Ion mirror and ion-optical lens for imaging

Abstract

An ion mirror is disclosed comprising an ion entrance electrode section (62) at the ion entrance to the ion mirror, an energy focussing electrode section (66) for reflecting ions back along a longitudinal axis towards said ion entrance, and a spatial focussing electrode section (64) arranged between the ion entrance electrode section (62) and the energy focussing electrode section (66) for spatially focussing the ions. One or more DC voltage supply is provided to apply a DC potential to the ion entrance electrode section (62) that is intermediate the DC potential applied to the spatial focussing electrode section (64) and the DC potential applied to the energy focussing electrode section (66). The ion mirror further comprises: (i) at least one first transition electrode (68) arranged between said ion entrance electrode section (62) and said spatial focussing electrode section (64), wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section (62) and the DC potential applied to the spatial focussing electrode section (64); and (ii) at least one second transition electrode (69) arranged between said energy focussing electrode section (66) and said spatial focussing electrode section (64), wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one second transition electrode (69) that is intermediate the DC potential applied to the spatial focussing electrode section (64) and the DC potential applied to the ion entrance electrode section (62).

Claims

1. An ion mirror comprising: an ion entrance electrode section at the ion entrance to the ion mirror; an energy focussing electrode section for reflecting ions back along a longitudinal axis towards said ion entrance; a spatial focussing electrode section arranged between the ion entrance electrode section and the energy focussing electrode section for spatially focussing the ions; one or more DC voltage supply configured to apply different DC voltages to the ion entrance electrode section, the spatial focussing electrode section and the energy focussing electrode section, and to apply a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the energy focussing electrode section; wherein at least one first transition electrode is arranged between said ion entrance electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and wherein at least one second transition electrode is arranged between said energy focussing electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one second transition electrode that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the ion entrance electrode section.

2. The ion mirror of claim 1, wherein the DC voltage supply is configured to apply multiple different DC potentials to different electrodes of the energy focussing electrode section for reflecting ions back along the longitudinal axis towards said ion entrance; and wherein the DC voltage supply is configured to apply a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the lowest DC potential applied to the energy focussing electrode section.

3. The ion mirror of claim 1, wherein the spatial focussing electrode section focuses ions in a dimension (Y-dimension) that is orthogonal to said longitudinal axis (X-dimension).

4. The ion mirror of claim 1, wherein the energy focussing electrode section comprises at least two electrodes at different positions along the longitudinal axis, wherein the DC voltage supply is configured to apply a different potential to each of the at least two electrodes so as to provide an electric potential profile along the energy focussing electrode section for reflecting ions along the longitudinal axis towards said ion entrance.

5. The ion mirror of claim 1, wherein said at least one first transition electrode comprises m first transition electrodes arranged at different positions along the longitudinal axis, wherein m is selected from the group comprising: 2; 3; 4; 5; 6; 7; 8; 9; and 10.

6. The ion mirror of claim 5, wherein the voltage supply is configured to apply a different DC potential to each of the m first transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the ion entrance section.

7. The ion mirror of claim 1, wherein said at least one second transition electrode comprises n second transition electrodes arranged at different positions along the longitudinal axis, wherein n is selected from the group comprising: 2; 3; 4; 5; 6; 7; 8; 9; and 10.

8. The ion mirror of claim 7, wherein the voltage supply is configured to apply a different DC potential to each of the n second transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the spatial focussing section to the energy focussing electrode section.

9. An ion mirror comprising: an ion entrance electrode section at the ion entrance to the ion mirror; an energy focussing electrode section for reflecting ions back along a longitudinal axis towards said ion entrance; a spatial focussing electrode section arranged between the ion entrance electrode section and the energy focussing electrode section for spatially focussing the ions; one or more DC voltage supply configured to apply different DC voltages to the ion entrance electrode section, the spatial focussing electrode section and the energy focussing electrode section, and to apply a DC potential to the spatial focussing electrode section that is intermediate the DC potential applied to the ion entrance electrode section and a DC potential applied to the energy focussing electrode section; and wherein at least one first transition electrode is arranged between said ion entrance electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and wherein at least one second transition electrode is arranged between said energy focussing electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one second transition electrode that is below the DC potential applied to the spatial focussing electrode section.

10. A mass spectrometer comprising an ion mirror as claimed in claim 1; or comprising two ion mirrors, each of the type claimed in claim 1, wherein the spectrometer is configured such that, in use, ions are reflected between the two ion mirrors, wherein the spectrometer is a time of flight mass spectrometer.

11. A time of flight mass spectrometer comprising: a time of flight region for separating ions according to their mass to charge ratio; and an ion optical lens for spatially focussing ions arranged within the time of flight region, said lens comprising: an ion entrance electrode section and an ion exit electrode section at opposite ends of the lens, and a spatial focussing electrode section arranged between the ion entrance and ion exit electrode sections for spatially focussing ions passing through the lens; one or more DC voltage supply configured to apply DC voltages to the ion entrance electrode section, the spatial focussing electrode section and the ion exit electrode section; and to apply a DC potential to the spatial focussing electrode section that is either lower or greater than both the DC potential applied to the ion entrance electrode section and the DC potential applied to the ion exit electrode section; at least one first transition electrode arranged between said ion entrance electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and at least one second transition electrode arranged between said ion exit electrode section and said spatial focussing electrode section, wherein said one or more DC voltage supply is configured to apply a DC potential to said at least one second transition electrode that is intermediate the DC potential applied to the ion exit electrode section and the DC potential applied to the spatial focussing electrode section.

