode 36 into the region between the first pair of electrodes 26a and the second pair of electrodes 26b. Therefore, any voltage applied to the collector electrode 36 may not interfere with ions being trapped within the mass filter. Therefore, the voltage applied to the collector electrode 36 may be set to any amplitude, but should not be set so high that a discharge occurs. It has also been found that contamination that accumulates on the collector electrode 36 does not interfere with ions being trapped radially within the mass filter. If charge builds up on a contaminated portion of the collector electrode, for example due to impacts from ions being mass filtered, this does not affect the voltage switching speed or any other aspect of the mass filter's performance. Furthermore, the collector electrode can be easily replaced if necessary.

[0086] For illustrative purposes only, examples of voltages that may be applied to the mass filter are described here. An RF voltage having a frequency of 1.6 MHz and an amplitude of 700 V (peak-to-peak) may be applied to the first pair of electrodes 26 a and the second pair of electrodes 26 b, and also to the collector electrode 36. The RF voltages applied to the first pair of electrodes 26 a and the collector electrode 36 may be in phase, while the RF voltage applied to the second pair of electrodes 26 b may be 180 degrees out of phase. The DC voltage applied to the first pair of electrodes 26 a may be −15 V. The DC voltage applied to the second pair of electrodes 26 b may be +15 V. The DC voltage applied to the collector electrode 36 may be −25 V. However, as noted above, these are only exemplary voltages, and voltages having alternative frequencies and amplitudes may be applied instead.

[0087] 6 shows a portion of one of the collector electrodes 36 according to one embodiment, configured to cause ions filtered by the mass filter to impinge on different discrete regions of the collector electrode at different times. The surface of the collector electrode facing the slot 34 comprises a plurality of discrete conductive regions 60, 62, 64 that can be electrically biased independently of one another, thereby causing filtered ions to impinge on a selected one of the regions at any given time. For example, during a first duration, the first region 60 can be electrically biased with a DC voltage to create a DC potential difference between the first region 60 and the adjacent electrode 26a with the slot 34 therein, such that ions are focused onto the first region 60. Different DC voltages can be applied to the other discrete regions 62, 64 so that fewer ions or substantially no ions impinge on those other discrete regions during the first duration.

[0088] After a certain duration, it may be desired to have the filtered ions impinge on the second discrete region 62 and not on the first discrete region 60. For example, if ions are filtered by a mass filter at high velocity, the filtered ions may impinge on the first discrete region 60, building up a charge to a level that allows the filtered ions to be repelled from the collector electrode and placed back onto the slotted electrode 26a. When it is desired to have the filtered ions impinge on the second discrete region 62 during the second duration, the second region 62 may be electrically biased with a DC voltage to create a DC potential difference between the second region 62 and the slotted electrode 26a such that the ions are focused onto that second region. Different DC voltages may be applied to the other discrete regions 60, 64 so that fewer ions or substantially no ions impinge on those other discrete regions during the second duration.

[0089] Similarly, after a period of time, it may be desired to have the filtered ions impinge on the third discrete region 64 and not on the first and second discrete regions 60 and 62. At this time, the third region 64 may be electrically biased with a DC voltage to create a DC potential difference between the third region and the slotted electrode 26a so that ions are focused onto that third region. Different DC voltages may be applied to the other discrete regions 60, 62 so that fewer ions impinge on those other discrete regions, or substantially no ions impinge on those other discrete regions.

[0090] Providing such discrete impact regions 60-64 can be useful for several reasons. For example, as explained above, it may be desirable to direct filtered ions to different regions of collector electrode 36 at different times to prevent excessive charge buildup in any particular region.

[0091] The collector electrode 36 may be connected to a charge detection circuit 66 configured to determine the charge on the discrete regions 60-64 to which filtered ions are directed, and if the determined charge reaches a threshold level, then the mass filter may switch the voltages applied to the discrete regions 60-64 so that the filtered ions are directed to a different one of the discrete regions 60-64. The charge on the different discrete regions may also be monitored, and if a threshold level is reached, then the mass filter may again switch the voltages applied to the discrete regions 60-64 so that the filtered ions are directed to another one of the discrete regions 60-64.

[0092] Alternatively, or in addition to charge detection techniques, the spectrometer may be configured to switch the voltages applied to the discrete regions 60-64 so that filtered ions are directed into each of the discrete regions for a preselected maximum amount of time.

