Abstract
Disclosed herein is a multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in an X direction, each mirror elongated along a drift direction Y orthogonal to the direction X, and an ion injector for injecting ions as an ion beam into the space between the ion mirrors at an inclination angle to the X direction. Along a first portion of their length in the drift direction Y the ion mirrors converge with a first degree of convergence, and along a second portion of their length in the drift direction Y the ion mirrors converge with a second degree of convergence or are parallel, the first portion of their length being closer to the ion injector than the second portion and the first degree of convergence being greater than the second degree of convergence.
Claims
1. A multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in an X direction, each mirror elongated generally along a drift direction Y, the X direction being orthogonal to the drift direction Y, and an ion injector for injecting ions as an ion beam into the space between the ion mirrors at an inclination angle to the X direction, wherein along a first portion of their length in the drift direction Y the ion mirrors converge with a first degree of convergence and along a second portion of their length in the drift direction Y the ion mirrors converge with a second degree of convergence or are parallel, the first portion of their length being closer to the ion injector than the second portion and the first degree of convergence being greater than the second degree of convergence.
2. The multi-reflection mass spectrometer of claim 1 wherein the first degree of convergence is such that the drift velocity of the ions in the direction Y is reduced across the first portion of length by at least 5% after the ions undergo one or more reflections in the ion mirrors in the first portion of length.
3. The multi-reflection mass spectrometer of claim 1 wherein the ions exhibit a greater average reduction in their drift velocity in the direction Y per reflection in at least one of the ion mirrors in the first portion of length compared to the average reduction in their drift velocity in the direction Y per reflection in the ion mirrors in the second portion of length.
4. The multi-reflection mass spectrometer of claim 1 wherein a return pseudo-potential gradient is generated by the converging mirrors along the first portion of the length that is greater than a return pseudo-potential gradient generated by the converging mirrors along the second portion of the length.
5. The multi-reflection mass spectrometer of claim 1 wherein, in use, the ion injector injects ions from one end of the mirrors into the space between the mirrors such that ions are reflected from one opposing mirror to the other a plurality of times whilst drifting along the drift direction away from the ion injector so as to follow a generally zigzag path within the mass spectrometer.
6. The multi-reflection mass spectrometer of claim 1, wherein the ion injector is located proximate to one end of the opposing ion-optical mirrors in the drift direction Y.
7. The multi-reflection mass spectrometer of claim 1, further comprising a detector located in a region adjacent the ion injector.
8. The multi-reflection mass spectrometer of claim 1, wherein along the first and/or second portions of its length the elongation generally in the drift direction Y of each mirror is linear.
9. The multi-reflection mass spectrometer of claim 1, wherein along the first and second portions of its length the elongation generally in the drift direction Y of each mirror is non-linear.
10. The multi-reflection mass spectrometer of claim 1, wherein at least one ion mirror curves towards the other mirror along at least one of the first and second portions of its length in the drift direction.
11. The multi-reflection mass spectrometer of claim 1, wherein both ion mirrors are shaped so as to produce in one or both of the first and second portions of length a curved reflection surface following a polynomial shape.
12. The multi-reflection mass spectrometer of claim 1, wherein along the second portion of their length in the drift direction Y, the ion mirrors are substantially non-parallel.
13. The multi-reflection mass spectrometer according to claim 1 wherein along the second portion of their length in the drift direction Y, the ion mirrors are substantially parallel.
14. The multi-reflection mass spectrometer of claim 1 wherein both mirrors are symmetrical to each other and both mirrors are curved along their first and/or second portions of length to follow a parabolic shape so as to curve towards each other as they extend in the drift direction.
15. The multi-reflection mass spectrometer of claim 1 wherein no portion of the ion beam is within an ion mirror when the ion beam passes between the first and second portions of the length in the direction Y.
16. The multi-reflection mass spectrometer of claim 1 wherein the transition between the first and second portions of the length in the direction Y occurs between first and second reflections in the opposing ion mirrors following injection.
17. The multi-reflection mass spectrometer of claim 1 wherein a distance between two adjacent envelopes of the ion beam within a mirror on either side of a transition between the first and second portions of the length is not smaller than 0.5*H, where H is local height of the mirror at the transition.
18. The multi-reflection mass spectrometer of claim 1 wherein one or more correction electrodes are mounted through the ion mirrors to reduce an electric field sag at the transition between the first and second portions of the length in the direction Y.
19. The multi-reflection mass spectrometer of claim 1 wherein the transition between the first and second portions of the length in the direction Y is a smooth curve.
20. The multi-reflection mass spectrometer of claim 1 wherein the first and second portions of the length in the direction Y are provided by the same continuous electrodes.
21. The multi-reflection mass spectrometer of claim 1 wherein the first and second portions of the length in the direction Y are electrically separated.
22. The multi-reflection mass spectrometer of claim 1 further comprising one or more compensation electrodes extending along at least a portion of the drift direction in or adjacent the space between the mirrors.
23. The multi-reflection mass spectrometer according to claim 22 comprising a pair of opposing compensation electrodes, each electrode being located either side of a space extending between the opposing mirrors.
24. The multi-reflection mass spectrometer according to claim 23 in which each of the compensation electrodes has a surface substantially parallel to the X-Y plane and having a polynomial profile in the X-Y plane such that the surfaces extend towards each mirror a lesser distance in the regions near one or both the ends of the mirrors than in the central region between the ends.
25. The multi-reflection mass spectrometer according to claim 22 in which each of the compensation electrodes has a surface substantially parallel to the X-Y plane and having a polynomial profile in the X-Y plane such that the surfaces extend towards each mirror a greater distance in the regions near one or both the ends of the mirrors than in the central region between the ends.
26. The multi-reflection mass spectrometer according to claim 22 in which the one or more compensation electrodes are, in use, electrically biased so as to produce, in at least a portion of the space extending between the opposing mirrors, an electrical potential offset which varies as a function of the distance along the drift length.
27. The multi-reflection mass spectrometer according to claim 22 in which the one or more compensation electrodes are, in use, electrically biased so as to compensate for at least some of the time-of-flight aberrations generated by the opposing mirrors.
28. The multi-reflection mass spectrometer according to claim 22 in which the one or more compensation electrodes are, in use, electrically biased so as to compensate for a time-of-flight shift in the drift direction generated by the opposing mirrors and so as to make a total time-of-flight shift of a system substantially independent of variations of an initial ion beam trajectory inclination angle in the X-Y plane.
29. The multi-reflection mass spectrometer according to claim 1 in which the motion of ions along the drift direction is opposed by an electric field resulting from convergence of the mirrors towards each other along the first and second portions of their lengths in the drift direction.