12. The spectrometer of claim 11, wherein said at least one first transition electrode comprises p first transition electrodes arranged at different positions along the longitudinal axis, wherein p is selected from the group comprising: 2; 3; 4; 5; 6; 7; 8; 9; and 10; and/or wherein said at least one second transition electrode comprises q second transition electrodes arranged at different positions along the longitudinal axis, wherein q is selected from the group comprising: 2; 3; 4; 5; 6; 7; 8; 9; and 10.

13. The spectrometer of claim 12, wherein the voltage supply is configured to apply a different DC potential to each of the p first transition electrodes so as to provide an electric potential profile that either progressively decreases in a direction along said longitudinal axis from the ion entrance electrode section to the spatial focussing section, and wherein the voltage supply is configured to apply a different DC potential to each of the q second transition electrodes so as to provide an electric potential profile that either progressively decreases in a direction along said longitudinal axis from the ion exit electrode section to the spatial focussing section; or wherein the voltage supply is configured to apply a different DC potential to each of the p first transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the ion entrance electrode section to the spatial focussing section, and wherein the voltage supply is configured to apply a different DC potential to each of the q second transition electrodes so as to provide an electric potential profile that progressively increases in a direction along said longitudinal axis from the ion exit electrode section to the spatial focussing section.

14. The spectrometer of claim 11, comprising a plurality of ion lenses, each lens configured as claimed in claim 11.

15. The spectrometer of claim 14, wherein the plurality of ion lenses are arranged adjacent to one another with their longitudinal axes in parallel and extending in a direction between first and second ion mirrors.

16. The spectrometer of claim 15, wherein one or more shielding electrodes is arranged laterally between adjacent ion lenses for providing an electric field free-region between the adjacent lenses and such that, in use, ions travel through the electric field free-region in between travelling through the laterally adjacent lenses; and wherein an apertured or slotted member is provided in the electric field free-region for blocking the flight paths of ions that have diverged in the direction perpendicular to the longitudinal axis by more than a threshold amount, and for transmitting ions through the aperture or slot that have flight paths which have diverged in the direction perpendicular to the longitudinal axis by less than a threshold amount.

17. A method of reflecting ions or mass spectrometry comprising: supplying ions to the ion entrance electrode section of an ion mirror as claimed in claim 1; applying a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the energy focussing electrode section; and at least one of: (i) applying a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and/or (ii) applying a DC potential to said at least one second transition electrode that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the ion entrance electrode section.

18. A method of reflecting ions or mass spectrometry comprising: supplying ions to the ion entrance electrode section of an ion mirror as claimed in claim 9; applying a DC potential to the ion entrance electrode section that is intermediate the DC potential applied to the spatial focussing electrode section and the DC potential applied to the energy focussing electrode section; and at least one of: (i) applying a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and/or (ii) applying a DC potential to said at least one second transition electrode that is below the DC potential applied to the spatial focussing electrode section.

19. A method of time of flight mass spectrometry comprising: providing a spectrometer as claimed in claim 11; separating ions according to their mass to charge ratio in the time of flight region; spatially focussing ions within the time of flight region using the ion optical lens by: applying a DC potential to the spatial focussing electrode section that is either lower or greater than both the DC potential applied to the ion entrance electrode section and the DC potential applied to the ion exit electrode section; and at least one of: (i) applying a DC potential to said at least one first transition electrode that is intermediate the DC potential applied to the ion entrance electrode section and the DC potential applied to the spatial focussing electrode section; and/or (ii) applying a DC potential to said at least one second transition electrode that is intermediate the DC potential applied to the ion exit electrode section and the DC potential applied to the spatial focussing electrode section.

20. The spectrometer of claim 11 wherein the ion optical lens is arranged between and spaced apart from two ion mirrors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic of a prior art MR-TOF-MS instrument;

(3) FIGS. 2A-2B show schematic views of a prior art MR-TOF-MS instrument having periodic lenses;

(4) FIG. 3 illustrates the ion mapping properties of an MR-TOF-MS instrument;

(5) FIG. 4 shows a simplified schematic of a prior art MR-TOF-MS instrument having periodic lenses;

(6) FIG. 5A shows the focal properties of an ion optical element having aberrations, and FIG. 5B shows the focal properties of an ion optical element having no aberrations;

(7) FIG. 6A shows a schematic of a prior art ion mirror; FIG. 6B shows a schematic of an ion mirror according to an embodiment of the present invention; FIG. 6C shows the potential profiles along the longitudinal axes of the prior art ion mirror and the ion mirror according to the embodiment of the present invention; FIG. 6D shows the potential profiles along the longitudinal axes of the prior art ion mirror and an ion mirror according to another embodiment of the present invention;

(8) FIG. 7A shows a schematic of a prior art ion optical lens; FIG. 7B shows a schematic of an ion lens according to an embodiment of the present invention; FIG. 7C shows the potential profiles along the longitudinal axes of the prior art ion lens and the ion lens according to the embodiment of the present invention; FIG. 7D shows the potential profiles along the longitudinal axes of the prior art ion lens and an ion lens according to another embodiment of the present invention;

(9) FIG. 8 shows a simplified schematic of an MR-TOF-MS instrument having ion mirrors and periodic lenses according to embodiments of the present invention;

(10) FIGS. 9A and 9B illustrate the performance of the analyser according to FIG. 8 in a macroscopic ion mapping mode; and

(11) FIG. 10 illustrates the performance of the analyser according to FIG. 8 in a microscopic ion mapping mode.

