ION MIRROR FOR MULTI-REFLECTING MASS SPECTROMETERS

Abstract

Improved ion mirrors 30 (FIG. 3) are proposed for multi-reflecting TOF MS and electrostatic traps. Minor and controlled variation by means of arranging a localized wedge field structure 35 at the ion retarding region was found to produce major tilt of ion packets time fronts 39. Combining wedge reflecting fields with compensated deflectors is proposed for electrically controlled compensation of local and global misalignments, for improved ion injection and for reversing ion motion in the drift direction. Fine ion optical properties of methods and embodiments are verified in ion optical simulations.

Claims

1. An ion minor comprising: a plurality of electrodes and at least one voltage supply connected thereto that are configured to generate an electric field region that reflects ions in a first dimension (X-dimension), and wherein at least part of the electric field region through which ions travel in use has equipotential field lines that diverge or converge as a function of position along a second, orthogonal dimension (Z-direction).

2. The ion mirror of claim 1, wherein said least part of the electric field region having equipotential field lines that diverge or converge is configured to tilt the time front of ions being reflected in the ion mirror; optionally wherein said at least part of the electric field region is configured to tilt the time front of ions being reflected in the ion mirror by a first angle, in the X-Z plane, that is greater than a second angle by which the electric field region steers the average ion trajectory, in the X-Z plane.

3. The ion mirror of claim 1 or 2, wherein said least part of the electric field region is arranged at or proximate an end of the ion mirror, in the second dimension, and wherein the equipotential field lines converge as a function of distance, in the second dimension, away from said end.

4. The ion mirror of any preceding claim, comprising one or more electrodes defining an opening through which the ions pass, wherein the opening has a width in a third dimension (Y-dimension) orthogonal to the first and second dimensions that varies as a function of position along the second dimension (Z-direction) for generating said equipotential field lines that diverge or converge.

5. The ion mirror of any preceding claim, comprising electrodes arranged on opposing sides of the ion minor in a third dimension (Y-dimension) that is orthogonal to the first and second dimensions, wherein the ion mirror comprises one or more voltage supply configured to apply different voltages to different ones of these electrodes for generating said equipotential field lines that diverge or converge.

6. The ion mirror of claim 5, comprising one or more first electrode arranged on a first side of the ion mirror, in the third dimension, and a plurality of second electrodes arranged on a second opposite side of the ion mirror; wherein the ion mirror is configured to apply different voltages to different ones of the second electrodes for generating said equipotential field lines that diverge or converge.

7. The ion mirror of claim 5 or 6, wherein said one or more first electrode and/or said plurality of second electrodes are arranged on a printed circuit board (PCB).

8. The ion mirror of any preceding claim, comprising a voltage supply and electrodes configured to apply a static electric field in an ion acceleration region adjacent, in a direction in which the ions are reflected, said part of the electric field region having equipotential field lines that diverge or converge; said ion acceleration region having parallel equipotential field lines for accelerating the ions out of the ion mirror.

9. The ion mirror of any preceding claim, wherein the ion mirror has a first length in the second dimension that comprises said at least part of the electric field region having equipotential field lines that diverge or converge, and a second length in the second dimension that includes only parallel equipotential field lines for reflecting ions; optionally wherein the ion mirror has a third length in the second dimension that comprises said at least part of the electric field region having equipotential field lines that diverge or converge.

10. The ion mirror of claim 9, wherein the first length is arranged at a first end of the ion mirror; and optionally wherein the third length is arranged at a second opposite end of the ion mirror (in the second dimension), with the second length between the first and third lengths.

11. A mass spectrometer comprising: a time-of-flight mass analyser or electrostatic ion trap having at least one ion mirror as claimed in any preceding claim and a pulsed ion accelerator for pulsing ion packets into the ion mirror.

12. The spectrometer of claim 11, comprising: a multi-pass time-of-flight mass analyser or electrostatic ion trap having at least one ion mirror as claimed in any one of claims 1-10, and electrodes arranged and configured so as to provide an ion drift region that is elongated in a drift direction (z-dimension) and to reflect or turn ions multiple times in an oscillating dimension (x-dimension) that is orthogonal to the drift direction; optionally wherein the drift direction (z-dimension) corresponds to said second dimension and/or wherein the oscillating dimension (x-dimension) corresponds to said first dimension.

13. The spectrometer of claim 12, wherein: (i) the multi-pass time-of-flight mass analyser is a multi-reflecting time of flight mass analyser having two ion mirrors that are elongated in the drift direction (z-dimension) and configured to reflect ions multiple times in the oscillation dimension (x-dimension), wherein at least one of said two ion mirrors is an ion mirror according to any one of claims 1-10; or (ii) the multi-pass time-of-flight mass analyser is a multi-turn time of flight mass analyser having an ion mirror according to any one of claims 1-10 and at least one electric sector configured to reflect and turn ions multiple times in the oscillation dimension (x-dimension).

14. The spectrometer of claim 12 or 13, comprising an ion deflector configured to back-steer the average ion trajectory of the ions, in the drift direction, thereby tilting the angle of the time front of the ions.

15. The spectrometer of claim 14, wherein the ion deflector is located at substantially the same position in the drift direction as said at least part of the electric field region having equipotential field lines that diverge or converge.

16. The spectrometer of claim 14 or 15, wherein said electric field region having equipotential field lines that diverge or converge is configured to tilt the time front of the ions passing therethrough so as to at least partially counteract a tilting of the time front by the ion deflector.

17. The spectrometer of claim 14, 15 or 16, wherein the ion deflector is configured to generate a quadrupolar field for controlling the spatial focusing of the ions in the drift direction.

