Ion guide within pulsed converters

Abstract

Elongation of orthogonal accelerators is assisted by ion spatial transverse confinement within novel confinement means, formed by spatial alternation of electrostatic quadrupolar field (22). Contrary to prior art RF confinement means, the static means provide mass independent confinement and may be readily switched. Spatial confinement defines ion beam (29) position, prevents surfaces charging, assists forming wedge and bend fields, and allows axial fields in the region of pulsed ion extraction, this way improving the ion beam admission at higher energies and the spatial focusing of ion packets in multi-reflecting, multi-turn and singly reflecting TOF MS or electrostatic traps.

Claims

1. A pulsed ion accelerator for a mass spectrometer comprising: an ion guide portion having electrodes arranged to receive ions travelling along a first dimension, including a plurality of DC electrodes spaced along the first dimension; DC voltage supplies configured to apply different DC potentials to different ones of said DC electrodes such that when ions travel through the ion guide portion along the first dimension they experience an ion confining force, generated by the DC potentials, in at least one dimension orthogonal to the first dimension; and a pulsed voltage supply configured to apply a pulsed voltage to at least one electrode for pulsing ions in a second dimension substantially orthogonal to the first dimension.

2. The pulsed ion accelerator of claim 1, wherein the ion guide portion comprises a first pair of opposing rows of said DC electrodes on opposing sides of the ion guide portion, wherein each row extends in the first dimension, and wherein the DC voltage supplies are configured to maintain at least some of the adjacent DC electrodes in each row at potentials having opposite polarities.

3. The pulsed ion accelerator of claim 2, wherein the ion guide portion comprises a second pair of opposing rows of said DC electrodes on opposing sides of the ion guide portion, wherein each row extends in the first dimension, and wherein the DC voltage supplies are configured to maintain at least some of the adjacent DC electrodes in each row at potentials having opposite polarities.

4. The pulsed ion accelerator of claim 1, wherein the DC voltage supplies are configured to maintain the DC electrodes at potentials so as to form an electrostatic quadrupolar field in a plane orthogonal to the first dimension, wherein the polarity of the quadrupolar field alternates as a function of distance along the first dimension.

5. The pulsed ion accelerator of claim 1, wherein the DC electrodes are arranged to form a quadrupole ion guide that is axially segmented in the first dimension, and wherein the DC voltage supplies are configured to maintain DC electrodes that are axially adjacent in the first dimension at opposite polarities, and DC electrodes that are adjacent in a direction orthogonal to the first dimension at opposite polarities.

6. The pulsed ion accelerator of claim 1, wherein the DC electrodes are arranged on one or more printed circuit board (PCB), insulating substrate, or insulating film.

7. The pulsed ion accelerator of claim 1, wherein the DC voltage supplies are configured to apply different DC voltages to the DC electrodes so as to form a voltage gradient in the first dimension that increases the ion confining force as a function of distance in the first dimension.

8. The pulsed ion accelerator of claim 1, wherein the DC electrodes are arranged in rows that are spaced apart in at least one dimension orthogonal to the first dimension for confining the ions between the rows, and wherein the DC electrodes are spaced apart in said at least one dimension by an amount that decreases as a function of distance in the first dimension.

9. The pulsed ion accelerator of claim 1, configured to control the DC voltage supplies to switch off at least some of said DC potentials applied to the DC electrodes and then subsequently control the pulsed voltage supply to apply the pulsed voltage for pulsing ions out of the ion accelerator; and/or wherein the pulsed ion accelerator is configured to control the DC voltage supplies to progressively reduce the amplitudes of the DC potentials applied to the DC electrodes with time, and then subsequently control the pulsed voltage supply to apply the pulsed voltage for pulsing ions out of the ion accelerator.

10. The pulsed ion accelerator of claim 1, comprising electrodes spaced apart in the second dimension on opposite sides of the ion guide portion; wherein these electrodes are spaced apart in said second dimension by an amount that decreases as a function of distance in the first dimension.