DETAILED DESCRIPTION

(12) The present invention provides an improved ion mirror and improved ion lens that may be used to improve ion mapping in a MR-TOF-MS.

(13) In order to assist the understanding of the embodiments of the present invention, a prior art instrument will now be described with reference to FIG. 1. FIG. 1 shows a schematic of the folded path planar MR-TOF-MS. The planar MR-TOF-MS 11 comprises two electrostatic mirrors 12, each composed of three electrodes that are extended in the drift Z-direction. Each ion mirror forms a two-dimensional electrostatic field in the X-Y plane. An ion source 13 (e.g. pulsed ion converter) and an ion detector 14 are located in the drift space between said ion mirrors 12 and are spaced apart in the Z-direction. Ion packets are produced by the source 13 and are injected into the time of flight region at a small inclination angle to the X-axis. The ions therefore have a velocity in the X-direction and also have a drift velocity in the Z-direction. The ions are reflected between the ion mirrors 12 multiple times as they travel in the Z-direction from the source 13 to the receiver 14. The ions thus have substantially sinusoidal or jigsaw ion trajectories 15,16,17 through the device.

(14) The ions advance in the drift Z-direction by an average distance of Z.sub.RC*sin for each mirror reflection, where C is the distance in the X-direction between the ion reflection points. The ion trajectories 15 and 16 represent the spread of trajectories caused by the initial ion packet width Z.sub.S in the ion source 13. The trajectories 16 and 17 represent the angular divergence of the ion packet, which increases the ion packet width by dZ at the detector 14. The overall spread of the ion packet by the time that it reaches the detector 14 of represented by Z.sub.D.

(15) The MR-TOF-MS 11 provides no ion focusing in the drift Z-direction, thus limiting the number of reflection cycles that can be performed before the beam becomes overly dispersed by the time it reaches the detector 14. This arrangement therefore requires an ion trajectory advance per mirror reflection Z.sub.R that is above a certain value in order to avoid ion trajectories overlapping and causing spectral confusion. As such, the number of ion reflections for an instrument of practical length in the Z-direction is limited to a relatively low value.

(16) It is known to introduce periodic lenses into the field-free region between the ion mirrors in order to limit the divergence of the ion beam in the Z-dimension, so as to overcome the above-described problem, e.g. as described in WO 2005/001878.

(17) FIGS. 2A and 2B illustrate a prior art instrument that is the same as that shown in FIG. 1, except that periodic lenses 23 are introduced into the field-free region between the ion mirrors. The instrument is therefore a multi-reflecting mass spectrometer 20 comprising a pair of planar mirrors 21, a drift space 22, a periodic lens array 23, a pulsed ion source 24 and a detector 26. FIG. 2A shows a view in the instrument in the X-Z plane and FIG. 2B shows a view in the instrument in the X-Y plane. The ions are pulsed into the drift space 22 between the ion mirrors 21 such that they perform multiple reflections between the ion mirrors 21 as they drift in the z-direction to the detector 26. The multiple mirror reflections extend the flight path of the ions, which improves mass resolution. The periodic lens 23 confine the ion packets along the main sinusoidal or zig-zag trajectory 25. The number of ion reflections shown in the drawings is for illustrative purposes and although the number of ion reflections illustrated in FIG. 2A is fewer than the number shown in FIG. 1 this is not intended to be significant. To the contrary, the provision of the periodic lenses shown in FIG. 2A enable a greater number of ion reflections per given distance in the Z-dimension as described in the Background section above.

(18) The inventors of the present invention have recognised that the MR-TOF-MS instrument has useful stigmatic or ion mapping properties that may be useful for imaging an ion source, or multiple ion sources, onto a detector. The spatial focusing and image mapping properties instruments having (e.g. gridless) planar ion mirrors have not previously been appreciated and have not been used for multiple practical reasons.

(19) FIG. 3 schematically illustrates the ability of the MR-TOF-MS analyzer to map ions from regions of a source of ions to corresponding regions on an array of regions downstream of the time of flight region. The coordinate system shown in FIG. 3 is the same coordinate system used in FIGS. 1-2. As described previously, ion reflections and time of flight separation primarily occur in the X-dimension, enabling the mass to charge ratios of the ions to be determined from the times of flight from the source of ions to the detector. However, the inventors have recognised that some degree of spatial information in the Y and Z dimensions is also retained as the ions pass from the source of ions to the downstream end of the time of flight region, i.e. the instrument maps ions. A position sensitive detector can therefore be provided downstream of the time of flight region such that ions are mapped from an array of regions on the source of ions to a corresponding array of regions on the position sensitive detector. Pixelated detectors, such as those disclosed in U.S. Pat. No. 8,884,220, may be used to record time-of-flight signals from a matrix of individual pixels in the detector by using an array channel data system.