18. An ion mirror comprising: a plurality of electrodes and at least one voltage supply connected thereto that are configured to generate an electric field region that reflects ions in a first dimension (X-dimension), and wherein at least part of the electric field region through which ions travel in use has equipotential field lines that diverge, converge or curve as a function of position along a second, orthogonal dimension (Z-direction); wherein the ion mirror comprises tuning electrodes arranged on opposing sides of the ion mirror in a third dimension (Y-dimension) that is orthogonal to the first and second dimensions, and voltage supplies configured to apply different voltages to different ones of the tuning electrodes for generating said equipotential field lines that diverge, converge or curve; and wherein the voltage supplies are configured to be adjustable so as to adjust the voltages applied to the tuning electrodes.

19. The ion mirror of claim 18, comprising one or more first electrode arranged on a first side of the ion mirror, in the third dimension, and a plurality of second electrodes arranged on a second opposite side of the ion mirror; wherein the ion mirror is configured to apply different voltages to different ones of the second electrodes and/or first electrodes for generating said equipotential field lines that diverge, converge or curve.

20. The ion mirror of claim 18 or 19, comprising electrodes that are tilted at an angle with respect to each other in a plane defined by the first and second dimensions (X-Z plane); and/or comprising one or more electrodes that are bent in a plane defined by the first and second dimensions (X-Z plane).

21. A method of mass spectrometry comprising: providing an ion mirror or mass spectrometer as claimed in any preceding claim; applying voltages to electrodes of the ion mirror so as to generate said electric field region having equipotential field lines that diverge, converge or curve as a function of position along the second dimension (Z-direction); and reflecting ions in the ion mirror in the first dimension (X-dimension).

22. A method of tuning an ion mirror comprising: providing an ion mirror as claimed in claim 18, 19 or 20; and adjusting the voltage supplies as a function of time so as to vary the voltages applied to the tuning electrodes and the divergence, convergence or curvature of said equipotential field lines.

23. Within electrostatic isochronous mass analyzer, an electrostatic gridless ion mirror comprising means for generating at least one electrically adjustable wedge or curved wedge field in the ion retarding region with equipotential lines diverging or converging in the first Z-direction, said direction being perpendicular to the second X-direction of ion reflection from the mirror at the XZ-plane of ion motion in the mirror.

24. The ion mirror as in claim 23, further comprising a set of parallel electrodes to form a flat post-acceleration field with equipotential lines parallel to said first Z-direction.

25. The ion mirror as in claim 23 or 24, wherein electrodes of said gridless ion mirror are substantially elongated in the first Z-direction and form substantially two-dimensional electrostatic field in the orthogonal XY-plane.

26. The mirror as in claims 23 to 25, wherein said means for generating said wedge or curved wedge field comprise one of the group: (i) a wedge slit electrode oriented substantially orthogonal to electric field lines of said wedge field; (ii) at least one electrode being tilted relative to other mirror electrodes; and (iii) a printed circuit board with multiple conductive pads interconnected by a resistive chain, said conductive pads are aligned with the direction of field lines divergence in said wedge field.

27. The ion mirror as in claims 23 to 26, wherein said isochronous mass analyzer is one of the group: (i) time-of-flight mass spectrometer; (ii) an open trap mass spectrometer; and (iii) an ion trap mass spectrometer with an image current detector.

28. The ion mirror as in claims 23 to 27, wherein electrodes of said ion mirror are made of printed circuit boards (PCB) with partially conductive surface, and wherein said wedge or curved wedge field is electrically adjusted to compensate for tilt and bow of said electrodes at standard accuracy of the PCB technology.

29. Within a method of mass spectral analysis in electrostatic fields of an isochronous mass analyzer, an electrostatic field of gridless ion mirror comprising at least one electrically adjustable wedge or curved wedge field in the ion retarding region with equipotential lines, diverging or converging in the first Z-direction, said direction being perpendicular to the second X-direction of ion reflection from the mirror at the XZ-plane of ion motion in the mirror, said wedge or curved wedge field followed by a region of a flat post-acceleration field with equipotential lines parallel to said first Z-direction.

30. The field as in claim 29, substantially elongated in the first Z-direction and two dimensional in the orthogonal XY-plane.

31. The field as in claim 29 or 30, wherein said method of mass spectral analysis comprises one of the group: (i) time-of-flight mass analysis; (ii) mass analysis within an open ion trap; and (iii) mass analysis within an ion trap mass spectrometer with an image current detector.

31. The method as in claims 29 to 31, wherein said wedge field is electrically adjusted to tilt time front of ion packets, used for one purpose of the group: (i) compensating the time front tilt at ion ray steering by deflectors or lenses; (ii) compensating the time front tilt at ion ray steering by trans-axial deflectors or lenses; (iii) for compensating unintentional misalignments of ion mirror electrodes; and (iv) for compensating misalignments of mass spectrometer components, such as ion sources, accelerators and deflectors.

32. A multi-reflecting mass spectrometer comprising: (a) a pulsed ion source or a pulsed converter generating ion packets substantially elongated in the first Z-direction; (b) a pair of parallel gridless ion mirrors separated by a drift space; electrodes of said ion mirrors are substantially elongated in the Z-direction to form an essentially two-dimensional electrostatic field in an orthogonal XY-plane; said field provides for an isochronous repetitive multi-pass ion motion and spatial ion confinement along a zigzag mean ion trajectory lying within the XY symmetry plane; (c) an ion detector; (d) at least one electrically adjustable electrostatic deflector, arranged for steering of ion trajectories for angle ψ, associated with equal tilting of ion packets time front; (e) at least one electrode structure to form at least one electrically adjustable wedge electrostatic field with equipotential lines diverging or converging in said Z-direction in the retarding region of said ion mirror, followed by electrostatic acceleration in a flat field with equipotential lines parallel to said Z-direction; said at least one wedge field is arranged for the purpose of adjusting the time-front tilt angle γ of said ion packets, associated with steering of ion trajectories at much smaller (relative to said angle γ) inclination angle φ; (f) wherein said steering angles ψ and φ are arranged for either denser folding of ion trajectories, and/or for bypassing rims of said source or of said deflector or of said detector by ion packets, and/or for reverting ion drift motion; and (g) wherein said time-front tilt angle γ and said ion steering angles ψ are electrically adjusted for compensating the T|Z and/or T/ZZ time-of-flight aberrations at said detector.