11. The pulsed ion accelerator of claim 1, comprising electrodes spaced apart in the second dimension on opposite sides of the ion guide portion; and wherein the average DC potential of said DC potentials is negative relative to said electrodes spaced apart in the second dimension so as to form a quadrupolar field that compresses the ions in the second dimension.

12. A mass spectrometer comprising: a time-of-flight mass analyser or electrostatic ion trap having the pulsed ion accelerator of claim 1, and electrodes arranged and configured to reflect or turn ions.

13. The mass spectrometer of claim 12, comprising: a multi-pass time-of-flight mass analyser or electrostatic ion trap having the pulsed ion accelerator and electrodes arranged and configured so as to provide an ion drift region that is elongated in a drift dimension and to reflect or turn ions multiple times in an oscillating dimension that is orthogonal to the drift dimension.

14. The spectrometer of claim 13, 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 dimension and configured to reflect ions multiple times in the oscillation dimension, wherein the pulsed ion accelerator is arranged to receive ions and accelerate them into one of the ion mirrors; or (ii) the multi-pass time-of-flight mass analyser is a multi-turn time of flight mass analyser having at least two electric sectors configured to turn ions multiple times in the oscillation dimension, wherein the pulsed ion accelerator is arranged to receive ions and accelerate them into one of the sectors.

15. The spectrometer of claim 13, comprising an ion deflector located downstream of said pulsed ion accelerator, and that is configured to back-steer the average ion trajectory of the ions, in the drift dimension, thereby tilting the angle of the time front of the ions received by the ion deflector.

16. The spectrometer of claim 13, comprising an ion source and a lens system between the ion source and pulsed ion accelerator for telescopically expanding the ion beam from the ion source.

17. The spectrometer of claim 13, comprising an ion source in a first vacuum chamber and the pulsed ion accelerator in a second vacuum chamber, wherein the vacuum chambers are separated by a wall and are configured to be differentially pumped, and wherein the ion guide portion protrudes from the second vacuum chamber through an aperture in the wall and into the first vacuum chamber.

18. A method of mass spectrometric analysis within an isochronous electrostatic field, comprising the following step: (a) forming electrostatic quadrupolar field in the XY-plane, which is spatially alternated along the orthogonal Z-direction; (b) passing an ion beam along the Z-direction; (c) pulsed accelerating of the moving ions in the X-direction, thus forming ion packets.

19. A mass spectrometer, comprising: (a) An ion source, generating an ion beam along a first drift Z-direction at some initial energy; (b) An orthogonal accelerator, admitting said ion beam into a storage gap, pulsed accelerating a portion of said ion beam in the second orthogonal X-direction, thus forming ion packets with a smaller velocity component in the Z-direction and with the major velocity component in the X-direction; (c) An electrostatic multi-pass (multi-reflecting or multi-turn) mass analyzer, built of ion mirrors or electrostatic sectors, substantially elongated in said Z-direction to form an electrostatic field in an XY-plane orthogonal to said Z-direction; said two-dimensional field provides for a field-free ion drift in the Z-direction towards a detector, and for an isochronous repetitive multi-pass ion motion within an isochronous mean ion trajectory surface—either symmetry s-XY plane of said ion mirrors or curved s-surface of electrostatic sectors; (d) within said storage gap of said orthogonal accelerator, an ion guide composed of electrodes, symmetrically surrounding said ion beam; said electrodes are energized by at least two distinct DC potentials to form an electrostatic quadrupolar field in the XY-plane, which is spatially alternated along the Z-direction.

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 prior art methods of ion beam spatial confinement within storage gaps of elongated orthogonal accelerators;

(3) FIG. 2 illustrates method of an embodiment of the present invention of ion beam spatial confinement by spatially alternated electrostatic quadrupolar fields;

(4) FIG. 3 shows electrode details and improved boundaries of the quadrupolar field of FIG. 2

(5) FIG. 4 shows construction principles to form the novel quadrupolar electrostatic guide within orthogonal accelerators.

