Multi-reflection mass spectrometer

Abstract

A multi-reflection mass spectrometer comprising two ion mirrors spaced apart and opposing each other in a direction X, each mirror elongated generally along a drift direction Y, the drift direction Y being orthogonal to the direction X, a pulsed ion injector for injecting pulses of ions into the space between the ion mirrors, the ions entering the space at a non-zero inclination angle to the X direction, the ions thereby forming an ion beam that follows a zigzag ion path having N reflections between the ion mirrors in the direction X whilst drifting along the drift direction Y, a detector for detecting ions after completing the same number N of reflections between the ion mirrors, and an ion focusing arrangement at least partly located between the opposing ion mirrors and configured to provide focusing of the ion beam in the drift direction Y, such that a spatial spread of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection having a number between 0.25N and 0.75N, wherein all detected ions are detected after completing the same number N of reflections between the ion mirrors.

Claims

1. A multi-reflection mass spectrometer comprising: two ion mirrors spaced apart and opposing each other in a direction X, each mirror elongated generally along a drift direction Y, the drift direction Y being orthogonal to the direction X; a pulsed ion injector for injecting pulses of ions into the space between the ion mirrors, the ions entering the space at a non-zero inclination angle to the X direction, the ions thereby forming an ion beam that follows a zigzag ion path having N reflections between the ion mirrors in the direction X whilst drifting along the drift direction Y; a detector for detecting ions after completing the same number N of reflections between the ion mirrors; and an ion focusing arrangement at least partly located between the opposing ion mirrors and configured to provide focusing of the ion beam in the drift direction Y, such that a spatial spread of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection having a number between 0.25N and 0.75N, wherein all detected ions are detected after completing the same number N of reflections between the ion mirrors.

2. The multi-reflection mass spectrometer of claim 1 wherein the spatial spread of the ion beam in the drift direction on the first reflection is substantially the same as the spatial spread of the ion beam in the drift direction on the N-th reflection.

3. The multi-reflection mass spectrometer of claim 1 wherein the spatial spread of the ion beam in the drift direction Y passes through a single minimum that is substantially halfway along the ion path between the ion focusing arrangement and the detector.

4. The multi-reflection mass spectrometer of claim 1 wherein the ion focusing arrangement comprises a drift focusing lens or pair of drift focusing lenses for focusing the ions in the drift direction Y.

5. The multi-reflection mass spectrometer of claim 4 wherein at least one drift focusing lens is a converging lens.

6. The multi-reflection mass spectrometer of claim 5 wherein the converging lens focuses the ions such that the spatial spread of the ion beam in the drift direction Y has a maximum at the converging lens that is 1.2-1.6 times, or about 2 times, the spatial spread at the minimum.

7. The multi-reflection mass spectrometer of claim 5 wherein the spatial spread of the ion beam in the drift direction Y has a maximum at the converging lens that is in the range 2 to 20 an initial spatial spread of the ion beam in the drift direction Y at the ion injector.

8. The multi-reflection mass spectrometer of claim 1 wherein the ion beam undergoes K oscillations between the ion mirrors from the ion injector to the ion detector and K is a value within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around an optimum value, K.sub.(opt) given by: $K\left(\mathrm{opt}\right)={\left(\frac{D_L^2}{4W}\right)}^{1/3}$ wherein D.sub.L is the drift length travelled by the ion beam in the drift direction Y, is the phase volume wherein =.sub.i.Math.x.sub.i and .sub.i is the initial angular spread and x.sub.i is the initial spatial spread of the ion beam at the ion injector, and W is the distance between the ion mirrors in the X direction.

9. The multi-reflection mass spectrometer of claim 1 wherein the angular spread of the ion beam, , after focusing by the ion focusing arrangement is within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around an optimum value, .sub.(opt) given by: $\left(\mathrm{opt}\right)=\sqrt{\frac{2}{WK\left(\mathrm{opt}\right)}}.$

10. The multi-reflection mass spectrometer of claim 1 wherein the ion focusing arrangement is located before a reflection having a number less than 0.25N in the ion mirrors.

11. The multi-reflection mass spectrometer of claim 1 wherein the initial spatial spread of the ion beam in the drift direction Y at the ion injector, x.sub.i, is 0.25-10 mm or 0.5-5 mm.

12. The multi-reflection mass spectrometer of claim 1 wherein the ion focusing arrangement comprises a drift focusing lens positioned after a first reflection and before a fifth reflection in the ion mirrors.

13. The multi-reflection mass spectrometer of claim 12 wherein the ion focusing arrangement comprises a drift focusing lens positioned after a first reflection in the ion mirrors and before a second reflection in the ion mirrors.

14. A multi-reflection mass spectrometer of claim 12 wherein the drift focusing lens is the only drift focusing lens positioned between the first reflection and the ion detector.

15. The multi-reflection mass spectrometer of claim 12 wherein the drift focusing lens comprises a trans-axial lens, wherein the trans-axial lens comprises a pair of opposing lens electrodes positioned either side of the beam in a direction Z, wherein direction Z is perpendicular to directions X and Y.

16. The multi-reflection mass spectrometer of claim 15 wherein each of the opposing lens electrodes comprises a circular, elliptical, quasi-elliptical or arc-shaped electrode.

17. The multi-reflection mass spectrometer of claim 15 to wherein each of the pair of opposing lens electrodes comprises an array of electrodes separated by a resistor chain to mimic a field curvature created by an electrode having a curved edge.

18. The multi-reflection mass spectrometer of claim 15 wherein the drift focusing lens comprises a multipole rod assembly or an Einzel lens.

19. The multi-reflection mass spectrometer of claim 15 wherein the lens electrodes are each placed within an electrically grounded assembly.

20. The multi-reflection mass spectrometer of claim 15 wherein the lens electrodes are each placed within a deflector electrode.

21. The multi-reflection mass spectrometer of claim 20 wherein the deflector electrodes have an outer trapezoid shape that acts as a deflector of the ion beam.

22. The multi-reflection mass spectrometer of claim 1 wherein the ion focusing arrangement comprises a first drift focusing lens positioned before the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the first drift focusing lens is a diverging lens, and a second drift focusing lens positioned after the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens.

23. The multi-reflection mass spectrometer of claim 1 wherein the ion focusing arrangement comprises at least one injection deflector positioned before the first reflection in the ion mirrors.

24. The multi-reflection mass spectrometer of claim 23 when dependent on claim 22, wherein the first drift focusing lens is placed within the at least one injection deflector.

25. The multi-reflection mass spectrometer of claim 1 wherein the inclination angle to the X direction of the ion beam is determined by an angle of ion ejection from the pulsed ion injector relative to the direction X and/or a deflection caused by the injection deflector.

26. 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 Y in or adjacent the space between the mirrors for minimising time of flight aberrations.

27. The multi-reflection mass spectrometer of claim 1 further comprising a reversing deflector located at a distal end of the ion mirrors from the ion injector to reduce or reverse the drift velocity of the ions in the direction Y.

28. The multi-reflection mass spectrometer of claim 27 further comprising a further drift focusing lens located between the opposing ion mirrors one, two or three reflections before the reversing deflector to focus the ion beam to a focal minimum within the reversing deflector.

29. The multi-reflection mass spectrometer of claim 27 further comprising a further drift focusing lens positioned within the reversing deflector to focus the ion beam to a focal minimum within one of the ion mirrors at the next reflection after the reversing deflector.

