Multireflection mass spectrometer

Abstract

Multireflection mass spectrometer (2) comprising: two ion mirrors (6, 8, 906, 908) spaced apart and facing each other in a direction X, each mirror generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X; a pulsed ion injector (4, 40, 104, 204, 904) for injecting ion pulses into the space between the ion mirrors (6, 8, 906, 908), the ions entering the space at a non-zero inclination angle to the direction X, the ions thereby forming an ion beam (5) which follows a zigzag ion path with N reflections between the ion mirrors (6, 8) in the direction X while drifting along the drift direction Y; a detector (14, 74, 114, 210, 974) for detecting ions after completion of the same number N of reflections between the ion mirrors, and an ion focusing arrangement,which is at least partially arranged between the opposing ion mirrors and configured to provide the focusing of the ion beam in the drift direction Y, such that a spatial distribution of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection with a number between 0.25N and 0.75N, wherein all detected ions are detected after completion of the same number N of reflections between the ion mirrors, wherein the ion focusing arrangement comprises a drift focusing lens (12, 72, 92, 106, 972) for focusing the ions in the drift direction Y, and wherein the drift focusing lens is a converging lens.

Description

Field of the InventionThis invention relates to the field of mass spectrometry, in particular time-of-flight mass spectrometry and electrostatic trap mass spectrometry. The invention is particularly concerned with time-of-flight mass spectrometry and electrostatic trap mass spectrometry which employ multi-reflection techniques for extending the ion flight path.BackgroundTime-of-flight (ToF) mass spectrometers are often used to determine the mass / charge ratio (m / z) of ions based on their time-of-flight along a flight path. In ToF mass spectrometry, short ion pulses are generated by a pulsed ion injector and directed along a predetermined flight path through an evacuated space to reach an ion detector. The detector then detects the arrival of the ions and provides an output to a data acquisition system. The ions in a pulse are separated based on their time of flight along the flight path after their m / z and arrive as time-separated short ion packets to the detector.Various arrangements are known which use multi-reflection to lengthen the flight path of ions within mass spectrometers. Flight path extension is desired to increase the time-of-flight separation of ions within time-of-flight (ToF) mass spectrometers or to increase the trapping time of ions within electrostatic trap (EST) mass spectrometers. In both cases, this improves the ability to distinguish small mass differences between ions. Improved resolution and advantages of increased mass accuracy and sensitivity typically associated therewith are an important attribute for a mass spectrometer for a wide range of applications, particularly for life science applications such as proteomics and metabolomics.Mass resolution in time-of-flight mass spectrometers is known to increase in proportion to the length of the flight path of the ions, assuming that the ion focusing characteristics remain constant. Unfortunately, ion energy distributions and space charge interactions can result in ions spreading in flight which, in long systems, can result in their being lost from the analyzer or reaching the detector with a much different flight time.Giles and Gill disclosed in U.S. Pat. No. 9,136,100 B2 that additional focusing lenses at an intermediate position within the flight tube of a conventional, single-reflection ToF analyzer, as shown in FIG. 1, were sufficient to greatly reduce the beam divergence at the ion mirror and detector and to allow an extension of the ion flight path.Nazerenko et al. disclosed in SU 1725289 A1 a multi-reflection time-of-flight analyzer (MR-ToF) consisting of two opposing ion mirrors elongated in a drift direction. Ions oscillate between the mirrors as they drift in the drift direction along the length of the system to a detector, so that the ions follow a zig zag flight path which is reflected between the mirrors and thereby results in folding a long flight path into a relatively compact volume, as shown in Figure 2. One problem is that the system does not have ways to reduce divergence of the ion beam in the drift direction, so that only few reflections are possible until the beam is wider than any detector. Another problem with uncontrolled beam propagation is that it may become possible for ions from different numbers of reflections to reach the detector and thus generate additional "harmonic" peaks for ions of a single m / z. To solve this problem, Verenchikov proposed in GB 2478300 A to allow or induce beam divergence in such a system and to use signal processing to generate single peaks from the data. A long focus lens between the ion source and the detector is used to change the number and / or position of harmonics.A solution to the problem of drift divergence has been demonstrated by Verenchikov in GB 2403063 A. The solution uses periodically spaced lenses located within the field free region between the two parallel elongated opposing mirrors as shown in Figure 3. The periodic lenses provide regular drift focusing after each reflection, every second reflection, or every few reflections.Instruments based on this design exhibited high resolutions of 50,000-100,000 and higher. A major drawback is that the ion path is accurately defined by the lens position and requires accurate alignment of the many elements to minimize ToF aberrations and ion losses. In this arrangement, the number of reflections is adjusted by the position of the lenses, and there is no way to change the number of reflections and hence the flight path length by changing the ion injection angle. The limited spatial acceptance of the lenses also requires a very close focused beam, making the system relatively sensitive to space charge effects at higher ion populations. To further increase the path length, it has been suggested that a deflector should be placed at the end of the mirror structure remote from the ion injector so that the ions can be deflected back through the mirror structure and the flight path length is doubled. However, the use of a deflector in this manner is susceptible to introducing beam aberrations that ultimately limit the maximum resolution performance that can be obtained.Sudakov also disclosed in WO 2008 / 047891 A2 a system comprising two opposing mirrors elongated in a drift direction, but he proposed an alternative means for both doubling the flight path length by recirculating the ions along the drift distance and simultaneously inducing steel convergence in the drift direction. Sudakov proposed segmenting the opposing mirrors to produce a superimposed third mirror in the drift direction, as shown in FIG. 4A, so that ions were distributed at widely varying drift speeds and could then be reflected back to a focus at the front of the mirror. The third mirror was thus oriented perpendicular to the opposing mirrors and was arranged at the end of the opposing mirrors remote from the ion detector. The ions in such a system may diverge in the drift direction from the ion injector as they pass through the analyzer, but the third ion mirror reverses this divergence. After reflection in the third mirror, the ions are converged again in the drift direction when they return to the vicinity of the ion injector. This advantageously allows the ion beam to be distributed in space during most of its path through the analyzer, which reduces space charge interactions and also avoids the use of multiple periodic structures along or between the mirrors for ion focusing. The third mirror also induces spatial focusing with respect to the initial ion energy in the drift direction. However, the third mirror is necessarily incorporated into the structure of the two opposing elongated mirrors and effectively partitions the elongated mirrors, i.e., the elongated mirrors are no longer continuous. Such a system was theoretically very advantageous because it more than doubled the flight path, and the high beam divergence meant good space charge tolerance as well as the ability to change the injection angle and with few intrinsic ToF aberrations (e.g., as induced by periodic lenses or by the use of a strong deflector that rotates the ion beam back in the drift direction). Unfortunately, the strong electric fields between the segments of the opposing mirrors required to integrate the third mirror into the electrode structure result in scattering of the ion beam, which is an effect that can only be limited with a large number of segments, making the mirror construction very complex.Grinfeld and Makarov disclosed in US 9,136,101 B2 a practical way to achieve reflection in the drift direction in a system consisting of two opposing ion mirrors elongated in the drift direction. They disclosed a reflection in the drift direction provided by converging opposing mirrors that generate a pseudopotential gradient along the drift direction that acts as an ion mirror to reverse the ion drift velocity and spatially focus the ions in the drift direction onto a focal point at which a detector is placed. A specially shaped central correction or compensation electrode serves to correct for ToF aberrations induced by the non-constant mirror separation. This arrangement, shown in FIG. 4B, avoids the scattering of the ion beam and obviates both the need for a complex mirror construction and the need for a third ion mirror, as proposed by Sudakov. However, the balance between mirror convergence and correction electrode potential still requires high mechanical accuracy.In view of the foregoing, it will be appreciated that improvements are still desired in multi-reflection time of flight (MR-ToF) and electrostatic trap (MR-EST) mass spectrometers. Among the desired characteristics of such spectrometers is the lengthening of the flight path in a time-of-flight analyzer to provide high resolution (e.g., >50K) while maintaining relatively compact size, high ion transfer, robust construction with tolerance to small mechanical variations.WO 2019 / 030476 A1 discloses a multi-pass time-of-flight or electro-trap mass spectrometer with an orthogonal accelerator, which is applicable to mirror-based multi-reflection or multi-revolution analyzers. The orthogonal accelerator is tilted and after the first ion reflection or reversal, the ion packets are redirected by the same angle α with a compensated deflector to compensate for time front steering and chromatic angle spreads.SummaryThe present invention provides in one aspect a multi-reflection mass spectrometer comprising:two ion mirrors spaced apart from each other and opposing each other in a direction X, each mirror being generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X,a pulsed ion injector for injecting ion pulses into the space between the ion mirrors, the ions entering the space at a non-zero angle of inclination to the direction X, the ions thereby forming an ion beam following a zig zag ion path with N reflections between the ion mirrors in the direction X as it drifts along the drift direction Y,a detector for detecting ions after completion of the same number N of reflections between the ion mirrors, andan ion focusing arrangement at least partially disposed between the opposing ion mirrors and configured to provide focusing of the ion beam in the drift direction Y such that a spatial distribution 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 by the detector after completion of the same number N of reflections between the ion mirrors.The ion focusing arrangement ensures that the detector detects only ions that have completed exactly the same number N of reflections between the ion mirrors, i.e., N reflections between exiting the ion injector and detecting by the detector.Due to the focusing characteristics of the ion focusing arrangement, the ion beam width in the drift direction Y at the ion detector is preferably substantially the same as in the ion focusing arrangement. The spatial distribution of the ion beam in the drift direction in the first reflection is preferably substantially the same as the spatial distribution of the ion beam in the drift direction in the Nth reflection. Preferably, the spatial distribution of the ion beam in the drift direction Y passes through a single minimum substantially halfway along the ion path between the ion focusing arrangement and the detector.The ion focusing arrangement comprises a drift focusing lens or optionally a pair of drift focusing lenses for focusing the ions in the drift direction Y. At least one drift focusing lens is a converging lens (i.e. has a converging effect on the ion beam width, in particular in the drift direction Y). Preferably, the converging lens focuses the ions such that the spatial distribution 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 the spatial distribution at the minimum. Moreover, preferably, the spatial distribution of the ion beam in the drift direction Y has a maximum at the converging lens that is in the range of 2 to 20 times the initial spatial distribution of the ion beam in the drift direction Y at the ion injector. The drift focusing lens (or lenses) is preferably located centrally in the space between the ion mirrors, i.e., midway between the ion mirrors, in the X direction, although in some embodiments the lens(s) may be spaced from this central position in the X direction.The ion beam undergoes K oscillations in total between the ion mirrors from the ion injector to the ion detector. In each oscillation, the ions travel a distance twice the distance between the mirrors and thus K is equal to N / 2, where N is the total number of reflections between the mirrors. The value K is preferably a value within a range that + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, K (opt), given by: where D L is the drift distance travelled by the ion beam in the drift direction Y, Π is the phase volume, where Π = δα i. δx i and δα i is the initial angular distribution and δx i is the initial spatial distribution of the ion beam at the ion injector, and W is the distance between the ion mirrors in the X direction. It is preferable that the angular distribution of the ion beam, δα, after focusing by the ion focusing assembly is within a range that + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, δα (opt), given by:Preferably, the initial spatial distribution of the ion beam in the drift direction Y at the ion injector is δx i, 0,25-10 mm or 0.5-5 mm.The ion focusing arrangement is preferably arranged before the N / 4 -ten reflection in the ion mirrors or before a reflection with a number of less than 0.25N. In some preferred embodiments, the ion focusing arrangement comprises a drift focusing lens positioned after a first reflection and before a fifth reflection in the ion mirrors (in particular before a fourth, third or second reflection). More preferably, 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. In some preferred embodiments, the ion focusing arrangement comprises only a single drift focusing lens positioned after the first reflection and in front of the detector. In such embodiments, the single drift focusing lens is preferably positioned in the ion mirrors after the first reflection and before a second reflection.Preferably, the drift focusing lens, or lenses in cases where there is more than one drift focusing lens, comprises a transaxial lens, the transaxial lens comprising a pair of opposing lens electrodes positioned on either side of the beam in a direction Z, the direction Z being perpendicular to the directions X and Y. Preferably, each of the opposed lens electrodes comprises a circular, elliptical, quasi-elliptical, or arcuate electrode. In some embodiments, each of the pair of opposing lens electrodes includes an array of electrodes separated by a resistor string to mimic a field curvature generated by an electrode having a curved edge. In some embodiments, the opposing lens electrodes are each disposed within an electrically grounded array. In some embodiments, the lens electrodes are each disposed within a deflector electrode. More preferably, each deflector electrode is disposed within an electrically grounded array. The deflector electrodes preferably have an outer trapezoidal shape that acts as a deflector of the ion beam.In some embodiments, the drift focusing lens comprises a multipolar rod arrangement. In some embodiments, the drift focusing lens comprises a single lens (a series of electrically biased apertures).In some preferred embodiments, the ion focusing arrangement comprises a first drift focusing lens that is a diverging lens in the drift direction Y (i.e. has a diverging effect on the ion beam width, in particular in the drift direction Y) and a second drift focusing lens that is a converging lens in the drift direction Y, wherein the second drift focusing lens is located downstream of the first drift focusing lens. In some preferred embodiments, the ion focusing arrangement comprises a first drift focusing lens positioned prior to the first reflection in the ion mirrors to focus 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 to focus the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens (i.e. has a converging effect on the ion beam width, in particular in the drift direction Y).In some embodiments, the ion focusing arrangement comprises at least one injection ion deflector positioned in