(6) FIG. 5 shows an MRTOF embodiment employing an elongated accelerator, novel confinement means and a method of side bypassing of the elongated accelerator by ion packets;

(7) FIG. 6 shows embodiments of trans-axial lens/wedge, used for spatial focusing of elongated ion packets produced in MRTOF of FIG. 5; and

(8) FIG. 7 illustrates methods of spatial ion packet focusing for MRTOF of FIG. 5, arranged by spatial-temporal correlation of ions in the novel confinement means used for spatial focusing of elongated ion packets.

DETAILED DESCRIPTION

(9) Referring to FIG. 1, prior art orthogonal accelerators (OA) 15 and 17 are shown in the XZ-view 10 and the XY-view 11. OA 15 employs a radio-frequency (RF) field for ion beam confinement, and OA 17 employs a DC field for ion beam confinement. Both OA 15 and 17 sequentially comprise: push electrode P; auxiliary confining electrodes 13; grounded mesh G; pull mesh N; and a set of electrodes for DC acceleration denoted DC with the mesh covered exit electrode.

(10) Continuous ion beam 11 propagates along the Z-axis and enters the space between push P and mesh G electrodes. Within this space, confining electric field 12 is arranged with the aid of auxiliary electrodes 13, connected to some electric signal U, either RF (in device 15) or DC (in device 17). Periodic pulses are applied to electrodes P and N to extract ion packets 14 out of continuous beam 11 for injection into a TOF MS mass analyser.

(11) OA 15 of prior art U.S. Pat. No. 5,763,878 or 8,373,120, proposes the spatial confinement of the ion beam 11 by radiofrequency RF radial field 16, generated by applying an RF signal to side electrodes 13. Optionally, the RF field is switched off before ion extraction pulses are applied (to P and N). Both the effective potential well of the RF field and the micro-oscillations of the ions depend on ion mass to charge ratio m/z=μ. Parameters of the ion beam 11 and of pulsed ion packets 14 depend on μ, on the RF phase at switching off, and on the time delay to pulses. In addition, OA 15 has two major drawbacks: (a) the RF field limits the transmitted mass range and (b) the extraction pulses induce strong oscillations onto resonant RF generators, thus impeding transmission, resolution and mass accuracy of TOF MS.

(12) OA 17, proposed in RU2013149761 employs a rectilinear electrostatic quadrupolar field 18, formed by applying a negative DC potential to electrodes 13. A weak electrostatic quadrupolar field focuses and confines the ion beam in the critical TOF X-direction (towards the ion mirror), while defocusing the ion beam in the non-critical transverse Y-direction. The method allows moderate elongation of ion packets 14, estimated to a length in the z-direction of about L.sub.Z≤50 mm. Longer OAs suffer strong ion losses in the Y-direction.

(13) Referring to FIG. 2, there are shown XZ 20, XY 25 and XYZ 26 views of an embodiment 20 of the present invention, depicting a gridless orthogonal accelerator with novel means for ion beam spatial confinement. Embodiment 20 comprises: a push electrode P; a pair of pull electrodes N with a slit S in-between; a set of electrodes 23 forming an ion guide for spatial ion beam confinement, located in the space between plates P and N and connected to at least two DC signals DC1 and DC2; a DC acceleration stage DC; and a lens L for terminating DC field at nearly zero ion packet divergence in the XY-plane. All electrodes of the OA may be aligned with the drift Z-axis. The OA 20 may be preceded by an ion source 27 generating an ion beam at specific energy per charge UZ and by a lens system 28.

(14) In operation, downstream of ion source 27, lens system 28 may expand the ion beam telescopically and form a nearly parallel ion beam 21 along the Z-axis. The telescopic expansion is preferably used to optimize so-called phase balancing of the ion beam 21 within ion guide 23, where initial angular divergence and width of the ion beam 21 provide for about equal impact onto thickness of the confined ion beam 29.