(20) The stigmatic, imaging or ion mapping performance of such an analyser can be used in two different regimes; a macroscopic mode or a microscopic mode. In the macroscopic mode, ions may be mapped from a relatively large area, e.g. 1010 mm, onto a position sensitive detector. This enables the instrument, for example, to map multiple input ion beams to the detector. In the microscope mode, ions may be mapped from a smaller area, e.g. 11 mm, to the detector. In this mode the ions may be mapped at much higher spatial resolutions. The input ion beam(s) used for the two modes of operation may have different characteristics. For example, the macroscopic mode may use ion beams having a more diffuse set of characteristics representative of the input conditions to be expected from multiple ion beam sources. The ions beam(s) used in the microscope mode may have a brighter set of characteristics, e.g. such as would be expected from a SIMS or MALDI source.

(21) As described above, the inventors have recognised that the MR-TOF-MS instrument has useful stigmatic or ion mapping properties that may be useful for imaging an ion source, or multiple ion sources, onto a detector. However, the inventors have also recognised that the stigmatic or ion mapping performance may be improved by reducing the aberrations associated with components of the instrument. Embodiments of these improvements will now be described using the known MR-TOF-MS analyser shown in FIG. 4 as an illustrative example.

(22) FIG. 4 shows a schematic of the known analyser shown in FIGS. 2A-2B, albeit with a greater number of periodic lenses 23. More specifically, FIG. 2A only shows five periodic lenses 23, whereas FIG. 4 shows twelve periodic lenses 23, each defining an ion Z-focussing region f. The electrode geometry is described above in relations to FIGS. 1 and 2, and also for example in WO 2013/063587. The analyser is optimised for high order time and energy focussing, meaning that it can achieve a relatively high isochronicity, i.e. a high time of flight resolution for incoming ion beams having a relatively large energy spread. In instrument configurations using an orthogonal accelerator 24 to inject ions into the time of flight region, the energy spread is caused by the spatial spread of the ions in the orthogonal acceleration region since ions at different spatial positions pick up different energies during the acceleration step. The ion mirror 21 is able to accept an ion beam having an energy spread of over 10% of the average energy of ions in the fight tube (which may be 6 keV for this analyser).

(23) Despite the excellent energy acceptance of this analyser due to the elimination of higher order energy aberration coefficients, its stigmatic or ion mapping performance is limited. For example, for the given input ion beam conditions, the smallest spot size in the Y-dimension that could be expected to be mapped to the detector 26 (e.g. as shown in FIG. 3) is about 2 mm in diameter. If the mapping field is 8 mm, then the mapping capacity is limited to only four spots. The number of reflections in the ion mirrors 21 may be reduced (e.g. to eight) in order to reduce the spatial blurring at the image plane. However, this would strongly compromise the time-of-flight resolution of the instrument.

(24) The ion mapping resolution in the Z-dimension is even lower than in the Y-dimension, due to the spatial aberration characteristics of the periodic lenses. For example, in a commercial Pegasus MR-TOF-MS instrument, the periodic lenses 23 are densely packed in order to enable a total of 32 or 44 reflections from the ion mirrors 21. The ion trajectories fill over 70% of the lens windows, and the lenses 23 are set to refocus ion packets every two or three ion mirror reflections. At such settings, the analyser fully smears the Z-spatial information of the ion packets due to high order aberrations of the lenses. The width of each lens 23 may be increased, the strength of each lens 23 may be reduced and the number of ion mirror reflections may be reduced (although this compromises the time-of-flight resolution) in order to improve the mapping capacity of the instrument. For example, an instrument having lenses 23 of twice the width, half the strength and a quarter of the ion mirror reflections may enable one to reach a spatial mapping capacity of 4 to 5.

(25) FIGS. 5A and 5B illustrate the concept of spatial aberrations. FIG. 5A shows how the spatial aberrations of an imperfect ion lens do not focus the ions to the same point, leading to blurring of the image in the image plane (i.e. at the ion detector 26). In contrast, FIG. 5B shows the use of an ion lens having no spatial aberrations and that focuses the ions to the same point, resulting in a non-blurred image at the image plane (i.e. detector 23). Embodiments of the present invention serve to minimise the distortions created by spatial aberrations.

(26) The present invention may be employed in MR-TOF-MS instruments of the type shown and described in relation to FIGS. 1-4. Embodiments of the present invention serve to minimise spatial aberrations caused by the ion mirrors 21 and/or the periodic lenses 23.

(27) The spatial aberrations caused by ions mirrors will now be described.

(28) FIG. 6A shows a schematic of a cross-section in the X-Y plane of a known ion mirror, e.g. such as an ion mirror of the type described in relation to FIGS. 1, 2 and 4. Ions enter the ion mirror from a time of flight region 60 at the right side of the mirror, pass through the ion mirror to the left (in the X-dimension), are reflected and then pass to the right (in the X-dimension) and out of the mirror. The rightmost side of the mirror comprises an ion entrance electrode section 62 that is maintained at a DC potential that defines the potential of the time of flight region (i.e. the flight tube potential). A Y-focussing electrode section 64 is provided adjacent to this for spatially focussing ions in the Y-dimension. This electrode section 64 is maintained at a lower DC voltage than the ion entrance electrode section (or at a higher DC voltage, depending on the polarity of the ions) so as to form an ion focussing section that initially accelerates ions. An energy focussing electrode section 66 is arranged adjacent to the Y-focussing electrode section 64. The energy focussing electrode section 66 comprises three electrode sections and an end cap electrode. These electrodes 66 are maintained at higher DC voltages than both the Y-focussing electrode section 64 and the ion entrance electrode section 62 (or at lower DC voltages, depending on the polarity of the ions) so as to decelerate the ions that have entered the ion mirror and reflect them back towards and out of the entrance to the ion mirror. The DC potential profile 61 along the X-dimension of the known ion mirror is shown in FIG. 6C as the solid line. The horizontal broken line represents the potential of the flight tube potential.