30. The multi-reflection mass spectrometer according to claim 1 in which an electric field causes the ions to reverse their direction and travel back towards the ion injector.
31. A method of mass spectrometry comprising injecting ions from an ion injector into a space between two opposing ion mirrors of a multi-reflection mass spectrometer, wherein the ions are repeatedly reflected back and forth between the mirrors whilst they drift down a general direction of elongation, and detecting at least some of the ions during or after their passage through the mass spectrometer, the two ion mirrors opposing each other in an X direction, each mirror elongated generally along a drift direction Y, the X direction being orthogonal to the drift direction Y, wherein along a first portion of their length in the drift direction Y the ion mirrors converge with a first degree of convergence and along a second portion of their length in the drift direction Y the ion mirrors converge with a second degree of convergence or are parallel, the first portion of their length being closer to the ion injector than the second portion and the first degree of convergence being greater than the second degree of convergence.
32. The method of mass spectrometry according to claim 31 wherein the first degree of convergence is such that the drift velocity of the ions in the direction Y is reduced across the first portion of length by at least 5% after the ions undergo one or more reflections in the ion mirrors in the first portion of length.
33. The method of mass spectrometry according to claim 31 wherein the ions exhibit a greater average reduction in their drift velocity in the direction Y per reflection in at least one of the ion mirrors in the first portion of length compared to the average reduction in their drift velocity in the direction Y per reflection in the ion mirrors in the second portion of length.
34. The method of mass spectrometry according to claim 31 in which the amplitude of motion along X direction decreases along at least a portion of the drift length as ions proceed away from the ion injector.
35. The method of mass spectrometry according to claim 31 in which ions are injected into the multi-reflection mass spectrometer from one end of the opposing ion-optical mirrors in the drift direction.
36. The method of mass spectrometry according to claim 31 in which the ions are turned around after passing along a drift length in direction Y and proceed back along the drift length towards the location of ion injection.
37. The method of mass spectrometry according to claim 31 wherein no portion of the ion beam is within an ion mirror when the ion beam passes between the first and second portions of the length in the direction Y.
38. A multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in an X direction, each mirror elongated generally along a drift direction Y, the X direction being orthogonal to the drift direction Y, and an ion injector for injecting ions into the space between the ion mirrors at an inclination angle to the X direction, wherein at least one of the ion mirrors along a first portion of its length in the drift direction Y has a first non-zero angle of inclination to the direction Y and along a second portion of its length in the drift direction Y has a second non-zero angle of inclination to the direction Y that is less than the first non-zero angle of inclination to the direction Y or has zero angle of inclination to the direction Y, the first portion of length being closer to the ion injector than the second portion.
39. A multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in an X direction, each mirror elongated generally along a drift direction Y, the X direction being orthogonal to the drift direction Y, and an ion injector for injecting ions as an ion beam into the space between the ion mirrors at an inclination angle to the X direction, such that ions injected into the spectrometer are repeatedly reflected back and forth in the X direction between the mirrors whilst they drift down the Y direction of mirror elongation so as to follow a zigzag path, wherein the ion mirrors along a first portion of their length in the drift direction Y provide a first return pseudo-potential gradient for reducing the ion drift velocity in the drift direction Y, and the ion mirrors along a second portion of their length in the drift direction Y provide a second return pseudo-potential gradient for reducing the ion drift velocity in the drift direction Y or along the second portion of their length do not provide a return pseudo-potential, wherein the first return pseudo-potential gradient is greater than the second return pseudo-potential gradient and the first portion of length is closer to the ion injector than the second portion.
40. A multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in an X direction, each mirror elongated generally along a drift direction Y, the X direction being orthogonal to the drift direction Y, and an ion injector for injecting ions as an ion beam into the space between the ion mirrors at an inclination angle to the X direction, such that ions injected into the spectrometer are repeatedly reflected back and forth in the X direction between the mirrors whilst they drift down the Y direction of mirror elongation so as to follow a zigzag path, wherein the ion mirrors along a first portion of their length in the drift direction Y provide a first rate of deceleration of the ion drift velocity in the drift direction Y, and the ion mirrors along a second portion of their length in the drift direction Y provide a second rate of deceleration of the ion drift velocity in the drift direction Y or along the second portion of their length do not provide a deceleration of the ion drift velocity in the drift direction Y, wherein the first rate of deceleration of the ion drift velocity is greater than the second rate of deceleration of the ion drift velocity and the first portion of length is closer to the ion injector than the second portion.
Description
DESCRIPTION OF THE FIGURES
(1) FIG. 1A and FIG. 1B are schematic diagrams of a multi-reflection mass spectrometer comprising two parallel ion-optical mirrors elongated linearly along a drift length, illustrative of prior art analysers, FIG. 1A in the X-Y plane, FIG. 1B in the X-Z plane.
(2) FIG. 2 is a schematic diagram of a multi-reflection mass spectrometer illustrative of further prior art analysers, comprising opposing ion-optical mirrors elongated parabolically along a drift length.
(3) FIG. 3 is a schematic diagram of a section in the X-Z plane of an embodiment of multi-reflection mass spectrometer comprising two ion-mirrors, together with ion rays and potential plots.
(4) FIG. 4 is a graph of the oscillation time, T plotted against the beam energy, , calculated for mirrors of the type illustrated in FIG. 3.
(5) FIG. 5A is a schematic diagram of a multi-reflection mass spectrometer, comprising opposing ion-optical mirrors elongated parabolically along a drift length and further comprising parabolically shaped compensation electrodes, some of them biased with a positive voltage. FIG. 5B is a schematic diagram of a section through the spectrometer of FIG. 5A. FIGS. 5C and 5D illustrate analogous embodiments with asymmetrical shapes of the mirrors.
(6) FIGS. 6A and 6B are schematic diagrams of multi-reflection mass spectrometers, comprising opposing ion-optical mirrors elongated linearly along a drift length and arranged at an inclined angle to one another, further comprising compensation electrodes with concave (FIG. 6A) and convex (FIG. 6B) parabolic shape. FIG. 6C is a schematic diagram of further multi-reflection mass spectrometer, comprising opposing ion-optical mirrors elongated linearly along a drift length and arranged parallel to one another, further comprising parabolic compensation electrodes.
(7) FIG. 7 is a graph showing a comparison of a two stage potential gradient of an embodiment of the invention with that of a simple, single-stage linear ramp of the prior art.
(8) FIG. 8 is a schematic diagram of a mass spectrometer embodying the present invention having two opposing ion mirrors that converge in two different linear stages.
(9) FIG. 9 is a schematic diagram showing detail of the mass spectrometer of FIG. 8 in which the ion trajectory shows ions initially entering the ion mirrors with an inclination angle to the X direction.