30. The multi-reflection mass spectrometer of claim 29 wherein the detector is located at an opposite end of the ion mirrors in the drift direction Y from the ion injector and wherein the ion mirrors diverge from each other along a portion of their length in the direction Y as the ions travel towards the detector.

31. The multi-reflection mass spectrometer of claim 30 wherein, starting from the end of the ion mirrors closest to the ion injector, the ion mirrors converge towards each other along a first portion of their length in the direction Y and diverge from each other along a second portion of their length in the direction Y, the second portion of length being adjacent the detector.

32. The multi-reflection mass spectrometer of claim 1 wherein the ion detector is an imaging detector.

33. A method of mass spectrometry comprising: injecting ions into a space between two ion mirrors that are spaced apart and opposing each other in a direction X, each mirror elongated generally along a drift direction Y, the drift direction Y being orthogonal to the direction X, the ions entering the space at a non-zero inclination angle to the X direction, the ions thereby forming an ion beam that follows a zigzag ion path having N reflections between the ion mirrors in the direction X whilst drifting along the drift direction Y, focusing the ion beam in the drift direction Y using an ion focusing arrangement at least partly located between the opposing ion mirrors, such that a spatial spread of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection having a number between 0.25N and 0.75N, wherein all detected ions are detected after completing the same number N of reflections between the ion mirrors, and detecting ions after the ions have completed the same number N of reflections between the ion mirrors.

34. The method of mass spectrometry of claim 33 wherein the focusing is such that the spatial spread of the ion beam in the drift direction on the first reflection is substantially the same as the spatial spread of the ion beam in the drift direction on the N-th reflection.

35. The method of mass spectrometry of claim 33 wherein the focusing is such that the spatial spread of the ion beam in the drift direction Y passes through a single minimum that is substantially halfway along the ion path between the ion focusing arrangement and the detector.

36. The method of mass spectrometry of any claim 33 wherein the ion beam undergoes K oscillations between the ion mirrors and K is a value within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around an optimum value, K.sub.(opt) given by: $K\left(\mathrm{opt}\right)={\left(\frac{D_L^2}{4W}\right)}^{1/3}$ wherein D.sub.L is the drift length travelled by the ion beam in the drift direction Y, is the phase volume wherein =.sub.i.Math.x.sub.i and .sub.i is an initial angular spread and x.sub.i is an initial spatial spread of the ion beam, and W is the distance between the ion mirrors in the X direction.

37. The method of mass spectrometry of claim 33 wherein the angular spread of the ion beam, , after focusing is within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around an optimum value, .sub.(opt) given by: $\left(\mathrm{opt}\right)=\sqrt{\frac{2}{WK\left(\mathrm{opt}\right)}}.$

38. The method of mass spectrometry of claim 33 wherein the focusing is performed using an ion focusing arrangement located before a reflection having a number less than 0.25N in the ion mirrors.

39. The method of mass spectrometry of claim 33 wherein an initial spatial spread of the ion beam in the drift direction Y at an ion injector, x.sub.i, is 0.25-10 mm or 0.5-5 mm.

40. The method of mass spectrometry of claim 33 wherein the ion focusing arrangement comprises a drift focusing lens positioned after a first reflection in the ion mirrors and before a fifth reflection in the ion mirrors.

41. The method of mass spectrometry of claim 33 further comprising deflecting the ion beam using a deflector positioned after a first reflection in the ion mirrors and before a fifth reflection in the ion mirrors.

42. The method of mass spectrometry of claim 33 wherein the ion focusing arrangement comprises a first drift focusing lens positioned before the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the first drift focusing lens is a diverging lens, and a second drift focusing lens positioned after the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens.

43. The method of mass spectrometry of claim 33 further comprising adjusting the inclination angle to the X direction of the ion beam by deflecting the ion beam using an injection deflector positioned before the first reflection in the ion mirrors.

44. The method of mass spectrometry of claim 33 further comprising applying one or more voltages to respective one or more compensation electrodes extending along at least a portion of the drift direction Y in or adjacent the space between the mirrors to minimise time of flight aberrations.

45. The method of mass spectrometry of claim 33 further comprising deflecting the ion beam using a reversing deflector at a distal end of the ion mirrors from the injection to reduce or reverse the drift velocity of the ions in the direction Y.

46. The method of mass spectrometry of claim 45 further comprising focusing the ion beam to a focal minimum within the reversing deflector.

47. The method of mass spectrometry of claim 45 further comprising providing a focusing lens within the reversing deflector and focusing the ion beam to a focal minimum within one of the ion mirrors at the next reflection after the reversing deflector.

48. The method of mass spectrometry of claim 33 wherein the detecting comprises forming a 2-D image of an ion source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically an embodiment according to the prior art.

(2) FIG. 2 shows schematically another embodiment according to the prior art.

(3) FIG. 3 shows schematically a further embodiment according to the prior art.

(4) FIGS. 4A and 4B show schematically still further embodiments according to the prior art.

(5) FIG. 5 shows schematically a multi-reflection mass spectrometer according to an embodiment of the present invention.

(6) FIG. 6 shows schematically an ion mirror electrode configuration and applied voltages.

(7) FIG. 7A shows schematically shaped drift focusing lenses having circular shape.

(8) FIG. 7B shows schematically shaped drift focusing lenses having elliptical shapes.

(9) FIG. 7C shows a lens integrated into a prism-like deflector.

(10) FIGS. 8A, 8B, and 8C schematically alternative structures for drift focusing lenses.

(11) FIG. 9 shows schematically an embodiment of an extraction ion trap.

(12) FIG. 10 shows schematically an embodiment of an injection optics scheme.

(13) FIG. 11 shows schematically a multi-reflection mass spectrometer according to another embodiment of the present invention.

(14) FIG. 12A shows simulated arrival time of an initial 2 mm wide thermal ion packet at the detector using the system mass spectrometer in FIG. 11.

(15) FIG. 12B shows drift spatial distribution of an initial 2 mm wide thermal ion packet at the detector using the system mass spectrometer in FIG. 11.

(16) FIG. 13A shows simulated trajectories for a beam of ions with a single focusing lens arrangement.

(17) FIG. 13B shows simulated trajectories for a beam of ions with a two lens arrangement.

(18) FIG. 14 shows schematically a representation of an ion beam width x as ions progress along the drift dimension.

(19) FIG. 15 shows graphs illustrating the effects of varying the initial ion beam width x.sub.0, drift length (D.sub.L) and mirror separation (W) on the achievable ion flight path length.

(20) FIG. 16 shows schematically an embodiment of a multi-reflection ToF configuration incorporating a reversing deflector to return the ion beam back to a drift zero position.

(21) FIG. 17 shows ion trajectories near the end of a mass analyser incorporating a drift reversing deflector and a focusing lens positioned one reflection before the reversing deflector.

(22) FIG. 18 shows simulated ion trajectories with thermal drift divergence through a complete analyser incorporating first and second deflectors to reduce initial drift energy and a third deflector to reverse the ion drift back to a detector with minimised time aberration.

(23) FIG. 19 shows ion trajectories near the end of a mass analyser incorporating a drift reversing deflector for reversal of ion trajectories by two passes through the deflector, in which the deflector incorporates a converging lens for minimisation of time-of-flight aberrations.

(24) FIG. 20 shows schematically an embodiment having mirror convergence and divergence to maximise the number of oscillations within the mirror space and beam divergence at the detector.