(29) The Y-focussing electrode section 64 provides a two dimension accelerating field in the X-Y plane. Such a field is necessary to enable the efficient transmission of ions, especially over the very large flight paths of MR-TOF-MS analysers. However, as known MR-TOF-MS instruments have not previously been recognised as being useful for ion mapping and have conventionally been used with non-position sensitive ion detectors (e.g. with a single point ion detector), no attention was paid to the stigmatic or ion mapping properties of the ion mirror. The inventors of the present invention realised that instrument is useful for ion mapping and that the image produced by the ion mapping could be improved (e.g. reduced image blurring at the ion detector) by graduating the electric field between the ion mirror electrodes more progressively. More specifically, the inventors recognised that it is desirable, at least for ion mapping applications, to graduate the change in potential difference between the Y-focussing electrode section 64 and the adjacent ion entrance electrode section 62 more progressively; and to graduate the change in potential difference between the Y-focussing electrode section 64 and the adjacent energy focussing electrode section 66 more progressively. The ion beam cross-section in the Y-dimension is typically at its widest within section 64 of the ion mirror. Progressive graduation of the electric field in this section smoothes the field distribution so that the mirror has a virtual aperture that is much larger in the Y-dimension that the real aperture. This essentially reduces the ratio of the beam cross-section to the virtual mirror aperture and thus allows the aberrations of the ion mirror to be reduced.

(30) FIG. 6B shows a schematic of an ion mirror according to an embodiment of the present invention. The ion mirror is substantially the same as that shown in FIG. 6A, except that first transition electrodes 68 are arranged between the ion entrance electrode section 62 and the Y-focussing electrode section 64, and second transition electrodes 69 are arranged between the Y-focussing electrode section 64 and the energy focussing electrode section 66. DC voltages are applied to the first transition electrodes 68 that have amplitudes between the amplitude of the DC voltage applied to the ion entrance electrode section 62 and the amplitude of the DC voltage applied to the Y-focussing electrode section 64. The different DC voltages applied to the respective different first transition electrodes progressively decrease in a direction from the ion entrance electrode section 62 to the Y-focussing electrode section 64 (or increase, depending on the polarity of the ions) so that the Y-focussing electrode section 64 initially accelerates the ions. DC voltages are applied to the second transition electrodes 69 that have amplitudes between the amplitude of the DC voltage applied to the Y-focussing electrode section 64 and the amplitude of the DC voltage applied to the closest of the energy focussing electrodes 66. The different DC voltages applied to the respective different second transition electrodes 69 progressively increase in a direction from the Y-focussing electrode section 64 to the energy focussing electrode section 66 (or decrease, depending on the polarity of the ions). The DC potential profile along the X-dimension 63 of the ion mirror is shown in FIG. 6C. The potential profile 63 substantially corresponds to the conventional potential profile 61, except that it differs in the region between the ion entrance electrode section 62 and the energy focussing electrode section 66, as shown by the curved dashed line.

(31) As can be seen from FIG. 6C, the inclusion of the first and second transition electrodes 68,69 smoothes out the voltage transition between the electrodes of the ion mirror, as compared to the conventional mirror. This reduces the spatial aberrations caused by the ion mirror and improves the ion mapping properties of the instrument.

(32) The ion mirror of this embodiment employs a potential profile for focussing ions in the Y-focussing section 64 that initially accelerates the ions. It is also possible to focus ions using a potential profile for focussing ions in the Y-focussing section 64 that initially decelerates the ions, although this is generally less preferred.

(33) FIG. 6D shows the conventional potential profile 61 shown in FIG. 6C and also a potential profile 65 along the X-dimension of an ion mirror according to an embodiment of the present invention in which a potential profile that initially decelerates the ions is used for focussing ions in the Y-focussing section 64. The ion mirror is the same as that shown in FIG. 6B, but different DC voltages are applied to the electrodes. In this embodiment, the DC voltage applied to the Y-focussing electrode section 64 is greater than the DC voltage applied to the ion entrance electrode section 62, but less than the greatest of the DC voltages applied to the energy focussing electrode section 66. DC voltages are applied to the first transition electrodes 68 that have amplitudes between the amplitude of the DC voltage applied to the ion entrance electrode section 62 and the amplitude of the DC voltage applied to the Y-focussing electrode section 64. The different DC voltages applied to the respective different first transition electrodes progressively increase in a direction from the ion entrance electrode section 62 to the Y-focussing electrode section 64 (or decrease, depending on the polarity of the ions). DC voltages are applied to the second transition electrodes 69 that have amplitudes between the amplitude of the DC voltage applied to the Y-focussing electrode section 64 and the amplitude of the DC voltage applied to the closest of the energy focussing electrodes 66. The different DC voltages applied to the respective different second transition electrodes 69 progressively decrease in a direction from the Y-focussing electrode section 64 to the energy focussing electrode section 66 (or increase, depending on the polarity of the ions). It will be appreciated that the potentials applied to the energy focussing electrode section 66 and the Y-focussing electrode section 64 are selected in order to ensure that ions which enter the ion mirror are able to pass through the Y-focussing electrode section 64, pass into the energy focussing electrode section 66, be reflected, pass back through the Y-focussing electrode section 64, and back out of the mirror.