33. The spectrometer as in claim 32, wherein for the purpose of controlling spatial defocusing or focusing of said at least one deflector, an additional quadrupolar field is formed within said deflector by at least one electrode structure of the group: (i) Matsuda plates; (ii) gate shaped deflecting electrode; (iii) side shields of the deflector with the aspect ratio under 2; (iv) toroidal sector deflection electrodes; and (v) additional electrode curvature within a trans-axial wedge deflector.

34. The spectrometer as in claim 32 or 33, wherein said reflecting wedge field within ion retarding region of at least one ion mirror is arranged with one electrode structure of the group: (i) a wedge slit oriented in the ZY-plane and located between mirror electrodes; (ii) at least one printed circuit board with discrete electrodes aligned in the Z-direction, connected via resistive divider and located between mirror electrodes; (iii) a locally tilted portion of at least one electrode of said ion mirror; and (iv) at least one split portion of at least one electrode of said ion mirror, connected to a separate potential.

35. The spectrometer as in claims 32 to 34, for the purpose of electrically compensating unintentional minor inaccuracy of misalignments of said ion mirrors, further comprising at least one printed circuit board, located between said mirror electrodes; said board forms discrete electrodes, connected via resistive chain to form a wedge or an arc shaped electrostatic wedge field within the ion retarding region of at least one ion mirror.

36. The spectrometer as in claims 32 to 35, wherein said pulsed ion source or said pulsed converter comprises one of the group: (i) a MALDI source; (ii) a SIMS source; (iii) a mapping or imaging ion source; (iv) an electron impact ion source; (v) an orthogonal accelerator; (vi) a pass-through orthogonal accelerator with an electrostatic ion guide; and (vii) a radio-frequency ion trap with radial pulsed ion ejection.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0156] Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0157] FIG. 1 shows prior art U.S. Pat. No. 6,717,132 planar multi-reflecting TOF with gridless orthogonal pulsed accelerator OA, and;

[0158] FIG. 2 illustrates problems of dense trajectory folding and limitations set by mechanical precision of the analyzer of FIG. 1;

[0159] FIG. 3 shows novel amplifying reflecting wedge field of an embodiment of the present invention used for electrically adjustable tilt of ion packets time-front; shows one mirror wedge achieved with a wedge slit; and presents simulated field structure with bent retarding equipotential;

[0160] FIG. 4 shows another embodiment of the present invention of the amplifying wedge mirror field, achieved with an auxiliary printed circuit board (PCB), and shows compensation of unintentional misalignment of ion mirror electrodes;

[0161] FIG. 5 shows one embodiment of PCB ion mirror of the present invention;

[0162] FIG. 6 shows another embodiment of PCB ion mirror of the present invention and shows technological improvements for PCB ion mirrors;

[0163] FIG. 7 illustrates novel methods of compensated ion steering of embodiments of the present invention used for improved ion injection and for improved reversal of ion drift motion, both being achieved with novel wedge mirror fields in combination with novel compensated deflectors;

[0164] FIG. 8 shows results of ion optical simulations verifying improvements of FIG. 7.

DETAILED DESCRIPTION

[0165] Referring to FIG. 1, a prior art multi-reflecting TOF instrument 10 according to U.S. Pat. No. 6,717,132 is shown having an orthogonal accelerator (OA-MRTOF). MRTOF 10 comprises: an ion source 11 with a lens system 12 to form a substantially parallel ion beam 13; an orthogonal accelerator (OA) 14 with a storage gap to admit the beam 13; a pair of gridless ion mirrors 16, separated by a field-free drift region, and a detector 17. Both OA 14 and mirrors 16 are formed with plate electrodes having slit openings, oriented in the Z-direction, thus forming a two dimensional electrostatic field, symmetric about the XZ symmetry plane (also denoted as s-plane). Accelerator 14, ion mirrors 16 and detector 17 are parallel to the Z-axis. In operation, ion source 11 generates a continuous ion beam. Commonly, ion sources 11 comprise gas-filled radio-frequency (RF) ion guides (not shown) for gaseous dampening of ion beams. Lens 12 forms a substantially parallel continuous ion beam 13, entering OA 14 along the Z-direction. An electrical pulse in OA 14 ejects ion packets 15, which travel in MRTOF at small inclination angle a (to the X-dimension), controlled by the ion source bias Uz.

[0166] Referring to FIG. 2, simulation examples 20 and 21 illustrate multiple problems of prior art MRTOF 10, if pushing for higher resolutions and denser trajectory folding. Exemplary MRTOF parameters are: D.sub.X=500 mm cap-cap distance; D.sub.Z=250 mm wide portion of non-distorted XY-field; acceleration potential is U.sub.X=8 kV, OA rim=10 mm and detector rim=5 mm.

[0167] In the example 20, to fit 14 reflections (i.e. L=7 m flight path) the source bias is set to U.sub.Z=9V. Parallel rays with an initial width in the z-direction of Z.sub.0=10 mm and no angular spread Δα=0 start hitting rims of OA 14 and of detector 17. In example 21, the top ion mirror is tilted by λ=1 mrad, representing the realistic overall effective angle of mirror tilt, accounting for built up faults of stack assemblies, standard accuracy of machining and moderate electrode bend by internal stress at machining. Every “hard” ion reflection in the top ion mirror then changes the inclination angle α by 2 mrad. The inclination angle α grows from α.sub.1=27 mrad to α.sub.2=41 mard, gradually expanding the central trajectory. To hit the detector after N=14 reflections, the source bias has to be reduced to U.sub.Z=6V. The angular divergence is amplified by the mirror tilt and increase the ion packets width in the z-direction to ΔZ=18 mm, inducing ion losses on the rims. Obviously, slits in the drift space may be used to avoid trajectory overlaps and spectral confusion, however, at a cost of additional ionic losses.