(15) Ion beam 21 enters the P-N gap and becomes spatially confined in the region 22 by a set of alternating electrodes with distinct DC voltages DC1 and DC2, generating a spatially alternating quadrupolar DC field E(X,Y), approximated at the field axis by a transverse field distribution:
E(X,Y)=E.sub.0*(X−Y)/R*sin(2πZ/H)  (Eq.1)

(16) where E, Y and Z are the dimensions of the ion guide; H is spatial period of quadrupolar field alternation, and R is the characteristic field radius.

(17) For ions having mass to charge ratio μ=m/z at specific axial (along Z-axis) energy U.sub.Z, the axial velocity is V.sub.Z=(2U.sub.Z/μ).sup.0.5. The spatial alternation of the quadrupolar DC field is sensed by ions moving through the DC field as if a periodic RF signal was being applied, which is known to radially confine ions to the field axis. The frequency of the sensed RF field F=H/V.sub.Z is inversely proportional to μ.sub.0.5. Then the effective potential well D(r) of the sensed RF field depends on the ion radial position r (where r.sup.2=X.sup.2+Y.sup.2). It is important to note that D(r) is independent of the ion mass to charge ratio μ:
D(r)=E.sub.0.sup.2*(r.sup.2/R.sup.2)/μ(2πF).sup.2=[E.sub.0.sup.2H.sup.2/2πU.sub.Z]*(r.sup.2/R.sup.2)  (Eq. 2)
[For reference: D(r)=E.sub.0.sup.2*(r.sup.2/R.sup.2)/μ(2πF).sup.2=E.sub.0.sup.2*(r.sup.2/R.sup.2)/μV.sub.Z.sup.2(2π/H).sup.2]

(18) Thus, the novel electrostatic ion guide equally confines ions of all mass to charge ratios μ, e.g. assuming they have similar axial and radial energies.

(19) The alternating quadrupolar field indefinitely (per Z) confines ion beam 29 in both transverse directions (i.e. X and Y directions), producing a spatially tight ion beam within substantially elongated orthogonal accelerators or other pulsed converters. Electrical pulses may be applied to electrodes P and N to convert the continuous ion beam 29 into pulsed ion packets 24 by orthogonal pulsed extraction. Preferably, voltages DC1 and DC2 are switched to zero or to different setting U1 and U2 at the time of the pulsed ion ejection so as to improve the electric field distribution at ion ejection.

(20) The novel electrostatic quadrupolar ion guide 23 provides for indefinite ion beam confinement. Relative to the RF confinement of prior art device 15 (see FIG. 1), the novel electrostatic confinement provides multiple advantages: it is mass independent; it does not require resonant RF circuits and can be readily switched on and off; the strength and shape of the transverse confining field can be readily varied along the guide length (i.e. along the z-direction); it can provide an axial gradient, slight wedge or curvatures of the confining field without constructing complex RF circuits.

(21) Referring to FIG. 3, the electrode structure of ion guide 23 for quadrupolar electrostatic ion confinement within OA 20 (of FIG. 2) is illustrated in multiple views 30, 31, and 32 of embodiment 30. The ion guide 23 in embodiment 30 comprises four rows 33 (in the z-direction) of electrodes 34,35. Electric potential DC1 is applied to alternating electrodes 35 in each row, as shown by the darker coloured electrodes 35. Electric potential DC2 is applied to alternating electrodes 34 in each row, as shown by the lighter coloured electrodes 34. Electrodes 34 and 35 are interleaved in the z-direction.

(22) In operation, as best seen in 3D view 31, electrodes 34 and 35 form a local quadrupolar electrostatic field 22 in every XY-cross section. The polarity of the quadrupolar field changes when shifting in the Z-direction. Ion beam 21 at specific mean energy U.sub.Z may be formed in an ion source 27, and may be shaped by lens 28. Ion beam 21 enters quadrupolar field 22 along the Z-axis. From this point the ion beam is denoted by number 29. Because of the periodically spatially alternating DC quadrupolar field, ions moving along the Z-axis sense a quadrupolar field that periodically changes with time, which is known to provide radial ion confinement towards the field axis (in a similar manner to an RF field acting on a static ion). The ion beam stays spatially confined in the x-y plane at limited angular divergence, without limits on the Z-length. The beam 29 is refocused multiple times by the quadratic field, eventually mixing ions within a limited phase space.