(25) FIG. 21 shows simulated ion trajectories with differing source position and energy, showing that the return position is correlated to the start position.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(26) Various embodiments of the invention will now be described with reference to the figures. These embodiments are intended to illustrate features of the invention and are not intended to be limiting on the scope of the invention. It will be appreciated that variations to the embodiments can be made while still falling within the scope of the invention as defined by the claims.

(27) A multi-reflection mass spectrometer 2 according to an embodiment of the present invention is shown in FIG. 5. Ions generated from an ion source (e.g. ESI or other source), which is not shown, are accumulated in a pulsed ion injector, in this embodiment in the form of ion trap 4. In this case, the ion trap is a linear ion trap, such as a rectilinear ion trap (R-Trap) or a curved linear ion trap (C-trap) for example. An ion beam 5 is formed by extracting a packet of trapped thermalized ions, which has for example less than 0.5 mm width in the drift direction Y, from the linear ion trap 4 and injecting it at high energy (in this embodiment 4 kV) into the space between two opposing parallel mirrors 6, 8 by applying an appropriate accelerating/extraction voltage to electrodes of the ion trap 4 (e.g. pull/push electrodes). Ions exit the ion trap via the slot 10 in the ion trap 4. The ion beam enters the first mirror 6 and is focused in the out-of-plane dimension by lensing effected by the first electrode pair 6a of the mirror 6, and reflected to a time focus by the remaining electrodes 6b-6e of the mirror. In this example, the available space between mirrors (i.e. the distance in direction X between the first electrodes (6a, 8a) of each mirror) is 300 mm and the total effective width of the analyser (i.e. the effective distance in the X direction between the average turning points of ions within the mirrors) is 650 mm. The total length (i.e. in direction Y) is 550 mm to form a reasonably compact analyser.

(28) Suitable ion mirrors such as 6 and 8 are well understood from the prior art (e.g. U.S. Pat. No. 9,136,101). An example configuration of ion mirror, like that shown in FIG. 5, is a mirror that comprises a plurality of pairs of elongated electrodes spaced apart in the X direction, such as five pairs of elongated electrodes, the first electrode pair (6a, 8a) of the mirror being set to ground potential. In each pair, there is one electrode positioned above the ion beam and one electrode below the beam (in Z direction shown). Example of voltages for the set of electrodes (6a-6e, 8a-8e) in order to provide a reflecting potential with a time focus for ions is shown in FIG. 6 with applied voltages being suitable for focusing 4 keV positive ions. For negative ions the polarities can be reversed.

(29) After the first reflection in the first ion mirror 6, the ion beam expands substantially under thermal drift to about 8 mm in width in the drift direction and meets an ion focusing arrangement in the form of a drift focusing lens 12, which focuses the ion beam in the drift direction Y. The drift focusing lens 12 is located in the direction X centrally in the space between the mirrors, i.e. halfway between the mirrors. The drift focusing lens 12 in this embodiment is a trans-axial lens comprising a pair of opposing lens electrodes positioned either side of the beam in a direction Z (perpendicular to directions X and Y). Specifically, the drift focusing lens 12 comprises a pair of quasi-elliptical plates 12a, 12b located above and below the ion beam. The lens may be referred to as a button-shaped lens. In this embodiment, the plates are 7 mm wide and 24 mm long with about 100V applied. In some embodiments, the pair of opposing lens electrodes may comprise circular, elliptical, quasi-elliptical or arc-shaped electrodes. The drift focusing lens 12 has a converging effect on the ion beam by reducing an angular spread of the ions in the drift direction Y.

(30) After focusing by the focusing lens 12, the ion beam 5 proceeds to undergo multiple further reflections between the ion mirrors in the direction X whilst drifting along the drift direction Y so as to follow a zigzag ion path in the X-Y plane between the ion mirrors (there being a total of N mirror reflections in the system). After completing N reflections (i.e. N/2 oscillations, where an oscillation is equal to twice the distance between consecutive reflections in the direction X), the ions are detected by an ion detector 14 to permit the time of flight of the ions to be detected. A data acquisition system comprising a processor (not shown) is interfaced to the detector and enables a mass spectrum to be produced. In the embodiment shown, the ions undergo 22 reflections (N=22), giving a total flight path of more than 10 metres. The detector is preferably a fast time response detector such as a multi-channel plate (MCP) or dynode electron multiplier with magnetic and electric fields for electron focusing.

(31) Important factors for the positioning of the drift focusing lens 12 have been determined. Firstly, the ion beam should preferably have expanded sufficiently so that by the time it reaches the focusing lens the effect of the lens on the drift energy or angular spread is maximised relative to its effect on the spatial spread. This means that the ion beam must be allowed to expand before it reaches the drift focusing lens. Thus, it is preferable to position the lens after the first reflection in the ion mirror 6 (unless the mirror separation is very large, for example 500 mm). Secondly, for an injection of an ion beam at a 2 degree inclination angle to the direction X into a mass spectrometer system of this size, the reflections of the central ion trajectory (i.e. centre of the ion beam) are separated by less than 25 mm, and it is important that the focusing lens not be so large as to interfere with adjacent ion trajectories. Without drift focusing, the ion beam would be already 20 mm wide by the third reflection and by the fourth reflection trajectories nearly start to overlap with those of other reflections. The optimum position for the drift focusing lens is therefore preferably after the first but before the fourth or fifth reflection in the system, i.e. it is positioned relatively early in a system such as this, which has a total of 22 reflections (N=22). The optimum position for the drift focusing lens is preferably before the reflection with a number less than 0.25N or less than 0.2N. The optimum position for the drift focusing lens is more preferably after the first reflection but before the second or third reflection (especially before the second).

(32) The concept of placing button shaped electrodes (e.g. circular, oval, elliptical or quasi-elliptical) above and below the ion beam to generate drift focusing in a multi-turn ToF instrument, albeit in a periodic manner and constructed within an orbital geometry, is described in US 2014/175274 A, the contents of which is hereby incorporated by reference in its entirety. Such lenses are a form of transaxial lens (see P. W Hawkes and E Kasper, Principles of Electron Optics Volume 2, Academic Press, London, 1989, the contents of which is hereby incorporated by reference in its entirety). Such lenses have an advantage of having a wide spatial acceptance, which is important to control such an elongated ion beam. The lenses need to be wide enough to both accommodate the ion beam and so that the 3D field perturbation from the sides of the lens does not damage the focal properties. The space between the lenses should likewise be a compromise between minimising these 3D perturbations and accommodating the height of the beam. In practice, a distance of 4-8 mm can be sufficient.

(33) A variation in lens curvature from a circular (button) lens to a narrow ellipse shaped lens is possible. A quasi-elliptical structure taking a short arc reduces the time-of-flight aberrations compared to a wider arc or full circle as the path through it is shorter but it requires stronger voltages and at extremes will start to induce considerable lensing out-of-plane. This effect may be harnessed for some combination of control of drift and out-of-plane dispersion in a single lens, but will limit the range of control over each property. As an adjunct, areas where strong fields are already applied, such as the ion extraction region at the ion trap 4, may be exploited via curvature of the ion trap pull/push electrodes to either induce or limit drift divergence of the ion beam. An example of this is the commercial Curved Linear Ion Trap (C-trap) described in US 2011-284737 A, the contents of which is hereby incorporated by reference in its entirety, where an elongated ion beam is focused to a point to aid injection into an Orbitrap mass analyser.

