Patent Application
20210242007
This application claims priority from and the benefit of United Kingdom patent application No. 1806507.8 filed on 20 Apr. 2018. The entire content of this application is incorporated herein by reference.
The invention relates to the area of multi-reflecting time-of-flight mass spectrometers and electrostatic ion traps, and is particularly concerned with improved electric fields in gridless ion mirrors.
TOF-MS with ion mirrors: Time-of-flight mass spectrometers (TOF MS) are widely used for their combination of sensitivity and speed. An ion mirror with two stages separated by grids has been introduced by Mamyrin in SU198034. The mirror folds the ion trajectories and allows reaching second order time per energy focusing, this way improving mass resolving power of TOF MS. Since then, vast majority of TOF MS employ ion mirrors. To eliminate ion losses and ion scattering on grids, gridless (grid-free) ion mirrors with moderate ion optical quality were introduced in U.S. Pat. No. 4,731,532A.
Multi-reflecting TOF MS: Introduction of Multi-reflecting TOF (MRTOF) MS greatly improves resolution and mass accuracy of TOF MS. Resolution improves primarily due to substantial extension of ion path, say, L=20-50 m in MRTOF versus L=2-5 m of singly reflecting TOF. To fit a reasonable instrument size, the ion path is densely folded between gridless ion mirrors, where grids can not be used because of devastating ion losses at multiple grid passages, as described in SU1725289, U.S. Pat. Nos. 6,107,625, 6,570,152, GB2403063, U.S. Pat. No. 6,717,132, incorporated herein by reference.
E-Traps: As exampled by U.S. Pat. No. 6,744,042, WO2011086430, US2011180702 and WO2012116765, incorporated herein by reference, multi-reflecting analyzers are proposed for use as electrostatic ion traps (E-traps). Ions are trapped between ion mirrors, oscillate at a mass dependent frequency, and the oscillation frequency is recorded with image current detectors. WO2011107836 proposes an open trap—a hybrid between TOF and E-trap.
Ion Mirrors: Most of MRTOF and E-traps employ similar electrostatic analyzers composed of two parallel gridless ion mirrors, separated by a drift space. Coaxial gridless ion mirrors were introduced in H. Wollnik, A. Casares, Int. J. Mass Spectrom. 227 (2003) 217-222 while planar gridless ion mirrors with improved third-order energy isochronicity and second-order spatial isochronicity were introduced in GB2403063. Further improvements in WO2013063587 and WO2014142897 have brought the energy isochronicity to fifth-order and spatial isochronicity to full third-order, including cross terms on energy, angular and spatial spreads. It is of significant relevance that gridless ion mirrors of high ion-optical quality have been constructed of very few thick electrodes, either rings or frames to generate desired field distributions.
PCB ion mirrors: Since the 1980s, printed circuit board (PCB) technology was proposed for making electrodes and electrode assemblies for mass spectrometers, as exampled in U.S. Pat. Nos. 4,390,784, 4,855,595, 5,834,771, 5,994,695, 6,614,020, 6,580,070, 7,498,569, EP1566828, U.S. Pat. Nos. 6,316,768, 7,675,031 and 8,373,120, incorporated herein by reference. However, the field structures of those mirrors were copying known mirror designs and were concerned with the construction method rather than with improved fields. As far as is known, there were no PCB mirrors proposed with an improved ion optical quality of ion mirrors, matching or exceeding the ion optical quality of best thick electrode mirrors.
The present invention provides an ion mirror for reflecting ions along an axis (X) comprising: a first axial segment (E2), within which the turning points of the ions are located in use, and a second axial segment (E3), wherein the first and second axial segments are adjacent each other in a direction along said axis (X); wherein at least the first axial segment comprises a plurality of electrodes that are spaced apart from each other along said axis (X), wherein the electrodes in at least the first axial segment have substantially the same lengths along said axis and adjacent pairs of these electrodes are spaced apart by substantially the same spacing such that these electrodes are arranged so as to have a pitch P along said axis; wherein said plurality of electrodes define windows arranged in a plane (Y-Z plane) orthogonal to said axis (X) through which the ions travel in use, wherein the windows have a minimum dimension H in said plane (Y-Z plane); and wherein P≤H/5.
The mirror may have a first axial end for receiving ions into the ion mirror, and a second axial end that the ions travel towards and are then reflected back towards (and out of) the first axial end. The second axial segment may be arranged closer to said first axial end of the ion mirror (i.e. the entrance/exit end) than the first axial segment.
The mirror may comprise voltage supplies for applying different voltages to different electrodes of the ion mirror for generating electric fields for performing said reflecting of the ions. At least the first axial segment may be defined between inter-segment electrodes that are spaced apart along said axis, each of said inter-segment electrodes being an electrode to which one of said voltage supplies is connected to. Said plurality of electrodes in the first axial segment may be arranged between the inter-segment electrodes, and may be electrically connected thereto and interconnected with each other by electronic circuitry such that when the voltage supplies apply voltages to the inter-segment electrodes, this causes the plurality of electrodes to be maintained at different potentials so as to generate said electric fields.
The term “inter-segment electrodes” refers to the electrodes at the axial ends of each axial segment, such as between adjacent segments. The “knot” electrodes referred to elsewhere herein are embodiments of the inter-segment electrodes.
The inter-segment electrodes defining the first axial segment may be connected to voltage supplies such that they are supplied with first and second potentials respectively, wherein a mean potential of the first and second potentials may equal a mean energy K0 of an ion to be reflected in the mirror divided by the charge q of that ion. This may ensure that the ions are reflected in the first axial segment.
The plurality of electrodes in the first axial segment may be interconnected to each other by a chain of resistors.