the ion mirrors prior to the first reflection, for example for adjusting the tilt angle of the ion beam during injection. Preferably, the angle of inclination 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 positioned in the ion mirrors prior to the first reflection. In certain embodiments, the first drift focusing lens may be placed within the at least one injection deflector. In some embodiments, the ion focusing arrangement comprises at least one ion deflector arranged after the first reflection in the ion mirrors, but preferably before the fourth, third or most preferred second reflection, optionally in addition to an injection ion deflector arranged before the first reflection. The ion deflector positioned after the first reflection may be used to adjust or optimize ion beam alignment. In some preferred embodiments, the mass spectrometer further comprises one or more compensation electrodes extending along at least a portion of the drift direction Y in or adjacent to the space between the mirrors for minimizing time-of-flight aberrations, e.g. caused by beam deflections.In some embodiments, a reverse deflector is located at an end of the ion mirrors remote from the ion injector to reduce or reverse the drift velocity of the ions in the direction Y. In such embodiments, another drift focusing lens is disposed between the opposing ion mirrors one, two, or three reflections before the reverse deflector to focus the ion beam to a focal minimum within the reverse deflector. In some cases, another drift focusing lens is positioned within or near (adjacent to) the inverting deflector to focus the ion beam to a focal minimum within one of the ion mirrors at the next reflection after the inverting deflector. In such embodiments, the ion beam preferably passes twice through the reverse deflector, receiving at each pass half of the deflection necessary to fully reverse the ion drift velocity, such that the ion drift velocity is fully reversed after the second pass.In some embodiments where the detector is disposed at an end of the ion mirrors opposite the ion injector in the drift direction Y, the ion mirrors diverge along a portion of their length in the direction Y as the ions move toward the detector. In some embodiments, the ion mirrors converge (thereby decreasing the distance between the mirrors) along a first portion of their length in the direction Y from the end of the ion mirrors closest to the ion injector and diverge (thereby increasing the distance between the mirrors) along a second portion of their length in the direction Y, the second portion of the length being adjacent the detector.In some embodiments, the mass spectrometer may be used for imaging, wherein the detector is an image detector, such as a 2D or pixel detector, i.e., a position sensitive detector.In another aspect, the present invention provides a method of mass spectrometry. The mass spectrometer of the present invention may be used to perform the method. The properties of the mass spectrometer thus also apply analogously to the method. The method of mass spectrometry comprises:injecting ions into a space between two ion mirrors spaced apart from each other and opposing each other in a direction X, each mirror being generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X, the ions entering the space at a non-zero angle of inclination to the direction X, the ions thereby forming an ion beam following a zig zag ion path with N reflections between the ion mirrors in the direction X as it drifts along the drift direction Y,focusing the ion beam in the drift direction Y using an ion focusing arrangement located at least partially between the opposing ion mirrors such that a spatial distribution of the ion beam in the drift direction Y passes a single minimum at or immediately after a reflection having a number between 0.25N and 0.75N, and detecting ions after the ions have completed the same number N of reflections between the ion mirrors. Thus, all detected ions are detected after completion of the same number N of reflections between the ion mirrors, and no harmonics are detected.Preferably, the focusing is such that the spatial distribution of the ion beam in the drift direction in the first reflection is substantially the same as the spatial distribution of the ion beam in the drift direction in the Nth reflection. Preferably, the focusing is such that the spatial distribution 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. Preferably, 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 (opt), given by: where D L is the drift distance travelled by the ion beam in the drift direction Y, Π is the phase volume, where Π = δα i ·δx i and δα i is an initial angular distribution and δx i is an initial spatial distribution of the ion beam, and W is the distance between the ion mirrors in the X direction. Preferably, the angular distribution of the ion beam, δα, after focusing is in a range that + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, δα (opt), given by:Preferably, the focusing is performed with an ion focusing arrangement that is arranged in the ion mirrors before a reflection with a number of less than 0.25N. Preferably, an initial spatial distribution of the ion beam in the drift direction Y at an ion injector is δx i, 0,25-10 mm or 0.5-5 mm.The ion focusing arrangement comprises a drift focusing lens which is preferably positioned after a first reflection in the ion mirrors and before a fifth reflection in the ion mirrors.In some embodiments, the method further comprises 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.In some embodiments of the method, the ion focusing arrangement comprises a first drift focusing lens positioned in the ion mirrors prior to the first reflection to focus 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 in the ion mirrors after the first reflection to focus the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens.In some embodiments, the method includes deflecting the ion beam using an injection deflector positioned in the ion mirrors prior to the first reflection.In some embodiments, the method further includes adjusting the angle of inclination to the X direction of the ion beam by deflecting the ion beam using the injection deflector.In some embodiments, the method further comprises applying one or more voltages to each one or more compensation electrodes extending along at least a portion of the drift direction Y in or adjacent to the space between the mirrors to minimize time-of-flight aberrations.In some embodiments, the method further comprises deflecting the ion beam using a reverse deflector at an end of the ion mirrors remote from the injection to reduce or reverse the drift velocity of the ions in the direction Y. In some of these embodiments, the method further comprises focusing the ion beam to a focal minimum within the reverse deflector. In some embodiments, the method further comprises a focusing lens within or near (adjacent to) the inverting deflector and focusing the ion beam to a focal minimum within one of the ion mirrors at the next reflection after the inverting deflector. In such embodiments, the ion beam preferably passes twice through the reverse deflector, receiving at each pass half of the deflection necessary to fully reverse the ion drift velocity, such that the ion drift velocity is fully reversed after the second pass.In some embodiments, the detecting includes forming a 2D image of an ion source, e.g., on an image detector such as a 2D or pixel detector.Problems with multi-reflection time-of-flight mass spectrometers with extended flight paths can arise from the need to control the divergence of the ion beam within the analyzer because ions are lost from the system or can reach the detector at different times, which affects sensitivity and resolution or complicates the mass spectrum. Prior art methods have achieved some success in this regard, but generally require the highest mechanical precision and alignment and / or complicated designs. GB 2478300 A proposed allowing beam divergence in such a system and using signal processing to generate individual peaks from the data. This prior art mentions the possibility of using a long focus lens between the ion source and the detector to change the number and position of harmonics (by changing the drift focusing characteristics), while the present disclosure describes using a drift focusing arrangement to cancel harmonics. Moreover, the present disclosure does not comprise any regular or periodic focusing lenses after each reflection, every second reflection or every few reflections, e.g. of the type of periodic focusing lenses shown in GB2403063 A. Compared to periodic focusing, the present invention is simpler, more tunable and easier to align, and at the same time allows a more diffuse ion beam and thus better space charge performance.This disclosure describes the use of a long drift focus ion lens or, in some embodiments, a pair of ion lenses (e.g., in a telescopic configuration in which a first diverges the beam and a second converges the beam) to reduce the drift distribution of an ion beam within a multi-reflection ToF (MR-ToF) analyzer or multi-reflection electrostatic traps (MR-EST) analyzer. In this manner, approximately all ions from an ion source or injector are delivered to a detector over a reasonably long ion flight path, e.g., >10 meters, and without substantial introduced ToF aberrations. As a result, a high mass resolution and a high ion transmission can be achieved. The use of a further drift focusing lens within the ion injection region is also advantageous since the combination of two lenses enables doubling of the initial spatial distribution of the ion beam or alternatively doubling of the flight path before alternating flight paths overlap.The present invention is also designed to be more tolerant of mechanical defects than the converging mirror system disclosed in US 9,136,101 B2.Preferably, methods of mass spectrometry utilizing the present invention comprise injecting ions into the multi-reflection mass spectrometer from one end of the opposing ion optical mirrors, the ions having a velocity component in the drift direction Y.A pulsed ion injector injects ion pulses into the space between the ion mirrors at a non-zero tilt angle to the X direction, the ions thereby forming an ion beam that follows a zig zag ion path N reflections between the ion mirrors in the direction X as it drifts along the drift direction Y. N is an integer value of at least 2. thus, the ion beam undergoes at least 2 reflections between the ion mirrors in the direction X while drifting along the drift direction Y.Preferably, the number N of ion reflections in the ion mirrors along the ion path from the ion injector to the detector is at least 3, or at least 10, or at least 30, or at least 50, or at least 100.Preferably, the number N of ion reflections in the ion mirrors along the ion path from the ion injector to the detector is 2 to 100, 3 to 100, or 10 to 100, or above 100, e.g. one of the groups: (i) from 3 to 10; (ii) from 10 to 30; (iii) from 30 to 100; (iv) above 100.Ions injected into the spectrometer are preferably repeatedly reflected back and forth between the mirrors in the direction X while drifting down in the direction Y of the mirror extension (in the direction +Y). Overall, the ion movement follows a zig zag path.In certain embodiments, as described below, after a number of reflections (typically N / 2), the ions may be reversed in drift velocity along Y and then repeatedly reflected back and forth in the X direction between the mirrors as they again drift upward in the Y direction.For convenience, in this document, the drift direction is referred to as the Y direction, the opposing mirrors are spaced apart from each other at a distance referred to as the X direction, where the X direction is orthogonal to the Y direction, which distance may be the same (such that the ion mirrors are substantially parallel to each other), or may vary at different locations in the Y direction. The ion flight path, hereinafter referred to simply as an ion path, generally occupies a spatial volume extending in the X and Y directions, where the ions are reflected between the opposing mirrors (in the X direction) and simultaneously travel along the drift direction Y. Generally, the ion beam experiences a mean shift dYin the drift direction Y per single ion reflection.The mirrors are typically smaller in dimension in the perpendicular Z direction (Z is perpendicular to X and Y), the spatial volume occupied by the ion flight path is typically a slightly distorted rectangular cuboid, the smallest dimension of which is preferably in the direction Z. For better understanding of the description herein, ions are injected into the mass spectrometer with initial velocity components in the +X and +Y directions, initially traveling toward a first ion mirror disposed in a +X direction and along the drift distance in a +Y direction. Thus, after the first reflection in the first ion mirror, the reflected ions continue to move in the -X direction toward the second ion mirror at a speed in the +Y direction. After the second reflection, the ions again move in the +X and +Y directions and so on. The average value of the ion velocity component in the Z direction is preferably zero.The dissolution power depends on the initial ion injection angle into the space between the mirrors (referred to herein as the tilt angle, which is the ion injection angle to the X direction in the X-Y plane) which determines the drift velocity and hence the total flight time. Ideally, this injection tilt angle should be minimized to maximize the number of reflections and thus the ion path length and mass resolution, but such minimization of the tilt angle may be limited by mechanical requirements of the injection device and / or detector, particularly in more compact designs. Advantageously, aspects of the present invention allow for a change in the number of ion oscillations within the mirror structure and thereby the total flight path length by changing the ion injection angle.In some embodiments, a deflector may be positioned between the mirrors to reduce the drift velocity after ion injection. In further embodiments, a delay stage as described in US 2018 / 0138026 A1 may be incorporated into the mirror structure itself in order to reduce the drift velocity, e.g. after the first one or two reflections, and thus to enable an increase in the flight time and subsequent resolution. In such embodiments, an additional deflector may not be required to be incorporated between the mirrors, which reduces the number of parts and the cost.The ion injector generally receives ions from an ion source, whether directly or indirectly via one or more optical devices (e.g., one or more of an ion guide, a lens, a mass filter, a collision cell). The ion source ionizes sample species to form the ions. Suitable ion sources are known in the art, e.g., electrospray ionization, chemical ionization, atmospheric pressure chemical ionization, MALDI, etc. In some embodiments, the ion injector itself may be the ion source (e.g., MALDI source). The ion source may ionize multiple types of samples, e.g., from a chromatograph, to form the ions.The ion injector is generally a pulsed ion source, i.e., it injects non-continuous pulses of ions rather than a continuous ion current. As is known in the art of ToF mass spectrometry, the pulsed ion injector forms short ion packets comprising at least a portion of the ions from the ion source. Typically, an accelerating voltage is applied by the ion injector to inject the ions into the mirrors, which may be several kV, such as 3 kV, 4 kV or 5 kV.The ion injector may comprise a pulsed ion injector, such as an ion trap, orthogonal accelerator, MALDI source, secondary ion source (SIMS source), or other known ion injection means for a ToF mass spectrometer. Preferably, the ion injector comprises a pulsed ion trap, more preferably a linear ion trap, such as a rectilinear ion trap or a curved linear ion trap (C-trap). The ion injector is preferably located in the position Y=0. The detector may be similarly positioned at Y=0 in some embodiments where the ion flight is reversed after a series of reflections in the Y direction.The ion injector preferably injects ion pulses with a limited initial width in the drift direction Y. In one embodiment, the ion pulse can be generated from an ion cloud accumulated in an ion trap. It is then ejected into the ion mirrors in a pulsating manner. The trap may provide an ion cloud of limited width in the drift direction. In preferred embodiments, the ion cloud in the ion injector, which is injected in the direction of the ion mirrors, has a width in the drift direction Y of 0.25 to 10 mm, or 0.5 to 10 mm, preferably 0.25 to 5 mm or 0.5 to 5 mm, e.g. 1 mm or 2 mm or 3 mm or 4 mm. This defines an initial ion beam width.The ion injector injects ions from one end of the mirrors into the space between the mirrors at an angle of inclination to the X axis in the X-Y plane such that ions are reflected multiple times from an opposing mirror to the other as they drift away from the ion injector along the drift direction to generally follow a zig zag path within the mass spectrometer.The ion injector is preferably disposed directly at an end of the opposing ion optical mirrors in the drift direction Y, so that ions can be injected into the multi-reflection mass spectrometer from an end of the opposing ion optical mirrors in the drift direction (injection in the +Y direction).The ion injector for injecting ions as an ion beam into the