(34) FIG. 7 shows different embodiments (A, B) of drift focusing lenses comprising circular 20 and quasi-elliptical 22 lens plates (electrodes) along with grounded surrounding electrodes 24 for each plate. The lens electrodes 20, 22 are insulated from the grounded surrounding electrodes 24. Also shown (C) is the integration of a lens 22 (in this case of the quasi-elliptical shape but could be circular etc.) into a deflector, which in this embodiment comprises a trapezoid shaped, prism-like electrode structure 26 arranged above and below the ion beam that serves as a deflector by presenting the incoming ions with a constant field angle rather than a curve. The deflector structure comprises a trapezoid shaped or prism-like electrode arranged above the ion beam and another trapezoid shaped or prism-like electrode arranged below the ion beam. The lens electrodes 22 are insulated from the deflector, i.e. trapezoid shaped, prism-like electrodes, in which they are located, which in turn is insulated from the grounded surrounding electrodes 24. Placement of the lens within a wide spatial acceptance deflector structure is a more space efficient design. Other possible embodiments of suitable lens are shown in FIG. 8, for example: an array (A) of mounted electrodes 30 (e.g. mounted on a printed circuit board (PCB) 32) separated by a resistor chain to mimic the field curvature created by shaped electrodes; a multipole rod assembly (B) to create a quadrupole or pseudo-quadrupole field, such as a 12-rod based lens having pseudo-quadrupole configuration with relative rod voltages (V) shown; and a aperture-based lens, such as a normal aperture Einzel-lens structure (C). Such embodiments of drift focusing lens, e.g. as shown in FIGS. 7 and 8, may be applicable to all embodiments of the multi-reflection mass spectrometer.

(35) An extraction ion trap 40 suitable for use as the ion trap 4 is shown in FIG. 9. This is a linear quadrupole ion trap, which may receive ions generated by an ion source (not shown) and delivered by an interfacing ion optical arrangement (e.g. comprising one or more ion guides and the like) as well understood in the art. The ion trap 4 is composed of a multipole (quadruple) electrode set. The inscribed radius is 2 mm. Ions are radially confined by opposing RF voltages (1000V at 4 MHz) applied to respective opposite pairs 41, 42 and 44, 44 of the multipole electrodes; and axially confined by a small DC voltage (+5V) on the DC aperture electrodes (46, 48). Ions introduced into the ion trap 4 are thermalized by collisional cooling with background gas present in the ion trap (<510.sup.3 mbar). Before extraction of the cooled ions into the ion mirrors of the mass analyser, the trap potential is raised to 4 kV and then an extraction field is applied by applying1000V to the pull electrode 42 and +1000V to the push electrode (41), causing positive ions to be expelled through a slot (47) in the pull electrode into the analyser in the direction shown by the arrow A. Alternatively, the rectilinear quadrupole ion trap shown could be replaced by a curved linear ion trap (C-trap).

(36) In addition to the ion trap 4, 40, it is preferred to have several further ion optical elements to control the injection of ions into the analyser (injection optics). Such ion injection optics may be considered part of the ion focusing arrangement. Firstly, it is beneficial to have out of plane focusing lenses (i.e. focusing in a direction out of the X-Y plane, i.e. in the direction Z) along the path between the ion trap 4 and the first mirror 6. Such out of plane focusing lenses can comprise elongated apertures that improve the transmission of ions into the mirror. Secondly, a portion, e.g. half, of the injection angle of the ion beam to the X direction as it enters the mirror can be provided by the angle of the ion trap to the X direction, and the remainder, e.g. the other half, can be provided by at least one deflector located in front of the ion trap (a so-called injection deflector). The injection deflector is generally positioned before the first reflection in the ion mirrors. The injection deflector can comprises at least one injection deflector electrode (e.g. a pair of electrodes positioned above and below the ion beam). In this way, the isochronous plane of the ions will be correctly aligned to the analyser rather than being 2 degrees misaligned with corresponding time-of-flight errors. Such a method is detailed in U.S. Pat. No. 9,136,101. The injection deflector may be a prism type deflector of the types shown in FIG. 7, with or without incorporating a drift focusing lens as shown in FIG. 7. In such embodiments, the injection deflector (e.g. prism type) for setting the injection angle can be provided in addition to a deflector (e.g. prism type) that can be mounted with or adjacent the drift focusing lens 12 after the first reflection in the ion mirror. In some embodiments, all or a major portion of the injection angle can be provided by an injection deflector. In addition, it will be appreciated that more than one injection deflector can be used (e.g. in series) to achieve a required injection angle (i.e. it can be seen that the system can include at least one injection deflector electrode, optionally two or more injection deflector electrodes). An example embodiment of an injection optics scheme is shown schematically in FIG. 10, along with suitable applied voltages. The ion trap 4 is a linear ion trap, to which the above described +1000V push and 1000V pull voltages are applied to the 4 kV trap to extract the ion beam. The beam then passes though in sequence ion optics comprising a ground electrode 52, first lens 54 held at +1800V, deflector 56 (+70V) of prism type with integrated elliptical lens (+750 V), second lens 58 held at +1200V and finally a ground electrode 60. The first and second lenses 54, 58 are apertured lenses (rectangular Einzel lenses) for providing out of plane focusing. The deflector 56 provides an inclination angle of the ion beam to the X-axis and the integrated elliptical lens can provide for controlled ion beam divergence in the drift direction Y.

(37) It has been found that this additional drift focusing lens, mounted between the extraction ion trap 4 (or optionally incorporated into the ion trap itself by utilising for example a curved pull/adjacent ground electrode) and the first reflection and operated in a diverging manner is beneficial as it allows control of the ion beam divergence before the beam reaches the converging lens 12. Even more beneficially, the additional drift focusing lens mounted between the extraction ion trap 4 and the first reflection can be mounted within an injection deflector as described above and shown in the injection optics scheme of FIG. 10. In certain embodiments, therefore, the ion focusing arrangement can comprise a first drift focusing lens positioned before the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the first drift focusing lens is a diverging lens, and a second drift focusing lens positioned after the first reflection in the ion mirrors for focusing the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens. The diverging drift focusing lens can be constructed as for the converging lens, e.g. as a trans-axial lens with circular, elliptical or quasi-elliptical shape, such as shown in FIG. 7, or as one of the other types of lens shown in FIG. 8. However, diverging drift focusing lens will have a different voltage applied to the converging drift focusing lens and acts on a different width of ion beam so as to provide different focusing properties to the converging drift focusing lens.

(38) It is preferable that the converging drift focusing lens 12, mounted after the first reflection, also incorporates an ion deflector, e.g. the prism type shown in FIG. 7 (embodiment C). This deflector can be tuned to adjust the injection angle to a desired level and/or to correct for any beam deflection imposed by mechanical deviations in the mirrors. Furthermore, errors in mirror manufacture or mounting can induce a small time-of-flight error with every reflection, as ions on one side of the beam see a shorter flight path than the other, and these can preferably corrected by the addition of two compensation electrodes within the space between the mirrors as described above.