The chain of resistors may be configured to form a substantially linear potential gradient at and along the plurality of electrodes within the segment.
The electrodes at the axial ends of the plurality of electrodes in the first axial segment may be electrically connected to the adjacent inter-segment electrodes, e.g. via resistors, so that the application of the voltages to the inter-segment electrodes causes voltages to be applied to the plurality of electrodes.
This enables the number of voltage supplies to be reduced. The precision of the resistors described above may be set at 1% or better, e.g. to sustain an optimal simulated field strength ratio E2/E1.
The second axial segment may also be bounded by inter-segment electrodes and may comprise a plurality of electrodes between them. These plurality of electrodes may be connected to each other and to the inter-segment electrodes using resistors, as described above in relation to the first axial segment.
The mirror may be configured such that the distance (X3) along said axis from the mean ion turning point in the first axial segment to the inter-segment electrode nearer to the mirror entrance/exit is ≤2H; ≤1.5H; ≤1H; ≤0.5H; in the range 0.2H≤X3≤1.7H; or in the range 0.1H≤X3≤1H.
The distance may be 0.2H≤X3≤1.7H in the case of a mirror having planar symmetry or may be 0.1H≤X3≤1H in the case of a mirror having cylindrical mirror symmetry.
The mirror may comprise voltage supplies and be configured to apply electric potentials to the electrodes of the first axial segment for generating a first linear electric field of a first strength E2 within the first axial segment, and to apply electric potentials to electrodes of the second axial segment for generating a second linear electric field of a second strength E3 within the second axial segment; wherein the ratio of field strengths E3/E2 is related to the distance X3 by the relationship E3/E2=A*[0.75+0.05*exp((4X3/H)−1)], where 0.5≤A≤2.
This relationship may be for ion mirror with planar symmetry.
The ratio E3/E2 may be one of the group: (i) 0.8≤E3/E2≤2 at 0.2≤X3/H≤1; (ii) 1.5≤E3/E2≤10 at 1≤X3/H≤1.5; and (iii) E3/E2≥10 at 1.5≤X3/H≤2.
The ion mirror may comprise a third axial segment arranged further from an entrance end of the ion mirror than the first axial segment. The mirror may comprise voltage supplies configured to apply electric potentials to the electrodes of the first axial segment for generating a first linear electric field of a first strength E2 within the first axial segment, and to apply electric potentials to electrodes of the third axial segment for generating a third linear electric field of a third strength E1 within the third axial segment; wherein E1<E2. The mirror may be configured such that the distance (X2) along said axis from the mean ion turning point within the first axial segment to the inter-segment electrode further from the mirror entrance is 0.2≤X2/H≤1.
The ion mirror may comprise voltage supplies and may be configured to apply electric potentials to the electrodes of the first axial segment for generating a first linear electric field (E2) of a first strength within the first axial segment, and to apply electric potentials to electrodes of the second axial segment for generating a second linear electric field (E3) of a second strength within the second axial segment; wherein the electrodes are configured such that the second linear electric field (E3) penetrates into the first axial segment so that the axial electric field in an axial portion of the first axial segment is non-linear where the turning points of the ions are located.
The axial electric field strength (E0) at the mean ion turning point may therefore be slightly different to the first strength of the first linear electric field (E2).
The electric fields described above may be the axial electric fields along the central axis of the mirror (i.e. away from the electrodes).
An axial electric field strength E0 at a mean ion turning point within the first axial segment may be related to the strength of the first linear electric field E2 by a relationship from the group comprising: (i) 0.01≤(E0−E2)/E2≤0.1; and (ii) 0.015≤(E0−E2)/E2≤0.03.
The electrodes may be configured such that the second linear electric field (E3) penetrates into the first axial segment so that the equipotential field lines in the first axial segment are curved where the turning points of the ions are located.
The different field strengths in said first and second axial segments may produce curved equipotential field lines in a transition region between the first and second axial segments.
Electrodes in the second axial segment may have substantially the same lengths along said axis and adjacent pairs of these electrodes may be spaced apart by substantially the same spacing such that these electrodes are arranged so as to have a pitch P along said axis. The plurality of electrodes may define windows in a plane (Y-Z plane) orthogonal to said axis (X) through which the ions travel in use, wherein the windows have a minimum dimension H in said plane (Y-Z plane). The ratio of said pitch to height may be given by P≤H/5.
Although two axial segments of the ion mirror have been described, the ion mirror may comprise more than two axial segments.
The mirror may comprise a third axial segment (E1) adjacent to the first axial segment (E2) in a direction along said axis (X); wherein the third axial segments comprises a plurality of electrodes that are spaced apart from each other along said axis (X).
The third axial segment may be arranged further from the first axial end of the ion mirror (the entrance end) than the first axial segment.
Electrodes in the third axial segment may have substantially the same lengths along said axis and adjacent pairs of these electrodes may be spaced apart by substantially the same spacing such that these electrodes are arranged so as to have a pitch P along said axis. The plurality of electrodes may define windows in a plane (Y-Z plane) orthogonal to said axis (X) through which the ions travel in use, wherein the windows have a minimum dimension H in said plane (Y-Z plane). The ratio of said pitch to height may be given by P≤H/5.
The mirror may comprise voltage supplies and may be configured to apply electric potentials to the electrodes of the third axial segment for generating a third linear electric field (E1) of a third strength within the third axial segment. The electrodes may be configured such that the third linear electric field (E1) penetrates into the first axial segment so that the axial electric field in an axial portion of the first axial segment is non-linear where the turning points of the ions are located.