space between the ion mirrors at an inclination angle to the X direction is preferably located in the X-Y plane. Thereafter, the injected ions follow their zig-zag path between the ion mirrors in the X-Y plane. However, the ion injector may be out of the X-Y plane such that ions are injected toward the X-Y plane and deflected by a deflector when they reach the X-Y plane to subsequently follow their zig-zag path between the ion mirrors in the X-Y plane. In some embodiments, C-shaped isochronous ion interfaces or sectors may be used for ion injection as disclosed in US 7,326,925 B2.The ion focusing assembly is generally disposed on the ion path. The ion focusing assembly is generally positioned along the ion path between the ion injector and the detector. The ion focusing assembly is preferably positioned along the ion path that is closer to the ion injector than the detector. For example, it is preferred to arrange the ion focusing arrangement along the ion path between first and fifth reflection or first and fourth reflection or first and third reflection or more preferably between first and second reflection.The ion focusing arrangement is arranged at least partially between the opposing ion mirrors. In some embodiments, the ion focusing arrangement is arranged entirely between the mirrors (i.e., in the space between the mirrors), and in other embodiments, the ion focusing arrangement is arranged partly between the mirrors and partly outside the space between the mirrors. For example, one lens of the ion focusing arrangement can be arranged outside the space between the ion mirrors, while another lens of the ion focusing arrangement is arranged between the ion mirrors.The ion focusing arrangement is configured to enable focusing of the ions in the drift direction. Typically, the ion focusing arrangement comprises a focusing lens that causes the ion beam to converge in the direct direction Y, referred to hereafter as a converging lens. The ion focusing arrangement or lens has a long focal length that provides a single focal minimum (i.e., a minimum spatial distribution) in the drift direction Y along the ion path at or immediately after a reflection (i.e., before the next reflection) of a number between 0.25 N and 0.75 N, i.e., the spatial distribution of the ion beam in the drift direction Y passes through a single minimum at or immediately after a reflection of a number between 0.25 N and 0.75 N. Typically, a single focal minimum occurs approximately or substantially halfway between the first and last (Nth) reflections. This means, for example, that the single focal minimum (minimum spatial distribution) in the drift direction Y along the ion path may occur at a point that is halfway between the first and the Nth reflections + / - 20% or + / - 10% or + / - 5% of the total ion path length between the first and the Nth reflections. In this way, the ion focusing arrangement may generally provide that the single focal minimum (minimum spatial distribution) in the drift direction Y occurs approximately or substantially halfway along the ion path between the ion focusing arrangement (i.e. the converging lens of the ion focusing arrangement) and the detector. For example, the single focal minimum (minimum spatial distribution) in the drift direction Y along the ion path may occur at a point that is midway between the ion focusing array (i.e., the converging lens of the ion focusing array) and the detector + / - 20% or + / - 10% of the total ion path length between the ion focusing array and the detector. Thus, unlike prior art periodic focusing arrangements, the ion focusing arrangement according to the present disclosure does not provide multiple focal minima (spatial distribution minima) in the drift direction Y along the ion path.Moreover, the ion focusing arrangement ensures by these focusing characteristics that the spatial distribution of the ions in the drift direction Y in the first reflection is substantially the same (e.g., within + / - 30%, + / - 20%, or preferably + / - 10%) as the spatial distribution of the ions in the drift direction Y in the Nth reflection. The spatial distribution in the first (or Nth) reflection in this document means the spatial distribution of the ions in the drift direction Y immediately downstream of the reflection, e.g. in the first crossing of the center point between the ion mirrors in the direction X after the first (or Nth) reflection. Analogously, it can thereby be ensured that the spatial distribution of the ions in the drift direction Y at the detector is substantially the same (e.g. within + / - 30%, + / - 20%, or preferably + / - 10%) as the spatial distribution of the ions in the drift direction Y at the ion focusing arrangement (i.e. the converging lens of the ion focusing arrangement). The spatial distribution of the ions in the drift direction Y at the converging lens of the ion focusing arrangement (and preferably at the final, Nth reflection and / or at the detector) for an initial ion beam width range of 0.25-10 mm or 0.5-5 mm (i.e. spatial distribution in the drift direction Y) is 5-25 mm or 5-15 mm. In preferred embodiments, the ion beam width in the drift direction Y at its maximum at the converging lens of the ion focusing assembly is in the range of 2 to 20 times (2x to 20x) the initial ion beam width (e.g., initial ion beam width from the pulses of ions at the ion injector, at an ejection point from the ion injector). This is determined by the phase volume of the ion beam determined by the ion injector, and the dimensions of the mirrors (mirror spacing (W) and mirror length in the drift direction Y). In embodiments, the ion beam width or spatial distribution of the ions in the drift direction Y at the single minimum (focal minimum or so-called "schluff") is generally about the maximum ion beam width at the lens (e.g., 0.65-0.75 or ~0.7 of the maximum ion beam width at the lens). In other words, the converging lens focuses the ions such that the spatial distribution of the ion beam in the drift direction Y has a maximum at the converging lens that is 1.2-1.6 times or 1.3-1.5 times or about the spatial distribution at the minimum.It is advantageous that the focusing properties of the ion focusing arrangement ensure that substantially all or all detected ions are detected after the completion of the same number of reflections N between the ion mirrors. In this way, no harmonics are detected, i.e. ions which have undergone a different number of reflections (more or less than N) in the ion mirrors.In some embodiments, at least one focusing lens (a so-called drift focusing lens that focuses ions at least or mainly in the drift direction Y) is arranged on the ion path. In some embodiments, at least two focusing lenses are arranged on the ion path, for example a pair of lenses. In some of these embodiments, a first focusing lens may be positioned prior to the first reflection of the ions in the ion mirrors and a second focusing lens may be positioned prior to the first reflection of the ions in the ion mirrors (e.g., between the first and fifth reflections, preferably between the first and fourth reflections, or between the first and third reflections, or most preferably between the first and second reflections). In some embodiments, the first focusing lens may be a lens that generates divergence (larger spatial distribution) of the ions in the drift direction Y (i.e., defocusing lens). A second focusing lens is then provided as a focusing lens which produces a convergence of the ions in the drift direction Y, in which the minimum of the spatial distribution of the ions in the drift direction Y takes place substantially halfway along the ion path between the second lens of the ion focusing arrangement and the detector. Thus, the ion focusing arrangement may comprise one or more ion focusing lenses. In some embodiments where the ion focusing arrangement comprises a plurality of focusing lenses, the last lens on the ion path produces a convergence of the ions in the drift direction Y, where the minimum of the spatial distribution of the ions in the drift direction Y is substantially halfway along the ion path between the last lens of the ion focusing arrangement and the detector.The present disclosure further provides a method of mass spectrometry comprising the steps of injecting ions into the multi-reflection mass spectrometer, e.g. in the form of a pulsed ion beam as known in ToF mass spectrometry, and detecting at least some of the ions during or after their passage through the mass spectrometer using the ion detector.Ion detectors known from the art of ToF mass spectrometry may be used. Examples include SEM detectors or Microchannel Plate (MCP) detectors or detectors with SEM or MCP in combination with a scintillator / photodetector. In some embodiments, the detector may be disposed at the opposite end of the ion mirrors in the drift direction Y to the ion injector. In other embodiments, the detector may be positioned in a region adjacent to the ion injector, for example, substantially at or near the same Y position as the ion injector. In such embodiments, the ion detector may be positioned, for example, within a distance (center to center) of 50 mm, or within 40 mm, or within 30 mm, or within 20 mm from the ion injector.Preferably, the ion injector is arranged to have a detector surface that is parallel to the drift direction Y, i.e. the detector surface is parallel to the Y axis. The detector may in some embodiments have a degree of tilt to the Y-direction, preferably by an amount corresponding to the angle of the isochronous plane of the ions, for example a degree of tilt of 1 to 5 degrees, or 1 to 4 degrees, or 1 to 3 degrees. The detector may be arranged in the direction X at a location between the ion mirrors, e.g. centrally or halfway between the ion mirrors.The multi-reflection mass spectrometer may constitute the whole or a part of the multi-reflection time-of-flight mass spectrometer. Preferably, in such embodiments of the invention, the ion detector arranged in a region adjacent to the ion injector is arranged to have a detection surface which is parallel to the drift direction Y, i.e. the detection surface is parallel to the Y axis. Preferably, the ion detector is arranged such that ions that have passed through the mass spectrometer by moving back and forth along the drift direction between the mirrors as described in this document are incident on the ion detection surface and detected. The ions may undergo an integer or non-integer number of complete oscillations K between the mirrors before striking a detector. Advantageously, the ion detector detects all ions after they have completed exactly the same number N of reflections between the ion mirrors.The multi-reflection mass spectrometer may constitute the entire multi-reflection electrostatic trap mass spectrometer or a part thereof, as will be described later. In such embodiments of the invention, the detector preferably comprises one or more electrodes arranged to be close to the ion beam as it passes but positioned not to interrupt it, the detection electrodes connected to a sensitive amplifier enabling measurement of the imaging current induced in the detection electrodes.The ion mirrors may comprise elongated ion mirrors of any known type. The ion mirrors are typically electrostatic ion mirrors. The mirrors may be grating mirrors or the mirrors may be gratingless. The mirrors are preferably gridless. The ion mirrors are typically planar ion mirrors, in particular electrostatic planar ion mirrors. In many embodiments, the planar ion mirrors are parallel to each other, for example, over most or all of the length in the drift direction Y. In some embodiments, the ion mirrors may not be parallel (e.g., at their input end closest to the ion injector, as in US 2018 / 0138026 A1) over a short length in the drift direction Y. The mirrors are typically substantially the same length in the drift direction Y. The ion mirrors are preferably separated by a portion of the electric field clearance.The ion-optical mirrors are opposed to each other. "Opposing mirrors" means that the mirrors are oriented such that ions directed into a first mirror are reflected from the first mirror to a second mirror and ions entering the second mirror are reflected from the second mirror to the first mirror. Thus, the opposing mirrors have electric field components directed generally in opposite directions and facing each other.Each mirror is preferably comprised of a plurality of elongated parallel rod electrodes, the electrodes being generally elongated in the direction Y. Mirror structures of this type are known from the prior art, for example as described in SU 172528 A or US 2015 / 0028197 A1. The extended electrodes of the ion mirrors may be provided as mounted metal rods or as metal traces on a printed circuit board base. The extended electrodes may be made of a metal having a low coefficient of thermal expansion, such as invar, so that flight time resists temperature changes in the instrument. The electrode shape of the ion mirrors can be accurately machined or obtained by wire electric discharge machining.The mirror length (total length of the first and second stages) is not specifically limited in the invention, but preferred practical embodiments have a total length in the range of 300 to 500 mm, more preferably 350 to 450 mm.The multi-reflection mass spectrometer comprises two ion mirrors, each mirror being mainly elongated in a direction Y. The extension may be linear (i.e., straight), or the extension may be non-linear (e.g., curved, or comprising a series of small steps to approximate a curve), as described below. The extension shape of each mirror may be the same or different. Preferably, the extension shape of each mirror is the same. Preferably, the mirrors are a symmetrical mirror pair. If the extension is linear, the mirrors may be parallel to each other, although in some embodiments the mirrors may not be parallel to each other.As described herein, the two mirrors are aligned with each other so as to lie in the X-Y plane and so that the longitudinal dimensions of both mirrors are generally in the drift direction Y. The mirrors are spaced apart from each other and face each other in the X direction. The distance or gap between the ion mirrors can be suitably arranged to be constant as a function of drift distance, i.e. as a function of Y, the longitudinal dimension of the mirrors. In this way, the mirrors are arranged parallel to each other. However, in some embodiments, the distance or gap between the mirrors may be arranged to vary depending on the drift distance, i.e., as a function of Y, the longitudinal dimensions of both mirrors will not be exactly in the Y direction, and therefore the mirrors will be described as being generally elongated along the drift direction Y. Thus, "generally elongated along the drift direction Y" may also be understood as "mainly or substantially elongated along the drift direction Y". In some embodiments of the invention, the longitudinal dimension of at least one mirror may be at an angle to the direction Y for at least a portion of its length.Here, the distance between the opposing ion mirrors in the X direction means an effective distance in the X direction between the average reversal points of the ions within the mirrors. An accurate definition of the effective distance W between the mirrors, which generally have a field-free region therebetween, is the product of the average ion velocity in the field-free region and the time period between two successive reversal points which is independent of the mass / charge ratio of the ion. An average reversal point of ions within a mirror herein denotes the maximum point or distance in the + / -X direction within the mirror that the ions that have average kinetic energy and average initial angular divergence characteristics, i.e., the point at which these ions are reversed in the X direction before moving back and forth out of the mirror. Ions with a given kinetic energy in + / -X direction are reversed at an equipotential surface within the mirror. The locus of such points at all positions along the drift direction Y of a particular mirror defines the reversal points for that mirror, and the locus is referred to below as a mean reflection surface. In the specification and claims, reference to the distance between the opposed ion optical mirrors means the distance between the opposed central reflecting surfaces of the mirrors as just defined. In the present invention, the ions, immediately before entering the respective opposing mirrors at a point along the elongate length of the mirrors, have their original kinetic energy in the + / -X direction. The distance between the opposing ion mirrors may therefore also be defined as the distance between opposing equipotential surfaces where the nominal ions (those having the average kinetic energy and the average initial angle of incidence) invert in the X direction, the equipotential surfaces extending along the elongated length of the mirrors.In the present invention, the mechanical construction of the mirrors, even when surface inspected, may appear to maintain a constant distance from each other in X as a function of Y, while the mean reflective surfaces may actually have different distances from each other in X as a function of Y. For example, one or more of the opposing ion mirrors may be formed from conductive traces deposited on an insulating build-up disc (such as a printed circuit board), and the build-up disc of such a mirror may be disposed at a constant distance from an opposing mirror along