[0168] In example 21, the inclination of ion mirror introduces yet another and much more serious problem—the time-front 15 of the ions becomes tilted by angle γ=14 mrad in-front of the detector. The total ion packet spreading in the time-of-flight X-direction ΔX=ΔZ*γ=0.3 mm limits mass resolution to R<L/2ΔX=11,000 at L=7 m flight path, being low even for a regular TOF and too low for MRTOF. To avoid the limitation, the electrode precision has to be brought to non-realistic realistic levels: λ<0.1 mrad, translated to better than 10 um accuracy and straightness of individual electrodes.

[0169] Summarizing the problems of prior art MRTOF, attempts of increasing flight path require much lower specific energies U.sub.Z of continuous ion beam and larger angular divergences Δα of ion packets, which induce ion losses on component rims and may produce spectral overlaps. Most important, small mechanical imperfections strongly affect MRTOF resolution and require unreasonably high precision.

[0170] Embodiments of the present invention propose to arrange wedge-shaped electrostatic fields with equipotential lines diverging in the Z-direction in the reflecting region of electrostatic gridless ion mirrors of either MRTOF or E-traps for effective and electrically adjustable control over the ion packets time-front tilt angle γ.

[0171] Referring to FIG. 3, a model gridless ion mirror 30 according to an embodiment of the present invention is shown and comprises a wedge reflecting field 35 and a flat post-accelerating field 38. An ion packet 34 (formed with any pulsed converter or ion source) is initially aligned with the Z-axis, as shown by a line for the time front. The ion packet 34 initially has a mean (average) ion energy K.sub.0 and energy spread ΔK. The ion packet 34 passes through field 38 and enters the wedge-shaped field 35 in the ion mirror at an inclination angle α (to the X-dimension). The ions are then reflected by the ion mirror (in the X-direction) and pass through the accelerating field 38.

[0172] Flat field 38 has equipotential lines arranged parallel to the Z-axis within potential boundaries corresponding to mean energies K.sub.1 and K.sub.1 of the ions, where K.sub.0>K.sub.1. Model wedge field 35 may be arranged with uniformly diverging equipotential lines in the XZ-plane, where the field strength E(z) is independent on the X-coordinate, and within the ion passage Z-region the field E(z) is reverse proportional to the Z-coordinate: E(z)˜1/z. Wedge field 35 starts at equipotential corresponding to K=K.sub.1 and continues at least to the ion retarding equipotential 36 (K=0), tilted to Z-axis at λ.sub.0 angle. This arrangement causes the time-front of the ion packet to be tilted by angle γ relative to the Z-axis, and the average trajectory of the ion packet (relative to the X-dimension) to be altered by steering angle ϕ.

[0173] While applying standard mathematics a non-expected and previously unknown result was arrived at: in ion mirror 30 with wedge field 35, the time-front tilt angle γ and the ion steering angle φ are controlled by the energy factor K.sub.0/K.sub.1 as:

[00001]$\gamma =4{\lambda }_0*{\left(K_0/K_1\right)}^{0.5}=4{\lambda }_0*u_0/u_1$$\varphi =4{\lambda }_0/3*{\left(K_1/K_0\right)}^{0.5}=4{\lambda }_0/3*u_1/u_0$$i.e.\gamma /\varphi =3K_0/K_1>>1$

where K.sub.1 and K.sub.0 are the mean ion kinetic energies at the exit of the wedge field 35 (index 1) and at the exit of flat field (index 0) respectively, and u.sub.1 and u.sub.0 are the corresponding mean ion velocities. The angle ratio γ/φ=3K.sub.0/K.sub.1 may be practically reaching well over 10 or 30 and is controlled electronically.

[0174] At K.sub.0/K.sub.1=1 (i.e. without acceleration in the field 38), the wedge field already provides twice larger time front tilt γ compared to fully tilted ion mirrors (γ=4λ.sub.0 Vs γ=2λ.sub.0), while producing a smaller steering angle (ϕ4/3λ.sub.0 Vs ϕ=2λ.sub.0). The angle ratio γ/ϕ further grows with the energy factor as K.sub.0/K.sub.1 because the angles are transformed with ion acceleration in the field 38: both flight time difference dT and z-velocity w are preserved with the flat field 38, where the time front tilt dT/u grows with ion velocity u and the steering angle dw/u decreases with ion velocity u. By arranging larger K.sub.0/K.sub.1 ratio, the combination of wedge field with post-acceleration provides a convenient and powerful tool for adjustable steering of the time fronts of ion packets, accompanied by negligibly minor steering of ion rays. Again referring to FIG. 3, one embodiment 31 of ion mirror with amplifying reflecting wedge field comprises a regular structure of parallel mirror electrodes, all aligned in the Z-direction, where C denotes the cap electrode, and E1 denotes the first mirror frame electrode. Although only one mirror frame electrode E1 is shown, a plurality of such mirror frame electrodes may be provided stacked in the Z-direction (e.g. usually, from 4 to 8 such electrodes). Mirror 31 further comprises a thin wedge electrode W, located between cap electrode C and first electrode E1. The wedge electrode W has a constant thickness in the X-direction and is aligned parallel with the Z-axis. However, as shown in the lower part of embodiment 31, the wedge electrode has a wedge-shaped (tapered) window in the YZ-plane for variable attenuation of the field due to the cap electrode C potential. Such wedge window appears sufficient for minor curving of reflecting equipotential lines 36 in the XZ-plane, while having minor effect on the structure and curvatures of the XY-field, which is important for ion optical quality of the ion mirror—high order (up to full 3rd order) isochronicity, up to 5th order time per energy focusing, spatial quality and low spatial aberrations.