The axial electric field strength (E0) at the mean ion turning point may therefore be slightly different to the first strength of the first linear electric field (E2).
The length of the first axial segment along said axis may be ≤5H; ≤4H; ≤3H; or ≤2H.
Providing a relatively short first axial segment enables the electric fields from the adjacent axial segments to penetrate to the ion turning point.
The mirror may comprise voltage supplies and may be configured to apply electric potentials to the electrodes of the first axial segment for generating a first linear electric field (E2) of a first strength within the first axial segment, and to apply electric potentials to electrodes of the second axial segment for generating a second linear electric field (E3) of a second, different strength within the second axial segment; so as to form a non-uniform axial electric field at the boundary between the first and second axial segments.
The electrode windows described herein may have no mesh or grid electrodes located therein. The entirety of the ion mirror may have no mesh or grid electrodes located therein.
The plurality of electrodes (and inter-segment electrodes) may be apertured electrodes that have their apertures aligned along said axis, wherein the apertures are said windows. The apertures may be rectangular, circular or another shape. The apertures may have the same size and/or shape throughout the mirror.
Alternatively, each axial segment may comprise rows of electrodes, wherein the rows are spaced apart orthogonally to the axis of reflection. Each of these rows may comprise said plurality of electrodes that are spaced apart from each other along said axis. The electrodes in the rows define windows in a plane (Y-Z plane) orthogonal to said axis (X) through which the ions travel in use. The minimum dimension H of the windows in said plane (Y-Z plane) may correspond to the distance between the rows.
The mirror may have voltage supplies and be configured to apply electric potentials to the electrodes of the first axial segment for generating a first linear electric field of a first strength E2 within the first axial segment, wherein 4.3U0/D<E2<5U0/D, where U0 is equal to a mean energy K0 of an ion to be reflected in the mirror divided by the charge q of that ion, and D is the distance from the mean ion turning point to a first order energy focusing time focal point of the mirror.
The mirror may be configured such that 15≤D/H≤25.
The mirror may comprise an entrance lens, the entrance lens optionally comprising one of the group: (i) an accelerating lens; (ii) a retarding lens; (iii) a multistage lens; (iv) a dual lens formed on both ends of an elongated lens electrode; and (v) an immersion lens.
The potentials and dimensions of the axial segments may be optimized per particular entrance lens to provide spatial ion focusing, for at least full second order spatial isochronicity and optionally high order time per energy isochronicity of the list: (i) at least third-order energy isochronicity; (ii) at least forth-order of energy isochronicity; (iii) at least fifth-order energy isochronicity; and (iv) at least sixth-order energy isochronicity. Small energy aberrations of particular order may be left at residual level for partial compensation of higher order aberrations.
The axial segments may be made using thin conductive electrodes, which may be either metal, carbon filled epoxy protrusion profiles, or conductive coated insulators. The electrodes may be attached to one or more insulating substrate, such as plastics, printed circuit boards (or PCB substrate), epoxy, ceramics, or quartz, or may be clamped with insulating spacers.
The positioning accuracy and straightness of the electrodes may be improved by either slots in the insulating substrates or by multiple connecting pins; or by using precision spacers, and/or by technological fixtures at electrode attachment to the substrate.
At least some of the electrodes of the ion mirror are conductive strips of a printed circuit board (PCB).
The PCB substrate may be made of either epoxy-based material, ceramics, quartz, glass, or Teflon.
The PCB may be provided with antistatic surface properties.
This may be provided by the residual conductance of the substrate, conductive lines on the substrate (other than the electrodes), by an antistatic or resistive coating on the substrate (e.g. of GOhm to TOhm range), or by maintaining the spacing between electrode strips as <1 mm.
An antistatic coating may be either deposited on top of or under the conductive strips. The antistatic coating may be produced by one of the group: (i) depositing onto a surface an insulator (e.g. polymer or metal oxide) coated with conductive particles; (ii) (thin) coating a surface with low conductance material such as SnO2, InO2, TiO2, or ZrO2; and (iii) exposing a surface to glow discharge at intermediate gas pressures with deposition of metal atoms or metal oxide molecules onto said PCB surface.
The mirror may comprise two parallel printed circuit boards that are spaced apart by said minimum dimension H, and which comprise said plurality of electrodes in the form of a periodic structure of conductive strips aligned on the PCBs orthogonal to said axis and with a period P≤H/5.
The strips may be interconnected by resistive chains as described above.
The inter-segment electrodes may be conductive strips on the PCBs. These inter-segment electrodes may form at least two or three axial segments, as described above.
The printed circuit board may be provided with antistatic properties by providing a periodic structure of parallel conductive lines between said conductive strips and/or an antistatic coating (e.g. with a resistance in the range from 1 GOhm/square to 10 TOhm/square).
The conductive strips may be curved in the plane of the PCB, optionally for forming trans-axial electric fields.
The axial segments may be formed with flexible printed circuit boards, e.g. such as either thin epoxy, Teflon, or Kapton based boards.
The topology of the ion mirror may be one of the group: (i) a 2D-planar mirror with slit windows; (ii) a 2D-circular mirror with ring windows; (iii) 2D-cylindrical mirror with electrodes arced around the Y-axis; and (iv) arc bent with circular Z-axis.
According to some embodiments, the electrodes in the first axial segment (and/or other axial segments) need not have the same lengths along the axis, and/or adjacent pairs of these electrodes may not be spaced apart by substantially the same spacing. Alternatively, or additionally, these electrodes may not have a pitch P along the axis that satisfies P≤H/5.