[0093] Another advantage of using multiple separate conductive regions is that one region 60-64 can be used to detect and analyze the ion signal of filtered ions, while another of the regions can be a non-detection region that simply receives the filtered ions. This allows, for example, filtered ions to generally be directed to the non-detection region, but occasionally, to analyze the ion signal of the filtered ions, the ions can be directed to the detection region instead. Ions can be directed to the detection region for a shorter percentage of time than they are directed to the non-detection region, allowing the detection region to remain relatively clean and therefore accurately detect the ion signal.

[0094] For example, the detection region may be connected to an ion current or charge detection circuit 66 that detects the ion current or charge in the detection region due to filtered ions impinging on the detection region. The detected current or charge may be used to determine the ion impingement rate at the collector electrode, for example, to determine the rate of contamination and therefore the remaining operational life before the non-detection region of the collector electrode should be cleaned or replaced. For example, the mass spectrometer may compare the detected ion current or charge to calibration data relating the ion current or charge value to the remaining operational life of the non-detection region. The spectrometer may be configured to display the remaining operational life and / or provide an alert indicating that the operational life has expired and the collector electrode should be replaced or cleaned.

[0095] Alternatively, or additionally, the detected ion current or charge may be used to determine the operational life of one or more components of the spectrometer downstream (or further upstream) of the mass filter, as the detected current or charge is related to the ion flux passing through the spectrometer and therefore the rate of contamination of components downstream (or upstream) of the mass filter. The detected ion current or charge may be compared to calibration data relating the value of the ion current or charge to the remaining operational life of components downstream (or upstream) of the mass filter, such as apertured components through which the ions pass, or ion optics such as ion lenses, further mass filters or ion guides. The spectrometer may be configured to display the remaining operational life of a component and / or provide an alert indicating that its operational life has expired and that the component should be replaced or cleaned.

[0096] Although three regions 60-64 have been described, it will be appreciated that only two regions or more than three regions may be provided instead, and it will also be appreciated that the operational life monitoring systems described herein may be used in embodiments where the collector electrode has only a single sensing region.

[0097] FIG. 7 shows a portion of one of the collector electrodes 36 according to an embodiment in which the surface of the collector electrode upon which filtered ions impinge is not flat but has a three-dimensional profile. In the depicted embodiment, the surface of the collector electrode facing the slotted electrode has a ridge 70 pointing toward the slotted electrode 26a. The ridge preferably extends longitudinally along the collector electrode, parallel to the slot 34 of the slotted electrode. The peak of the ridge is preferably positioned to align with the center of the slot (in the width direction of the slot, i.e., perpendicular to the length of the slot). Thus, the surface of the collector electrode may be tapered such that its distance from the slotted electrode 26a increases as a function of the distance away from the ridge 70 in the width dimension of the slot 34. Such a raised profile of the collector electrode may enhance the ion-focusing and attracting properties of the collector electrode.

[0098] Although the ridges 70 are shown as having the same height and shape along the length of the collector electrode 36, it is contemplated that the height of the ridges may vary as a function of length along the collector electrode, for example, such that the electric field between the collector electrode and the slotted electrode varies as a function of length.

[0099] The surface of the collector electrode facing the slotted electrode 26a is shown as tapering gradually and continuously from the ridge, however, it is contemplated that the surface may vary in other ways.

[0100] FIG. 8 illustrates a portion of one of the collector electrodes 36 according to one embodiment, in which at least a portion of the surface of the collector electrode facing the slotted electrode has a modified surface 80, making it easier for an operator to see contamination on that surface due to impingement of filtered ions than would be the case without the modified surface. This can help an operator determine that filtered ions are impinging on the collector electrode and also when the collector electrode needs to be cleaned or replaced. The at least a portion of the modified surface 80 can be a discrete portion of the surface facing the slotted electrode 26a that is positioned adjacent to the slot 34. For example, the topography of the portion of the surface 80 facing the slotted electrode can be modified to have, for example, a different texture, profile, or adhesive properties than other portions of the surface facing the slotted electrode. It is contemplated that the at least a portion of the surface can be modified to have a lower reflectivity to visible light. For example, the at least a portion of the surface can be etched or coated to reduce the reflectivity of visible light.

[0101] Embodiments are contemplated in which the collector electrode is a plate removably mounted to the support member, or in which the collector electrode comprises a conductive plate positioned facing the slot and removably mounted to the remainder of the collector electrode. In such embodiments, the plate may be removed from the mass filter to analyze contamination on the plate due to filtered ions impinging on it. For example, the contamination may be analyzed using surface analysis techniques such as scanning electron microscopy (SEM) or mass spectrometry techniques such as matrix assisted laser desorption ionization (MALDI). It will be appreciated that the provision of such a plate facilitates such analysis. The plate may be rigid or flexible.