[0175] A simulated ion optical model for a realistic ion mirror with wedge electrode W of embodiment 31 is illustrated by icons 32 and 33, where icon 32 shows the electrode structure (C, W and E1) around the ion reflection region and also shows equipotential lines in the XY-plane at one particular Z-coordinate. Icon 33 illustrates a slight bending of retarding equipotential 36 in the XZ-middle plane at strong disproportional compression of the picture in the Z-direction, so that the slight curvature of the line 36 can be seen. Dark vertical strips in icon 33 correspond to ion trajectories, arranged at relative energy spread δ=0.05, so that angled tips illustrate the range of ion penetration into the mirror. Icon 33 shows that the wedge field 35 is spread in the Z-direction in the region for several ion reflections, which helps distributing the time-front tilting at yet smaller bend and smaller displacement of equipotential 36.

[0176] Simulations have confirmed that: (i) adjustments of the amplifying factor of 4(K.sub.0/K.sub.1).sup.0.5 allows strong tilting of the time-front at small wedge angles λ.sub.0, thus not ruining the structure of electrical fields, which are optimized for reaching overall isochronicity and spatial focusing of ion packets; (ii) the time front tilt angle can be electronically adjusted from, for example, from 0 to 6 degrees if using wedge W in both opposite ion mirrors; (iii) the compensation of the time-front tilting for deflectors (see FIG. 7) is reached simultaneously with compensation of chromatic dependence of the Z-velocity, as illustrated in FIG. 7.

[0177] Referring to FIG. 4, yet another embodiment 40 of ion mirror with an amplifying wedge reflecting field is shown comprising conventional ion mirror electrodes (cap electrode C, first frame electrode E1, and optional further frame electrodes E2, etc.) and further comprising a printed circuit board 41, placed between cap C and first mirror electrode E1. PCB 41 may either be composed of two aligned parallel PCB plates or may be one PCB with a constant size (z-independent) window, being a wider window than the one in the first frame electrode El to prevent the board 41 being charged by stray ions.

[0178] To produce a desired curvature or bend of the ion retarding equipotential 46, the PCB 41 carries multiple conductive pads, connected via surface mounted resistive chain 42, energized by several power supplies U.sub.1 . . . U.sub.j 43. Preferably, absolute voltages of supplies 43 are kept low, say under 1 kV, which is to be achieved at ion optical optimization of the mirror electrode structure. The net of resistors 42 and power supplies 43 allows adjusting the voltage distribution on PCB 41 flexibly and electronically, thus generating a desired tilt or curvature of retarding equipotential 46, either positive or negative, either weak or strong, either local or global, as illustrated by dashed lines 45. Flexible electronic control over tilt and curvature of the retarding line 46 is a strong advantage of the PCB wedge embodiment 40.

[0179] Again referring to FIG. 4, an exemplary embodiment 44 illustrates the case of mirror cap electrode C being unintentionally tilted by angle λ.sub.C to the Z-axis, this angle being expected to be a fraction of 1 mrad at realistic accuracy of mirror manufacturing. A printed circuit board 41 may be used for recovering the straightness of the reflecting equi-potential 47, primarily designed for local compensation of the time-front tilting by unintentional mirror faults. Similarly, a second (opposing) ion mirror may have another PCB with a quadratic distribution of PCB potentials for electronically controlled correction of unintentional overall bend of ion mirror electrodes. Exemplary retarding equi-potentials 48 and 49 illustrate the ability of forming a compensating wedge or curvature.

[0180] Some practical aspects of using and tuning of PCB wedge are considered. Optionally, PCB electrodes 41 may be used at manufacturing tests only. The occurred inaccuracy of ion mirrors may be determined when measuring the required PCB compensation at recovered MRTOF resolution, which in turn could be used for calibrated mechanical adjustment of individual ion mirrors. Alternatively, the number of regulating power supplies 43 may be potentially reduced and the strategy of analyzer tuning may be optimized for constant use. It is expected that a pair of auxiliary power supplies may be used for simultaneous reaching of: creating preset wedge fields at far and near Z-edge, compensating electrodes faulty tilts, and compensating electrodes faulty bends. Indeed, all wedge fields may produce the same action—they tilt the time front of ion packets, and it is expected that a generic distribution of PCB potential may be pre-formed for each mirror, while controlling overall tilt and bow of wedge fields by a pair of low voltage power supplies 43.

[0181] Compared to wedge slit W in FIG. 3, PCB wedge mirrors 40 and 41 of FIG. 4 look more attractive for being more flexible. Adjusting potentials allows adjusting amplitude and sign of bend or tilt of the reflecting equipotential 46.

[0182] The proposed compensation mechanism of FIG. 3 and FIG. 4 relaxes the precision requirements onto parallelism and precision of ion mirror electrodes from the tens of microns range (as described in FIG. 1) to 100-300 um range and, hence, may allow using lower precision technologies. Embodiments of the invention propose ion mirrors manufactured with more robust, reproducible, and lower cost technology of printed circuit boards (PCBs) at standard (for PCB) precision, being notably lower compared to precision obtainable at standard electrode machining, while using PCB wedge compensation.

[0183] Referring to FIG. 5, one embodiment 50 of a PCB ion mirror of the present invention comprises: PCB electrodes 51 each having a conductive window 54, attachment ribs 52, and optional aligning holes 53; a base support 55; stiffing ribs 56 and/or stiffing supports 59; a compensating PCB 57 with multiple conductive pads; and an optional spacing electrode 58. PCB ion mirror 50 incorporates features to solve deficiencies of standard PCB technology:

[0184] It is important that compensating PCB 57 is used to form an electronically controlled wedge reflecting field (e.g. as described in FIG. 4) for the purpose of compensating electrodes 51 misalignments and limited parallelism, specified at 0.1 mm in PCB technology. It is believed that PCB ion mirrors are unable to operate in practice without this feature.

[0185] The internal edge of window 54 is made conductive, similarly to standard PCB vias (usually made electrolytic). The preferred coating is Nickel, referred to in PCB industry as soft gold. The conductive rim may be at least three times wider than the gaps between electrodes 51 to minimize the insulator exposure and to avoid field effects of charged surfaces.