From another aspect, the present invention provides an ion mirror for reflecting ions along an axis (X) comprising: a first axial segment, within which the turning points of the ions are located in use, and a second axial segment, wherein the first and second axial segments are adjacent each other in a direction along said axis (X); and voltage supplies configured to apply electric potentials to electrodes of the first axial segment for generating a first linear electric field of a first strength within the first axial segment, and to apply electric potentials to electrodes of the second axial segment for generating a second linear electric field of a second strength within the second axial segment; wherein the voltage supplies and electrodes are configured such that the second linear electric field penetrates into the first axial segment so that the axial electric field in an axial portion of the first axial segment is non-linear where the turning points of the ions are located, and such that an axial electric field strength E0 at a mean ion turning point within the first axial segment is related to the strength E2 of the first linear electric field by the relationship 0.01≤(E0−E2)/E2≤0.1.
The mirror according to this aspect may have any one, or combination, of the features described above and elsewhere herein.
For example, the relationship may be 0.015≤(E0−E2)/E2≤0.03.
From another aspect, the present invention provides an ion mirror for reflecting ions along an axis (X) comprising: an entrance end for receiving ions; a first axial segment (E2), within which the turning points of the ions are located in use, and a second axial segment (E3) adjacent the first axial segment in a direction along said axis (X); and voltage supplies for applying different voltages to different electrodes of the ion mirror for generating electric fields that perform said reflecting of the ions; wherein at least the first axial segment is defined between inter-segment electrodes that are spaced apart along said axis, each of said inter-segment electrodes being an electrode to which one of said voltage supplies is connected to, wherein the first axial segment comprises a plurality of electrodes spaced apart from each other along said axis (X) and arranged between the inter-segment electrodes, wherein the plurality of electrodes are electrically connected to the inter-segment electrodes and interconnected with each other by electronic circuitry such that when the voltage supplies apply voltages to the inter-segment electrodes, this causes the plurality of electrodes to be maintained at different potentials so as to generate said electric fields; wherein said plurality of electrodes define windows arranged in a plane (Y-Z plane) orthogonal to said axis (X) through which the ions travel in use, wherein the windows have a minimum dimension H in said plane (Y-Z plane); and wherein the mirror is configured such that the distance (X3) along said axis from a mean ion turning point within the first axial segment to the inter-segment electrode nearer to an entrance end of the mirror is selected from the group of: ≤2H; ≤1.5H; ≤1H; ≤0.5H; in the range 0.2H≤X3≤1.7H; or in the range 0.1H≤X3≤1H.
The mirror according to this aspect may have any one, or combination, of the features described above and elsewhere herein.
From another aspect, the present invention provides a mass spectrometer comprising: at least one ion mirror as described herein; an ion source for providing ions into the ion mirror; and an ion detector.
The mass spectrometer may be either: (i) a time of flight mass spectrometer, optionally a multi-reflecting time of flight mass spectrometer comprising two of said ion mirrors arranged to reflect ions between the ion mirrors multiple times; or (ii) an electrostatic trap mass spectrometer.
From another aspect, the present invention provides a method of mass spectrometry comprising: providing an ion mirror or spectrometer as described herein; supplying ions into said ion mirror; reflecting ions at ion turning points within said first axial segment (E2); and detecting the ions.
The method may be operated to perform any of the functions described herein.
Embodiments of the invention provide a particular range of ion optical designs of ion mirrors for reaching an unprecedented ion optical quality of gridless ion mirrors, found to provide mass resolving powers above 100,000 for an unusually wide energy spread—above 20%. This allows improving so-called turn-around time of ion packets by applying stronger extraction fields within ion sources for obtaining higher resolutions per flight path.
The improvement is based on a novel qualitative realization—energy acceptance of ion mirrors improves by using an ion reflecting field with a weak non-uniformity at the ion turning region, where a controlled slight curvature of the axial field distribution is achieved by penetration of an external field into an open region of an initially uniform field. A controlled and weak non-uniformity of the electric field allows keeping the flight time independent of the position of the ion turning point in a wide energy range while, by Laplace law, the non-linearity of the axial field also generates a spatial curvature of equipotential lines to improve time per spatial and angular aberrations.
Ion mirrors are then improved by constructing the entire ion mirror, or at least the ion mirror's reflecting part of open connected segments, having linear potential distributions on segment electrodes, i.e. each segment separately generating fundamentally uniform fields. Field penetration between segments generates slight field curvatures, while not generating strong oscillations of field strength and of higher field derivatives, unavoidable in prior art designs of gridless ion mirrors, constructed of thick electrodes. Embodiments of the invention provide a range of optimal geometries and conditions (sweet spot) to form the desired uniformity and slight controlled curvature of ion mirror fields. Preferred embodiments illustrate examples of such geometries and of such fields.
The approach perfectly falls into PCB methods of making ion mirrors, since generation of the linear field segments may be formed using narrow strip electrodes, energized via dividing resistive chains. Embodiments of the invention use PCB boards with conductive strips at the inner surface of ion mirrors. To avoid electrically charging the insulators, the inner surface may be coated by a resistive or antistatic coating, e.g. at GOhm to TOhm range, sufficing at moderate and technologically reasonable uniformity. Alternatively, substrate materials may be made with controlled impurities to generate a limited substrate conductance.
Novel mirror fields may be also formed with separate thin electrodes frames or electrode rods, interconnected by resistive chains, which is considered a less preferred method for reasons of higher making and assembly cost, however, reducing risks of substrate charging. To support parallelism of thin electrodes, embodiments of the invention provide a range of constructing methods and designs, such as aligning grooves or use of technological jigs at electrode assembly.