(10) FIG. 10 is a schematic diagram showing a two stage mirror of a mass spectrometer according to the present invention, incorporating a field compensation PCB at the interface of the stages.
(11) FIG. 11 is a schematic diagram showing a two stage mirror of a mass spectrometer according to the present invention, incorporating a correcting distortion at the interface of the stages.
(12) FIG. 12 is a schematic diagram showing a two stage mirror of a mass spectrometer according to the present invention, incorporating axial field correcting electrodes at the interface of the stages.
(13) FIG. 13 is a schematic diagram showing a mass spectrometer according to the present invention, incorporating a mirror set including a curved first stage of higher degree of convergence and a curved second stage of lower degree of convergence.
(14) FIG. 14 is a schematic diagram showing a construction of ion mirror comprising bar electrodes with voltages applied.
(15) FIG. 15 is a schematic diagram showing a mass spectrometer according to the present invention, incorporating a mirror set including a curved first stage of higher degree of convergence and a curved second stage of lower degree of convergence and having a central stripe compensation electrode.
(16) FIG. 16 is a graph showing the dimensionless sum of return pseudopotentials of the converging ion mirrors and a compensation electrode positioned therebetween.
(17) FIG. 17 is a schematic diagram of an ion injection optical arrangement for use with an embodiment of the invention with applied voltages shown.
(18) FIG. 18 is a plot of a simulated ion trajectory of an embodiment of the invention.
(19) FIG. 19 is a graph of the time dispersion of ions with m/z=195 arriving at the detector in an embodiment of the present invention.
(20) FIG. 20 is a graph of the spatial dispersion in direction Y of ions with m/z=195 arriving at the detector in an embodiment of the present invention.
(21) FIG. 21 is a schematic diagram depicting the spacing between adjacent beam envelopes within the mirror in the vicinity of the transition in the degree of convergence.
DETAILED DESCRIPTION
(22) Various embodiments of the present invention will now be described by way of the following examples and the accompanying figures.
(23) FIG. 1A and FIG. 1B are schematic diagrams of a multi-reflection mass spectrometer comprising parallel ion-optical mirrors elongated linearly along a drift length, illustrative of prior art analysers. FIG. 1A shows the analyser in the X-Y plane and FIG. 1B shows the same analyser in the X-Z plane. Opposing ion-optical mirrors 11, 12 are elongated along a drift direction Y and are arranged parallel to one another. Ions are injected from ion injector 13 with angle to axis X and angular divergence , in the X-Y plane. Accordingly, three ion flight paths are depicted, 16, 17, 18. The ions travel into mirror 11 and are turned around to proceed out of mirror 11 and towards mirror 12, whereupon they are reflected in mirror 12 and proceed back to mirror 11 following a zigzag ion flight path, drifting relatively slowly in the drift direction Y. After multiple reflections in mirrors 11, 12 the ions reach a detector 14, upon which they impinge, and are detected. In some prior art analysers the ion injector and detector are located outside the volume bounded by the mirrors. FIG. 1B is a schematic diagram of the multi-reflection mass spectrometer of FIG. 1A shown in section, i.e. in the X-Z plane, but with the ion flight paths 16, 17, 18, ion injector 13 and detector 14 omitted for clarity. Ion flight paths 16, 17, 18 illustrate the spreading of the ion beam as it progresses along the drift length in the case where there is no focusing in the drift direction. As previously described, various solutions including the provision of lenses in between the mirrors, periodic modulations in the mirror structures themselves and separate mirrors have been proposed to control beam divergence along the drift length. However it is advantageous to allow the ions to spread out as they travel along the drift length so as reduce space charge interactions, so long as they can be brought to some convergence where necessary to be fully detected.
(24) A preferred feature of the present invention is to provide an elongated opposing ion-mirror structure in which a smooth returning force is produced. FIG. 2 is a schematic diagram of a multi-reflection mass spectrometer described in US2015/0028197, comprising opposing ion-optical mirrors 31, 32 elongated generally along a drift length Y and having the shapes of parabolas converging towards each other in the distal end from the ion injector 33. This can be an arrangement for the second portion of length of the ion mirrors in the present invention. The disclosure of US2015/0028197 is hereby incorporated by reference in its entirety (however, where anything in the incorporated reference contradicts anything stated in the present application, the present application prevails). The injector 33 may be a conventional ion injector known in the art, for example an ion trap, orthogonal accelerator, MALDI ion source etc. Ions are accelerated by the acceleration voltage V and injected into the multi-reflection mass spectrometer from ion injector 33, at an angle in the X-Y plane and with an angular divergence , in the same way as was described in relation to FIG. 1. Accordingly three ion flight paths 36, 37, 38 are representatively shown in FIG. 2. As already described, ions are reflected from one opposing mirror 31 to the other 32 a plurality of times whilst drifting along the drift direction away from the ion injector 33 so as to follow a generally zigzag paths within the mass spectrometer. The motion of ions along the drift direction is opposed by an electric field resulting from the non-constant distance of mirrors 31, 32 from each other along their lengths in the drift direction, and the said electric field causes the ions to reverse their direction and travel back towards the ion injector 33. Ion detector 34 is located in the vicinity of ion injector 33 and intercepts the ions. The ion paths 36, 37, 38 spread out along the drift length as they proceed from the ion injector due to the spread in angular divergence as previously described in relation to FIG. 1A, but upon returning to the vicinity of the ion injector 33, the ion paths 36, 37, 38 have advantageously converged again and may conveniently be detected by ion-sensitive surface of detector 34 which is oriented orthogonal to the X axis.
(25) The embodiment of FIG. 2 comprising opposing ion-optical mirrors 31, 32 is an example in which parabolic elongation of both mirrors is utilized. As already noted, in embodiments of the present invention the elongation may be linear (i.e. the mirrors are straight, possibly positioned at an angle towards each other), or the elongation may be non-linear (i.e. comprising curved mirrors), the elongation shape of each mirror may be the same or it may be different and any direction of elongation curvature may be the same or may be different. The mirrors may become closer together along the whole of the drift length, or along only a portion of the drift length, e.g. only at an injection end, or only at an injection end and a distal end (from the injector end), of the drift length of the mirrors.