(23) Preferably, lens 28 reshapes the phase space of the ion beam 21 entering the ion guide 23 for optimal balance between width and divergence of the confined ion beam 29. Preferably, the average potential (DC1+DC2)/2 is slightly negative relative to P and N electrodes to form a combination of the alternating quadrupolar field 22 with a constant per Z quadrupolar field, thus providing stronger compression of the ion beam 29 in the X-direction Vs Y-direction.

(24) Embodiment 30 is further improved by arranging so-called “adiabatic entrance” 36 and “adiabatic exit” 37 conditions for ion beam 29.

(25) For adiabatic entrance 36, there is arranged a smooth spatial rise of quadrupolar DC field, spread for at least 2-3 spatial periods of the DC field alternation. The smooth rise of the quadrupolar field may be arranged either by the illustrated Y-spreading of ion guide 23 electrodes, and/or by narrowing of the storage gap between electrodes N and P in the X-direction, and/or by arranging a gradient of DC voltages in the Z-direction, e.g. by resistive dividers.

(26) Ions staying on axis of the guide 23 experience zero transverse field and have zero micro-motion, however, radially distant ions do not. For “adiabatic exit” 37 of radially distant ions at pulsed extraction of ion packets, embodiments of the invention initially maintain the DC1 and DC2 amplitudes constant and then switch the amplitudes to gradually decrease with time, e.g. as shown for DC1 in graph 37. The switching time may correspond to the time after the ion has passed through several DC alternations of the ion guide 23, as shown in plot 37 by time variation 38 of sensed quadrupolar field for some probe ion. This adiabatic switching reduces the energy of “micro-motion” of the ions within the confined ion beam 29 before pulsed ejection.

(27) Referring to FIG. 4, multiple construction principles 40 to 46 are proposed for forming confining means 23 within OA 20 of FIG. 2.

(28) One particular embodiment 40 of the static quadrupolar guide 23 comprises a set of four parallel-aligned printed circuit boards (PCB) 47. Conductive pads on each board 47 form a row of alternated electrodes 34 and 35, distinct in the drawing by color coding as described above. Two DC potentials are interconnected with the conductive pads through displaced PCB vias, DC1 to electrodes 35 and DC2 to electrodes 34. Each side (in the Y-direction) of ion guide 40 is formed by a pair of boards 47, separated by an insulating plate, which is preferably also a PCB. Alternatively, the pair may be arranged within a single thick multilayer PCB for better precision. Since boards 47 are set distant from spatially confined ion beam 29, only limited care shall be used to shield insulating surfaces from stray ions. Since DC1 and DC2 potentials are expected to be in the range of several tens of Volts, the insulating ridges may be thin. Still, edge slots and edge conductive coatings are preferred for the ion guide robustness against the charging by stray ions.

(29) Another particular embodiment 41 employs conductive electrodes 34 and 35 attached to both sides of a single PCB support 47. This is equivalent to one pair of boards 47 shown in embodiment 40. Another PCB support 47 with conductive electrodes 34 and 35 attached to both sides thereof would be required to form the ion guide 23 according to embodiment 41.

(30) Yet another particular embodiment 42 comprises a row of alternating electrodes 34 and 35 constructed of two thin electrode plates that are spaced apart by a thin insulator such as a film, say, PTFE or Kapton film. Extending electrode ribs appear mutually displaced in the X-direction by the thickness of the insulator, which is expected to generate only minor Z-modulation of the quadrupolar field on the beam 29 axis. This is equivalent to one pair of boards 47 shown in embodiment 40. Another corresponding structure would be required to form the ion guide 23 according to embodiment 42.

(31) Ion guides 42-44 are preferred for their compatibility with heating to approximately 150-200° C. for robust operation of the guide, for preventing built-up of insulating coatings or deposition of droplets from ESI sources.