The proposed making methods pose an additional limitation by surface leakage. PCB and plastics start leaking at field strengths above 1 kV/mm and safe design requires keeping field strengths under 500V/mm, reduced to 300V/mm for ultra conservative design. Embodiments of the invention account for this limit at ion optical design and propose a subset of sweet spot geometries and conditions in forming high quality ion mirrors with uniform field segments.
Improved ion mirrors can be constructed of planar and cylindrical symmetry and are applicable for a range of isochronous electrostatic analyzers, such as electrostatic traps, open ion traps and TOF mass spectrometers. A planar version allows stacking multiple low cost mirrors into an array. Those arrays are proposed for improving duty cycle of orthogonal accelerator and for various multiplexing schemes, already known in mass spectrometry.
According to one aspect of the invention, within time-of-flight, or multi-reflecting time-of-flight, or isochronous electrostatic trap mass spectrometers, there is provided an isochronously reflecting gridless ion mirror comprising:
(a) within Cartesian XYZ coordinates, a set of parallel conductive electrodes having or forming mutually aligned windows oriented orthogonal to the ion reflection axis X to form a two dimensional electrostatic field in an XY plane; the characteristic smallest transverse size H of said windows is defined as either a window diameter for ring electrodes, or a smaller Y-dimension for rectangular windows;
(b) electrodes are grouped into at least two segments denoted as E2 and E3; wherein the segments E2 and E3 are adjacent and are separated by a “knot” electrode with an open window, not having mesh; wherein distinct potentials are applied to “knot” electrodes on segments boundaries; and wherein electrodes of each segment are interconnected with a uniform resistive chain to form a linear potential distribution on electrodes within segments with corresponding potential gradients E2 and E3 on electrodes; wherein the segment E3 is located upstream of the E2 segment, i.e. closer to the mirror exit;
(c) potentials U2 and U3, applied to “knot” electrodes surrounding the E2 segment, are chosen to contain the mean potential U0, also defining the X-axis origin X=0: U2>U0>U3, U0=K0/q, where K0 is the mean ion energy and q is the ion charge, this way ensuring that the mean ion turning point is contained within the E2 segment;
(d) wherein said applied potentials are chosen to provide non equal potential gradients in the segment E2 and E3 to form a non uniform axial field at segments' boundary;
(e) at least in the E2 segment, the electrodes thickness and spacing in the X-direction are uniform and the spatial period P of electrodes is P≤H/5;
(f) wherein to provide for advanced isochronous and spatially focusing properties at ion reflection, the mirror satisfies the following set of conditions:
(i) field strength E2 is 4.3U0/D≤E2≤5U0/D, where D is the distance from mean ion turning point to a first order energy focusing time focal point;
(ii) the distance X3 (in the positive X-direction from the ion turning point to the exit from the ion mirror) from the mean ion turning point (X=0; U=U0) to the nearest downstream “knot” electrode plane is 0.2H≤X3≤1.7H in case of the planar mirror symmetry and is 0.1H≤X3≤1H in case of the cylindrical mirror symmetry;
(iii) the ratio of field strengths E3/E2 is linked to the X3 distance by the relation E3/E2=A*[0.75+0.05*exp((4XT/H)−1)] for ion mirror with planar symmetry, where 0.5≤A≤2 to provide for a controlled non-linearity of axial field distribution, demonstrated to enhance the energy acceptance of the ion mirror.
Preferably, the ratio E3/E2 may be linked to the X3 distance as: (i) 0.8<E3/E2≤2 at 0.2<X3/H≤1; (ii) 1.5<E3/E2≤5 at 1<X3/H≤1.5; and (iii) E3/E2>5 at 1.5<X3/H≤2. Preferably, said mirror may further comprise an E1 segment with a field strength E1, located upstream of said segment E2 (in the negative X-direction) and separated from the adjacent segment E2 by a “knot” electrode with an open window, not having mesh; wherein E1<E2; and wherein the distance |X2| from the mean ion turning point to the separating “knot” electrode may be 0.2≤X2/H≤1.
Preferably, in order to provide for a non-linearity of the axial field distribution at the mean ion turning point (X=0) and this way to enhance the energy acceptance of the ion mirror, the axial field strength E0 at the mean ion turning point with X=0, U=U0, and E=E0 may be slightly different from the E2 potential gradient in the E2 segment, containing the mean ion turning point, occurring due to the field penetration of surrounding segments E1 and E3; wherein said field non linearity may be contained in one range of the group: (i) 0.01≤(E0−E2)/E2≤0.1; and (ii) 0.015≤(E0−E2)/E2≤0.03.
Preferably, 15≤D/H≤25.
Preferably, the mirror may further comprise an entrance lens, formed by either thick electrodes or by segments with uniform electric field on the walls; said entrance lens may comprise one of the group: (i) accelerating lens; (ii) retarding lens; (iii) a multistage lens; (iv) a dual lens formed on both ends of an elongated lens electrode; and (v) an immersion lens. Preferably, potentials and dimensions of said segments may be optimized per particular entrance lens to reach for spatial ion focusing, for at least full second order second order spatial isochronicity and high order time per energy isochronicity of the list: (i) at least third-order energy isochronicity; (ii) at least forth-order of energy isochronicity; (iii) at least fifth-order energy isochronicity; and (iv) at least sixth-order energy isochronicity; and wherein small energy aberrations of particular order may be left at residual level for partial compensation of higher order aberrations.
Preferably, the number of said connected power supplies may be reduced by using auxiliary resistors connected between said “knot” electrodes; wherein the precision of said auxiliary resistors is set at 0.1% or better to sustain optimal simulated field strength ratio E2/E1.