[0102] FIG. 9 shows a portion of one of the collector electrodes 36 according to one embodiment, comprising a stack of conductive peel strips 90 disposed thereon to receive filtered ions from the slotted electrode 26a. The stack 90 may be disposed on the body 92 of the collector electrode and configured such that applying a voltage to the body of the collector electrode transmits the voltage to all of the peel strips. For example, multiple peel strips in the stack may be adhered to each other, and optionally to the body of the collector electrode, using a conductive adhesive. During use of the mass filter, filtered ions impinge on the outermost peel strip 94 in the stack of peel strips, and therefore contamination accumulates thereon. The outermost peel strip 94 may then be removed from the stack to expose a clean, uncontaminated peel strip 96. For example, if a conductive adhesive is used to adhere the peel strips together in the stack, the adhesive may be provided at a level of adhesion such that the outermost peel strip can be manually removed from the remainder of the stack. This process may be repeated each time it is desired to remove the contaminated surface of the collector electrode. It is contemplated that the release strip may be flexible, such as conductive tape, or may be a rigid plate.

[0103] Figure 10 shows a portion of a mass filter in one embodiment in which each collector electrode 36 can be rotated so that different surfaces of the collector electrode can be positioned to face the slotted electrode 26a at different times. While only some of the mass filter's electrodes 26a, 26b, 36 are shown, it will be understood that the mass filter of this embodiment also includes the other electrodes described in connection with Figures 4A-4D. The collector electrode 36 can be configured so that it can be removed from the structural support 38 (e.g., as described in connection with Figure 4D) and replaced within the structural support with a different surface of the collector electrode facing the slotted electrode 26a. For example, if the collector electrode has a rectangular cross-sectional shape as shown in Figure 10, the collector electrode can be rotated one or more times so that up to four different surfaces of the collector electrode can be positioned to face the slotted electrode. However, it is contemplated that the collector electrode can have other cross-sectional shapes to provide a fewer or greater number of surfaces that can be positioned to face the slotted electrode. For example, the collector electrode may have at least three, at least five, or at least six surfaces that may be positioned to face the slotted electrode.

[0104] It is contemplated that the collector electrode may be mounted within the mass filter by one or more structural supports 38 that are configured to allow the collector electrode to be rotated so that different sides of the collector electrode face the slotted electrode without having to disassemble the mass filter to remove the collector electrode. For example, the collector electrode may be mounted on a pivot member of the structural support so that it may be rotated. The pivot member may be slidably mounted within the structural support so that the collector electrode may be moved away from the slotted electrode to provide space for rotating the collector electrode. The structural support may also include a locking mechanism 100 for releasably locking the collector electrode in a plurality of positions with different sides facing the slotted electrode.

[0105] Although the collector electrode has been described as being rotated to expose different portions of its surface to the slotted electrode, it is contemplated that the collector electrode may alternatively be slid vertically and / or horizontally relative to the slotted electrode so that different portions of the same surface of the collector electrode are positioned adjacent the slots at different respective times.

[0106] Additionally or alternatively to the collector electrode being rotatable and / or slidable in the manner described above, it is contemplated that the structural support 38 may allow the collector electrode to be moved toward and away from the slotted electrode 26a, and optionally fixed at a number of different distances from the slotted electrode.

[0107] It is contemplated that the mass filters described herein may be used to filter ions that would otherwise impinge on and contaminate components of a mass spectrometer downstream of the mass filter. For example, the mass filters described herein may be apertured components through which ions pass, or sacrificial mass filters positioned to protect downstream components such as ion optics such as ion lenses, further mass filters, or ion guides. Thus, the mass filters described herein may be bandpass mass filters having mass transmission windows that are capable of transmitting multiple ion species simultaneously at any given time. For example, the mass transmission window may have a width of ≥ 50 Da, ≥ 100 Da, ≥ 150 Da, or ≥ 200 Da, although mass transmission windows of other widths are also contemplated.

[0108] The mass filters described herein may be located in relatively high pressure regions of the spectrometer, e.g., relatively upstream regions, so as to perform pre-filtering of ions to protect downstream components. For example, the mass filter may be located in a 10 -2 mbar~10 -3 mbar, etc. -4The pressure may be at or above mbar, however it is contemplated that the mass filter may be used at lower pressures.

[0109] Although the present invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as set forth in the appended claims.

[0110] For example, although embodiments are described herein in connection with different figures, it is contemplated that features from the various embodiments may be combined.