(32) Yet another particular embodiment 43 comprises machined (say by EDM) electrodes with bent extending electrode ribs. Optionally, ribs may be slightly bent in embodiment 42 as well.

(33) Yet another particular embodiment 44 may have a curved Z axis, e.g. for reducing gas flux, for removal of charged droplets from ESI ion source, for removal of light and metastable particles from EI source, or for convenience of instrumental packaging. Initially turned electrodes may be machined by EDM.

(34) Again referring to FIG. 4, in embodiment 45, electrostatic quadrupolar guides 40-44 may be further improved by seamless extending of the ion guides beyond the ion OA ion storage gap of electrodes N and P, e.g. so as to guide ions passed gaseous RF ion guides or passed ion optics, already forming a nearly parallel ion beam. Preferably, the ion guiding PCBs 47 (or set of conductive electrode 34 and 35) may pass through a wall 48 that separates differentially pumped stages of the spectrometer, with the pumping denoted by white arrows. The guide is expected to operate in the pressure range of, for example, up to 0.1-1 mTorr. Beyond this pressure threshold, ions may start losing their kinetic energy and may be lost on the ion guide walls.

(35) Again referring to FIG. 4, in embodiment 46 an array of ion guides 49 may be formed for operating with multiple ion sources, or multiple beam 21 fractions for increased throughput of mass spectral analyses with various TOF MS, or for mapping or imaging MRTOF, e.g. for use with the systems as described in WO2017091501, WO2017087470, and WO2017087456.

(36) Referring to FIG. 5, an OA-MRTOF embodiment 50 according to the present invention is shown in two variants: 50L—with linear Z-axis and 50C—with circular Z-axis, where functionally similar components are denoted with the same numbers between variants. Embodiment 50L comprises the novel electrostatic quadrupolar ion guide 51 for ion beam spatial confinement within a Z-elongated orthogonal accelerator 52. Embodiment 50 further comprises a pair of parallel gridless ion mirrors M, separated by a floated field-free drift space to form a multi-reflecting analyzer. Electrodes of OA 52 and of ion mirrors M are substantially elongated in the linear drift Z-direction to provide a two-dimensional electrostatic field in the X-Y plane, symmetric around s-XZ symmetry plane of isochronous trajectory surface and having zero field component in the Z-direction. Embodiment 50 further comprises: a continuous ion source 27; a lens system 28 to form a substantially parallel ion beam 21; an isochronous Z-focusing trans-axial lens 53; a set of dual Y-deflectors 54 and 55; and a TOF detector 59. Preferably, ion source 27 comprises an RF ion guide with pulsed exit gate, denoted by RF and by pulse symbol.

(37) In operation, ion beam 21 is generated by source 27, formed by ion optics 28, and entering OA 51 along the Z-direction. Ion beam is transverse confined with guide 51, as described in FIGS. 2 to 4, becoming a confined portion 29 of the ion beam 21. Pulsed OA 52 extracts elongated ion packets 58. The mean ion trajectory of the ion packets 58 moves at a small inclination angle α to the x-axis, which is controlled by the U.sub.Z specific energy of ion beam 21 and by the acceleration voltage U.sub.X of the drift space.

(38) Downstream of OA 51, elongated ion packets 58 are pulsed displaced in the Y direction by deflectors 54 and 55, thus bypassing the Y-displaced OA 52 and returning to the axis of ion mirrors M (best seen in the X-Y plane view). Ions are reflected between ion mirrors M in the X-direction within the s-XZ symmetry plane while drifting towards the detector 59 in the z-direction. Since ion packets are focused by trans-axial lens 53 in the Z-direction, they reach the face of detector 59 without hitting the rims of the detector. The duty cycle of the OA-MPTOF 50 may be improved, e.g. to above 50% from the several percent in conventional MPTOFs. The method becomes possible because of ion beam spatial confinement within the OA by the novel quadrupolar electrostatic ion guides. While embodiment 50 depicts multi-reflecting TOF MS (MR TOF), similar improvements are applicable to sector multi-turn TOF MS (MT TOF) and to singly reflecting TOF MS. The injection scheme of circular embodiment 50C may be useful for ion injection into cylindrical electrostatic traps.