the entire drift path, while the conductive traces deposited on the build-up disc may be at a non-constant distance from the electrodes in the opposing mirror. Even if electrodes of both mirrors are at a constant distance from each other along the whole drift distance, different electrodes may be biased with different electrical potentials within the one or both mirrors along the drift distance, causing the distance between the opposing central reflecting surfaces of the mirrors to vary along the drift distance. Thus, the distance between the opposing ion optical mirrors in the X direction varies along at least a portion of the length of the mirrors in the drift direction.Preferably, the variation in distance between the opposing ion optical mirrors in the X direction varies uniformly as a function of the drift distance. In some embodiments of the present invention, the variation in distance between the opposing ion mirrors in the X direction varies linearly as a function of drift distance, or in two linear steps, i.e., the distance between the opposing ion optical mirrors in the X direction varies as a first linear function of drift distance for the first span and varies as a second linear function of drift distance for the second span, the first linear function having a higher gradient than the second linear function (i.e., the distance between the opposing ion optical mirrors in the X direction varies more than a function of drift distance for the first linear function than the second). In some embodiments of the present invention, the variation in distance between the opposing ion optical mirrors in the X direction varies nonlinearly as a function of drift distance.The two elongated ion optical mirrors may be similar to each other or may be different. For example, one mirror may comprise a grating while the other may not comprise a grating; one mirror may comprise a curved portion while the other mirror may be straight. Preferably, both mirrors are gridless and similar to each other. Most preferably, the mirrors are gridless and symmetrical.The mirror structures may be contiguous in the drift direction Y, i.e., non-divided, and this eliminates ion beam scattering associated with the stepwise change in electric field in the gaps between such divisions.Advantageously, embodiments of the present invention may be constructed without including additional lenses or diaphragms in the region between opposing ion optical mirrors. However, additional lenses or diaphragms could be used in the present invention to influence the phase space volume of ions within the mass spectrometer and embodiments are designed that include one or more lenses and diaphragms disposed in the space between the mirrors.In some embodiments, the mass spectrometer of the present invention includes one or more compensation electrodes in space between the mirrors to minimize the effects of time-of-flight aberrations caused, for example, by mirror misalignment. The compensation electrodes extend along at least a portion of the drift direction in or adjacent to the space between the mirrors.In some embodiments of the present invention, compensation electrodes having opposing ion optical mirrors elongated generally along the drift direction are used. In some embodiments, the compensation electrodes are used in combination with non-parallel ion mirrors. In some embodiments, the compensation electrodes generate electric field components that oppose ion movement along the +Y direction along at least a length of the ion optical mirrors in the drift direction. These electric field components preferably provide or contribute to a feedback force on the ions as they move along the drift direction.The one or more compensation electrodes may be of any shape and size relative to the mirrors of the multi-reflection mass spectrometer. In preferred embodiments, the one or more compensation electrodes comprise extended surfaces parallel to the X-Y plane facing the ion beam, the electrodes being displaced in the + / -Z direction out of the ion beam flight path, i.e. each one or more electrodes preferably have a surface substantially parallel to the X-Y plane and which, when two such electrodes are present, are preferably located on both sides of a space extending between the opposing mirrors. In a further preferred embodiment, the one or more compensation electrodes are elongated in the Y-direction along a substantial portion of the drift path, each electrode being arranged on a side of the space extending between the opposing mirrors. In this embodiment, preferably the one or more compensation electrodes are elongated in the direction Y along a substantial portion, the substantial portion being at least one or more of: 1 / 10; 1 / 5; 1 / 4; 1 / 3; 1 / 2; 3 / 4 of the total drift distance. In some embodiments, the one or more compensation electrodes comprise two compensation electrodes elongated in the Y direction along a substantial portion of the drift path, the substantial portion being at least one or more of: 1 / 10; 1 / 5; 1 / 4; 1 / 3; 1 / 2; 3 / 4 of the entire drift path, one electrode being displaced out of the ion beam flight path in the +Z direction, the other electrode being displaced out of the ion beam flight path in the -Z direction, and the two electrodes being thereby arranged on both sides of a space extending between the opposing mirrors. However, other geometries are known in the art. The one or more compensation electrodes may be elongated in the direction Y along substantially the first and second length portions in the direction Y (i.e. along the two different mirror convergence stages), or e.g. substantially along only the second length portion. Preferably, the compensation electrodes are electrically biased in use so that the total time of flight of the ions is substantially independent of the angle of incidence of the ions. Since the total drift distance travelled by the ions is dependent on the angle of incidence of the ions, the total time of flight of the ions is substantially independent of the drift distance travelled.Compensation electrodes may be biased with an electrical potential. When a pair of compensation electrodes is used, the same electrical potential may be applied to each electrode of the pair, or different potentials may be applied to the two electrodes. When two electrodes are present, the electrodes are preferably symmetrically disposed on both sides of a space extending between the opposing mirrors, and both electrodes are electrically biased at substantially equal potentials.In some embodiments, in one or more pairs of compensation electrodes, each of the individual electrodes in the pair may be biased at the same electrical potential, and this electrical potential may be zero volts with respect to the potential referred to herein as an analyzer reference potential. Typically, the analyzer reference potential will be ground potential, but it will be understood that the analyzer potential can arbitrarily increase, i.e., the entire analyzer potential can be shifted up or down with respect to ground. Typically, the analyzer reference potential is applied, for example, to a shield such as electrodes used to confine mirrors, and as defined herein, is the potential in the drift space between the opposing ion optical mirrors in the absence of all electrodes other than those comprising the mirrors.In preferred embodiments, two or more pairs of opposing compensation electrodes are provided. In such embodiments, some pairs of compensation electrodes in which each electrode is biased with zero volts are referred to herein as non-biased compensation electrodes, and other pairs of compensation electrodes to which non-zero electrical potentials are applied are referred to herein as biased compensation electrodes. Typically, the non-biased compensation electrodes confine the fields of biased compensation electrodes. In one embodiment, surfaces of at least one pair of compensation electrodes have a profile in the X-Y plane such that the surfaces extend towards each mirror at a greater distance in the regions near one or both end(s) of the mirror than in the central region between the ends. In another embodiment, at least one pair of compensation electrodes have surfaces with a profile in the X-Y plane such that the surfaces extend towards each mirror at a smaller distance in the regions near one or both end(s) of the mirror than in the central region between the ends. In such embodiments, preferably, the pair(s) of compensation electrodes extend along the drift direction Y from a region adjacent to an ion injector at an end of the elongated mirrors, and the compensation electrodes have substantially the same length in the drift direction as the elongated mirrors and are disposed on both sides of a space between the mirrors. In alternative embodiments, the compensation electrode surfaces may be composed of multiple discrete electrodes as just described.Preferably, in all embodiments of the present invention, the compensation electrodes do not comprise ion-optical mirrors in which the ion beam impinges on a potential barrier which is at least as great as the kinetic energy of the ions in the drift direction. However, as has already been stated and will be described below, they preferably generate components of an electric field which counteract the ion movement along the direction +Y along at least one length section of the ion-optical mirrors in the drift direction.Preferably, the one or more compensation electrodes are electrically biased in use to compensate for at least a portion of the time-of-flight aberrations generated by the opposing mirrors. If more than one compensation electrode is present, the compensation electrodes may be biased with the same electrical potential or they may be biased with different electrical potentials. If more than one compensation electrode is present, one or more of the compensation electrodes may be biased with a non-zero electrical potential, while other compensation electrodes may be maintained at another electrical potential, which may be a zero potential. In use, some compensation electrodes may serve the purpose of limiting the spatial extent of the electric field of the other compensation electrodes.In some embodiments, one or more compensation electrodes may include a plate coated with a material having an electrical resistance and to which a different electrical potential is applied at different ends of the plate in the Y direction, thereby creating an electrode whose surface has an electrical potential varying thereover as a function of the drift direction Y. Accordingly, electrically biased compensation electrodes cannot be maintained at a single potential. Preferably, the one or more compensating electrodes are electrically biased in use to compensate for a time-of-flight shift in the drift direction produced by misalignment or manufacturing tolerances of the opposing mirrors and to make the total time-of-flight shift of the system substantially independent of such misalignment or manufacturing.The electrical potentials applied to the compensation electrodes can be kept constant or can be varied over time. Preferably, the potentials applied to the compensation electrodes are held constant in time as ions pass through the multi-reflection mass spectrometer. The electric bias voltage applied to the compensation electrodes may be such as to cause ions flying near a compensation electrode thus biased to decelerate or accelerate, the shapes of the compensation electrodes being correspondingly different, and examples thereof will be described below. As described herein, the term "width" as applied to compensation electrodes refers to the physical extent of the biased compensation electrode in the + / -X direction. It is understood that potentials (i.e. electrical potentials) and electrical fields provided by the ion mirrors and / or potentials and electrical fields provided by the compensation electrodes are present when the ion mirrors and compensation electrodes are electrically biased, respectively.The biased compensation electrodes disposed adjacent or in the space between the ion mirrors may be positioned between two or more non-biased (grounded) electrodes in the X-Y plane, also disposed adjacent the space between the ion mirrors. The shapes of the non-biased electrodes may be complementary to the shape of the biased compensation electrodes.In some preferred embodiments, the space between the opposing optical mirrors in the X-Z plane is open at both ends of the drift path. "open" in the X-Z plane means that the mirrors are not bounded by electrodes in the X-Z plane that fully or substantially bridge the gap between the mirrors.Embodiments of the multi-reflection mass spectrometer according to the present invention may constitute the entire multi-reflection electrostatic trap mass spectrometer or a part thereof. A preferred electrostatic trap mass spectrometer comprises two multiple reflection mass spectrometers arranged end-to-end symmetrically about an X-axis such that their respective drift directions are collinear, the multiple reflection mass spectrometers defining a volume in which, in use, ions follow a closed path having isochronous properties in both the drift directions and an ion flight direction. Such systems are described in US 2015 / 0028197 A1 and illustrated in FIG. 13 of this document, the disclosure of which is hereby incorporated by reference in its entirety (however, where contradictions between statements in the incorporated reference and statements in the present application the present application has priority). Multiple pairs (e.g., four pairs of multi-reflection mass spectrometers arranged end-to-end) of strip-shaped detection electrodes can be used for reading an induced current signal at each passage of the ions between the mirrors. The electrodes in each pair are symmetrically separated in the direction Z and may be disposed in the planes of compensation electrodes or closer to the ion beam. The electrode pairs are connected to the direct input of a differential amplifier and the electrode pairs are connected to the inverse input of the differential amplifier, thereby providing a signal of the induced differential current that advantageously reduces noise. To obtain the mass spectrum, the induced current signal is processed in known ways using Fourier transform algorithms or a specialized comb sampling algorithm as described by J.B. Greenwood et al. in Rev. Sci. Instr. 82, 043103 (2011).The multi-reflection mass spectrometer of the present invention may constitute the entire multi-reflection time-of-flight mass spectrometer or a part thereof.A composite mass spectrometer may be formed comprising two or more multi-reflection mass spectrometers according to the invention oriented such that the X-Y planes of each mass spectrometer are parallel and optionally mutually displaced in a perpendicular Z direction, the composite mass spectrometer further comprising ion optical means for directing ions from one multi-reflection mass spectrometer to another. In such an embodiment of a composite mass spectrometer, a group of multi-reflection mass spectrometers are stacked on top of each other in the direction Z, and ions are passed from a first multi-reflection mass spectrometer in the stack to further multi-reflection mass spectrometers by means of deflection means such as electrostatic electrode deflectors, providing an extended flight path composite mass spectrometer in which ions follow the same path no more than once, so that TOF analysis is enabled over the whole mass range, since there is no overlap of ions. Such systems are described in US 2015 / 0028197 A1 and depicted in FIG. 14 of this document. In another such embodiment of a composite mass spectrometer, a group of multi-reflection mass spectrometers are arranged to lie in the same X-Y plane and ions are passed from a first multi-reflection mass spectrometer to further multi-reflection mass spectrometers by means of deflection means such as electrostatic electrode deflectors, providing an extended flight path composite mass spectrometer in which ions follow the same path no more than once, thus enabling TOF analysis over the whole mass range, since there is no overlap of ions. Other arrangements of multiple reflection mass spectrometers are planned, in which some of the spectrometers lie in the same X-Y plane and others are displaced in the perpendicular direction Z, with ion optical means adapted to pass ions from one spectrometer to another, providing an extended flight path compound mass spectrometer in which ions follow the same path no more than once. Preferably, when some spectrometers are stacked in the Z direction, the spectrometers have alternating orientations of the drift directions to avoid the need for deflection means in the drift direction.Alternatively, embodiments of the present invention may be used with further beam deflection means arranged to reverse ions once or more times and to return them back through the multi-reflection mass spectrometer or composite mass spectrometers and thereby multiply the flight path length, but at the expense of mass range.Analysis systems for MS / MS can be provided using the present invention, which include a multi-reflection mass spectrometer and an ion injector including an ion trap device upstream of the mass spectrometer, and a pulsed ion gate, a high energy collision cell, and a time of flight analyzer downstream of the mass spectrometer. Such systems are described in US 2015 / 0028197 A1 and depicted in Fig. 15 of that document. Moreover, the same analyzer could be used for both or more such analysis stages, thereby providing the capability of MS n by configuring the collision cell so that ions exiting the collision cell are directed back into the ion trap device.As a result of time-of-flight focusing in both the X-direction and the Y-direction, the ions arrive at substantially the same coordinates in the Y-direction at the detector after a designated number of oscillations between the mirrors in the X-direction. This achieves spatial focusing on the detector and the construction of the mass spectrometer is greatly simplified.DESCRIPTION OF THE FIGURESFIG. 