[0111] Although an embodiment is described in which each of the first pair of electrodes 26a has a gap / slot 34 therethrough, it is contemplated that only one of these electrodes may have a slot. Alternatively, or additionally, one or more of the second pair of electrodes 26b may have a gap / slot through which ions may be ejected, with a collector electrode provided behind such slot. This allows, for example, the DC voltages applied to the first and second pair of electrodes to be switched so that ions to be filtered pass through one or more slots in the second pair of electrodes rather than through the slots in the first pair of electrodes.

[0112] The embodiments described thus far have slots formed as slotted apertures in the electrodes, i.e., the slots are completely surrounded by the electrode within which they are located. However, it is contemplated that each slot may not be completely surrounded by the electrode material. For example, the slot may extend to the periphery of the electrode within which it is located. For example, the slot may extend parallel to the central axis of the mass filter, and the first end of the slot may extend to the upstream end of the electrode.

[0113] Alternatively, it is contemplated that each slot may be formed between two or more separate electrode portions rather than being defined within the electrode. For example, two electrode portions may be spaced apart to form a slot therebetween. The electrode portions may be connected to the same voltage source and, optionally, may be connected to one another to act as a single electrode.

[0114] Although a mass filter has been described in which the electrodes extend substantially the entire length of the mass filter, it is contemplated that at least some of the electrodes may be axially segmented. For example, the first pair of electrodes 26a and / or the second pair of electrodes 26b may be axially segmented, and different DC and / or RF voltages may be applied to different axial segments, for example, to force ions axially through the mass filter. This may allow, for example, the mass transmission window to be more quickly switched or scanned over different mass ranges. Additionally or alternatively, the collector electrode may be axially segmented, and different DC and / or RF voltages may be applied to different axial segments.

[0115] Embodiments have been described in which the mass filtering electrodes 26a, 26b are linear. For example, at least the radially inner surface of each of the first and second pairs of electrodes may be a substantially planar surface. The surface of the collector electrode that receives the filtered ions may also be a substantially planar surface. However, the present invention is not limited to such electrodes. For example, the electrodes of the first pair of electrodes 26a and / or the second pair of electrodes 26b may have a circular, hyperbolic, C-shaped, plate-like, or any other cross-sectional shape in a plane perpendicular to the central axis.

[0116] Although an embodiment has been described in which the mass filter is a quadrupole mass filter, the invention can be extended to other types of mass filters.

Claims

1. 1. A mass filter comprising: a plurality of mass filtering electrodes for mass filtering ions passing therethrough, at least a first of the electrodes having a first gap therethrough; a voltage source positioned and configured to apply a voltage to the mass filtering electrodes such that ions having a mass to charge ratio within a mass transmission window are confined by the electrodes and transmit through the mass filter, while at least some ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the first gap to be filtered by the mass filter; a first collector electrode positioned to receive the ions transmitting through the first gap, wherein the voltage source applies a DC potential difference between the first mass filtering electrode and the first collector electrode so as to urge the ions transmitting through the first gap onto the first collector electrode.

2. A mass filter according to claim 1, wherein the mass filter is a quadrupole mass filter.

3. 3. A mass filter as claimed in claim 1 or 2, wherein a second of the mass filtering electrodes, positioned opposite the first mass filtering electrode, has a second gap therethrough, and the voltage source is arranged and configured to apply the voltage to the mass filtering electrode such that ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the second gap, and the mass filter comprises a second collector electrode arranged to receive the ions that transmit through the second gap.

4. 4. A mass filter according to claim 3, wherein the voltage source is configured to (i) apply a first DC voltage to the first and second mass filtering electrodes, (ii) apply a second DC voltage of opposite polarity to two other opposing mass filtering electrodes, and (iii) apply a DC voltage of the same polarity as the first DC voltage but of greater magnitude to the first and second collector electrodes.

5. 5. A mass filter according to claim 3 or 4, wherein the voltage source is configured to (i) apply a first phase of an RF voltage to the first and second mass filtering electrodes, (ii) apply a second, different phase of the RF voltage to two other opposing mass filtering electrodes, and (iii) apply the first phase of the RF voltage to the first and second collector electrodes.

6. A mass filter according to any preceding claim, wherein an RF voltage is applied to the first and / or second collector electrodes.

7. 7. A mass filter according to any preceding claim, wherein each of the first and / or second collector electrodes comprises a plurality of conductive regions, and the voltage source is configured to apply different voltages to different conductive regions so as to urge the filtered ions onto at least one of the conductive regions.