(39) In U.S. Pat. No. 9,136,101, elongate electrodes (termed therein compensation electrodes) with a low voltage (e.g. 20V) are used to correct the time-of-flight error caused by the many hundreds of microns of mirror convergence. Similar electrodes, following linear or curved or even complex functions can be used in the present invention to correct for small misalignments or curvature of the mirror electrodes. One or more sets of compensation electrodes can be used wherein each set comprises a pair of elongate electrodes, one electrode positioned above the ion beam and one electrode positioned below the ion beam. The sets of compensation electrodes preferably extend for most of the length of the ion mirrors in the drift direction Y. Whilst such compensation electrodes can be considered for many error functions, the primary mechanical errors are likely to be non-parallelism of mirror electrodes and curvature around the centre, thus two sets of compensation electrodes should be sufficient, preferably each set of compensation electrodes having a different profile in the X-Y plane, e.g. one set having a profile in the X-Y plane that follows a linear function and one set with a profile in the X-Y plane that follows a curved function. The two sets of compensation electrodes are preferably placed side-by-side in the space between the ion mirrors. A set having a profile in the X-Y plane that follows a linear function, when biased, can correct for mirror tilt or misalignment. A set having a profile in the X-Y plane that follows a curved function, when biased, can correct for mirror curvature. The only disadvantage is that such compensation electrodes may add to any unwanted deflection of the ion beam, which can then be corrected by an appropriate voltage on the deflector, i.e. the deflector positioned between the mirrors after the first reflection.

(40) An example of a preferred embodiment, comprising ion injection optics, drift focusing lenses and deflectors, and compensation electrodes is shown schematically in FIG. 11. This embodiment shows the simulated trajectories 65 of ions encompassing the typical range of thermal energies. An extraction ion trap 4 is shown for injecting an ion beam represented by the ion trajectories 65 between parallel elongate ion mirrors 6 and 8 of the type shown in FIGS. 5 and 6. The ion beam is injected generally in the X direction but with a small, 2 degrees, inclination angle to the X axis direction, i.e. with a velocity component in the drift direction Y. In this way, a zigzag trajectory path through the analyser is achieved. The ion beam first passes through injection optics, the injection optics comprising first lens 64 for out of plane focusing, deflector 66 of the above described prism type having an integrated elliptical drift focusing lens 67 mounted therein and second lens 68 for out of plane focusing. The drift focusing lens 67 is preferably a diverging lens. The beam diverges in the drift direction Y as it leaves the ion injector (ion trap) 4 as it travels towards the first mirror 6. The drift focusing lens 67 can provide further desired divergence. The ions undergo the first of N reflections in the first mirror 6 and are thereby reflected back towards the second ion mirror 8. The diverging ion beam encounters a drift focusing lens 72. The drift focusing lens 72 in this embodiment is located after the first reflection in the ion mirrors and before the second reflection (i.e. a reflection in the second ion mirror 8). The lens 72 is an elliptical drift focusing lens as described above mounted within a deflector 76 of the above described prism type. While the first drift focusing lens 67 is a diverging lens (to diverge the width of the beam in the drift direction Y), the second drift focusing lens 72 is a converging lens (to converge the width of the beam in the drift direction Y). The ion focusing arrangement of the drift focusing lens 72 provides long focusing of the ion beam in the drift direction Y, such that a spatial spread of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection having a number between 0.25N and 0.75N, preferably approximately half-way between the first reflection and reflection N. Thus, the ion beam passes through a single minimum that is preferably substantially halfway along the ion path between the ion focusing lens 72 and the detector 74. Two sets of compensation electrodes 78 (one set of curved shape 78 and one set of linear shape 78) are provided in the shown embodiment to correct for any unwanted beam deflections of the ion beam as it undergoes its zigzag path, for example caused by mechanical or alignment deviations or unwanted curvature in the mirror construction. The two sets of compensation electrodes 78 are positioned side-by-side, although not in electrical contact, i.e. the sets are displaced from each other in the direction X. The set of curved shaped compensation electrodes 78 comprises a pair of elongate electrodes having a curved profile in the X-Y plane, one electrode above the ion beam and on electrode below the ion beam. The set of linear shaped compensation electrodes 78 comprises a pair of elongate electrodes having a linear profile in the X-Y plane, one electrode above the ion beam and on electrode below the ion beam. In FIG. 11, for each set of compensation electrodes 78 and 78 only one electrode of the pair is visible as the other electrode of the pair is located directly below the one shown. After N reflections between the two ion mirrors 6, 8 the ions are detected by the detector 74. Advantageously, due to the focusing properties of the drift focusing lens 72, whereby the ion beam width in the drift direction Y is substantially the same (e.g. +/30%, or +/20%, or +/10%) at the detector 74 as at the drift focusing lens 72, all ions are detected after completing exactly the same number N of reflections between the ion mirrors, i.e. there are no detected overtones. Furthermore, the detection of all ions after completing exactly the same number N of reflections can be achieved with a single focusing lens (converging lens) located early in the reflection system e.g. after the first but before the fourth, third or second reflections, or with a pair of focusing lenses (a diverging lens placed upstream of the converging lens). FIG. 12 shows simulated ion peaks in time (A) and drift space (B) at the detector plane formed by a representative ion packet of m/z=195, for the instrument configuration shown in FIG. 11. It can be seen that as well as maintaining good drift focusing, the build-up of time-of-flight aberrations is limited, giving a resolving power in excess of 100,000. In some embodiments, it may be beneficial to include further lenses along the ion path. The form of multi-reflection ToF spectrometer shown in FIG. 11 has the advantage of good tolerance to mechanical errors in the assembly and alignment of the mirrors, as the resultant broad deflection to the ion trajectory can be easily corrected by adjusting the deflector and/or compensation electrode voltage to compensate.

(41) It has been found that having a diverging lens located shortly after the ion injector (ion trap), preferably between the ion injector and the first reflection, is beneficial to optimise the expansion of the ion beam before it reaches the main drift focusing lens (the converging focusing lens). Thus, a telescopic lens system is preferred. The diverging lens preferably has a strong voltage applied to it as the beam is initially very narrow. In the embodiments described above with reference to FIGS. 5, 6 and 11, a voltage of +750V was found to optimise ion beam expansion to the second focusing lens positioned after the first reflection, which had 125V applied. To illustrate this, FIG. 13 shows the expansion of a thermal ion beam that is 2 mm wide in the drift direction Y at the ion injection trap (spatial and thermal divergence plotted) over 22 reflections for single lens (A) and telescopic two-lens (B) configurations. In single lens configuration (A), the converging lens 92 is an elliptical drift focusing lens as described above mounted within a deflector 96 of the above described prism type. There is a first deflector 86 provided before the first reflection to adjust the injection inclination angle but there no diverging lens. In the two lens configuration (B), the system is the same except that a diverging drift focusing lens 87 is provided before the first reflection, wherein lens 87 is an elliptical drift focusing lens mounted within the prism type deflector 86. It can be seen that ion reflections eventually start to overlap along the central axis in the single lens case (A), as the 2 mm initial beam width is too great, but not with the two lens configuration (B). Thus, the two lens configuration enables a greater number of total reflections N to be used. In some embodiments, it may be possible to have both diverging and converging lenses located before the first reflection in the ion mirrors, however such arrangements are much less preferable due to the constraints on the initial beam width and phase volume and the lens voltage that would be required.