[0186] The tracking distance of uncoated PCB is arranged at outer sides of PCB 51 to reduce surface gradient to under 300-500 V/mm, where surface discharges are known to start at 1 kV/mm. Yet a larger tracking distance may be obtained if avoiding direct contact between edges of electrode 51 and base plate 55.

[0187] Though base support 55, stiffing ribs 56 or stiffing supports 59 may be made of any mechanically stable material, preferably, we propose PCB material for matching the thermal expansion coefficient (TCE) of electrodes 51, e.g. being 4-5 ppm/C for wide spread FR-4 PCB material. Otherwise, large thermal variations (specified from −50 to +50 C) at instrument transportation may ruin the ion mirror precision and flatness. Optionally, one may use more expensive materials with close TCE, say Titanium or ceramics, however, authors are not aware of any low cost material with matching TCE, except G-10 (equivalent of FR-4 PCB), which is far less preferable for reasons of generating dust and chips. Moreover, PCB supports and ribs allow convenient soldering. Slits in supports 55 are aligned with electrode ribs 52, so that ribs could be soldered at outer sides of PCB 55.

[0188] Embodiment 50 may be designed to compensate for the expected moderate PCB flexing. PCB electrodes 51 are stiff in the X- and Y-direction. Multiple aligning ribs 52 are soldered to slits in the base support 55, providing stiffness in the Z-direction. Flexing of base PCB plate 55 in the Y-direction (harmful at precision assembly) is compensated by attaching stiffing PCB ribs 56, or stiffing supports 59. Supports 59 may be metal (say aluminium) if using a hole and slit mounting to overcome TCE mismatch. Thus, PCB flexing is prevented in the fully assembled ion mirror in all three directions, where initial parallelism before soldering may be improved by technological jigs.

[0189] Referring to FIG. 6, embodiment 60 further improves the straightness and stiffness of individual mirror electrodes 51 before the step of entire mirror assembly by soldering of PCB or metal ridges 61 between a pair of electrodes 51. Parallelism of external surfaces of electrodes 51 and mutual alignment of windows 54 may be improved with technological jigs, e.g. referenced with aligning holes 53. Optionally, the same jig may be used for both the attachment of ridges and the assembly of the entire ion mirror.

[0190] Again referring to FIG. 6, another important step is proposed for improving the precision of electrode mounting, which is very likely to be affected by large variations of PCB thickness, specified to 5% of PCB thickness and rarely controlled at PCB manufacturing. Embodiment 62 illustrates the approach with exemplary milled slot 63 machined in PCB base plate 55 for precision of matching between bottom surface of base 55 and the edge of electrode 51. It is assumed that the bottom surface of PCB 55 is pressed against a flat and hard surface at machining and then to rigid jig fixture or support 59 during assembly stage. Similar slots may be machined on ribs 52 for improved parallelism of electrodes 51.

[0191] Preferably, external edge and ribs 52 are milled simultaneously with internal window 54 to ensure their parallelism, specified at 0.1 mm in PCB industry, while typically being better. Yet preferably, simultaneously machined aligning holes 53 may serve for better alignment of the windows in the electrodes 51 windows.

[0192] Again referring to FIG. 6, another embodiment of PCB ion mirror 64 of the present invention comprises: a pair of parallel PCB plates 65, connected via side stands 68 and enforced by stiffing ribs 69; a compensating PCB 57 with multiple pads, interconnected by (not shown) a resistive chain; and an optional spacing electrode 58, which may also serve as a mirror cap. FIG. 6 shows the bottom half of ion mirror 64 in solid lines and upper plate 65 in dashed lines. Slits 67 are machined mutually parallel (at single installation) and aligned with not shown reference holes. Straightness and flatness of strips 66 is improved with PCB stiffing ribs 69, soldered at conductive pads, preferably on external side of ion mirror 64. Preferably, back side of PCB plate 65 has machined slots (similar to 63) for improved precision of ribs mounting, ensuring plate 65 straightness after the assembly.

[0193] Electrodes of ion mirror 64 are formed as follows. Plates 65 have multiple conductive coated strips 66, which are separated by slits 67 with partially conductive edges. To arrange electrical separation of adjacent electrodes, slits 67 are made partially conductive, for example by initially making fully conductive edges with PCB vias technology, and then disrupting the coating by making additional holes at far Z-edges of slits 67.

[0194] Without going into further details, the inventors claim that embodiment 64 also satisfies all measures of embodiment 60 for compensating deficiencies of standard PCB technology.

[0195] The inventors believe that known methods of making PCB ion optical components were missing most of those steps and could not provide precision, sufficient for ion mirrors.

[0196] Referring to FIG. 7, an embodiment of an improved MRTOF 70 of the present invention is shown comprising: a conventional ion source 11, generating ion beam 13 along the Z-axis; an orthogonal accelerator 14 (or any other pulsed source) aligned with the Z-axis; a pair of gridless ion mirrors with two-dimensional fields 38 aligned with the Z-axis and local wedge fields 35; and front and rear deflectors 71F and 71R. Ion packets are steered by deflectors 71 to control the ion packets inclination angle α with respect to the X-axis. The time front tilting angle γ of ion packets, introduced by deflectors 71 is compensated by the combination of mirror wedge fields 35 and post-accelerating flat field 38 to bring the ion packets time front 79 being parallel to face of detector 17. Yet strongly preferably, the time front compensation is arranged locally in close vicinity of every deflector, so that spatial mixing of ion packets would not affect MRTOF isochronicity. Ion packet steering and tilting at front and rear zones are shown below in zoom views 74 and 75.