(34) The DC potential profile 65 along the ion mirror of this embodiment is shown in FIG. 6D. The potential profile 65 substantially corresponds to the conventional potential profile 61, except that it differs in the region between the ion entrance electrode section 62 and the energy focussing electrode section 66, as shown by the curved dashed line.

(35) The spatial aberrations caused by a periodic lens will now be described.

(36) FIG. 7A shows a schematic of a cross-section in the X-Z plane of a known periodic lens, e.g. such as a periodic lens 23 of the type described in relation to FIGS. 2 and 4. As described previously, the lens is arranged between the ion mirrors such that ions pass from one of the ion mirrors to the lens, through the lens so as to be focussed in the Z-dimension as they pass therethrough, and then out of the lens towards the other ion mirror. The lens comprises three electrode sections 72,74,76 arranged along the device (in the X-dimension). A first ion entrance electrode section 72 is arranged at a first end of the device, an ion exit electrode section 74 is arranged at the opposite end of the device (in the X-dimension), and a Z-focussing electrode section 76 is arranged therebetween. In operation, the ion entrance and ion exit electrode sections 72,74 are maintained at the same DC potential as the ion entrance electrode sections of the ion mirrors. This maintains an electric field-free drift region 70 between the periodic lens and each of the ion mirrors. The Z-focussing electrode section 76 of the lens is maintained at a lower DC voltage than the ion entrance and ion exit electrode sections 72,74 of the lens so as to focus in the Z-dimension ions passing through the lens (or at a lower DC voltage, depending upon the polarity of the ions). The DC potential profile 71 along the X-dimension of the periodic lens is shown as the solid line in FIG. 7C and is formed such that the ions are initially accelerated by the potential profile.

(37) This conventional periodic lens is acceptable for known MR-TOF-MS instruments. However, the periodic lens has relatively poor stigmatic or ion mapping properties at its operating potentials, primarily due to the large potential differences between the electrode sections of the lens, and partly due to the relatively small size of the lens.

(38) FIG. 7B shows a schematic of a periodic lens according to an embodiment of the present invention. The lens is substantially the same as that shown in FIG. 7A, except that first transition electrodes 78 are arranged between the Z-focussing electrode section 76 and the ion entrance electrode section 72; and second transition electrodes 79 are arranged between the Z-focussing electrode section 76 and the ion exit electrode section 74. DC voltages are applied to the first transition electrodes 78 that have amplitudes between the amplitude of the DC voltage applied to the ion entrance electrode section 72 and the amplitude of the DC voltage applied to the Z-focussing electrode section 76. The different DC voltages applied to the respective different first transition electrodes 78 progressively decrease in a direction from the ion entrance electrode section 72 to the Z-focussing electrode section 76 (or increase, depending on the polarity of the ions). This creates a potential profile that initially accelerates the ions. DC voltages are applied to the second transition electrodes 79 that have amplitudes between the amplitude of the DC voltage applied to the Z-focussing electrode section 76 and the amplitude of the DC voltage applied to the ion exit electrode section 74. The different DC voltages applied to the respective different second transition electrodes 79 progressively increase in a direction from the Z-focussing electrode 76 to the ion exit electrode section 74 (or decrease, depending on the polarity of the ions). The DC potential profile 73 along the X-dimension of the ion lens is shown as the dashed line in FIG. 7C.

(39) Additionally, the whole lens is substantially increased in length (in the X-dimension) and width (in the Z-dimension), as compared to a known periodic lens. More specifically, the length of the Z-focussing electrode section 76 and the lengths of the ion entrance and ion exit electrode sections 72,74 have been increased in length, and the widths of these sections have been increased.

(40) As can be seen from FIG. 7C, the inclusion of the first and second transition electrodes 78,79 smoothes out the voltage transition between the electrode sections of the lens, as compared to the conventional lens. The larger size of the lens of the embodiment of the present invention also renders the variation in the potential profile 73 more gentle than that of the conventional potential profile 71. These features reduce the spatial aberrations caused by the lens and improve the ion mapping properties of the instrument.

(41) The lens of this embodiment employs a potential profile for focussing ions in the Z-focussing section 76 that initially accelerates the ions. It is also possible to focus ions using a potential profile for focussing ions in the Z-focussing section 76 that initially decelerates the ions, although this is generally less preferred.

(42) FIG. 7D shows the conventional potential profile 71 that is shown in FIG. 7C and also shows a potential profile 75 along the X-dimension of a lens according to an embodiment of the present invention in which a potential profile is used for focussing ions in the Z-focussing section 76 that initially decelerates the ions. The lens is the same as that shown in FIG. 7B, but different DC voltages are applied to the electrodes. In this embodiment, the DC voltage applied to the Z-focussing electrode section 76 is above the DC voltages applied to the ion entrance and ion exit electrode sections 72,74. DC voltages are applied to the first transition electrodes 78 that have amplitudes between the amplitude of the DC voltage applied to the ion entrance electrode section 72 and the amplitude of the DC voltage applied to the Z-focussing electrode section 76. The different DC voltages applied to the respective different first transition electrodes 78 progressively increase in a direction from the ion entrance electrode section 72 to the Z-focussing electrode section 76 (or decrease, depending on the polarity of the ions). This creates a potential profile that initially decelerates the ions. DC voltages are applied to the second transition electrodes 79 that have amplitudes between the amplitude of the DC voltage applied to the Z-focussing electrode section 76 and the amplitude of the DC voltage applied to the ion exit electrode section 74. The different DC voltages applied to the respective different second transition electrodes 79 progressively decrease in a direction from the Z-focussing electrode 76 to the ion exit electrode section 74 (or increase, depending on the polarity of the ions).