(39) Referring to FIG. 6, there are shown two embodiments 60 and 61 of Z-elongated gridless orthogonal accelerators (52 in FIG. 5) with quadrupolar electrostatic ion guide 23 (51 in FIG. 5). Both embodiments comprise push plate P, pull slit electrode N, slit electrodes DC for static acceleration, and a particular trans-axial lens 53. The trans-axial lens 53 may be a slit electrode (i.e. through which the ions may be pulsed) that is divided into two electrodes (in the x-direction) by a constant width gap that is curved in the X-Z plane at a curvature radius, e.g. R˜1 m. Trans-axial lens 53 may be chosen for being slim in the Y-direction, which useful for ion packet Y-displacement as shown in FIG. 5. Embodiments 61 and 60 differ by using curvature of extraction field 64, here depicted by trans-axially curved pull electrode P. Embodiment 61 further comprises an optional trans-axial wedge 62 for ion steering. The wedge 62 may be combined with lens 53, which also may be achieved by tilting lens 53 relative to the Z axis.

(40) The figures show iso-potential lines and ion trajectories. According to simulations, the trans-axial lens 53 serves for: (a) terminating the electrostatic DC accelerating field; (b) providing for ion spatial focusing in the XZ-plane to focal plane f2, in all cases simulated for F=5 m focal distance; and (c) providing substantial parallel beam in the XY-plane. Graph 63 shows time spreads introduced by spatial ion Z-focusing, simulated for 1000 amu ions. The trans-axial lens 53 alone in the embodiment 60 introduces positive T|ZZ aberration with additional time spread dT(z)=T|ZZ*z.sup.2. The long focal distance F=5 m helps keeping the aberration moderate and allows focusing L.sub.Z=20 mm long ion packets at dT(z)=0.3 ns amplitude.

(41) Use of curved extraction field 64 in the embodiment 61 allows reverting the sign of the overall T|ZZ aberration, which may be further optimized for complete mutual compensation of T|ZZ aberrations. Without describing exhaustive details of ion optical simulation, the novel quadrupolar electrostatic ion guide 23 was found an important part of the Z-focusing trans-axial system: it retains the ion beam at limited width and diameter; it controls initial starting position at acceleration; it helps forming a T|ZZ compensating curvature of extracting pulsed field; it helps forming spatially focusing in Y-directions, while eliminating multiple time per Y aberrations.

(42) Referring to FIG. 7, similar to FIG. 5, OA-MRT embodiment 70 of the present invention comprises: two parallel gridless ion mirrors M; an Z-elongated orthogonal accelerator OA 52, an optional trans-axial wedge/lens 53 for ion packet focusing; a dual Y-deflector 54 and 55 for the side OA bypassing by ion packets; and a detector 59. Ion beam 29 is retained within elongated OA 52 by any of described spatial confinement means 23/51.

(43) Within ion packets 58, ions retain the V.sub.Z velocity of ion beam in the z-direction. If forming a negative correlation between V.sub.Z and z-coordinate in guide 51, ion packets 58 would be naturally focused onto detector 59.

(44) Focusing condition 71 for a narrow range of mass to charge ratios μ=m/z may be achieved by pulsing of ion source or transfer optics, where V.sub.Z(z) is the ion axial velocity in guide 51, V.sub.Z0=V.sub.Z(z=0), and D.sub.Z is the OA-detector distance:
V.sub.Z(z)/V.sub.Z0=1−z/D.sub.z@μ=m/z  (eq. 3)

(45) For this purpose, the embodiment 70 may comprise one of the following means: an RF ion guide 73 with optional auxiliary electrodes 74 and an exit gate 75; a pulse generator; a time dependent U(t) signal generator.