(42) The difficulty in collimating an ion beam with lenses comes from ions initially having independent distributions in space and energy. A lens that controls expansion due to the initial ion energy spread will induce convergence from the initial spatial spread. This cannot be eliminated but may be minimised by allowing (or inducing) a large expansion in the beam width. As complete collimation is impossible, it has been found that having a small convergence of the ion beam after the focusing lens is preferable. In order to maximise the ion beam path length, the ion beam spatial spread in the drift direction passes through a single minimum at a mid-way point between the converging drift focusing lens and the detector. After the minimum the ion beam then begins to diverge until the ion beam strikes the detector plane with a similar spatial spread as the beam had at the drift focusing lens. The focusing system is represented schematically in FIG. 14. The ion injector 104, wherein ions have initial spatial spread dx.sub.i in the drift direction, injects ions to the converging drift focusing lens 106 located between the ion mirrors (e.g. between first and second reflections). The ions diverge in the beam expansion region a that is defined between the ion injector 104 and the drift focusing lens 106. The ion beam reaches its maximum spatial spread dx[0] in the drift direction Y at the drift focusing lens 106. Thereafter, the lens 106 focuses the ion beam so that it converges, over converging region b, to its focal minimum (minimum spatial spread) or gorge in the drift direction Y at position f. The focal minimum at position f occurs approximately at a distance halfway between the drift focusing lens 106 and the detector 114. After the focal minimum f, the ion beam again diverges, over diverging region c, until it reaches the detector 114, at which point the ion beam reaches its maximum spatial spread dx[0] in the drift direction Y again.

(43) An optimised analytical solution is now described. The mass resolving power of a ToF mass spectrometer is known to be proportional to the total flight length L. In a multi-reflection ToF mass spectrometer of the type described FIGS. 5, 6, 11 and 13, the total flight length L=KL.sub.0 where K is the number of oscillations between mirrors and L.sub.0 is length of a single oscillation, the latter is approximately double the distance between the mirrors, W. The value K is equal to half the total number of reflections (N), i.e. K=N/2. The drift step per one oscillation is:
.sub.D=W/sin

(44) where is the injection angle (the angle of the ion beam to the direction X as it enters the mirrors and thus reflects between the mirrors, around 2 degrees being typical). Accordingly, the number of oscillation on the whole drift length D.sub.L is:
K=D.sub.L/.sub.D

(45) This may be increased by choosing a smaller injection angle that leads to a smaller drift step .sub.D. The drift step has, nevertheless, a low limit .sub.D(min) determined by a minimal separation between neighbouring oscillations.

(46) The phase volume of the ion beam in the direction of drift is denoted as . As the phase volume is constant along a trajectory according to the Liouville's theorem, is determined by the ion injector and cannot be modified by any collimation optics. Such optics may, however, be used to prepare the ion beam before injection into the analyser by setting the optimal ratio between the spatial and the angular spreads and optimal correlation.

(47) There is a minimum of the ion-beam spatial spread x.sub.0 on the oscillation k.sub.0. As there are no optical elements for collimating the ion trajectories in the drift direction between the first and the last oscillations, the angular spread stays constant and the spatial spread on any oscillation k is:
x[k]={square root over (x.sub.0.sup.2+W.sup.2(kk.sub.0).sup.2.sup.2)}

(48) The optimization target consists in maximization of the total flight length with respect to .sub.D and the phase distribution of the ion beam, the optimum being subject to following restrictions: 1) The spatial spread on the first oscillation x[0].sub.D/2 to prevent overlap between the ion beam after first reflection and the ion source (or collimator) 2) The spatial spread after the last oscillation x[K].sub.D/2 to prevent overlap between the ion beam on the last but one (K1) oscillation and the ion detector 3) The phase volume in the direction of drift is x.sub.0= is fixed.

(49) It is easy to see that the optimal position of the ion beam's gorge (the minimum spatial spread) x.sub.0 is on the middle oscillation k.sub.0=K/2, which gives:

(50) ${x\left[0\right]}^2={x\left[K\right]}^2=\frac{{}^2}{{}^2}+{W^2\left(\frac{K}{2}\right)}^2{}^2\frac{{}_D^2}{4}={\left(\frac{D_L}{2K}\right)}^2$

(51) In the optimum case, the inequality turns to equality, and the optimal value of the angular spread to maximize the number of oscillations K is given by the equation dK=0

(52) $\left(opt\right)=\sqrt{\frac{2}{WK\left(\mathrm{opt}\right)}},K\left(\mathrm{opt}\right)={\left(\frac{D_L^2}{4W}\right)}^{1/3}$

(53) As an example, for a 1 mm wide (in Y) ion cloud at the ion injector, with reasonable inter-mirror distance and drift length given by Wand D.sub.L:

(54) $W=1000\mathrm{mm},D_L=500\mathrm{mm}$$=1\mathrm{mm}\sqrt{\frac{0.025\mathrm{eV}}{24000\mathrm{eV}}}=1\mathrm{mm}1.8\mathrm{mrad}1.810^{-3}\mathrm{mm}$

(55) The value 0.025 eV is the (thermal) energy spread of the ions and 4000 eV is the ion acceleration voltage.

(56) $K\left(\mathrm{opt}\right)={\left(\frac{500^2{\mathrm{mm}}^2}{41.810^{-3}\mathrm{mm}1000\mathrm{mm}}\right)}^{1/3}32.5$$\left(opt\right)\sqrt{\frac{21.810^{-3}\mathrm{mm}}{1000\mathrm{mm}32.5}}3.310^{-4}$$x_0=\frac{}{\left(opt\right)}\frac{1.810^{-3}\mathrm{mm}}{3.310^{-4}}=5.45\mathrm{mm}$$\begin{array}{c}x\left[0\right]=x\left[K\right]=\sqrt{x_0^2+{\left(W\frac{K\left(opt\right)}{2}\left(o\mathrm{pt}\right)\right)}^2}\\ \sqrt{5.45^2{\mathrm{mm}}^2+{\left(1000\mathrm{mm}\frac{32.5}{2}3.310^{-4}\right)}^2}7.6\mathrm{mm}\end{array}$${}_D=\frac{D_L}{2K}=\frac{500\mathrm{mm}}{32.5}=7.7\mathrm{mm}$

(57) The total flight length is thereby given by:
L=K(opt)W=32.51000 mm=32.5 m

(58) It can be seen in the example that the spatial spread on the first oscillation x[0] and the spatial spread after the last oscillation x[K] have a value 7.6 mm that is about 2 times the minimum spatial spread in the system x.sub.0 5.45 mm. In general, the converging lens preferably focuses the ions such that the spatial spread of the ion beam in the drift direction Y has a maximum at the drift focusing lens (and preferably the ion detector) that is 1.2-1.6 times, more preferably 1.3-1.5 times, or about 2 times, the minimum spatial spread.

(59) To provide an optimized system it follows that as the ion beam undergoes K oscillations between the ion mirrors from the ion injector to the ion detector, K preferably has a value within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around the above optimum value, K.sub.(opt) given by:

(60) $K\left(\mathrm{opt}\right)={\left(\frac{D_L^2}{4W}\right)}^{1/3}$

(61) Similarly, the angular spread of the ion beam, , after focusing by the drift focusing arrangement is preferably within a range that is +/50%, or +/40%, or +/30%, or +/20%, or +/10% around the above optimum value, .sub.(opt) given by:

(62) 0$\left(\mathrm{opt}\right)=\sqrt{\frac{2}{WK\left(\mathrm{opt}\right)}}$

(63) FIG. 15 shows graphs illustrating the effects of varying the initial ion beam width x.sub.0, (mirror) drift length (D.sub.L) and mirror separation (W) on the achievable flight path length based on this analytical approach. It is clear that very long flight paths are achievable with reasonably practical mirror arrangements (for example, 1.5 m long and 2 m wide may give a 60 m flight path). The graphs show (A) variation of flight path length with mirror separation W (base 1000 mm) and (B) variation of flight path length with drift length D.sub.L (base 500 mm), each for different initial ion population widths x.sub.0 (1 mm, 2 mm and 4 mm).