(43) As can be seen from FIG. 7D, the inclusion of the first and second transition electrodes 78,79 smoothes out the voltage transition between the electrode sections of the lens, as compared to the conventional lens. The larger size of the lens of the embodiment of the present invention also renders the variation in the potential profile 75 more gentle than that of the conventional potential profile 71. These features reduce the spatial aberrations caused by the lens and improve the ion mapping properties of the instrument.

(44) The lens of the embodiments of the present invention may not completely focus the ions in the Z-dimension, but provides sufficient Z-focusing to prevent the ion beam from diverging excessively.

(45) FIG. 8 shows a schematic of an analyser according to an embodiment of the present invention. The analyser is similar to that described in relation to FIG. 4, although it includes ion mirrors 87 and periodic lenses 89 according to the embodiments of the present invention described above. As each of the periodic lenses 89 has an increased width (in the Z-dimension), as compared to a conventional periodic lens 23, fewer periodic lenses are provided per unit length in the Z-dimension. In the illustrated embodiment, the periodic lenses 89 provide six Z-focussing regions F that focus the ions in the Z-direction as they pass therethrough. The embodiment of FIG. 8 also differs from the analyser show in FIG. 4 in that the embodiment of FIG. 8 includes a position sensitive ion detector 81 onto which the source of ions 83 is mapped.

(46) Also, a shielding electrode 80 is provided between the source of ions 83 and the adjacent periodic lens 89 such that ions exit the source 83 into a field-free region. A shielding electrode 82 is also provided between the detector 81 and the adjacent periodic lens 89 such that ions exiting the final periodic lens pass to the detector 81 through a field-free region. Additionally, shielding electrodes are provided in the centre (in the Z-dimension) of the array of periodic lenses so as to provide a field-free region 84. An aperture or slit 86 is provided in the field-free region 84 which only transmits ions that have not diverged excessively in the Z-dimension. This blocks the flight paths of ions that have diverged excessively in the Z-dimension and that would cause blurring of the image at the detector plane.

(47) In operation, ions are pulsed from the source of ions 83 towards a first of the ions mirrors 87a in the X-Z plane and at an acute inclination angle to the X-dimension. The ions therefore have a velocity in the X-dimension and also a drift velocity in the Z-dimension. The ions enter into the first of the ion mirrors 87a and are reflected towards the second of the ion mirrors 87b. The angle at which the ions are injected is selected such that the ions reflected by the first ion mirror 87a have a sufficient drift velocity in the Z-dimension that they pass into an entrance end of the first periodic lens 89a. This lens 89a serves to focus the ions in the Z-dimension so as to prevent the ion beam expanding excessively in the Z-dimension. The ions then exit the other end of the periodic lens 89a and travel into the second ion mirror 87b. The ions are reflected by the second ion mirror 87b and the drift velocity of the ions in the Z-dimension causes the ions to enter into the second periodic lens 89b, which focuses the ions in the Z-dimension. The ions then exit the other end of the second periodic lens 89b and travel into the first ion mirror 87a. The ions are reflected again by the first ion mirror 87a and the drift velocity of the ions in the Z-dimension causes the ions to enter into the third periodic lens 89c, which focuses the ions in the Z-dimension. The ions then exit the other end of the periodic lens 89c and travel again into the second ion mirror 87b. The ions are reflected by the second ion mirror 87b and the drift velocity of the ions in the Z-dimension causes the ions to enter into the field-free region 84. Ions which have not diverged excessively in the Z-dimension are transmitted through aperture or slit 86 and then exit the field-free region 84.

(48) The ions exiting the field-free region 84 travel into the first ion mirror 87a. The ions are reflected again by the first ion mirror 87a and the drift velocity of the ions in the Z-dimension causes the ions to enter into the fourth periodic lens 89d, which focuses the ions in the Z-dimension. The ions then exit the other end of the periodic lens 89d and travel into the second ion mirror 87b. The ions are reflected by the second ion mirror 87b and the drift velocity of the ions in the Z-dimension causes the ions to enter into the fifth periodic lens 89e, which focuses the ions in the Z-dimension. The ions then exit the other end of the fifth periodic lens 89e and travel into the first ion mirror 87a. The ions are reflected again by the first ion mirror 87a and the drift velocity of the ions in the Z-dimension causes the ions to enter into the sixth periodic lens 89f, which focuses the ions in the Z-dimension. The ions then exit the other end of the periodic lens 89f and travel again into the second ion mirror 87b. The ions are reflected by the second ion mirror 87b and the drift velocity of the ions in the Z-dimension causes the ions to impact on the position sensitive detector 81.