1 schematically shows an embodiment according to the prior art. FIG. 2 schematically shows a further embodiment according to the prior art. FIG. 3 schematically shows a further embodiment according to the prior art.FIGS. 4A and 4B schematically show still further embodiments according to the prior art. Figure 5 schematically shows a multi-reflection mass spectrometer according to an embodiment of the present invention. FIG. 6 schematically shows an ion mirror electrode configuration and applied voltages. FIG. 7 shows schematically shaped drift focusing lenses with circular (A) and elliptical (B) shapes and a lens integrated into a prism-shaped deflector (C). FIG. 8 schematically shows alternative structures for drift focusing lenses. Figure 9 schematically shows an embodiment of an extraction ion trap. FIG. 10 schematically shows an embodiment of an injection optics scheme. Fig. 11 schematically shows a multi-reflection mass spectrometer according to an embodiment of the present invention. FIG. 12 shows the simulated time of arrival (A) and the spatial drift distribution (B) of an initial 2 mm wide thermal ion packet at the detector using the system mass spectrometer in FIG. 11. FIG. 13 shows simulated trajectories for an ion beam with a single focusing lens arrangement (A) and a two-lens arrangement (B). FIG. 14 schematically shows an illustration of an ion beam width δx as ions travel along the drift dimension. FIG. 15 shows diagrams illustrating the effects of variation of the initial ion beam width δx 0, drift distance (D L) and mirror separation (W) on the achievable ion flight path length. FIG. 16 schematically illustrates an embodiment of a multi-reflection ToF configuration with a reverse deflector to restore the ion beam to a drift null position. FIG. 17 shows ion trajectories near the end of a mass analyzer with a drift reversal deflector and a focusing lens positioned a reflection before the reversal deflector. FIG. 18 shows simulated ion trajectories with thermal drift divergence through a full analyzer that includes first and second deflectors for reducing the initial drift energy and a third deflector for reversing the ion drift back to a detector with minimized time aberration. FIG. 19 shows ion trajectories near the end of a mass analyzer with a drift reversal deflector for reversing ion trajectories through two passes through the deflector, the deflector including a converging lens for minimizing time-of-flight aberrations. Figure 20 schematically shows an embodiment with mirror convergence and divergence to maximize the number of oscillations in mirror space and beam divergence at the detector. Figure 21 shows simulated ion trajectories with different source position and energy, showing that the return position correlates with the starting position.Detailed DescriptionVarious 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 limit the scope of the invention. It is to be understood that changes may be made to the embodiments, but still fall within the scope of the invention as defined by the claims.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), not shown, are collected in a pulsed ion injector, in this embodiment in the form of the ion trap 4. An ion beam 5 is formed by extracting a packet of trapped thermalized ions, for example, having a width of less than 0.5 mm in the drift direction Y, from the linear ion trap 4 and injecting it at high energy (4 kV in this embodiment) into the space between two opposing parallel mirrors 6, 8 by applying an appropriate accelerating / extracting voltage to the electrodes of the ion trap 4 (e.g., pull / push electrodes). Ions exit the ion trap via the slit 10 in the ion trap 4 The ion beam enters the first mirror 6 and is focused in the out-of-plane dimension by a lens carried out by the first electrode pair 6a of the mirror 6 and reflected by the remaining electrodes 6b-6e of the mirror to a time-focusing point. In this example, the available space between the 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 analyzer (i.e., the effective distance in direction X between the average reversal points of the ions within the mirrors) is -650 mm. The total length (i.e., in the direction Y) is 550 mm to form a relatively compact analyzer.Suitable ion mirrors such as 6 and 8, for example, are well understood from the prior art (e.g. U.S. Pat. No. 9,136,101 B2). An exemplary configuration of an ion mirror such as that shown in FIG. 5 is a mirror that includes multiple pairs of elongated electrodes spaced apart in the X direction, such as five pairs of elongated electrodes, with the first pair of electrodes (6a, 8a) of the mirror set at ground potential. In each pair, an electrode is located above the ion beam and an electrode is located below the beam (shown in the Z direction). An example of voltages for the electrode set (6a-6e, 8a-8e) to provide a reflection potential with a time focusing point for ions is shown in Figure 6, where the applied voltages are suitable for focusing 4 keV positive ions. For negative ions, the polarities may be reversed.After the first reflection in the first ion mirror 6, the ion beam expands substantially to a width of approximately 8 mm under thermal drift in the drift direction and impinges on 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 arranged 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 transaxial lens including a pair of opposed lens electrodes positioned on both sides of the beam in a direction Z (perpendicular to the directions X and Y). More specifically, the drift focusing lens 12 includes a pair of quasi-elliptical plates 12 a, 12 barranged 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 -100 V applied. In some embodiments, the pair of opposing lens electrodes may include circular, elliptical, quasi-elliptical, or arc-shaped electrodes. The drift focusing lens 12 convergingly acts on the ion beam by reducing an angular distribution of the ions in the drift direction Y.After focusing by the focusing lens 12, the ion beam 5 undergoes several more reflections between the ion mirrors in the direction X while drifting along the drift direction Y to follow a zig zag ion path in the X-Y plane between the ion mirrors (there are N total specular reflections in the system). After completion of N reflections (i.e., N / 2 "oscillations" in which an oscillation is equal to twice the distance between successive reflections in the direction X), the ions are detected by an ion detector 14 to detect the time of flight of the ions. A data acquisition system comprising a processor (not shown) is connected to the detector and enables the generation of a mass spectrum. In the illustrated embodiment, the ions 22 pass through reflections (N=22) resulting in an overall flight path of more than 10 meters. The detector is preferably a fast time response detector, such as a multichannel plate (MCP) or dynode electron multiplier with magnetic and electric fields for electron focusing.Important factors for the positioning of the drift focusing lens 12 have been determined. First, the ion beam should preferably have expanded to such an extent that, until reaching the focusing lens, the effect of the lens on the drift energy or the angular distribution is maximized relative to its effect on the spatial distribution. This means that the ion beam must be able to expand before it reaches the drift focusing lens. Therefore, it is preferable to position the lens in the ion mirror 6 after the first reflection (unless the mirror distance is very large, e.g., 500 mm). Second, for injection of an ion beam at an angle of inclination of 2 degrees to the direction X into a mass spectrometer system of this size, the reflections of the central ion trajectory (i.e., the center of the ion beam) are separated by less than 25 mm, and it is important that the focusing lens is not so large as to interfere with adjacent ion trajectories. Without drift focusing, the ion beam would already be 20 mm wide due to the third reflection and would almost begin to overlap with those of other reflections in the fourth reflection flight path. The optimum position for the drift focusing lens is therefore preferably positioned relatively early 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 optimal position for the drift focusing lens is preferably before reflection with a number of less than 0.25 N or less than 0.2 N. The optimal position for the drift focusing lens is preferably after the first reflection, but before the second or third reflection (in particular before the second).The concept of placing button-shaped electrodes (e.g., circular, oval, elliptical, or quasi-elliptical) above and below the ion beam to produce drift focusing in a multiturn ToF instrument, although constructed periodically and within an orbital geometry, is described in US 2014 / 175274 A1, the contents of which are incorporated herein by reference in their entirety. Such lenses are one 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 are incorporated herein by reference in their entirety). Such lenses have the advantage of wide spatial acceptance, which is important to control such an elongated ion beam. The lenses must be wide enough to both receive the ion beam and not interfere with the focusing characteristics by the 3D field disturbance from the sides of the lens. The distance between the lenses should also be a compromise between minimizing these 3D disturbances and recording the beam height. In practice, a distance of 4-8 mm may be sufficient.Variation of the lens curvature from a circular (button) lens to a narrow ellipsoidal lens is possible. A quasi-elliptical structure taking a short arc reduces time-of-flight aberrations compared to a wider arc or full circle because the path through it is shorter, but it requires more stress and begins to induce a considerable out-of-plane lens effect at extreme values. This effect can be made useful for a combination of off-plane drift control and dispersion in a single lens, but limits the control range over each property. In addition, regions in which strong fields are already applied, such as the region of ion extraction at the ion trap 4, can be used via the curvature of the ion trap pull / push electrodes in order either to induce or to limit the drift divergence of the ion beam. An example of this is the commercially available curved linear ion trap (C-trap) described in document US 2011 / 284737 A1, the content of which is incorporated in its entirety by reference into this document, wherein an elongated ion beam is focused on a point to facilitate injection into a Orbitrap™ mass analyzer.Figure 7 shows various embodiments (A, B) of drift focusing lenses comprising circular 20 and quasi-elliptical 22 lens plates (electrodes) and grounded enclosing electrodes 24 for each plate. The lens electrodes 20, 22 are insulated from the grounded enclosing electrodes 24. Also shown (C) is the integration of a lens 22 (in this case the quasi-elliptical shape, but could also be circular, etc.) into a deflector, which in this embodiment comprises a trapezoidal prismatic electrode structure 26 arranged above and below the ion beam serving as a deflector by presenting a constant field angle to the incoming ions instead of a curve. The deflector structure includes a trapezoidal or prismatic electrode disposed above the ion beam and another trapezoidal or prismatic electrode disposed below the ion beam. The lens electrodes 22 are insulated from the deflector, i.e., trapezoidal prismatic electrodes in which they are disposed, which in turn are insulated from the grounded enclosing electrodes 24. Placement of the lens within a wide spatial acceptance deflector structure is a more space saving design. Further possible embodiments of a suitable lens are illustrated in Figure 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 simulate the field curvature generated by shaped electrodes; a multipolar rod array (B) for generating a quadrupole or pseudo-quadrupole field, such as an illustrated 12 rod-based lens with pseudo-quadrupole configuration with relative rod voltages (V) and a diaphragm-based lens, such as a single lens structure with normal diaphragm (C). Such embodiments of the drift focusing lens, e.g. as shown in Figures 7 and 8, may be applicable to all embodiments of the multi-reflection mass spectrometer.An extraction ion trap 40 suitable for use as the ion trap 4 is illustrated in Figure 9. This is a linear quadrupole ion trap that can receive ions generated by an ion source (not shown) and supplied by an ion optical arrangement (e.g., consisting of one or more ion guides and the like), as is also well understood in the art. The ion trap 4 consists of a multipolar (quadruple) electrode set. The inscribed radius is 2 mm. Ions are radially constrained by opposing RF voltages (1000V at 4 MHz) applied to the respective opposing pairs 41, 42 and 44, 44 / of multipolar electrodes; and axially constrained by a small DC voltage (+5V) across the DC-gated electrodes (46, 48). The ions introduced into the ion trap 4 are thermalized by collision cooling with the background gas present in the ion trap (<5×10 -3 mbar). Prior to extraction of the cooled ions into the ion levels of the mass analyser, the trap potential is raised to 4kV and an extraction field is then applied by applying -1000V to the pull electrode 42 and +1000V to the push electrode (41), whereby positive ions are ejected into the analyser through a slot (47) in the pull electrode in the direction indicated by the arrow A. Alternatively, the illustrated rectilinear quadrupole ion trap could be replaced by a curved linear ion trap (C-trap).In addition to the ion trap 4, 40, a plurality of further ion-optical elements are preferably used for controlling the ion injection into the analyzer ("injection optics"). This ion injection optics can be considered part of the ion focusing arrangement. First, it is advantageous 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) on the path between the ion trap 4 and the first mirror 6. Such out-of-plane focusing lenses may include elongated apertures which enhance the transmission of ions into the mirror. Second, a portion, e.g., half, of the injection angle of the ion beam to the X direction when entering the mirror may be provided by the angle of the ion trap in the X direction, and the rest, e.g., the other half, may be provided by at least one deflector (a so-called injection deflector) disposed in front of the ion trap. The injection deflector is generally positioned in the ion mirrors prior to the first reflection. The injection deflector may include at least one injection deflector electrode (e.g., a pair of electrodes positioned above and below the ion beam). In this manner, the isochronous plane of the ions is correctly aligned with the analyzer, rather than being offset by 2 degrees with corresponding time-of-flight errors. Such a method is described in U.S. Pat. No. 9,136,101 B2. The injection deflector may be a prism 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 adjusting the injection angle may be provided in addition to a deflector (e.g., prism type) that may be mounted after the first reflection in the ion mirror with or adjacent to the drift focusing lens 12. In some embodiments, all or a large portion of the injection angle may be provided by an injection deflector. Moreover, it should be appreciated that more than one injection deflector (e.g., in series) may be used to achieve a required injection angle (i.e., it will be appreciated that the system may include at least one injection deflector electrode, optionally two or more injection deflector electrodes). An exemplary embodiment of an injection optics scheme is schematically illustrated in FIG. 10, along with suitable applied voltages. The ion trap 4 is a linear ion trap to which the above described +1000 V push and -1000 V pull voltages are applied to the 4 kV trap to extract the ion beam. The beam then passes successively through the ion optics comprising a ground electrode 52, a first lens 54 maintained at +1800V, a prism type integrated elliptic lens deflector 56 (+750V), a second lens 58 maintained at +1200V, and finally a ground electrode 60. The first and second lenses 54, 58 are aperture lenses (rectangular single lenses) for providing out-of-plane focus. The deflector 56 provides an angle of inclination of the ion beam to the X-axis and the integrated elliptical lens may provide controlled ion beam divergence in the drift direction Y.It has been found that this additional drift focusing lens mounted and divergently operated between the extraction ion trap 4 (or optionally integrated into the ion trap itself, for example using a curved pull / adjacent ground electrode) and the first reflection is advantageous because it allows control of ion beam divergence before the beam reaches the convergent lens 12. More advantageously, the additional drift focusing lens mounted between the extraction ion trap 4 and the first reflection, as described above and illustrated in the