8. 8. A mass filter as claimed in claim 7, wherein the voltage source is configured to operate (i) in a first mode in which a first voltage is applied to a first one of the conductive regions and one or more different voltages are applied to one or more other conductive regions, such that substantially all or a majority of the ions that strike the collector electrode strike the first conductive region, and (ii) in a second mode in which a voltage is applied to a second one of the conductive regions and one or more different voltages are applied to one or more other conductive regions, such that substantially all or a majority of the ions that strike the collector electrode strike the second conductive region.

9. 9. A mass filter according to claim 8, comprising a charge detection circuit configured to detect the charge on the first conductive region during the first mode, and to switch the mass filter to the second mode when the detected charge reaches a threshold level.

10. 10. A mass filter according to claim 8 or claim 9, wherein the mass filter is configured to switch from the first mode to the second mode after a maximum preselected amount of time operating in the first mode.

11. 9. A mass filter as claimed in claim 8, comprising a charge or current detection circuit configured to detect the charge or current on the second conductive region during the second mode, the mass filter being configured to determine a remaining operational life of the collector electrode based on the charge or ion current detected on the second conductive region in the second mode and then switch to the first mode.

12. 12. A mass filter according to any preceding claim, wherein a surface of the first collector electrode facing the first gap comprises a ridge towards the first gap and / or a surface of the second collector electrode facing the second gap comprises a ridge towards the second gap.

13. 13. A mass filter according to any preceding claim, wherein at least a part of the surface of the first and / or second collector electrodes facing the first and / or second gaps, respectively, has a modified surface so that it is easier for an operator to see contamination on that surface due to impingement of the ions than would be possible in the absence of the modified surface.

14. 14. A mass filter as claimed in claim 13, wherein at least part of the surface has a different topology and / or is modified relative to other parts of the collector electrode, such as by being etched or coated, to have a lower reflectivity to visible light.

15. 15. A mass filter according to any preceding claim, wherein the first and / or second collector electrodes are or comprise conductive plates arranged to face the first and / or second gaps, respectively.

16. 16. A mass filter according to any preceding claim, wherein each of the first and / or second collector electrodes comprises a stack of conductive stripping strips arranged to receive ions passing through the first and / or second gap, respectively.

17. 17. A mass filter according to claim 16, wherein the release strips in each stack are adhered to each other, and optionally to the body of the collector electrode, using a conductive adhesive.

18. 18. A mass filter according to any preceding claim, wherein the first and / or second collector electrodes comprise one or more structural supports configured to mount the first and / or second collector electrodes to the mass filtering electrode so as to be located adjacent the first and / or second gaps, respectively.

19. 19. A mass filter according to claim 18, wherein the one or more structural supports are configured to releasably mount the first and / or second collector electrodes in multiple or different orientations or positions relative to the mass filtering electrode so that each collector electrode is (i) rotatable so that different surfaces of the collector electrode receive the ions from the gap at different respective times, and / or (ii) movable so that different portions of the same surface on the collector electrode receive the ions from the gap at different respective times.

20. 20. A mass filter according to any preceding claim, wherein the first gap is a slot through the first mass filtering electrode and / or the second gap is a slot through the second mass filtering electrode.

21. 1. A mass filter comprising: a plurality of mass filtering electrodes for mass filtering ions passing therethrough, at least a first of the electrodes having a first gap therethrough; a voltage source positioned and configured to apply a voltage to the mass filtering electrodes such that ions having a mass to charge ratio within a mass transmission window are confined by the electrodes and transmit through the mass filter, while at least a portion of the ions having a mass to charge ratio outside the mass transmission window are unstable and pass through the first gap to be filtered by the mass filter; a first collector electrode positioned to receive ions transmitted through the first gap.

22. 22. A mass and / or ion mobility spectrometer comprising a mass filter according to any preceding claim, further comprising a detector or analyser for detecting or analysing ions within the mass transmission window that are transmitted by or derived from the mass filter.

23. the mass filter is a bandpass mass filter that filters ions that would otherwise impinge on components of the spectrometer downstream of the mass filter; and / or The mass filter is 10 －４ 23. The spectrometer of claim 22, wherein the spectrometer is maintained at a pressure of at least 1000 psi mbar.

24. 1. A method for mass filtering ions, comprising: Providing a mass filter according to any one of claims 1 to 21; applying the voltage to the mass filter such that the mass filter transmits only ions having a mass to charge ratio within the mass transmission window and filters ions having a mass to charge ratio outside the mass transmission window, and at least some of the filtered ions pass through the first and / or second gaps onto the first and / or second collecting electrodes, respectively.

25. 25. A method of mass and / or ion mobility spectrometry comprising the method of claim 24, further comprising detecting and / or mass analysing and / or ion mobility analysing ions transmitted by or derived from the mass filter.

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