(49) The ions separate, primarily in the X-dimension, according to their times of flight through the analyser. As such, ions of different mass to charge ratio arrive at the detector 81 at different times. The mass to charge ratio of any given ion can be determined from the duration between the time at which that ion was pulsed into the analyser by the source 83 and the time at which that ion was detected by the detector 81.

(50) The ions may be focused in the Z-dimension by the periodic lenses 89 in a parallel to point manner by the time that the ions reach the aperture or slit 86. The focusing in the Z-dimension of the downstream periodic lenses 89 may then be set to allow the ions to be focused in a point to parallel manner. For example, in the X-Z plane the ions may be initially injected as a substantially parallel beam at the source 83 and the periodic lenses 89 may focus the ions in a parallel to point manner such that the ions are at their most focused in the Z-dimension at the location of the aperture or slit 86. Downstream of the aperture of slit 86, the periodic lenses 89 may focus the ions in a point to parallel manner such that the ions are parallel at the location of the detector 81.

(51) Each reflection in each ion mirror 87 may focus the ions in the Y-dimension in a point to parallel manner. In other words, the ions may be focused in the Y-dimension by the ion mirrors 87 such that they have their narrowest width in the Y-dimension at a location between the ion mirrors 87. The ions may diverge as they travel from this focal point towards a given ion mirror 87 and may enter each ion mirror 87 as a substantially parallel ion beam (in the X-Y plane). The ion mirror 87 may then reflect and focus the ions back to the focal point between the ion mirrors 87. The ions may then diverge in the Y-dimension such that the ions may enter the next ion mirror 87 as a substantially parallel ion beam (in the X-Y plane). That ion mirror 87 may then reflect and focus the ions back to the focal point between the ion mirrors 87. This process may be repeated for each reflection for each ion mirror 87. Alternatively, each reflection in each ion mirror 87 may focus the ions in the Y-dimension in a parallel to point manner. In other words, the ions may be focused in the Y-dimension by the ion mirrors 87 such that they have their narrowest width in the Y-dimension within each ion mirror and are substantially parallel (in the X-Y pane) at a mid-way location between the ion mirrors 87.

(52) The analyser according to FIG. 8 maps ions from the source of ions 83 to the detector 81, in the manner shown schematically in FIG. 3.

(53) FIGS. 9A and 9B illustrate the performance of the analyser according to FIG. 8 in a macroscopic ion mapping mode. FIG. 9A shows an example of a simulation of the ions detected at the detector 81 when using a source of ions 83 that is a 2D array of macro-size pulsed ion beams. According to this example, a 66 array of pulsed ion beams (e.g. as shown in FIG. 3) was mapped from the source of ions to the position sensitive detector 81. Each ion beam in this simulation is generated so as to have a diameter of approximately 0.5 mm (in the Y-Z plane). The centres of adjacent ion beams in the array are initially separated from each other by 1 mm. The analyser then maps the image of this array, for example along a 10 m effective path length, to the detector plane almost without spatial distortions, as shown by FIG. 9A. Although the 2D array in this example was a 66 array of pulsed ion beams, only the ions detected from the ion beams having initial coordinates in the Y-Z plane of Y.sub.0=Z.sub.0=0, 1, 2, 3, 4 and 5 mm are shown. The ions detected from the other ion beams have been omitted from FIG. 9A for clarity, although a 66 array of ion beams would be detected at the detector 81.

(54) Due to the improved spatial resolution of the analyser, the ion packets from different ion beams at the source of ions 83 are able to be mapped to separate spots on the ion detector 81. This system therefore allows parallel independent acquisitions of an array of ion beams or ion packets, with minimal ion losses and without any signal interference at the detector 81. This leads to an improvement in the throughput of the analyser. Although a 66 array of ion beams has been described, arrays of higher numbers of ion beams and larger fields of view may be provided using the analyzer.

(55) The spatial resolution in the above example is around 750 microns, which is ideal for interfacing multiple input ion beams to the detector 81. Although the spatial resolution in this example is moderate in terms of the number of pixels resolved, TOF analysers are not conventionally able to sustain imaging properties at large fields of view. For example, the imaging field in a conventional TOF microscope is typically well under 1 mm.

(56) FIG. 9B shows time profiles for ion packets detected in FIG. 9A having a mass to charge ratio of 1000 amu. The flight time is approximately 290 s, while the FWHM aberration blurring of each ion packet is under 0.5 ns, allowing for initial time spreads of about 1 ns and a mass resolving power of about R100,000. This high value of resolution is unprecedented for multi-channel TOF mass spectrometers.

(57) FIG. 10 illustrates the performance of the analyser according to FIG. 8 in a microscopic ion mapping mode. The upper plot shown in FIG. 10 corresponds to that described in relation to FIG. 9A, except that each ion beam in this simulation is generated so as to have a smaller diameter (in the Y-Z plane), and the centres of adjacent ion beams in the array are initially separated from each other by 0.1 mm, rather than 1 mm. The lower three plots in FIG. 10 show expanded views of three of the spots on the detector 81 that are shown in the upper plot in FIG. 10. The spatial resolution in the microscope mode can be around 10 microns. This mode may be useful to simultaneously analyse ions from different areas of the same sample in parallel.

(58) The analyser is able to operate in the microscopic mode with a field of view having a spatial resolution of 1 mm.sup.2 and with a mass resolving power up to 100,000. Both of these values are superior over conventional TOF mass spectrometers.

(59) Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.