injection optics scheme in FIG. 10, may be mounted in an injection deflector. Thus, in certain embodiments, the ion focusing arrangement may include a first drift focusing lens positioned in the ion mirrors prior to the first reflection to focus 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 in the ion mirrors after the first reflection to focus the ion beam in the drift direction Y, wherein the second drift focusing lens is a converging lens. The diverging drift focusing lens may be constructed like the converging lens, for example, as a transaxial lens having a circular, elliptical, or quasi-elliptical shape as shown in FIG. 7, or as one of the other types of lenses shown in FIG. 8. However, the diverging drift focusing lens has a different voltage applied to the converging drift focusing lens and acts on a different width of the ion beam to impart different focusing characteristics to the converging drift focusing lens.It is preferable that the converging drift focusing lens 12 mounted after the first reflection also includes an ion deflector, for example, the prism type (embodiment C) illustrated in FIG. 7. This deflector can be tuned to adjust the injection angle to a desired level and / or to correct for the beam deflection caused by mechanical deviations in the mirrors. Moreover, errors in mirror manufacture or mounting at each reflection can cause a small time of flight error because ions on one side of the beam experience a shorter flight path than on the other, and these can preferably be corrected by adding two compensation electrodes in the space between the mirrors as described above.In US 9,136,101 B2, elongated electrodes (referred to herein as "compensation electrodes") having a low voltage (e.g., -20 V) are used to correct the time-of-flight error caused by the many hundred microns mirror convergence. Similar electrodes following linear or curved or even complex functions may be used in the present invention to correct for small displacements or curvatures of the mirror electrodes. One or more sets of compensation electrodes may be used, each set comprising a pair of elongate electrodes - an electrode disposed above the ion beam and an electrode disposed below the ion beam. The sets of compensation electrodes preferably extend over most of the length of the ion mirrors in the drift direction Y. While such compensation electrodes may be considered for many error functions, the primary mechanical errors are likely to be non-parallelism of the mirror electrodes and curvature about the center, so that 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., a set having a profile in the X-Y plane following a linear function and a set having a profile in the X-Y plane following a curved function. The two compensation electrode sets 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 can correct mirror tilt or misalignment when biased. A set having a profile in the X-Y plane following a curved function can correct the mirror curvature when biased. The only drawback is that such compensation electrodes may contribute to an undesired deflection of the ion beam, which may then be corrected by a corresponding voltage on the deflector, i.e. the deflector positioned between the mirrors after the first reflection.An example of a preferred embodiment comprising ion injection optics, drift focusing lenses and deflectors and compensation electrodes is schematically illustrated in Figure 11. This embodiment shows the simulated trajectories 65 of the ions comprising the typical range of thermal energies. An extraction ion trap 4 is shown for injecting an ion beam represented by ion paths 65 between parallel elongated ion mirrors 6 and 8 of the type shown in Figures 5 and 6. The ion beam is injected generally in the X direction but at a small inclination angle of 2 degrees to the X-axis direction, i.e., with a velocity component in the drift direction Y. In this manner, a zig zag trajectory path is achieved through the analyzer. The ion beam first passes through an injection optics, the injection optics comprising an out-of-plane focusing lens 64, a prism type deflector 66 of the type described above having an integrated elliptical drift focusing lens 67 mounted therein, and a second out-of-plane focusing lens 68. 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 moves toward the first mirror 6. The drift focusing lens 67 may provide further desired divergence. The ions pass through the first of N reflections in the first mirror 6 and are thereby reflected back to the second ion mirror 8. The diverging ion beam impinges on a drift focusing lens 72, The drift focusing lens 72 in this embodiment is arranged 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 a drift focusing elliptical lens as described above mounted in a deflector 76 of the prism type described above. 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 distribution 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 halfway between the first reflection and the reflection N. Thus, the ion beam passes through a single minimum, preferably substantially halfway along the ion path between the ion focusing lens 72 and the detector 74. Two sets of compensation electrodes 78 (a set of curved shape 78 / and a set of linear shape 78 / / ) are provided in the illustrated embodiment to correct for unwanted beam deflections of the ion beam during the zig-zag path caused, for example, by mechanical or alignment deviations or unwanted curvature in the mirror structure. The two sets of compensation electrodes 78 are positioned side by side, although not in electrical contact, i.e., the sets are displaced in the direction X relative to each other. The set of curved compensation electrodes 78 / includes a pair of elongated electrodes having a curved profile in the X-Y plane, an electrode above the ion beam, and an electrode below the ion beam. The set of linear shaped compensation electrodes 78 / / includes a pair of elongated electrodes having a linear profile in the X-Y plane, an electrode above the ion beam, and an electrode below the ion beam. In Figure 11, for each set of compensation electrodes 78 / and 78 / / only one electrode of the pair is visible, since the other electrode of the pair is located directly below that illustrated. After N reflections between the two ion mirrors 6, 8, the ions are detected by the detector 74. Advantageously, due to the focusing characteristics of the drift focusing lens 72, wherein 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 completion of the exactly same number N of reflections between the ion mirrors, i.e., no "harmonics" are detected. Moreover, the detection of all ions after the completion of the exactly equal number N of reflections can be achieved with a single focusing lens (converging lens) arranged early in the reflection system, e.g. after the first but before the fourth, third or second reflection, or with a pair of focusing lenses (a diverging lens arranged upstream of the converging lens). FIG. 12 shows simulated ion peaks in time (A) and drift space (B) in the detector plane, which were formed by a representative ion packet of m / z=195, for the device configuration illustrated in FIG. 11. It can be seen that, in addition to maintaining good drift focusing, the build-up of time-of-flight aberrations is limited, which leads to a resolution performance of more than 100,000. In some embodiments, it may be advantageous to include additional lenses along the ion path. The form of multi-reflection ToF spectrometer shown in Figure 11 has the advantage of a good tolerance to mechanical errors in the mounting and alignment of the mirrors, since the resulting wide deflection to the ion path can easily be corrected by adjusting the deflector and / or compensation electrode voltage.It has been found that the presence of a diverging lens located just after the ion injector (ion trap), preferably between the ion injector and the first reflection, is advantageous to optimize 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. A strong voltage is preferably applied to the diverging lens because the beam is initially very narrow. In the above-described embodiments with reference to FIGS. 5, 6, and 11, it was found that a voltage of +750 V optimizes the ion beam expansion to the second focusing lens positioned after the first reflection, to which -125 V was applied. To illustrate this, FIG. 13 shows the extension of a thermal ion beam 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 the single lens configuration (A), the converging lens 92 is a drift focusing elliptical lens as described above mounted in a deflector 96 of the prism type described above. Prior to the first reflection, a first deflector 86 is provided to adjust the injection tilt angle, but there is no diverging lens. In the two lens configuration (B), the system is the same except that prior to the first reflection, a diverging drift focusing lens 87 is provided, where the lens 87 is an elliptical drift focusing lens mounted within the prism deflector 86. It can be seen that in the single lens case (A), the ion reflections eventually begin to overlap along the central axis, since the initial beam width of 2 mm is too large, but not in the two lens configuration (B). Thus, the two lens configuration allows for the use of a greater number of total reflections N. In some embodiments, it may be possible to place both diverging and converging lenses in the ion mirrors prior to the first reflection, however, such arrangements are far less advantageous due to the constraints of the initial beamwidth and phase volume as well as the required lens voltage.The difficulty of collimating an ion beam with lenses results from ions first having independent distributions of space and energy. A lens that controls expansion due to the initial ion energy distribution induces convergence from the initial spatial distribution. This cannot be eliminated, but it can be minimized by allowing (or inducing) a large extension in beam width. Since complete collimation is not possible, it has been found that the presence of a small convergence of the ion beam after the focusing lens is preferable. To maximize the length of the ion beam path, the ion beam spatially distributed in the drift direction passes a single minimum at a point midway between the converging drift focusing lens and the detector. After the minimum, the ion beam then begins to diverge until the ion beam impinges on the detector plane, with a similar spatial distribution as the beam at the drift focusing lens. The focusing system is schematically shown in Fig. 14. The ion injector 104, where ions have an initial spatial distribution dx i in the drift direction, injects ions into the converging drift focusing lens 106 located between the ion mirrors (e.g., between first and second reflections). The ions diverge in the beam expanding region a defined between the ion injector 104 and the drift focusing lens 106. The ion beam reaches its maximum spatial distribution 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 to its focal minimum (minimum spatial extent) or its waist in the drift direction Y at position f via the converging region b. The focal minimum at position f is approximately at a distance midway between the drift focusing lens 106 and the detector 114. After the focal minimum f, the ion beam diverges again over the diverging region c until it reaches the detector 114, where the ion beam reaches its maximum spatial distribution dx[0] in the drift direction Y again.An optimized analytical solution will now be 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 in Figures 5, 6, 11 and 13, the total flight length is L = K × L 0, where K is the number of oscillations between the mirrors and L 0 is the length of a single oscillation, the latter being about twice as great as the distance between the mirrors, W. The value K corresponds to half the total number of reflections (N), That is, K=N / 2. the drift step per one oscillation is: where θ is the injection angle (the angle of the ion beam to direction X as it enters the mirrors and thus reflects between the mirrors, with about 2 degrees being typical). Accordingly, the number of oscillations on the entire drift distance D is L:This can be increased by the selection of a smaller injection angle, which leads to a smaller drift step Δ D. The drift step nevertheless has a lower limit Δ D(min), which is determined by a minimum separation between adjacent oscillations.The phase volume of the ion beam in the drift direction is referred to as π. Since the phase volume is constant along a trajectory following the Liouville Theorem, Π is determined by the ion injector and cannot be changed by collimating optics. However, such optics can be used to "prepare" the ion beam prior to injection into the analyzer by adjusting the optimum ratio between the spatial and angular distributions and the optimum correlation.There is a minimum of the spatial distribution δx 0 of the ion beam to the oscillation k 0. Since there are no optical elements for collimating the ion trajectories in the drift direction between the first and last oscillations, the angular distribution δα remains constant and the spatial distribution at each oscillation is k:The optimization objective is to maximize the total flight length with respect to Δ D and the phase distribution of the ion beam, the optimum being subject to the following constraints: 1) The spatial distribution in the first oscillation is δx[0]≤Δ D / 2, to avoid overlap between the ion beam after the first reflection and the ion source (or collimator) 2) The spatial distribution after the last oscillation is δx[K]≤Δ D / 2, to avoid overlap between the ion beam in the next to last (K-1) oscillation and the ion detector 3) The phase volume in the drift direction is δx 0 δα=π.It is easily seen that the optimum position of the "sluff" of the ion beam (the minimum spatial distribution) δx 0 lies on the mean oscillation k 0= K / 2, which results:In the optimum case, the inequality turns into equality, and the optimum value of the angle distribution for maximizing the number of oscillations K is given by the equation dK=0As an example of a 1 mm wide (in the Y direction) ion cloud at the ion injector, with an appropriate distance between the mirrors and drift distance given by W and D L:The value 0.025 eV is the (thermal) energy distribution of the ions and 4000 eV is the ion acceleration voltage.The total flight length is given by:In the example, it can be seen that the spatial distribution at the first oscillation δx[0] and the spatial distribution after the last oscillation δx[K] have a value of 7.6 mm, which corresponds to approximately √2 times the minimum spatial distribution in the system δx 05,45 mm. Generally, the converging lens preferably focuses the ions such that the spatial distribution 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 the minimum spatial distribution.To provide an optimized system, it follows that when the ion beam undergoes K oscillations between the ion mirrors from the ion injector to the ion detector, K preferably has a value in a range of + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around the above optimal value, K(opt), given by:Similarly, the angular distribution of the ion beam δα after focusing by the drift focusing arrangement is preferably in a range that is + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around the above-indicated optimal value, δα (opt), given by:FIG. 15 shows plots illustrating the effects of variation of the initial ion beam width δx 0, ( mirror) drift distance (D L) and mirror separation (W) on the achievable flight path length based on this analytical approach. It will be appreciated that very long flight paths can be achieved with relatively practical mirror arrangements (for example, 1.5 metres in length and 2 metres in width can result in a flight path of 60 metres). The diagrams show (A) the variation of the flight path length with mirror distance W (base 1000 mm) and (B) the variation of the flight path length with drift distance D L( base 500 mm), respectively for different initial ion population widths δx 0(1 mm, 2 mm and 4 mm).In a further embodiment, as long as the ion beam remains relatively well focused, it is possible to place a deflector or deflector / drift focusing lens combination (as described above) or other beam direction control means at the distal (more remote) end of the mirrors from the end at which the ion injector is located to reverse the drift speed of the ion beam. Hereinafter, such deflectors are referred to as end or reverse deflectors. This results in the reflection of the ions back to the beginning end of the mirrors where a detector can be placed. This allows multiplication (e.g., doubling) of the time of flight of the ions. It may also be possible, in some embodiments, to have a deflector in the mirrors on one side to reverse the beam again for multiplication of the time of flight of the ions. Such end or reversal deflectors preferably have broad spatial acceptance and operate isochronously. Another consideration is that positioning the detector near the ion injector introduces space constraints. One problem avoidance disclosed in U.S. Pat. No. 9,136,101 B2 is to inject ions at a large injection angle to improve the distance and then use a deflector that is located after the first reflection to reduce this injection angle. Another possible solution to the problem of space and injection angle is disclosed in US 7,326,925 B2 which uses sectors to perform ion injection at a small angle and optionally extraction to a detector. A