[0197] Again referring to FIG. 7, preferably, novel deflector 71 (F or R) of embodiments of the present invention comprise a pair of deflection plates 72 at potentials U and 0 (referenced to acceleration potential U.sub.ACC), or biased for symmetric potentials +U/2 and −U/2) and side plates 73 set at different potential U.sub.Q. Side plates are known as Matsuda plates in sectors. Side plates 73 generate an additional quadrupolar field. The Z-component of the overall electric field becomes, for example, E.sub.Z=U/H−2U.sub.Q*z/H.sup.2, where H and D are distance and effective length of the deflecting field, and z is coordinate within ion packet. The quadrupolar field compensates to first order the variations of the ion steering angleψ, produced by ions slowing down in the region of higher deflection potential and removes the over-focusing effect of conventional deflectors. As a result, deflector 71 is capable of compensating for the angular dispersion of conventional deflectors, is capable of steering ion rays for the same angle ψ independent on the Z-coordinate (i.e. focal distance F.fwdarw.∞), and tilts the time front for constant angle γ=−ψi.e. keeps time fronts straight. Alternatively, deflector 71 is capable of controlling the focal distance F independent of the steering angle ψ.

[00002]$E_{Z}U/H-E_Q*z/H,\psi =D/2H*U/K;\gamma =-\psi$$1/F=2{\psi }^2/D-K/U_{Q}D=1/D\left(2{\varphi }^2-K/U_Q\right)$

where K is the mean energy of ion packets.

[0198] Compensated deflectors 71 nicely fit MRTOF. The quadrupolar field in the Z-direction generates an opposite focusing or defocusing field in the transverse Y-direction. Below simulations prove that the focal properties of MRTOF analyzers are sufficient to compensate for the Y-focusing of deflectors 71 without any significant TOF aberrations.

[0199] Alternatively, compensated deflectors may be trans-axial (TA) deflectors, formed by wedge electrodes. The invention proposes using a second order correction, produced by an additional curvature of TA-wedge. Controlled focusing/defocusing may be also generated by combination of the TA-wedge and TA-lens, arranged separately or combined into a single TA-device. For a narrower range of deflection angles, the compensated deflector may be arranged with a single potential while selecting the size of Matsuda plates or with a segment of toroidal sector.

[0200] Again referring to FIG. 7, zoom views 74 and 75 of embodiment 70 illustrate methods and embodiments of (a) compensated ion injection at front end (74); and (b) compensated ion packet steering and drift reversal at the rear end (75).

[0201] View 74 illustrates the method of compensated ion injection. Ion injection mechanism into MRTOF of the embodiments of the present invention comprises: a “flat” orthogonal accelerator (OA) 14 aligned with the Z-axis; an ion mirror with a “flat” field 38 at higher ion energies; a reflecting wedge field 35 with retarding equipotential 36 tilted at λ.sub.0 angle; and a compensated deflector 71, preferably located along the ion path and after first ion mirror reflection.

[0202] Ion beam 13 propagates along the Z-axis at elevated (compared to FIG. 1) energies (e.g. 20-50 V) to enhance ion admission into OA 14, to increase the inclination angle α1 of ion rays, thus, improving ion packet bypassing the OA rim, and to reduce the ion packets angular divergence Δα. The time-front 76 of ejected ion packets is parallel to the Z-axis, since both ion beam 13 and OA 14 are parallel to the Z-axis. After ion reflection within the wedge mirror field 35 and after post-acceleration in the flat field 38, ion packets' time-front 77 becomes tilted at angle γ>>λ.sub.0, as has been explained in FIG. 3. Ion rays are then steered back in compensated deflector 71F by angle ψ=−γ, so that the inclination angle α.sub.2=α.sub.1−ψ notably reduced, allowing for denser folding of ion rays in MRTOF (for the purpose of higher resolution), while the orientation of the time front 78 is recovered for γ=0.

[0203] Again referring to FIG. 7, view 75 illustrates the method and mechanism of compensated back-end steering in MRTOF with wedge field. The back end of ion mirror comprises a similar “flat” entrance field 38, and a wedge reflecting field 35 with retarding equipotential line 36 tilted at an angle λ.sub.0. Ion packets 76 arrive to the far Z-end after multiple reflections in MRTOF, where they traveled at an inclination angle α.sub.2 and with the time-front 76 being parallel to the Z-axis, i.e. γ=0. After ion reflection in mirror wedge field 35 and after post-acceleration in flat field 38, ion packets time-front 77 becomes tilted for relatively large (say, 3 deg) angle γ=2α.sub.2. Ion rays are steered back by angle ψ=−γ=2α.sub.2 in compensated deflector 71R, so that the inclination angle becomes −α.sub.2, while orientation of the time front 78 is recovered for γ=0. As a result, ion drift motion in the Z-direction is reverted without tilting of the time-front, which helps to achieve about twice denser folding of ion rays in MRTOF 70.

[0204] Similar time front compensation occurs in-front of the detector 17. Ions arrive at the inclination angle α.sub.2, deflector 71F steers ion rays and tilts time front, since deflector 71F is set static and it was set in deflecting state at the ion injections stage 74. Wedge field 35 with flat post-acceleration field 38 tilts the time front to compensate for the tilt at ray steering. The resulting time front 79 is then set parallel to the Z-axis, which simplifies the detector installation.

[0205] Alternatively, the front deflector 71F may be pulsed for trapping ion packets for multiple Z-passages, this way increasing the ion flight time and flight path with the purpose of increased resolution.

[0206] Table 2 below presents formulas for time front tilt angles γ, for ray steering angles ϕ and for chromatic dependence d(Δw)/ dδ of the Z-component of ion velocity w induced by wedge ion mirror and by deflectors. Table 3 below shows conditions for compensating the time front tilt and the chromatic dependence of the Z-velocity in the combined system, which may be achieved simultaneously.