(26) After a pair of reflections in mirrors 31 and 32, the inclination angle changes by the value =2(Y), where =L(Y) is convergence angle of the mirrors with the effective distance L(Y) between them. This angle change is equivalent to the inclination angle change on the 2L(0) flight distance in the effective returning potential .sub.m(Y)=2V[L(0)L(Y)]/L(0). Parabolic elongation L(Y)=L(0)A Y.sup.2, where A is a positive coefficient, generates a quadratic distribution of the returning potential in which the ions advantageously take the same time to return to the point of their injection Y=0 independent of their initial drift velocity in the Y direction. The mirror convergence angle (Y) is advantageously small and doesn't affect the isochronous properties of mirrors 31, 32 in the X direction as will be described further in relation to FIGS. 3 and 4. FIG. 2 is an example in which both an extended flight path length and spatial focusing of ions in the drift (Y) direction is accomplished by use of non-parallel mirrors. This embodiment advantageously needs no additional components to both double the drift length and induce spatial focusingonly two opposing mirrors are utilised. The use of opposing ion-optical mirrors elongated generally along the drift direction Y such that the mirrors are not a constant distance from each other along at least a portion of their lengths in the drift direction has produced these advantageous properties and these properties are achieved by alternative embodiments in which the mirrors are elongated linearly, for example. In this particular embodiment the opposing mirrors are curved towards each other with parabolic profiles as they elongate away from one end of the spectrometer adjacent an ion injector and this particular geometry further advantageously causes the ions to take the same time to return to their point of injection independent of their initial drift velocity.
(27) FIG. 3 is a schematic diagram of a multi-reflection mass spectrometer comprising two preferred ion-mirrors 41, 42, together with ion rays 43, 44, 45, 46 and electrical potential distribution curves 49. Such ion mirrors can be employed with the present invention. Mirrors 41, 42 are shown in cross section, in the X-Z plane. Each mirror comprises a number of electrodes, and the electrode dimensions, positions and applied electrical voltages are optimized such that the oscillation time, T, of ions between the mirrors, is substantially independent of the ion energy, , in the interval .sub.0+/(/2), where .sub.0=qV is the reference energy defined by the acceleration voltage V and the ion charge q. The ion charge is hereafter assumed positive without loss of generality of the invention's applicability to both positive and negative ions. Electrical potential distribution curve 49 illustrates that each mirror has an accelerating region to achieve spatial focusing of ion trajectories in the X-Z plane parallel (43, 44) to point (45, 46) after a first reflection, and from point to parallel after a second reflection, providing ion motion stability in the X-Z plane. Ions experience the accelerating potential region of the mirror twice on each reflection: once on entry and once on exiting the mirror. As is known from prior art, this type of spatial focussing also helps to eliminate some time-of-flight aberrations with respect to positional and angular spreads in the Z direction.
(28) As known from the prior art, mirrors of this design can produce highly isochronous oscillation time periods for ions with energy spreads /.sub.0>10%. FIG. 4 is a graph of the oscillation time, T plotted against the beam energy, , calculated for mirrors of the type illustrated in FIG. 3. It can be seen that a highly isochronous oscillation time period is achieved for ions of 2000 eV+/100 eV. Gridless ion mirrors such as those illustrated in FIG. 3 could be implemented as described in U.S. Pat. No. 7,385,187 or WO2009/081143 using flat electrodes that could be fabricated by well-known technologies such as wire-erosion, electrochemical etching, jet-machining, electroforming, etc. They could be also implemented on printed circuit boards.
(29) FIG. 5A is a schematic diagram of a multi-reflection mass spectrometer described in US2015/0028197, comprising opposing ion-optical mirrors elongated parabolically along a drift length, further comprising compensation electrodes. Parabolically shaped ion mirrors and/or compensation electrodes can be employed with the present invention as described herein. In particular, this mirror system can be an arrangement for the second portion of length of the ion mirrors in the present invention. As a more technological implementation, parabolic shapes could be approximated by circular arcs (which could be then made on a turning machine). Compensation electrodes allow further advantages to be provided, in particular that of reducing time-of-flight aberrations. The embodiment of FIG. 5A is similar to that of FIG. 2, and similar considerations apply to the general ion motion from the injector 63 to the detector 64 the ions undergoing a plurality of oscillations 60 between mirrors 61, 62. As the ion beam approaches the distal end of mirrors 61, 62, the beam's angle of inclination in the X-Y plane gets progressively smaller until its sign is changed in the turning point and the ion beam starts its return path towards detector 64. The ion beam width in the Y dimension reaches its maximum near the turning point and the trajectories of ions having undergone different numbers of oscillations overlap thus helping to average out space charge effects. The ions come back to the detector 64 after a designated integer number of full oscillations between mirrors 61 and 62. Three pairs of compensation electrodes 65-1, 65-2 as one pair, 66-1, 66-2 as another pair and 67-1, 67-2 as a further pair, comprise extended surfaces in the X-Y plane facing the ion beam, the electrodes being displaced in +/Z from the ion beam flight path, i.e. each compensation electrode 65-1, 66-1, 67-1, 65-2, 66-2, 67-2 has a surface substantially parallel to the X-Y plane located either side of a space extending between the opposing mirrors as shown in FIG. 5B. FIG. 5B is a schematic diagram showing a section through the mass spectrometer of FIG. 5A. In use, the compensation electrodes 65 are electrically biased, both electrodes having voltage offset U(Y)>0 applied in case of positive ions and U(Y)<0 applied in case of negative ions. Hereafter we assume the case of positive ions for this and the other embodiments if not stated otherwise. Voltage offset U(Y) is, in some embodiments, a function of Y, i.e. the potential of the compensation plates varies along the drift length, but in this embodiment the voltage offset is constant. The electrodes 66, 67 are not biased and have zero voltage offset. The compensation electrodes 65, 66, 67 have, in this example, a complex shape, extending in X direction a varying amount as a function of Y, the width of biased electrodes 65 in the X direction being represented by function S(Y). The shapes of unbiased electrodes 66 and 67 are complementary to the shape of biased electrodes 65. The extent of the compensation electrodes in the X direction is, in some embodiments, a width that is constant along the drift length, but in this embodiment the width varies as a function of the position along the drift length. The functions S(Y) and U(Y) are chosen to minimize the most important time-of-flight aberrations, as will be further described.
(30) In use, the electrically biased compensation electrodes 65 generate potential distribution u(X, Y) in the plane of their symmetry Z=0, which is shown with schematic potential curve 69 in FIG. 5B. The potential distribution 69 is restricted spatially by the use of the unbiased compensation electrodes 66 and 67. The returning electric field E.sub.y=u/Y makes the same change of the trajectory inclination angle as the effective potential distribution .sub.ce(Y)=L(0).sup.1u(X,Y)dXU(Y)S(Y) averaged over the effective distance between the mirrors L(0). The last approximate equality holds if the separation between the compensation electrodes in Z-direction is sufficiently small. In the embodiment shown in FIGS. 5A and 5B, the compensation electrodes are parabolic in shape, so that S=B Y.sup.2, where B is a positive constant, and the voltage offset is constant U=constV sin.sup.2 V, where V is the accelerating voltage. (The accelerating voltage is with respect to the analyser reference potential.) Therefore, the set of compensation electrodes also generates a quadratic contribution to the effective returning potential, which, being additive with the same sign to the quadratic contribution of the parabolic mirrors, maintains the isochronous properties in drift direction. In embodiments with constant voltage offsets on biased compensation electrodes, the returning electric field E.sub.y is essentially non zero only near the edges of the compensation electrodes, which are non-parallel to the drift axis Y, and the ion trajectories thus undergo refraction every time they cross the edges.