Preferably, said segments may be made as a stack of thin conductive electrodes, either metal, or carbon filled epoxy protrusion profiles, or conductive coated insulators; wherein said electrodes are either attached to side insulating plates—plastic, printed circuit boards, ceramics, or quartz, or clamped with insulating spacers; and wherein the positioning accuracy and straightness of the electrodes may be improved by either slots in the side insulating substrates or by multiple connecting pins to the mounting holes in the side insulating substrates or by using precision spacers for electrode clamping, and/or by technological fixtures at electrode attachment to the substrate.
Preferably, at least a portion of mirror electrodes may be conductive stripes on a printed circuit board; said boards being made of either epoxy-based material, ceramics, quartz, glass, or Teflon; and wherein the antistatic surface properties may be arranged either with residual conductance of the substrate or with antistatic or resistive coatings from GOhm to TOhm range, or by keeping spacing between stripes<1 mm.
According to another aspect of the invention, there is provided an ion mirror for reflecting ions in an X-direction, and comprising:
(a) two parallel printed circuit boards, aligned in an XZ plane and spaced in the orthogonal Y direction by distance H; said boards are formed on either epoxy, ceramic, glass, quartz, or glass substrate;
(b) wherein said printed circuit boards have periodic structure of conductive stripes aligned with the Z axis with a period P being less than H/5;
(c) wherein said stripes are interconnected by a uniform resistive chain and wherein individual potentials are applied to selected “knot” conductive stripes, forming boundaries and separating at least three segments of uniform potential gradient; wherein said potential gradients are different between said segments;
(d) wherein the X-length of an intermediate segment is less than 2H and wherein potentials U2 and U3 on the boundaries of this intermediate segment are chosen to contain mean ion specific energy U0: U2>U0>U3; and
(e) wherein said printed circuit board has an antistatic feature formed either with periodic structure of parallel fine conductive lines between said conductive stripes and/or an antistatic coating with resistance in the range from 1 GOhm/square to 10 TOhm/square.
Preferably, said antistatic coating may be either deposited on top or under said conductive stripes; and wherein said antistatic coating may be produced by one of technology the group: (i) depositing into surface of insulator (polymer or metal oxide) coated conductive particles; (ii) thin coated with low conductance material such as SnO2, InO2, TiO2, or ZrO2; and (iii) exposed to glow discharge at intermediate gas pressures with deposition of metal atoms or metal oxide molecules onto said PCB surface.
Preferably, said conductive stripes are curved in the XZ plane to form trans-axial electric fields. Preferably, said ion segments may be formed with flexible printed circuit boards, either thin epoxy boards, or Teflon, or Kapton based boards, and wherein the topology of said ion mirror is one of the group: (i) 2D-planar with slit windows; (ii) 2D-circular with ring windows; (iii) 2D-cylindrical with electrodes arced around Y-axis; and (iv) arc bent with circular Z-axis.
According to another aspect of the invention, there is provided a multi-reflecting time-of-flight mass spectrometer with at least two ion mirrors comprising:
(a) at least two segments of uniform two-dimensional electric field in an XY-plane, formed within channels of equal height, merged and open to each other for mutual field penetration in the X-direction of ion reflection;
(b) wherein the ion energy, said segments dimensions and fields are chosen to provide for locating the ion turning point within said penetrating field and distant from the field boundary by less than N calibers of smallest channel transverse dimension H; and
(c) wherein N is one of the group: (i) N≤2; (ii) N≤1.5; (iii) N≤1; and (iv) N≤0.5.
Preferably, the spectrometer may further comprise one mean of isochronous ion packet focusing in the Z-direction of the group: (i) a trans-axial lens in front of the said mirror stack; (ii) a trans-axial lens arranged within said ion mirrors; (iii) an electrostatic wedge at ion reflection region of said ion mirror for compensating time-of-flight per spatial aberrations by any spatial focusing means.
Preferably, said at least two ion mirrors may be configured into two arrays of ion mirrors, mutually shifted in the Y direction for arranging ion trajectory shift in the Y-direction for every ion reflection within said ion mirror arrays.
According to the another aspect of the invention, there is provided a method of forming electrostatic field of isochronous ion mirror comprising the following steps:
(a) forming open and adjacent segments with uniform electrostatic field;
(b) forming different field strength in said segments to produce mutual field penetration and curvature of equipotential lines in transition regions between segments;
(c) arranging ion energy and field strength and lengths so that the ion turning point appears in a segment E2, and wherein for the purpose of high quality isochronicity and wide spatial acceptance of said reflecting field, the field penetration of at least one adjacent segment is arranged for the field E0 at ion reflecting point deviating from the field strength E2 in one range of the group: (i) 0.01≤(E0−E2)/E2≤0.1; (ii) 0.015≤(E0−E2)/E2≤0.03.
Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:
Prior Art Ion Mirrors: Referring to
Mirror 10 forms uniform electric fields E1 and E2 in the core volume of segments 11 and 12 without distorting field-free (E=0) conditions in the drift space D. Plot 16 shows potential distributions: 18—at electrodes and 19—at the mirror axis. Small steps of voltage between individual electrodes appear well smoothed at sufficient distance from electrodes, usually considered equal to a spatial period of the electrode structure. To provide for second order time per energy focusing, there exists an optimal ratio of field strength E1 and E2, which depends on the segments length. In case of ultimately short stage 12, U2 is ⅔ of the ion mean specific energy per charge. As known in TOF MS field, with elongation of the stage 12, the ratio of the field strengths E2/E1 varies form E2/E1>>1 to about E2/E1=1, while reducing the ion on mesh scattering at the price of gradual reduction of the energy acceptance. The grid-covered mirror 10 has an exceptional spatial acceptance, i.e. may operate with very wide ion packets. However, if used for multi-reflecting TOF, ion passages through mesh cause devastating ion losses.