(64) In a further embodiment, as long as the ion beam remains reasonably well focused, it is possible to place a deflector or a deflector/drift focusing lens combo (such as described above), or some other beam direction control means at the distal (far) end of the mirrors from the end at which the ion injector is location, in order to reverse the ion beam's drift velocity. Herein such deflectors are referred to as end or reversing deflectors. This results in reflection of the ions back to the starting end of the mirrors, where a detector can be placed. This enables multiplication (e.g. doubling) of the ions' time-of-flight. It can also be possible in some embodiments to have a deflector in the mirrors at one side to reverse the beam again for multiplication of the ions' time-of-flight. Such end or reversing deflectors, preferably have a wide spatial acceptance and operate in an isochronous manner. Another consideration is that positioning the detector proximate to the ion injector introduces space restrictions. One workaround disclosed in U.S. Pat. No. 9,136,101 is to inject ions with a high injection angle to improve the clearance and then use a deflector located after the first reflection to reduce this injection angle. Another possible solution to the problem of space and injection angle is disclosed in U.S. Pat. No. 7,326,925 which uses sectors to carry out ion injection at a small angle and optionally extraction to a detector. Increasing the ion mirror spacing is another possible solution.

(65) An embodiment of a system employing a reversing deflector at the distal end is shown in FIG. 16. However, this embodiment is less preferred as the time aberrations from both of these deflectors become damaging to resolving power. Ion injector 204, located at Y=0, injects ions and first and second deflectors 206, each with integrated drift focusing lenses, adjust the injection angle. Out of plane lenses 205 are also used in the injection optics. The second drift focusing lens focuses the ions as described above with a focal minimum halfway along the ion path. After N/2 reflections along a zigzag flight path, where N is the total number of reflections that ions undergo in the system, the ion beam's drift velocity is reversed along Y by a reversing deflector 208 located at the distal end of the mirrors 6, 8 from the ion injector 204. The deflector 208 is a trapezoid shaped, prism type as described above. This results in reflection of the ions back to towards the starting end of the mirrors, the ions undergoing a further N/2 reflections along the zigzag flight path until they reach an ion detector 210 placed proximate to the ion injector 204 at Y=0. Convergence of the ion mirrors at an entrance portion of their length could be used instead of a deflector to reduce the initial injection angle (e.g. a decelerating stage such as described in US 2018/0138026 A1), which in combination with a compensation electrode would eliminate the timing error from this first deflector completely. It is also possible to correct part of the aberration from the deflector which sets the injection angle with a dipole field placed immediately in front of the detector as in US 2017/0098533.

(66) The beam reversing deflector should preferably incorporate a mechanism to minimise time-of-flight aberration incurred across the width of the ion beam. Two methods to reduce this effect are now described.

(67) The first method is the minimisation of the ion beam width via a focusing lens the turn before beam drift-reversal. A lens can be positioned so that ions pass through it prior to reaching the reversing deflector, preferably one reflection prior to reaching the reversing deflector. The voltage of the lens can be set so that the (relatively wide) ion beam is focused almost to a point within the reversing deflector, thereby minimising ToF aberrations. Thus, the lens preferably has a point focus within the reversing deflector. The ion beam can then diverge to its original width on the return path along the drift direction Y as it passes through such lens a second time, as shown in FIG. 17. The beam can thereby be collimated for the return path by passing through the lens. FIG. 17 shows schematically the beam reflections near to the distal end of the mirrors. The forward direction of the ion beam is shown by arrow F and the reverse direction by arrow R. The reversing deflector 308 is shown positioned at the distal end of the ion mirrors. The trapezoid or prism-type structure of the electrodes of the reversing deflector 308 are shown positioned above and below the ion beam. An ion drift focusing lens 316, which in the shown embodiment is an elliptical shaped, trans-axial lens, is positioned one reflection prior to the reversing deflector 308, and acts to focus the ion beam almost to a point within the reversing deflector. The ion beam then diverges to its original width on the return path R and is collimated by passing through the lens 316 a second time. As an example, in keeping with the above embodiments, a voltage of +300 V can be applied to the reversing deflector 308 and a voltage of 160 V applied to the elliptical lens 316. FIG. 18 shows simulated ions trajectories of ions with a 3a thermal divergence travelling through a mass analyser according to the present invention that incorporates a reversing deflector. Greater than 200,000 resolution can be achieved with proper alignment of ion injector, detector and deflector voltages. First and second deflectors (prism deflectors) 406 reduce the initial drift energy of the ions from the injector 404 and third deflector 408 (reversing prism deflector) reverses the ion drift back to a detector with minimum time aberration. A preferred system using these components to achieve high resolution comprises to inject the ions into the analyser so that they exit the second deflector (i.e. after the first reflection) with a focal plane that is parallel to the drift direction Y, which minimises any focal plane tilt that might be imperfectly corrected on the ions' return journey back through the second deflector (prism). This can be achieved by suitably arranging the ion source, for example by turning the ion source back compared to the previous described embodiments, so as to eject ions from the source at a slightly negative drift (e.g. 1.5 degrees), then change the drift to positive by applying a large voltage on the first prism deflector (e.g. +375V). Ions then reach the second prism deflector (e.g. voltage 120V), which sets the injection angle and also aligns the focal plane to the drift axis Y. The downside of this approach is that ions may reach the detector with a linear focal plane tilt induced by the return journey through the second prism deflector, although that can be compensated either by correctly aligning the detector (with the focal plane tilt) or by providing a focal plane tilt correcting device. Thus, in some embodiments, the ion source may be arranged to eject ions in a negative drift direction (away from the mirrors) and a first ion deflector (generally before the first reflection) reverts the ions to a positive drift direction. A second ion deflector (generally after the first reflection) may adjust the inclination angle of the ion beam and/or align the focal plane of the ion beam to the drift direction Y.

(68) The second method for minimising time-of-flight aberration associated with use of a reversing deflector comprises self-correction of the time-of-flight aberration via two passes through the reversing deflector, which has a focusing lens integrated or in close proximity (e.g. not separated from the deflector by a reflection). For example, a deflector, such as a prism deflector for example, operated at half the voltage required to completely reverse the ions in the drift direction Y (impart opposite drift direction velocity), will instead reduce the ions' drift velocity to zero. Thus, when the ions exit the deflector and reach the ion mirror for the next reflection they will be reflected back into the deflector whereupon the deflection acts to change the ions' drift velocity from zero to the reverse drift velocity and the reversal of the ion trajectory is thereby completed. If a focusing lens is incorporated into the deflector, such as a prism type deflector, for example as described earlier and shown in FIG. 7C, or is just placed in proximity to the deflector, focusing can be applied such that when the ions return to the deflector on the opposite side of the deflector from which they enter, the time-of-flight aberration of the deflector for ions going through the deflector one way and the other cancel out. The deflector/lens assembly is thus self-correcting. However, the return angle should be designed to be slightly offset from the injection angle, so that the beam for example reaches a detector instead of simply returning to the ion injector. For example, a slightly lower voltage could be applied on the reversing deflector (so as to provide slightly less than 100% reflection, e.g. 95% instead of 100% reflection). An example of such a system is shown schematically in FIG. 19. Ions travelling in the drift direction from the ion injector first enter the reversing deflector 508 from the left side as shown by the arrow A. The deflector 508 is of the trapezoid, prism type as shown in the expanded drawing. The voltage applied (+150 V) to the deflector is half that applied to effect the full reversal of the drift velocity as shown in FIGS. 17 and 18. This reduces the ions' drift velocity substantially to zero and the ions enter the mirror (not shown) for the next reflection with zero drift velocity. The deflector has an integrated drift focusing lens 506 (e.g. elliptical shape). At the same time as the ions have their drift velocity reduced to zero by the deflector, they are focused to a focal point in the mirror (preferably at their turning point in the mirror). The lens 506 in this embodiment has a voltage 300 V applied to it. After reflection the ions begin to diverge and re-enter the deflector for a second time, this time from the opposite of the deflector as shown by the direction of arrow B. The deflection is thereby applied again, this time having the effect of completing the reversal of the ions' drift velocity. The lens 506 at the same time acts to collimate the ion beam for the return path.