further possible solution is to increase the distance of the ion mirrors.An embodiment of a system using a reversal deflector at the distal end is shown in Figure 16. However, this embodiment is less preferred because the time aberrations from these two deflectors become detrimental to the resolution performance. The ion injector 204 located at Y=0 injects ions and the 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 at a focal minimum halfway along the ion path as described above. After N / 2 reflections along a zig-zag flight path, where N is the total number of reflections ions experience in the system, the drift velocity of the ion beam along Y is reversed by a reverse deflector 208 at the end of mirrors 6, 8 remote from ion injector 204. The deflector 208 is a trapezoidal prismatic type as described above. This results in a reflection of the ions back to the beginning end of the mirrors, which ions pass through further N / 2 reflections along the zig zag flight path until they reach an ion detector 210 placed near the ion injector 204 at Y=0. Convergence of the ion mirrors at an input portion of their length could be used instead of a deflector to reduce the initial injection angle (e.g., a delay stage as described in US 2018 / 0138026 A1), which in combination with a compensation electrode would completely eliminate the time error of this first deflector. It is also possible to correct a part of the aberration by the deflector adjusting the injection angle with a dipole field placed immediately before the detector as in US 2017 / 0098533 A1.The beam reversal deflector should preferably include a mechanism to minimize time-of-flight aberration across the width of the ion beam. Two methods for reducing this effect will now be described.The first method is to minimize the ion beam width across a focusing lens in the curve prior to beam drift reversal. A lens may be positioned so that ions pass through it before reaching the reverse deflector, preferably a reflection before reaching the reverse deflector. The tension of the lens can be adjusted so that the (relatively wide) ion beam is focused near a point within the reverse deflector, thereby minimizing ToF aberrations. Thus, the lens preferably has a point focus within the reverse deflector. The ion beam may then diverge back to its original width along the drift direction Y while passing through this lens a second time, as shown in FIG. 17. This allows the beam to be collimated for return travel by passing through the lens. Figure 17 schematically shows the beam reflections near the distal end of the mirrors. The forward direction of the ion beam is indicated by arrow F and the reverse direction by arrow R. The reverse deflector 308 is shown positioned at the distal end of the ion mirrors. The trapezoidal or prismatic structure of the electrodes of the return deflector 308 is shown positioned above and below the ion beam. An ion drift focusing lens 316, which in the illustrated embodiment is an elliptically shaped transaxial lens, is positioned a reflection in front of the reverse deflector 308 and causes the ion beam to be focused near a point within the reverse deflector. The ion beam then diverges to its original width on return path R and is collimated by a second pass through lens 316. For example, with the foregoing embodiments remaining, a voltage of +300 V may be applied to the reverse deflector 308 and a voltage of -160 V may be applied to the elliptical lens 316. Figure 18 shows simulated ion trajectories of ions with a thermal divergence of ±3σ moving through a mass analyzer according to the present invention including a reverse deflector. With the correct alignment of the voltages of ion injector, detector and deflector, a resolution of more than 200,000 can be achieved. First and second deflectors (prism deflectors) 406 reduce the initial drift energy of the ions from the injector 404 and the third deflector 408 (reverse prism deflector) returns the ion drift to a detector with minimal time aberration. A preferred system using these components to achieve high resolution is to inject the ions into the analyzer so that they leave the second deflector (i.e. after the first reflection) with a focal plane parallel to the drift direction Y, thereby minimizing any focal plane tilt that could be incompletely corrected on the return of the ions by the second deflector (prism). This can be achieved by a suitable arrangement of the ion source, e.g. by rotating the ion source backwards compared to the previously described embodiments, so that ions with a slightly negative drift (e.g. -1.5 degrees) are ejected from the source and then the drift is changed by applying a high voltage to the first prism deflector (e.g. +375 V). Ions then reach the second prism deflector (e.g., voltage -120 V) which adjusts the injection angle and also aligns the focal plane with the drift axis Y. The drawback of this approach is that ions can reach the detector with a linear focal plane tilt induced by the return flight through the second prism deflector, although this can be compensated for either by a correct orientation of the detector (with the focal plane tilt) or by the provision of a focal plane tilt corrector. Thus, in some embodiments, the ion source may be arranged to emit ions in a negative drift direction (away from the mirrors) and a first ion deflector (generally prior to the first reflection) returns the ions in a positive drift direction. A second ion deflector (generally after the first reflection) may adjust the angle of inclination of the ion beam and / or align the focal plane of the ion beam with the drift direction Y.The second method of minimizing the time-of-flight aberration associated with using a reverse deflector comprises self-correcting the time-of-flight aberration over two passes through the reverse deflector having an integrated or proximate focusing lens (e.g., not separated by a reflection from the deflector). For example, a deflector, such as a prism deflector, that operates at half the voltage required to fully reverse the ions in the drift direction Y (imparting an opposite drift direction velocity) instead reduces the drift velocity of the ions to zero. Thus, as the ions leave the deflector and reach the ion mirror for the next reflection, they are reflected back into the deflector again, whereupon the deflection causes the drift velocity of the ions to change from zero to the reverse drift velocity, and the reversal of the ion trajectory is thereby completed. If a focusing lens is integrated into the deflector, such as a prism deflector, for example as described above and illustrated in FIG. 7C, or simply placed only near the deflector, focusing may be done such that when the ions return to the deflector on the side of the deflector opposite its entrance, the time-of-flight aberration of the deflector for ions passing through the deflector in one direction and the other will cancel. The deflector / lens arrangement is thus self-correcting. However, the return angle should be applied slightly offset from the injection angle so that the beam reaches a detector, for example, rather than simply returning to the ion injector. For example, a slightly lower voltage could be applied to the reverse deflector (to produce slightly less than 100% reflection, e.g., 95% rather than 100% reflection). An example of such a system is schematically illustrated in Fig. 19. Ions moving in the drift direction from the ion injector first enter the reverse deflector 508 from the left as indicated by the arrow A. The deflector 508 is the trapezoidal prism type as shown in the exploded drawing. The voltage applied to the deflector (+150V) is half the voltage applied to effect the complete reversal of the drift velocity, as shown in Figures 17 and 18. This reduces the drift velocity of the ions to substantially zero and the ions enter the mirror (not shown) for the next reflection at zero drift velocity. The deflector has an integrated drift focusing lens 506 (e.g., elliptical shape). As the drift velocity of the ions is reduced to zero by the deflector, they are simultaneously focused at a focus in the mirror (preferably at their inflection point in the mirror). A voltage of -300 V is applied to the lens 506 in this embodiment. After reflection, the ions begin to diverge and enter the deflector a second time, this time from the opposite side of the deflector as indicated by the direction of arrow B. The deflection is thereby re-applied, this time with the effect of completing the reversal of the drift velocity of the ions. Lens 506 simultaneously causes the ion beam to be collimated for return travel.The use of reverse deflectors to reverse the ion beam and double the flight path is known in the art, but this tends to damage resolution. The more isochronous deflection methods presented herein are useful to limit time-of-flight aberrations and to maintain resolution. Both are relatively simple structures. This problem is solved in the prior art either by cancelling the aberration by a mirror tilt which cooperates in combination with a deflector (US 9,136,101 B2) which is mechanically demanding), or by always compressing the ion beam with periodic lenses so that the aberration in the deflection is small (GB 2 403 063 A) but which suffers from a relatively poor space charge performance.Patent application US 2018 / 0138026 A1 describes the use of the curvature of the mirror electrodes along at least a portion of the drift path of the analyzer as a means for controlling the drift velocity and thus maximizing the number of reflections within the confined space of the analyzer. Figure 20 shows a modification of the apparatus of Figure 11 to incorporate this concept. The ion injection system and ion focusing arrangement are the same as described in FIG. 11 (i.e., they include ion injector 904, wherein the ion optics include out-of-plane lens 964, drift focusing lens integrated deflector 966 967 the second out-of-plane lens 968 and drift focusing lens integrated deflector 976 972. Mirrors 906, 908 first converge along a first portion of their length in the drift direction Y to reduce the ion drift velocity, as described, for example, in US 2018 / 0138026 A1, the contents of which are incorporated herein by reference in their entirety. The first portion of its length is adjacent to the ion injector. The mirrors preferably converge first following a curved function to reduce the drift velocity, although the convergence could be linear, for example. Thereafter, the ion mirrors are parallel (or nearly parallel) to maximize the number of reflections and then diverge to separate the various reflections and maximize the space for detector 974. The mirrors preferably diverge following a curved function, although the divergence could be, for example, linear. The convergence and divergence need not be coincident (symmetrical) and the central region may even be entirely flat (parallel). A set of elongated time-of-flight compensation electrodes 978 (one above and one below the ion beam) having a shape corresponding to 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. At an injection angle of 4 kV ions of 2 degrees, the specular convergence (difference between the widest specular separation and the shortest separation) should be <600 μm to prevent drift reflection of some ions. The more converging and diverging regions preferably have multi-reflections to prevent ion scattering (the deflection remains adiabatic). As described in US 2018 / 0138026 A1, the reduction of the ion drift velocity can be achieved by mirror convergence with flat oblique mirror surfaces instead of smoothly curved mirrors. The use of mirror tilt / divergence to maximize the number of turns within the mirror is obviously advantageous, but at the expense of defocusing the ion beam in the drift dimension. Moderate reductions in drift velocity (~25%) were considered feasible in the simulation before drift focusing became inconsistent even with higher order Gaussian functions. A converging mirror method is disclosed in US 9,136,101 B2, but requires ion reversal and involves placing the detector and ion source in the same space between the mirrors, which is not required in the embodiments described herein. Another method to achieve similar results using convergence / divergence of the distance between the mirrors in the drift direction Y would be to reduce / increase the height of the apertures of the electrodes (the height of the mirror apertures in the Z direction) in the direction / away from the center of the ion mirrors in the drift direction Y. A third possibility would be to disturb the mirror field by applying a disturbing 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, in order to increase the potential (for positive ions) towards the Y-center (towards the center of the ion mirrors in the drift direction Y or the center of the ion beam path) and to decrease it towards the drifting ends (towards the ends of the ion mirrors or towards 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 mounted between the ion mirror electrodes could be used to provide the interference potential (as shown in FIG. 3 of WO 2019 / 030472 A1). The extension of the wedge shape of the electrode changes along the drift direction Y and thus also its interference potential. Alternatively, straight (non-wedge-shaped) additional electrodes could also be used, which provide a disturbing potential that varies along the drift direction Y. A similar form of correction or compensation electrode not disclosed in the prior art would be an electrode extending along the back side of a mirror or each mirror, for example a wedge-shaped electrode, the height of which (and thus voltage disturbance of the reflecting part of the ion mirrors) increases along the drift direction Y. Such electrodes have an disproportionate influence on the time of flight compared to drift, so that they can best be coupled to a functionally appropriate strip-shaped compensation electrode between the mirrors to balance the two properties. However, such electrodes are generally not preferred because the field exponentially penetrates through the back of the mirror, leading to disproportionate effects on high energy ions and, consequently, loss of energy input through the mirror.Multi-reflection mass spectrometers of the present invention can 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 over many points and corresponding mass spectra. Thus, in some embodiments, ions from multiple spatially separated points on a sample may be generated sequentially in an ion source and a mass spectrum may be recorded from each point to obtain an image of the sample. Referring to the system shown in Figure 16 having the deflector of Figure 17, one of its characteristics is that the ion position at the end of the system is closely related to the ion position in the ion source. This shows that a multi-reflection ToF analyzer with a long-range focus lens and a reverse deflector may be suitable for "stigmatic imaging" with an image detector (e.g., a 2D detector array or pixel detector) in which the ion distribution can be imaged within a region along the source surface with a single extraction of ions. Simulated trajectories of ions with variation of the initial spatial and energetic components are illustrated in Figure 21 as they return to a detection plane with an energy focus. The focus is tunable in terms of energy. Ions exit the source plane 1004 from a point and pass through the ion focusing assembly, which includes a first deflector / lens assembly 1006 and a second deflector / lens assembly 1008 of the configuration shown in FIGS. 11 and 16. The initial direction of the ions is represented by arrow A, and the return ion beam after being inverted in the drift direction Y by an inverting deflector (not shown) is represented by arrow B. The ions return to the source plane at a corresponding point where a detector (not shown) can be placed in proximity.The embodiments presented above could also be implemented not only as ultra high resolution ToF instruments, but also as low cost medium performance analyzers. For example, if the ion energy and thus the applied voltages do not exceed a few kilovolts, the entire array of mirrors and / or compensation electrodes could be implemented as a pair of printed circuit boards (PCBs) arranged with their printed surfaces parallel to each other and facing each other, preferably flat and made of FR4 glass-filled epoxy or ceramic, spaced apart from each other by metal spacers and aligned by dowels. PCBs may be bonded or otherwise attached to more resilient material (metal, glass, ceramic, polymer) thereby stiffening the system. Preferably, electrodes on each PCB may be defined by laser cut grooves that provide sufficient isolation from breakdown while not significantly exposing the dielectric interior. Electrical connections may be implemented via the backside that is not opposite the ion beam, and may also integrate resistive voltage dividers or full power supplies.For practical implementations, the extension of the mirrors in the drift direction Y should not be too long to reduce the complexity and cost of the design. Preferably, means are provided for compensating the stray fields, for example using end electrodes (preferably located at a distance of at least 2-3 times the height of the mirror in the direction Z from the closest ion flight path) or end PCBs simulating the potential distribution of infinitely elongate mirrors. In the previous case, electrodes could use the same voltages as the mirror electrodes and could be implemented as flat