Referring to
Referring to
U.S. Pat. No. 6,384,410 provides a numerical example for dimensions and voltages. We analyzed the ion optical properties of the exemplary mirror 37, as illustrated by shape of electrodes and equi-potential lines. The mirror provides for second-order time per energy focusing and allows 7% energy acceptance at 1E-5 level of time isochronicity. The design compensates for spatial focusing/defocusing of transition fields T1 and T2 (as stated in U.S. Pat. No. 6,384,410), thus, returning a non-diverging ion beam, which may be expressed as Y|Y=1. However, it does not focus initially diverging ion packets and generates a substantial second order time per space aberration TYY, limiting packets width under 5 mm and angular divergence under 5 mrad for dT/T≤1E-5. Both shortages—absence of angular focusing and very small spatial acceptance do compromise use of mirror 30 for multi-reflecting TOF and E-traps.
The reflecting field E1 in the segment 31 may be highly uniform at the ion turning region, far-spaced from the “knot” electrode 33 by distance XT, which is specifically stressed in U.S. Pat. No. 6,384,410. Simulations of the numerical example 37 have confirmed that the field E1 penetrates at 1E-6 level only at the ion turning point XT=2.5D, where D is the electrode window diameter D=25 mm. The uniform field in the vicinity of the ion turning point strongly compromises the energy acceptance of ion mirrors. Besides, by nature of electric fields, highly uniform reflecting fields have no curvature of reflecting equipotential lines, thus, not providing for any means to improve the spatial isochronicity. As known in the field of ion optics, lenses always produce positive T|YY aberrations. Mirror 30 forms lens with T1 and T2 fields but has no means for compensating their time per space aberrations. If adding spatial focusing features to mirror 31 (say by making entrance lens T2 stronger), those time aberrations would increase further. Thus, the ion mirror 30 has low ion optical quality, not suitable for multi-reflecting TOF mass spectrometers and electrostatic traps.
Embodiments of the present invention improve the ion optical quality, the design and manufacturing technology of gridless ion mirrors, e.g. for MRTOF and E-Traps.
Principles of novel ion mirrors: Improved ion mirrors for Multi-reflecting TOF (MRTOF) and E-traps mass spectrometers according to embodiments of the invention shall be free of grids, shall provide spatial ion focusing, and shall be highly isochronous at wide energy and spatial acceptances.
Here we state that the ideal reflecting field near the ion turning point should have an optimal non-linearity of the field profile E(x) and a curvature of equi-potential lines, caused by the E(x) non-linearity to provide for two features of high quality ion mirrors: (A) compensation or minimizing of high-order time per energy aberrations; and (B) compensation of time per spatial spread aberrations. The weakly inhomogeneous field strength distribution in the area of the ion turning point leads to much better independence of the flight time with respect to energy, than both purely homogeneous and highly inhomogeneous fields of gridless ion mirrors.
The inventor has found that the quality of ion mirrors can be improved compared to the prior art by merging open regions of uniform fields, where mutual field penetration between segments allows the production of a monotonous and nearly uniform reflecting field at the ion turning point, with a controlled optimal non-linearity (of few percent) in order to provide for high order energy focusing and wider energy acceptance, also accompanied by providing spatial isochronicity. For yet better ion optical quality, the length of the ion reflecting segment shall be limited to allow for a sufficient field penetration from both ends, this way maximizing the energy acceptance.
Referring to
The axial segments described herein may be denoted by their fields Ei.
The structure of openly merged segments 41,42,43 etc. forms potential distribution U(x) 47 at the mirror symmetry axis (at Y=0, i.e. away from the electrodes) with nearly uniform fields in the axially central part of individual segments, and with transition fields at segment boundaries. The potential distribution 47 is characterized by an accelerating lens around the segment E5 for spatial ion focusing in the Y-direction, so as by a reflecting field in segments E1 to E4 to provide for isochronous ion reflection in the X-direction.
Alternative electrode structures may be used to generate the same structure of electrostatic field. Those structures may comprise a set of thin electrodes with rectangular or circular windows, a pair of parallel printed circuit boards (planar ceramic, epoxy or Teflon PCB, or a flexible kapton PCB, rolled into a cylinder) with conductive stripes and with high-Ohmic antistatic coating, a pair of resistive plates (or a cylinder) with conductive stripes for knot electrodes, or an insulating (planar or cylindrical) support with resistive coating, separated into segments by conductive stripes. While understanding that multiple known technologies may be used to form the desired fine electrode structure, embodiments of the invention are primarily concerned with the properties of the desired electrostatic field itself to form the optimal non linearity 48 and the optimal curvature 49 of the electrostatic field near the ion turning region.
The ion mean turning point is defined by the potential U=U0=K0/q at the mirror axis, corresponding to the full stop of ions with mean kinetic energy K0 and charge q. In the embodiment 40, let us distinguish one core segment (a first axial segment 42) with the field E2, wherein ions of mean energy are turned: U2>U0>U3. An important feature of embodiments of the present invention is the controlled penetration of surrounding uniform fields E1 and E3 (from second and third axial segments 41,43) into the E2 segment (42) and particularly to the location of the ion turning point (at X=0). As we found at ion optical modeling, the ion optical quality of the ion mirrors may be improved due to the penetration of the E3 field (from the second axial segment 43) into the E2 segment (42) to the location of the ion turning point. This provides for both: (a) slight and controlled non-linearity of E(x) curve as shown in icon 48; and (b) spatial curvature of equipotential lines in the region, surrounding the ion turning point at optimal X=0, as shown in the icon 49. Both non-linearity 48 and curvature 49 are mutually related by the nature of electrostatic fields. The optimal penetration of the E3 field corresponds to approximately 1-3% of E(x) variation=(E0−E2)/E2. In other words, the penetration of fields into the E2 segment (42) to the location of the ion turning point (X=0) may cause the field at that point E0 to differ from E2 by approximately 1-3% of E2. Allowing penetration of yet another field E1 (from the third axial segment 41) into the E2 segment (42) to the location of the ion turning region allows further improvement of the ion optical quality and provides for higher flexibility of controlling the field non-linearity in the E2 segment. Accordingly, the E1 and/or E3 field may be caused to penetrate to the ion turning region.