(69) The use of reversing deflectors to reverse the ion beam and double the flight path is known in prior art but these tend to harm resolution. The more isochronous deflection methods presented here are useful to limit the time-of-flight aberrations and preserve resolution. Both are relatively simple constructions. This problem is addressed in the prior art either by having the aberration cancelled out with mirror inclination working in combination with a deflector (U.S. Pat. No. 9,136,101), which is mechanically demanding), or by having the ion beam always compressed with periodic lenses so the aberration on deflection is small (GB2403063) but this suffers from relatively poor space charge performance.

(70) In patent application US 2018-0138026 A1 is described the use of curvature of the mirror electrodes along at least a portion of the drift length of the analyser as a means of controlling the drift velocity and thus maximising the number of reflections within the limited space of the analyser. FIG. 20 shows the apparatus of FIG. 11 modified to incorporate this concept. The ion injection system and ion focusing arrangement is the same as described in FIG. 11 (i.e. comprising ion injector 904, injection optics comprising out of plane lens 964, deflector 966 with integrated drift focusing lens 967, second out of plane lens 968 and deflector 976 with integrated drift focusing lens 972. Mirrors 906, 908 first converge along a first portion of their length in the drift direction Y to reduce ion drift velocity, for example as described in US 2018-0138026 A1, the contents of which is hereby incorporated by reference in its entirety. The first portion of their length is adjacent to the ion injector. The mirrors preferably first converge following a curved function to reduce drift velocity, although, the convergence could be linear for example. Thereafter, the ion mirrors run parallel (or close to parallel) to maximise the number of reflections and then diverge to separate the different reflections and maximise space for the detector 974. The mirrors preferably diverge following a curved function, although, the divergence could be linear for example. The convergence and divergence need not match (be symmetrical), and the central region may even be completely flat (parallel). A set of elongated time of flight compensation electrodes 978 (one above and one below the ion beam) with a shape matching the mirror curvature (or its inverse) is preferably positioned centrally between the ion mirrors to correct the time-of-flight aberrations of the mirror curvature. For a 2 degree injection angle of 4 kV ions, mirror convergence (difference between the furthest mirror separation and the shortest separation) should be <600 m to prevent drift reflection of some ions. The more strongly converging and diverging regions preferably incorporate multiple reflections to prevent ion scattering (deflection remains adiabatic). As described in US 2018-0138026 A1, reduction of ion drift velocity by mirror convergence may be achieved with flat angled mirror surfaces instead of smoothly curving mirrors. The use of mirror convergence/divergence to maximise the number of turns within the mirror is obviously advantageous, however comes at the cost of defocusing the ion beam in the drift dimension. Modest reductions in drift velocity (25%) were seen to be feasible in simulation before drift focusing became untenable, even with higher order gaussian functions. A converging mirror method is disclosed in U.S. Pat. No. 9,136,101 but it requires reversal of the ions and involves locating the detector and the ion source in the same space between the mirrors, which is not necessary in the embodiments described here. Another method to achieve similar results to applying convergence/divergence of the distance between the mirrors in the drift direction Y would be to reduce/increase the height of the electrodes' apertures (the height of the mirror apertures in the Z direction) towards/away from the centre of the ion mirrors in the drift direction Y. A third way would be to perturb the mirror field by applying a perturbation potential via additional electrodes within the mirrors, for example one or more additional electrodes between the electrodes of the mirrors, such as those described in WO 2019/030472 A1, so as to increase the potential (for positive ions) towards the Y centre (towards the centre of the ion mirrors in the drift direction Y or mid-point of the ion beam path) and decrease it towards the drift termini (towards the ends of the ion mirrors or the beginning and end of the ion beam path). For negative ions, the direction of such a potential would be reversed. As an example, additional wedge shaped electrodes located between the ion mirror electrodes could be used to provide the perturbation potential (such as shown in FIG. 3 of WO 2019/030472 A1). The extent of the wedge shape of the electrode changes along the drift direction Y and therefore so does its perturbation potential. Alternatively, straight (non-wedged) additional electrodes could be used that provide a perturbation potential that varies along the drift direction Y. A similar form of correction or compensation electrode, not disclosed in prior art, would be an electrode extending along the back of a mirror or each mirror, for example a wedge shaped electrode that increases in height (and thus voltage perturbation of the reflecting part of the ion mirrors) along the drift direction Y. Such electrodes have a disproportionate effect on time-of-flight compared to drift, so may be best paired with a function-matching stripe shaped compensation electrode between the mirrors to balance the two properties. However, such electrodes are not generally preferred as the field penetrates through the back of the mirror in an exponential manner, leading to disproportionate effects on ions with high energy and consequent loss of energy acceptance by the mirror.

(71) Multi-reflection mass spectrometers of the present invention may be combined with a point ion source such as laser ablation, MALDI etc for imaging applications, where each mass spectrum corresponds to a source point and images are built up over many points and corresponding mass spectra. Thus, in some embodiments, ions may be produced from a plurality of spatially separate points on a sample in an ion source in sequence and from each point a mass spectrum recorded in order to image the sample. Referring to the system shown in FIG. 16, incorporating the deflector of FIG. 17, one of its properties is that the ion position at the end of the system is strongly related to the ion position in the ion source. This shows that a multi-reflection ToF analyser with a long range focal lens and reversing deflector may be suitable for stigmatic imaging with an imaging detector (e.g. a 2D detector array or pixel detector), where the ion distribution within an area along the source surface may be imaged with a single extraction of ions. Simulated trajectories of ions with variation in initial spatial and energy components are shown returning to a detection plane with an energy focus in FIG. 21. The focal point is tuneable with respect to energy. Ions leave the source plane 1004 from a point and pass through the ion focusing arrangement comprising first deflector/lens arrangement 1006 and second deflector/lens arrangement 1008 of the configuration shown in FIGS. 11 and 16. The ions' initial direction is shown by arrow A and the return ion beam after being reversed in the drift direction Y by a reversing deflector (not shown) is shown by arrow B. The ions return to the source plane at a corresponding point, where a detector (not shown) may be located in proximity.

(72) The 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 a 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 may be defined by laser-cut grooves that provide sufficient isolation against breakdown, whilst at the same time not significantly exposing the dielectric inside. Electrical connections may be implemented via the rear surface which does not face the ion beam and may also integrate resistive voltage dividers or entire power supplies.

(73) For practical implementations the elongation of the mirrors in the drift direction Y should not be too long in order to reduce the complexity and cost of the design. Preferably means are provided for compensating the fringing fields, for example 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.

(74) The spectrometer according to the invention in some embodiments may be used as a high resolution mass selection device to select precursor ions of particular mass-to-charge ratio for fragmentation and MS2 analysis in a second mass spectrometer. For example, in the manner shown in FIG. 15 of U.S. Pat. No. 9,136,101.

(75) 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.

(76) 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.

(77) 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 as defined by the claims. 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.

(78) 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.