(31) The time-of-flight aberration of the embodiment in FIG. 5A results from two factors: the mirror convergence and the time delay of ions whilst travelling in between the compensation electrodes. When summed up, these two factors give the oscillation time T(Y)=T(0)[L(Y)+S(Y)U/2V]/L(0) being a function of drift coordinate. In terms of components of the effective returning potential, T(Y)T(0)=T(0) [.sub.ce(Y).sub.m(Y)]/2V. The coefficients A and B which define the parabolic shapes of the mirrors 61, 62 and the compensation electrodes 65, 66, 67, correspondingly, are preferably chosen in certain proportions to make the components of the returning force equal .sub.ce(Y)=.sub.m(Y), so that the time per oscillation T(Y) is advantageously constant along the entire drift length and thus eliminates time-of-flight aberrations with respect to the initial angular spread. So, the decrease of the oscillation time at the position distant from the injection point due to the mirror convergence is completely compensated by decelerating the ions while travelling through the region between the compensating electrodes with increased electric potential. In this embodiment, both components of the effective potential contribute equally to the returning force that drives the ion beam back to the point of injection.
(32) The embodiment in FIGS. 5A and 5B can be generalized by introduction of a polynomial representation of the effective returning potential components .sub.m=(V sin.sup.2 ).sub.m and .sub.ce=(V sin.sup.2 ).sub.ce where .sub.m=m.sub.1y+m.sub.2y.sup.2 and .sub.ce=c.sub.0+c.sub.1y+c.sub.2y.sup.2+c.sub.3y.sup.3+c.sub.4y.sup.4 are dimensionless functions of dimensionless normalized drift coordinate y=Y/Y.sub.0*, and Y.sub.0* is the designated drift penetration depth of an ion with mean acceleration voltage V and mean injection angle . Therefore, the sum of coefficients m.sub.1+m.sub.2+c.sub.1+c.sub.2+c.sub.3+c.sub.4 equals to 1 by definition. Consider an ion which reaches its turning point in drift direction Y=Y.sub.0 that is a function of the ion's injection angle + defined by condition .sub.m(y.sub.0)+.sub.ce(y.sub.0)c.sub.0=sin.sup.2(+)/sin.sup.2 , where y.sub.0=Y.sub.0/Y.sub.0* is the normalized turning point coordinate. The return time taken for this ion to come back to the injection point Y=0 is proportional to integral
(33)
whilst the time-of-flight offset of the moment when an ion with given normalized turning point coordinate y.sub.0 impinges the detector's plane X=0 after a designated number of oscillations between the mirrors is proportional to integral
(34)
(35) The deviation of function (y.sub.0) from (1) thus determines the time-of-flight aberration with respect to the injection angle.
(36) Values of the coefficients m and c are to be found from the following conditions: (1) the integral is substantially constant (not necessarily zero) in the vicinity of y.sub.0=1, which corresponds to slow time-of-flight dependence on the injection angle in the interval /2, and (2) the integral has vanishing derivative (1) to ensure at least first-order spatial focusing of the ions on the detector. The embodiment represented schematically in FIG. 5A with parabolic mirrors and parabolic compensation electrodes corresponds to the values of coefficients m and c as in the first column in Table 1. Since the effective returning potential is quadratic, (y.sub.0)1 and the ion beam is ideally spatially focused onto the detector. At the same time, (y.sub.0)0 which corresponds to complete compensation of the time-of-flight aberration with respect to the injection angle. Alternative embodiments may compromise these ideal properties for the sake of mirror fabrication feasibility. A preferred embodiment comprising only straight mirrors elongated along the drift direction and tilted towards each other with a small convergence angle is a particular case, straight mirrors being more easily manufactured than curved mirrors (or even circular arcs). The embodiments with straight mirrors are characterized by linear dependence of the .sub.m component of the effective returning force, therefore the coefficients m.sub.1>0 and m.sub.2=0. Curved mirrors might be asymmetric as shown for example in FIG. 5C and FIG. 5D, with one mirror 62 being straight (FIG. 5C) or both mirrors may be curved in the same direction (FIG. 5D). In both cases, however, separation between the mirrors at the distal end is smaller than separation between the mirrors at the end next to the injector 63 and detector 64. These examples are only some of the possible mirror arrangements which may be utilised with the present invention for the second portion of the mirror length.
(37) FIG. 6A is a schematic diagram of a multi-reflection mass spectrometer described in US2015/0028197, comprising opposing straight ion-optical mirrors 71, 72 elongated along a drift length and tilted by small angle towards each other. This can be an arrangement for the second portion of length of the ion mirrors in the present invention. The linear part of the total effective returning potential =.sub.m+.sub.ce is zero because m.sub.1=c.sub.1, and is a quadratic function of the drift coordinate (save for the inessential constant resulting from c.sub.0). Therefore exact spatial focusing of the ion beam 70 originating from injector 73, takes place on the detector 74. The value of coefficient c.sub.0 may be an arbitrary positive value greater than .sup.2/64 to make the width function S(Y) of positively biased (in the case of positively charged ions) compensation electrodes 75 strictly positive along the drift length. The narrowest part of the biased compensation electrodes 75 is located at the distance (/8)Y.sub.0* from the point of ion injection. Two pairs of unbiased compensation electrodes 76 and 77 have their shapes complementary with the shapes of electrodes 75 and. serve to terminate the electric field from the biased compensation electrodes 75.
(38) FIG. 6B is a schematic diagram of a multi-reflection mass spectrometer similar to that shown in FIG. 6A, with like components having like identifiers, but with negative offset U<0 on the biased compensating electrodes 75 (in case of positively charged ions). This can be an arrangement for the second portion of length of the ion mirrors in the present invention. It will be appreciated that for negative ions the polarities of the applied potentials will be opposite to those described here. The choice of coefficient c.sub.0</41 makes the dimensionless function .sub.ce(y)<0 along the whole drift length, so that the electrode width S(Y) is strictly positive. In this embodiment, the biased compensating electrodes 75 have convex parabolic shapes with their widest parts located at the distance (/8)Y.sub.0* from the point of ion injection.
(39) The value of the mirror convergence angle is expressed through the coefficient m.sub.1=/4 with formula =m.sub.1L(0) sin.sup.2 /2Y.sub.0*. With the effective distance between the mirrors L(0) being comparable with the drift distance Y.sub.0* and the injection angle =50 mrad, the mirror convergence angle can be estimated as 1 mrad 0. Therefore, FIGS. 6A and 6B show the mirror convergence angle, and other features, not to scale.