plates of suitable shape and attached to the mirror electrodes.The spectrometer of the invention may in some embodiments be used as a high resolution mass selection device to select precursor ions having a particular mass / charge ratio for fragmentation and MS2analysis in a second mass spectrometer. For example, in the manner shown in FIG. 15 of U.S. Pat. No. 9,136,101 B2.As used herein, including in the claims, singular forms of the terms in this specification are to be construed to include the plural form as well, and vice versa, unless the context suggests otherwise. For example, unless the context suggests otherwise, a singular reference herein, including in the claims, means "a" or "an", "a / one or more.".Throughout the specification and claims of this specification, the words "comprise," "including," "comprising," and "include," and the variants of the words, for example, "comprising" and "comprises," etc., mean "including, but not limited to," and are not intended to exclude (and not exclude) other components.It is to be understood that changes may be made to the above embodiments of the invention, but still fall within the scope of the invention as defined by the claims. Each feature disclosed herein may be replaced, unless otherwise indicated, by alternative features serving the same, equivalent or similar purpose. Thus, unless otherwise indicated, each disclosed feature represents an example of a generic series of equivalent or similar features.The use of any and all of the examples provided herein, or exemplary language ("for example," "such as," "for example," and the like), is intended merely to better illustrate the invention and is not intended to be limiting as to the scope of the invention unless otherwise claimed. No language wording in the specification shall be construed to indicate any non-claimed element as essential to the practice of the invention.The present invention also relates to the following embodiments: 1. mass spectrometry methods comprising:injecting ions into a space between two ion mirrors spaced apart from each other and opposing each other in a direction X, each mirror being generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X, the ions entering the space at a non-zero angle of inclination to the direction X, the ions thereby forming an ion beam following a zig zag ion path with N reflections between the ion mirrors in the direction X as it drifts along the drift direction Y,focusing the ion beam in the drift direction Y using an ion focusing arrangement located at least partially between the opposing ion mirrors such that a spatial distribution of the ion beam in the drift direction Y passes 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 completion of the same number N of reflections between the ion mirrors, anddetecting ions after the ions have completed the same number N of reflections between the ion mirrors,wherein the ion focusing arrangement comprises a drift focusing lens for focusing the ions in the drift direction, and wherein the drift focusing lens is a converging lens.2. The method of mass spectrometry according to Embodiment 1, wherein focusing is performed such that the spatial distribution of the ion beam in the drift direction in the first reflection is substantially the same as the spatial distribution of the ion beam in the drift direction in the Nth reflection. 3. method of mass spectrometry according to embodiment 1 or 2, wherein the focusing is performed such that the spatial distribution of the ion beam in the drift direction Y passes through a single minimum substantially halfway along the ion path between the ion focusing arrangement and the detector. 4. method of mass spectrometry according to any one of Embodiments 1 to 3, wherein the ion beam K undergoes oscillations between the ion mirrors, and K is a value within a range that + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, K (opt), given by: wherein D L is the drift distance travelled by the ion beam in the drift direction Y, Π is the phase volume, wherein Π = δα i. δx i and δα i is an initial angular distribution, and δx i is an initial spatial distribution of the ion beam, and W is the distance between the ion mirrors in the X direction. 5. method of mass spectrometry according to any one of Embodiments 1 to 4, wherein the angular distribution of the ion beam, δα, after focusing is within a range that is + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, δα (opt), given by: 6. method of mass spectrometry according to any one of Embodiments 1 to 5, wherein the focusing is performed using an ion focusing arrangement arranged before reflection with a number of less than 0.25 N in the ion mirrors. 7. method of mass spectrometry according to any one of Embodiments 1 to 6, wherein an initial spatial distribution of the ion beam in the drift direction Y at an ion injector is δx i, 0,25-10 mm or 0.5-5 mm. 8. The method of mass spectrometry according to any one of Embodiments 1 to 7, wherein the drift focusing lens is positioned after a first reflection in the ion mirrors and before a fifth reflection in the ion mirrors. 9. The method of mass spectrometry according to any one of Embodiments 1 to 8, further comprising deflecting the ion beam using a deflector disposed after a first reflection in the ion mirrors and before a fifth reflection in the ion mirrors. 10. method of mass spectrometry according to any one of embodiments 1 to 9, wherein the ion focusing arrangement comprises a first drift focusing lens positioned in the ion mirrors before the first reflection to focus 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 in the ion mirrors after the first reflection to focus the ion beam in the drift direction Y, wherein the second drift focusing lens is the converging lens. 11.The method of mass spectrometry according to any one of Embodiments 1 to 10, further comprising adjusting the inclination angle to the X direction of the ion beam by deflecting the ion beam using an injection deflector positioned in the ion mirrors before the first reflection. 12.A method of mass spectrometry according to any of embodiments 1 to 11, further comprising applying one or more voltages to each one or more compensation electrodes extending along at least a portion of the drift direction Y in or adjacent to the space between the mirrors to minimize time-of-flight aberrations. 13.The method of mass spectrometry according to any one of Embodiments 1 to 12, further comprising deflecting the ion beam using a reverse deflector at an end of the ion mirrors remote from the injection to reduce or reverse the drift velocity of the ions in the direction Y. 14. The method of mass spectrometry according to Embodiment 13, further comprising focusing the ion beam to a focal minimum within the reversal deflector. 15. The mass spectrometry method according to Embodiment 13, further comprising providing a focusing lens within the reverse deflector, and focusing the ion beam to a focal minimum within one of the ion mirrors at the next reflection after the reverse deflector. 16. The method of mass spectrometry according to any one of embodiments 1 to 12, wherein the detecting comprises forming a 2D image of an ion source.

Claims

A multi-reflection mass spectrometer (2) comprising: two ion mirrors (6, 8, 906, 908) spaced apart from each other and opposing each other in a direction X, each mirror being generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X; a pulsed ion injector (4, 40, 104, 204, 904) for injecting ion pulses into the space between the ion mirrors (6, 8, 906, 908), the ions entering the space at a non-zero angle of inclination to the direction X, the ions thereby forming an ion beam (5) following a zig zag ion path with N reflections between the ion mirrors (6, 8) in the direction X as it drifts along the drift direction Y; a detector (14, 74, 114, 210, 210, 974) for detecting ions after completion of the same number N of reflections between the ion mirrors, and an ion focusing arrangement arranged at least partially between the opposing ion mirrors and configured to provide the focusing of the ion beam in the drift direction Y such that a spatial distribution 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 completion of the same number N of reflections between the ion mirrors, wherein the ion focusing arrangement comprises a drift focusing lens (12, 72, 92, 106, 972) for focusing the ions in the drift direction Y, and wherein the drift focusing lens is a converging lens.The multi-reflection mass spectrometer according to claim 1, wherein the spatial distribution of the ion beam (5) in the drift direction Y in the first reflection is substantially the same as the spatial distribution of the ion beam in the drift direction Y in the Nth reflection.A multi-reflection mass spectrometer as claimed in claim 1 or 2, wherein the spatial distribution of the ion beam (5) in the drift direction Y is through a single minimum substantially halfway along the ion path between the ion focusing arrangement and the detector (14, 74, 114, 210, 974).A multi-reflection mass spectrometer as claimed in any preceding claim, wherein the ion focusing arrangement comprises a pair of drift focusing lenses for focusing the ions in the drift direction Y.The multi-reflection mass spectrometer of claim 4, wherein the converging lens focuses the ions such that the spatial distribution of the ion beam in the drift direction Y has a maximum at the converging lens (12, 72, 92, 106, 972) that is 1.2-1.6 times or about 2 - flat of the spatial distribution at the minimum.A multi-reflection mass spectrometer as claimed in claim 5, wherein the spatial distribution of the ion beam in the drift direction Y has a maximum at the converging lens (12, 72, 92, 106, 972) which is in the range of 2 to 20 times the initial spatial distribution of the ion beam (5) in the drift direction Y at the ion injector.A multi-reflection mass spectrometer as claimed in any preceding claim, wherein the ion beam undergoes K oscillations between the ion mirrors (6, 8, 906, 908) from the ion injector (4, 40, 104, 204, 904) to the ion detector (14, 74, 114, 210, 974), and K is a value within a range that is + / - 50% or + / - 40% or + / - 30% or + / - 20% or + / - 10% around an optimum value, K (opt), given by: K ( o p t ) = ( D L 2 4 Π W ) 1 / 3 wherein D L is the drift distance travelled by the ion beam in the drift direction Y, Π is the phase volume, where π = δα i · δx i and δα i is the initial angular distribution and δx i is the initial spatial distribution of the ion beam at the ion injector, and W is the distance between the ion mirrors in the X direction.The multi-reflection mass spectrometer according to any preceding claim, wherein the angular distribution of the ion beam, δα, 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, δα (opt), given by: δ α ( o p t ) = 2 Π W K ( o p t ) wherein K (opt) is the optimum value for the number of oscillations between the ion mirrors from the ion injector to the ion detector, Π is the phase volume, and W is the distance between the ion mirrors in the X direction.A multi-reflection mass spectrometer as claimed in any preceding claim, wherein the ion focusing arrangement is arranged prior to reflection at a number of less than 0.25 N in the ion mirrors (6, 8, 906, 908).The multi-reflection mass spectrometer of any preceding claim, wherein the initial spatial distribution of the ion beam in the drift direction Y at the ion injector is δx i, 0,25-10 mm or 0.5-5 mm.The multi-reflection mass spectrometer of any preceding claim, wherein the drift focusing lens is positioned in the ion mirrors after a first reflection and before a fifth reflection.The multi-reflection mass spectrometer of claim 11, wherein the drift focusing lens (12) is positioned after a first reflection in the ion mirrors and before a second reflection in the ion mirrors (6, 8, 906, 908).The multi-reflection mass spectrometer of claim 11 or 12, wherein the drift focusing lens (12) is the only drift focusing lens positioned between the first reflection and the ion detector (14).The multi-reflection mass spectrometer of any of claims 11 to 13, wherein the drift focusing lens (12) comprises a transaxial lens, the transaxial lens comprising a pair of opposing lens electrodes positioned on either side of the beam in a direction Z, the direction Z being perpendicular to the directions X and Y.The multi-reflection mass spectrometer of claim 14, wherein each of the opposed lens electrodes comprises a circular (20), elliptical (22), quasi-elliptical (12a, 12b), or arcuate electrode.The multi-reflection mass spectrometer of claim 14, wherein each of said pair of opposed lens electrodes comprises an array of electrodes (30) separated by a resistor string to mimic a field curvature generated by an electrode having a curved edge.The multi-reflection mass spectrometer of claim 14, wherein the drift focusing lens comprises a multipole rod assembly or a single lens.A multi-reflection mass spectrometer as claimed in any of claims 14 or 17, wherein the lens electrodes are each disposed within an electrically grounded array.A multi-reflection mass spectrometer as claimed in any one of claims 14 to 18, wherein the lens electrodes are each disposed within a deflector electrode.The multi-reflection mass spectrometer of claim 19, wherein the deflector electrodes have an outer trapezoidal shape acting as a deflector of the ion beam.The multi-reflection mass spectrometer of any preceding claim, wherein the ion focusing arrangement comprises a first drift focusing lens (67, 87, 967) positioned prior to the first reflection in the ion mirrors (6, 8, 906, 908) to focus the ion beam in the drift direction Y, the first drift focusing lens being a diverging lens, and a second drift focusing lens (72, 92, 972) positioned after the first reflection in the ion mirrors to focus the ion beam in the drift direction Y, the second drift focusing lens being the converging lens.The multi-reflection mass spectrometer of any preceding claim, wherein the ion focusing arrangement comprises at least one injection deflector (56, 66, 86, 966) positioned in the ion mirrors prior to the first reflection.The multi-reflection mass spectrometer of claim 22 when dependent on claim 21, wherein the first drift focusing lens (67, 87, 967) is disposed within the at least one injection deflector (66, 86, 966).The multi-reflection mass spectrometer of any preceding claim, wherein the angle of inclination 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.A multi-reflection mass spectrometer as claimed in any preceding claim, further comprising one or more compensation electrodes (78, 978) extending along at least a portion of the drift direction Y in or adjacent to the space between the mirrors to minimize time of flight aberrations.Multi-reflection mass spectrometer according to any of the preceding claims, further comprising a reverse deflector (208, 308, 408, 508) arranged at an end of the ion mirrors (6, 8) remote from the ion injector (204, 404), for reducing or reversing the drift speed of the ions in the direction Y.The multi-reflection mass spectrometer of claim 26, further comprising a further drift focusing lens (316) disposed between the opposing ion mirrors one, two or three reflections before the inverting deflector (308) to focus the ion beam to a focal minimum within the inverting deflector.The multi-reflection mass spectrometer of claim 26, further comprising a further drift focusing lens (506) positioned within the inverting deflector (508) for focusing the ion beam at the next reflection after the inverting deflector to a focal minimum within one of the ion mirrors.The multi-reflection mass spectrometer of any of claims 1 to 25, wherein the detector (974) is disposed at an opposite end of the ion mirrors in the drift direction Y from the ion injector (904), and wherein the ion mirrors (906, 908) diverge along a portion of their length in the direction Y as the ions move toward the detector.The multi-reflection mass spectrometer of claim 29, wherein, starting from the end of the ion mirrors (906, 908) closest to the ion injector (904), the ion mirrors converge along a first portion of their length in the direction Y and diverge along a second portion of their length in the direction Y, the second portion of the length being adjacent the detector.A multi-reflection mass spectrometer as claimed in any preceding claim, wherein the ion detector is an image detector.A method of mass spectrometry comprising: injecting ions into a space between two ion mirrors (6, 8, 906, 908) spaced apart from each other and opposing each other in a direction X, each mirror being generally elongated along a drift direction Y, the drift direction Y being orthogonal to the direction X, the ions entering the space at a non-zero angle of inclination to the direction X, the ions thereby forming an ion beam following a zig zag ion path with N reflections between the ion mirrors in the direction X as it drifts along the drift direction Y; focusing the ion beam in the drift direction Y using ion focusing arrangement located at least partially between the opposing ion mirrors, such that a spatial distribution of the ion beam in the drift direction Y passes 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 completion of 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, wherein the ion focusing arrangement comprises a drift focusing lens (12, 72, 92, 106, 972) for focusing the ions in the drift direction Y, and wherein the drift focusing lens is a converging lens.

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