(46) In one method, an ion extracting pulse is applied to gate 75. The extracting pulse is known to generate an ion bunch with an energy spread in spite of gaseous dampening at about 10 mTorr gas pressures. Deeper starting ions will arrive to the OA 52 at later time, appear at smaller z within the guide 51, but will have larger V.sub.Z. This produces ion packet compression 71 (eq. 3) at the detector 59. Though the method looks similar to the known Pulsar method, here ions are Z-compressed at the D.sub.Z distance of detector 59, rather than at the OA center of conventional TOF instruments. Note that the correlation 71 (eq. 3) occurs for narrow μ range only, controlled by the time delay between extraction and OA pulses. The embodiment is attractive for target analysis, where a narrow mass range is selected intentionally, while TOF data may be acquired at maximal OA frequency and at maximal dynamic range of the MRTOF detector.

(47) In another method, to arrange the correlation 71 (eq. 3), either ion guide 73 and/or extraction electrode 75 and/or lens 28 are arranged into an elevator system, whose reference potential is time variable U(t). The effect of the time variable elevator is very similar to the above described bunching effect, though the elevator exit may be set closer to the OA entrance and may allow somewhat wider μ range. In both above methods, a nearly unity duty cycle of OA is expected for narrow μ range, thanks to the novel confinement means 51, permitting substantial OA elongation.

(48) Yet in another method, to obtain focusing conditions for a wide mass range i.e. for all μ, the z-dependent specific energy U(z) (energy per charge) may be arranged with a resistive divider within confining means 51. For optimal ion packet compression onto detector 59, the U(z) shall satisfy condition 72, where U.sub.Z0=U(z=0):
U(z)/U.sub.Z0=(1−z/D.sub.Z).sup.2  (eq. 4)

(49) Ion beam 29 slows down in a Z-dependent axial potential distribution U(z) of confinement means 51. The desired z-focusing of ion packets is achieved for the entire ionic mass range, i.e. occurs for ions of all μ, while confinement means 51 provide mass independent radial confinement, as has been explained with equation Eq. 2. The method may be particularly attractive when using a “soft and prolonged” Pulsar mode, where open gate forms a prolonged quasi-continuous ion beams.

(50) Again referring to FIG. 7, particular OA-MRTOF embodiments 76 and 77 of the present invention employ components and methods of embodiment 70. Embodiment 76 is improved by using higher energies of continuous ion beam 21, the OA 52 is tilted at angle δ to the z-axis and ions are back steered (in the z-direction) within a trans-axial lens/wedge 53 and 63. Embodiment 77 also allows using higher beam energies with back deflection with trans-axial lens/wedge 53 and 63, however, to compensate for time-front tilting and bending by TA wedge/lens 53 and 63, the OA 52 remains straight, while a wedge pulsed accelerating field is arranged for compensating tilting of ion packets time fronts, similar to a co-pending PCT application having the same filing date as this application and entitled “ACCELERATOR FOR MULTI-PASS MASS SPECTROMETERS” (and claiming from GB 1712613.7 filed 6 Aug. 2017). In both embodiments 76 and 77, ion confinement means 51 are useful for confining ion beam 29 within a precisely defined region of accelerating field.

(51) Referring to FIG. 8, improved accelerator 52 with ion confining means 51 by spatially alternated electrostatic quadrupolar field is applicable to a wider variety of isochronous electrostatic analyzers, exampled here by embodiment 80 of multi-turn sector TOF MS, embodiment 81 of singly reflecting TOF MS, and embodiment 82 of circular (also referred as “elliptical”) electrostatic trap. All those embodiments comprise the same components of FIG. 5: continuous ion beam 21, quadrupolar electrostatic ion guide 51 for spatial confinement of ion beam 29, being a confined portion of beam 21, an orthogonal accelerator 52, a trans-axial wedge/lens 53, a deflector 54, and a detector 59.

(52) Annotations

(53) 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;

(54) 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;

(55) 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 ϵ—spread in steering angle in conventional deflectors;

(56) Aberration Coefficients T|Z, T|ZZ, T|δ, T|δδ, etc;

(57) indexes are defined within the text

(58) 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.