(40) FIG. 6C is a schematic diagram of a multi-reflection mass spectrometer similar to that shown in FIG. 6A, with like components having like identifiers, but with zero convergence angle, i.e. =0. This is an example of a mass spectrometer comprising two opposing ion-optical mirrors elongated generally along a drift direction (Y), each mirror opposing the other in an X direction and having a space therebetween, the X direction being orthogonal to Y, the mirrors being a constant distance from each other in the X direction along the whole of their lengths in the drift direction. This can be an arrangement for the second portion of length of the ion mirrors in the present invention. In this embodiment, the opposing mirrors are straight and arranged parallel to each other. Compensation electrodes similar to those already described in relation to FIG. 6A extend along the drift direction adjacent the space between the mirrors, each electrode having a surface substantially parallel to the X-Y plane, and being located either side of the space extending between the opposing mirrors, the compensation electrodes being arranged and biased in use so as to produce an electric potential offset having a different extent in the X direction as a function of the distance along the drift length (providing a return pseudopotential). The coefficient c.sub.2=1 for this embodiment, and the other coefficients m and c vanish. The biased compensation electrodes produce a quadratic distribution of the total effective returning potential (Y)=.sub.ce(Y), therefore, exact spatial focusing of the ion beam 70 originating from injector 73, takes place on the detector 74. The value of coefficient c.sub.0 may be an arbitrary positive value. Two additional pairs of unbiased compensation electrodes similar to electrodes 76 and 77, having their shapes complementary with the shape of biased compensation electrodes 75, serve to terminate the field from compensation electrodes 75. In this embodiment the compensation electrodes 75 are electrically biased to implement isochronous ion reflection in the drift direction; however, the time-of-flight aberrations with respect to the injection angle are not compensated.
(41) In a similar manner, a multi-reflection mass spectrometer similar to that shown in FIG. 6B may be formed, but once again with zero convergence angle, i.e. =0. In this embodiment, biased compensating electrodes have convex parabolic shape with negative offset U<0 applied to implement isochronous ion reflection in the drift direction.
(42) The present invention provides an improvement that can be utilised with the above described mirror arrangements and relates to high resolving power, along with the advantages in mass accuracy and sensitivity that come with it.
(43) The resolving power of the spectrometers described in the prior art above is dependent upon the initial angle of ion injection, which determines the drift velocity and thus the overall time of flight. Ideally this injection angle would be minimised, but it can be restricted by the mechanical requirements of the injection apparatus and of the detector, especially for more compact designs. A solution presented in the prior art is to use an additional deflector positioned between the mirrors to reduce the drift velocity after ion injection, but this introduces some mechanical restrictions and time-of-flight aberrations of its own, and adds to the complexity and cost of the instrument.
(44) Embodiments of the present invention comprise reducing the post-injection drift velocity by modifying the return pseudo-potential generated by two converging mirrors. According to one type of embodiment, there is provided a first drift region of low displacement from the injector in the drift direction Y wherein the mirrors converge relatively more sharply (relatively higher convergence angle of the mirrors), followed by a second drift region of higher displacement from the injector in the drift direction Y wherein the mirrors converge relatively less sharply (relatively lower convergence angle of the mirrors compared to the first drift region), preferably wherein the convergence angle of the mirrors is substantially smaller in the second drift region than in the first drift region. Thus, the potential gradient is provided in two stages. A comparison of this two stage potential gradient with that of a simple, single-stage linear ramp is shown in FIG. 7, which plots the relationship between the return pseudo-potential provided to the ions by the mirrors (vertical axis) and mirror drift length (from the end of the mirrors closest to the ion injector) (horizontal axis). Line 80 represents the return pseudo-potential for the simple, single-stage linear ramp of the prior art. In contrast, line 82 represents the return pseudo-potential for the first drift region or first portion of mirror length, in which the mirrors converge sharply (giving a higher return pseudo-potential gradient). Further, the line 82 represents the return pseudo-potential for the second drift region or second portion of mirror length, in which the mirrors converge with much lower convergence angle (giving a lower return pseudo-potential gradient). The ion drift velocity is consequently more rapidly reduced in the first drift region (i.e. in a first portion of mirror length along Y), allowing increased time of flight through the second drift region (i.e. in a second portion of mirror length along Y) and overall increased flight path.
(45) Referring to FIG. 8, there is shown a schematic diagram of a simple design embodying the present invention having two opposing ion mirrors 90, 92 that converge in two different linear stages. The return pseudo-potential provided by this embodiment is of the two linear stage type shown by lines 82, 84 in FIG. 7. First mirror 90 converges towards the other mirror in a first stage or portion 90.sup./ of higher degree of convergence and a second or portion stage 90.sup.// of lower degree of convergence. Second mirror 92 similarly converges in a first stage or portion 92.sup./ and a second stage or portion 92.sup.//. In other words, the first stage or portion 90.sup./, 92.sup./ of each mirror has a higher angle of inclination to the direction Y than the second stage or portion 90.sup.//, 92.sup.// of the mirror. Both mirrors are matched, i.e. are symmetric. In other embodiments, however, it could be designed so that only one mirror has the higher inclination angle in the first portion built into it, which would be the mirror that the ions strike first after leaving the ion injector (in this case, first mirror 90).
(46) In FIG. 8, a beam of ions is injected from an ion injector or ion source 94 (such as an ion trap, orthogonal acceleration injector or MALDI source) and follows a trajectory 98 into the space between two sets of inclined elongated ion mirrors 90, 92. As an ion trap for the ion injector in the present invention, an RF storage multipole can be used. Ions enter the storage multipole in the X-Y plane from an ion guide and are stored in it whilst at the same time losing their excessive energy (becoming thermalised) in collisions with a bath gas (preferably nitrogen) contained within the multipole. After a sufficient number of ions are accumulated, the RF is switched off as described in WO2008/081334 and a bipolar extraction voltage applied to all or some electrodes of the storage multipole to eject the ions towards the first mirror. For example, push-pull voltages can be applied to the multipole. Upon ejection from the multipole, the ions are accelerated by the acceleration voltage V, preferably in the range 5-30 kV. Alternatively, an orthogonal ion accelerator can be used to inject the ion beam into the mass spectrometer as described in the U.S. Pat. No. 5,117,107 (Guilhaus and Dawson, 1992).