TABLE-US-00001 TABLE 2 Chromatic dependence of Time-front Rays Steering Z-velocity Tilt Angle Angle d(Δw) / dδ Wedge Mirror [00003]${\gamma }_0^{\left(M\right)}=4{\lambda }_0\sqrt{\frac{K_0}{K_1}}$ [00004]${\phi }^{\left(M\right)}\approx +\frac{4{\lambda }_0}{3}\sqrt{\frac{K_1}{K_0}}$ [00005]$2{\lambda }_0{u}_0\sqrt{\frac{K_0}{K_1}}$ Deflector −ψ.sub.0 ψ.sub.0 [00006]$-\frac{1}{2}{u}_0{\psi }_0$

TABLE-US-00002 TABLE 3 Condition for the Condition for 1st order Compensating Time-front Tilt Chromatic Spread of Compensation Z-velocity Wedge Mirror + Deflector [00007]$4\lambda \sqrt{\frac{K_0}{K_1}}={\psi }_0$ [00008]$4\lambda \sqrt{\frac{K_0}{K_1}}={\psi }_0$

[0207] Overall, accounting for all above described methods of compensated ion steering, embodiment 70 allows: (i) a more efficient ion injection at higher energies; (ii) dense folding of ion rays for multiple reflections; (iii) reversal of ion rays for doubling ion path; (iv) compensating additional time-of-flight aberrations associated with steering of elongated (in the Z-direction) ion packets; (v) compensating chromatic angular spreads for reduced ion packet divergence; and (vi) compensating Y-related TOF and spatial aberrations of deflectors by spatial and isochronous properties of ion mirrors. Below described simulations do confirm those claimed positive effects.

[0208] The above described methods allow minor compensation of components (OA, mirrors detector) misalignments by adjusting ion injection energy, steering angles and strength of wedge fields. Wedges 35 may be combined with global compensation of ion mirror misalignment 44 of FIG. 4.

[0209] Referring to FIG. 8, there are presented results of ion optical simulations of MRTOF 70 with compensated ion injection and with compensated reversal of ion trajectory in the Z-direction. The exemplar simulated compact MRTOF 80 comprises: parallel ion mirrors with cap-cap distance D.sub.X=450 mm and useful length D.sub.Z=250 mm, separated by a drift space at U.sub.X=−8 kV acceleration voltage; an ion source 11 generating an ion beam 13 along Z-axis at U.sub.Z=54 V specific energy with ΔU.sub.Z=0.5 V spread; a straight orthogonal accelerator 14 OA aligned with the Z-axis; front and rear deflectors 71F and 71R with compensating Matsuda plates; a wedge electrode W at front and rear Z-end; and a detector 17 at front Z-end.

[0210] Example 80 illustrates spatial focusing of ion rays 81 for Z.sub.0=10 mm long ion packets, while not accounting angular spread of ion packets Δα=0 at ΔU.sub.Z=0 and not accounting relative energy spread of ion packets δ=ΔK/K=0 at ΔX=0. The chosen position of deflector 71F improves the ion packets bypassing of the deflector 71F and of detector 17 rim. Matsuda plates' voltages of the deflectors 71F and 71R are electrically adjusted for moderate spatial focusing of initially parallel rays onto detector 17, while being balanced for achieving optimal focusing in other examples of FIG. 8.

[0211] Example 82 illustrates angular divergence of ion rays 83 at ΔU.sub.Z=0.5V, while not accounting ion packets width Z.sub.0=0 and energy spread δ=0. Matsuda plate of the reversing deflector 71R is adjusted (being the same for all examples of FIG. 8) for spatial focusing of initially diverging rays onto detector 17.

[0212] Example 84 illustrates ion rays at all accounted spreads of ion beam. Though trajectories look filling most of the drift space, apparently, simulated ion losses are within 10%.

[0213] Example 86 presents the overall mass resolution R.sub.M=82,000 achieved in compact 450×250 mm analyzer while accounting all realistic spreads of ion beam and ion packets, so as DET=1.5 ns time spread. The outstanding performance and low level of analyzer aberrations prove the entire concept and confirms the claimed low TOF and spatial aberrations of MRTOF with the novel wedge ion mirror.

[0214] Yet higher resolutions are expected at larger size instruments, since the flight path L grows as product of instrument dimensions: L=2D.sub.X*D.sub.Z/L.sub.Z, where L.sub.Z is the ion advance per reflection. Note that the dense packing became available with the novel mechanism of compensated ion injection of the present invention.

ANNOTATIONS

[0215] Coordinates and Times:

x,y,z—Cartesian coordinates;
X, Y, Z—directions, denoted as: X for time-of-flight, Z for drift, Y for transverse;
Z.sub.0—initial width of ion packets in the drift direction;
ΔZ—full width of ion packet on the detector;
D.sub.X and D.sub.Z—used height (e.g. cap-cap) and usable width of ion mirrors
L—overall flight path
N—number of ion reflections in mirror MRTOF or ion turns in sector MTTOF
u—x-component of ion velocity;
w—z-component of ion velocity;
T—ion flight time through TOF MS from accelerator to the detector;
ΔT—time spread of ion packet at the detector;

[0216] Potentials and Fields:

U—potentials or specific energy per charge;
U.sub.Z and ΔU.sub.Z—specific energy of continuous ion beam and its spread;
U.sub.X—acceleration potential for ion packets in TOF direction;
K and ΔK—ion energy in ion packets and its spread;
δ=ΔK/K—relative energy spread of ion packets;
E—x-component of accelerating field in the OA or in ion mirror around “turning” point;
μ=m/z—ions specific mass or mass-to-charge ratio;

[0217] Angles:

α—inclination angle of ion trajectory relative to X-axis;
Δα—angular divergence of ion packets;
γ—tilt angle of time front in ion packets relative to Z-axis
λ—tilt angle of “starting” equipotential to axis Z, where ions either start accelerating or are reflected within wedge fields of ion mirror
θ—tilt angle of the entire ion mirror (usually, unintentional);
φ—steering angle of ion trajectories or rays in various devices;
ψ—steering angle in deflectors
Εspread in steering angle in conventional deflectors;

[0218] Aberration Coefficients

T|Z, T|ZZ, T|δ, T|δδ, etc;

[0219] Indexes are within the text

[0220] Although the present invention has been describing with reference to preferred embodiments, it will be apparent to those skilled in the art that various modifications in form and detail may be made without departing from the scope of the present invention as set forth in the accompanying claims.