(47) At low drift displacement, i.e. in the first portion of length, the mirrors have a higher degree of mirror convergence, i.e. in portion 90.sup./ and 92.sup./, leading to rapid loss of ion velocity in the drift direction Y. As shown in the detail of FIG. 9, the ions on trajectory 98 initially enter the ion mirrors with an inclination angle 1 to the X direction but after reflection in the first portion of the ion mirrors the rapid loss of ion velocity in the drift direction Y reduces the inclination angle to 2 (2<1). Subsequently, following a zig-zag path between the two mirrors, the ions enter the second portion of the mirrors having the lower degree of mirror convergence, wherein ion drift velocity continues to be lost but more slowly (i.e. on average a lower loss per reflection), before the ions are eventually reflected back up the drift length, following a reverse path between the mirrors that terminates with ions striking a detector 96 positioned adjacent the ion injector (at substantially the same Y coordinate).
(48) In the embodiment shown in FIG. 8, there is only one reflection of the ions in the first portion of the mirror length of higher convergence, which is in the first ion mirror 90.sup./. In other embodiments, further rapid reductions in ion drift velocity could be effected by arranging for one or more additional reflections in the first portion of the mirror length. For the two linear stage design, a main consideration is that no portion of the ion beam is arranged to be within the mirror structure when the beam is passing between the two stages of the mirrors. Where a portion of the ions reach the mirror in the low convergence stage (second stage) at the same time as the remaining ions reach the mirror in the high convergence stage (first stage), the drift energy divergence of the ion beam will increase and the ions scatter uncontrollably. This imposes a minimum drift velocity into the second stage that is dependent on the mirror separation and the spatial divergence of the ion beam at that point. As the ion beam diverges with increasing Y, it is preferable to have the ion beam transition between the stages as early as possible, and especially between the first and second reflections as shown in FIG. 8.
(49) A related problem that can arise in some embodiments is that a field sag between the two stages can cause some drift energy broadening, even at a distance to the corner that separates the two regions. It is therefore desirable to apply a correction to minimise this field disturbance. One way to accomplish this is to mount printed circuit board (PCB) based field correcting electrodes through the mirror at the corner where convergence changes. Such an embodiment of a two stage mirror with a field compensation PCB is shown in FIG. 10. The PCB 91 is held in place at its top and bottom edge (in Z direction) by recesses 95 in the mirror electrodes. The two faces (93, 93) of the field correcting PCB 91 are printed with electrode tracks, which have slightly different track extents and/or applied voltages to mimic continuation of the stages. Other embodiments of electrodes mounted or printed on opposite faces of an insulating substrate than PCB could be used. Another method is to incorporate a small distortion in the mirror surface at the corner, so that the first stage of higher mirror convergence ends with a small increase in convergence, and stage 2 commences with a small decrease. Such an embodiment is such in FIG. 11, wherein a correcting modification 97 to the mirror 90 is shown that provides a distortion in the mirror surface at the corner between the two mirror stages. This effect could also be mimicked using small pairs of electrodes 99 hung from the mirror electrodes 90 (e.g. with insulating mountings) at the transition point between the two stages as shown in FIG. 12.
(50) Each mirror is made of a plurality of elongated bar electrodes, the electrodes elongated generally in the direction Y (although not parallel to Y) as described in US2015/0028197. The elongated electrodes of the ion mirrors may be provided, for example, as mounted metal bars or as metal tracks on a PCB base. The elongated electrodes may be made of a metal having a low coefficient of thermal expansion such as Invar such that the time of flight is resistant to changes in temperature within the instrument. The electrode shape of the ion mirrors can be precisely machined or obtained by wire erosion manufacturing. The electrode dimensions, positions and applied electrical voltages are optimized such that the oscillation time, T, of ions between the mirrors, is substantially independent of the ion energy, , in the interval .sub.0+/(/2), where .sub.0=qV is the reference energy defined by the acceleration voltage V and the ion charge q. The ion charge is herein assumed positive without loss of generality of the invention's applicability to both positive and negative ions.
(51) In some embodiments, the two stages of the mirrors need not be formed by the same sets of bar electrodes. The elongated mirrors can instead be separated electrically at the transition point between the stages, or the mirrors can be built from entirely different structures at added cost and complexity. This electrical separation would have some advantage in allowing a partial retune of the instrument.
(52) It is most preferable for systems incorporating the invention to include compensation electrodes in or adjacent the space between the mirrors to minimise the impact of time of flight aberrations caused by the change in distance between the mirrors, as described above and in US2015/0028197 A1. One such embodiment is shown in FIG. 15 as described below.
(53) Neither the first nor second stages of the mirror convergence need be linear. Indeed the corner that is present at the transition between two linear stages shown in FIG. 8 is undesirable. The aberration introduced by the corner can be removed by blending the two stages together with a smooth curve, so that aberrations in drift energy dispersion are averaged out over multiple reflections. Embodiments can therefore be provided in which two linear stages are connected by a smooth curve. In some embodiments, for example in addition to the smooth curve joining the stages, the second stage of lower degree of convergence may be constructed with a portion (or its whole length) that follows a polynomial (preferably parabolic) shape so that the mirror has a convergence in the manner described in US2015/0028197 A1 or FIG. 5A above, which improves the Y spatial focus at the detector for ion beams with wide drift energy dispersion. This is preferable when handling decelerated ions as the drift energy dispersion increases substantially as a proportion of drift energy.
(54) FIG. 13 shows schematically a mass spectrometer according to the present invention, incorporating a mirror set including a curved first stage 101 of higher degree of convergence at low displacement along Y from the ion injector 94 for rapidly decelerating ions and allowing more reflections in the second stage, and a curved second stage of lower degree of convergence for reflecting the ions multiple times before the ions are eventually turned around by the pseudo potential of the curved mirrors to follow the return path to the detector 96.
(55) A set of suitable dimensions and voltages for an embodiment as shown in FIG. 13 are as follows. The two ion mirrors have internal dimensions 17545048 mm (i.e. mirror depth (in X)mirror length (in Y)mirror height (in Z)), and are set opposed to each other with an inter-mirror gap of 320 mm. The mirrors are each constructed from five bar electrodes with voltages applied in the manner shown in FIG. 14 (for positive ions), which shows the bar electrodes schematically as linear although they are actually parabolic. Convergence of the mirrors follows a function generated by a mathematical optimisation, from 0 mm at Y=0 to 0.362 mm at the desired ion turning point 375 mm in the drift direction, i.e. the inter mirror gap is 320 mm at Y=0 and is 320-0.362 mm at the turning point (Y=375 mm). This function (1) is shown below, and increases the time of flight by >50% relative to a parabolic converging mirror of the prior art without a first, decelerating stage. This is equivalent to 30 oscillations of ions between the mirrors versus 20 in a system without the decelerating stage of the invention.
(56)
(57) The space between the mirrors is shared by compensation electrodes, more specifically between a grounded electrode and a shaped stripe electrode that runs the length of the mirrors and has an applied potential of +24.11 V. The grounded and stripe electrodes are planar having surfaces substantially parallel to the X-Y plane and are located either side of the space extending between the opposing mirrors. This electrode serves to counter the time of flight perturbation of the mirror convergence. The width occupied by the compensation stripe electrode expands from near 0 mm at the injection point to 120 mm at the turning point at Y=375 mm, with a shape following the same function as the mirror convergence but curving in the opposite direction, as shown in FIG. 15 wherein the stripe-shaped central compensation or correcting electrode is denoted 103. The mirror and the stripe electrode each form a return pseudopotential, the dimensionless sum of which is shown in FIG. 16.
(58) In general, the compensation electrodes have a complex shape, extending in the X direction a varying amount as a function of the Y direction, the width of the biased stripe compensation electrodes in the X direction being represented by a function S(Y). The shapes of unbiased (grounded) electrodes are generally complementary to the shape of the biased electrodes. The biased compensation electrodes located adjacent or in the space between the ion mirrors can be positioned between two or more unbiased (grounded) electrodes in the X-Y plane that are also located adjacent or in the space between the ion mirrors.
(59) Injection of ions into the analyser in this embodiment is performed with a linear ion trap with a 2 mm inscribed radius, with sufficient axial potential well to constrain the trapped ion cloud within 3 mm. For the injection step, the trap is lifted to +4000 V and ions extracted by applying 500 V/mm extraction field. Ion divergence into the first mirror is controlled by a set of three electrodes (lenses), and a deflector is present for fine tuning. The centre of the trap is set centrally between the mirrors in X, and at the Y=0 position in the drift dimension, and the trap is set at an inclination of 2.64 degrees to set the ion injection angle. This ion injection optical arrangement with applied voltages is shown in FIG. 17.
(60) The detector plane is set 20 mm away from the trap in the lateral (X) direction, and at Y=0 in the drift direction, with a 2.6 degree tilt to match the angle of the ion isochronous plane. The simulated trajectory is traced in FIG. 18, with 30 turns or reflections in each mirror before the beam reaches the turning point in the Y direction.
(61) The key measures of the performance of the system are the overall time of flight, the ion time focus, and the ion spatial focus at the detector. The first two define resolution and the last item the transmission and the presence of overtones were ions strike the detector one or more turns early. Compared to a prior art system without an initial decelerating stage, with the system specifications above the flight time of ions with m/z=195 was expanded from 408 to 612 s, but the time focus (full width half maximum) also expanded slightly from 1 to 1.2 ns, giving an overall improvement in mass resolution from 200,000 to 255,000. The spatial spread along the detector also increased from a standard deviation of 0.95 to 1.16 mm, which is acceptable as nearly 100% of the ions should still strike within the confines of the detector. Plots of the time and Y spatial dispersion at the detector are shown in FIGS. 19 and 20 respectively.
(62) Higher decelerating stages can also be considered, for example with time of flight increases of 2 and 2.5 that of a mirror without a decelerating stage. However, these mirror arrangements may demonstrate poor spatial focusing of the ion beam onto the detector, as the increasing proportional energy spread of the ion cloud overwhelms that of the mirrors. The increase in the Y-spread (full width at 1% relative intensity) of the ion cloud as increasing levels of deceleration are applied could be compensated by reducing the Y energy and spatial spread of the initial ion cloud, either with a smaller trap, improved ion cooling, or use of lenses with a Y field component in the injection optics.
(63) Although the ion beam is represented schematically in most of the drawings herein as a line without a significant width, in reality the ion beam occupies a region of space termed the beam envelope. Another preferred condition for the ion beam in the vicinity of the transition between the first and second portions of the mirror length (transition in the degree of convergence) is that the distance between two adjacent beam envelopes (i.e. the distance between the beam envelope on either side of the transition) within a mirror should not be smaller than a) 0.5*H, b) 1*H, or c) 2*H, where H is the local height of the mirror (local height meaning the internal height within the mirror, in the Z direction, at the transition). This is shown in FIG. 21, where the distance d between the beam envelopes within a mirror either side of the transition in the degree of convergence is indicated.
(64) Multi-reflection mass spectrometers of the present invention are image-preserving and may be used for simultaneous imaging or for image rastering at a speed independent of the time of flight of ions through the spectrometer.
(65) All embodiments presented above could be also implemented not only as ultra-high resolution TOF instruments but also as low-cost mid-performance analysers. For example, if the ion energy and thus the voltages applied do not exceed few kilovolts, the entire assembly of mirrors and/or compensation electrodes could be implemented as a pair of printed-circuit boards (PCBs) arranged with their printed surfaces parallel to and facing each other, preferably flat and made of FR4 glass-filled epoxy or ceramics, spaced apart by metal spacers and aligned by dowels. PCBs may be glued or otherwise affixed to more resilient material (metal, glass, ceramics, polymer), thus making the system more rigid. Preferably, electrodes on each PCB are defined by laser-cut grooves that provide sufficient isolation against breakdown, whilst at the same time not significantly exposing the dielectric inside. Electrical connections are implemented via the rear surface which does not face the ion beam and may also integrate resistive voltage dividers or entire power supplies.
(66) For practical implementations the elongation of the mirrors in the drift direction Y should be minimised in order to reduce the complexity and cost of the design. This could be achieved by known means e.g. by compensating the fringing fields using end electrodes (preferably located at the distance of at least 2-3 times the height of mirror in Z-direction from the closest ion trajectory) or end-PCBs which mimic the potential distribution of infinitely elongated mirrors. In the former case, electrodes could use the same voltages as the mirror electrodes and might be implemented as flat plates of appropriate shape and attached to the mirror electrodes.
(67) With the present invention, the incorporation of a decelerating stage into the mirror structure itself in the invention allows for an increase of the flight time and consequent resolution to be made without the requirement for an additional deflector to be incorporated between the mirrors, thus reducing the number of parts and cost. Furthermore, the minimum drift energy requirement to steer the ion beam around a deflector as proposed in the prior art is also removed. Whilst some requirement is imposed in the case where a sharp corner is formed at the end of the first, rapid decelerating stage, a decelerating stage based on curved opposing mirrors becomes advantageous as it greatly reduces this issue and the minimum drift energy ceases to be a function of the initial beam width; depending solely on the drift energy dispersion versus the energy acceptance of the reflecting stage.
(68) As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as a or an means one or more.
(69) Throughout the description and claims of this specification, the words comprise, including, having and contain and variations of the words, for example comprising and comprises etc, mean including but not limited to and are not intended to (and do not) exclude other components.
(70) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
(71) The use of any and all examples, or exemplary language (for instance, such as, for example and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