Comparing novel and prior art gridless mirrors: Referring to
The difference between axial distributions 54 and 55 is barely visible on a crude scale. However, let us highlight one difference: the axial potential distribution 55 of ion mirrors 40 according to embodiments is much more linear near the ion turning point at U/U0=1, i.e. field strength variations E/E0 at the ion turning point are much smaller and more monotonic as compared to prior art mirrors 20 constructed of thick electrodes.
Referring to
Referring to
Referring to
Referring back to
Optimizing novel mirrors: To accelerate the analysis and the optimization of ion mirrors according to embodiments of the invention, the inventor came up with an analytical expression for the axial distribution of the electric field E(x) in the planar two-dimensional gap with height H, where two segments with the field strengths E1 and E2 are openly merged at X=0:
E(x)=E1+(E2−E1)*(2/π)*arctan(exp[−π*x/H])
At |X/H|>0.1 the expression may be approximated by:
E(x)=E1+(E2−E1)*(2/π)*exp(−πX/H)*[1+⅓*exp(−2πX/H)+⅕*exp(−4πX/H)]
Having an analytical expression strongly accelerates ion optical simulations and optimization procedures. Now we could vary parameters—channel height H, segments lengths Li and segments field strengths Ei at the walls, while optimizing a large set of low-order and high-order time and spatial aberrations for a variety of mirror systems which differ by the entrance lenses.
Optimization criteria: In optimization procedure we were setting acceptance criteria, comprising: spatial ion focusing (Y|Y=0 per one reflection); at least third-order time per energy (T|K=T|KK=T|KKK=0) focusing with low or zero higher order time per energy terms; full compensation of at least second-order time per spatial, angular and energy aberrations, including cross terms; and wider spatial and angular acceptances of model ion mirrors at about 1E-5 level of isochronicity.
Variety of novel mirrors: To provide for spatial ion focusing, the mirrors according to embodiments of the invention may have an entrance lens, preferably at an attracting potential |UL|<|UD|, which can be either a single stage lens or a multi-stage lens, or an immersion lens. The entrance lens part can be formed either with stepped field segments of thick electrodes. The reflecting fields of mirrors according to embodiments of the invention were constructed with segmented fields (stepped E) and were individually optimized per specific entrance lens. Varying the lens part of the ion mirror leads to minor adjustments of the mirror reflecting part if optimizing those ion mirrors for lowest aberrations and highest energy acceptances.
Referring to
Table of comparison of ion-optical parameters of E-Steps and T-Steps mirrors.
Sweet spot: While varying the lens part of novel ion mirrors, optimizing ion mirror aberrations, and analyzing parameters of field segments, we arrived to the following conclusions and rules:
1. Qualitative rules:
Referring to
Diagram 91 shows the normalized field strength at the ion turning point E0D/U0 for novel ion mirrors (E-steps) 92, and for prior art thick electrode mirrors (U-steps) 93. Data points are aligned by the ratio X2/H, which can not be defined in thick electrode systems and is set to 0 for displaying purposes. While E0D/U0 may widely vary for thick electrode mirrors, the optimal range is narrow and well defined for novel mirrors: 4.5<E0D/U0<5, with most of points clustered around E0D/U0=4.6. The result means that all novel mirrors reproduce similar optimal field distributions in the ion reflecting part.
Diagram 94 shows the normalized window height H/D for novel ion mirrors (E-steps) 95, and for prior art thick electrode mirrors (U-steps) 96. Data points are aligned by the ratio X2/H. While H/D ratio may widely vary for thick electrode mirrors, the optimal range is narrow and well defined for novel mirrors: 0.04<H/D<0.06, with most of points clustered around H/D=0.055, again meaning that novel mirrors reproduce similar optimal field distributions in the ion reflecting part.
Diagram 97 plots the field non linearity (E0−E2)/E2 for novel ion mirrors at ion mean turning point (X=0), aligned with the X2/H ratio (same as in diagrams 91 and 94). The plot illustrates the central point of the invention—novel ion mirrors composed of field segments should have a non-zero optimal non-linearity at the ion turning point to provide for a notable improvement of the energy acceptance. The useful range of the reflecting field non-linearity appears 0.01<(E0−E2)/E2<0.04 for all simulated cases of novel mirrors. Comparing energy and angular acceptances of all simulated cases, best results are obtained in the range 0.015<(E0−E2)/E2<0.03.
Diagrams 97 and 98 illustrate that to reach the optimal non-linearity of diagram 97, the steps in the surrounding field shall be linked to the depth of mutual field penetration. According to diagram 98, field strength of E1 segment shall be slightly smaller than E2: E1<E2; 1.02<E2/E1<1.08. E2−E1 step grows at deeper field penetration X2/H. The useful range of penetration depth X2/H is limited to 0.8.
According to diagram 99, the field strength E3 should be in general larger than E2 (E3>E2), and the E3/E2 ratio is linked to the penetration depth X3/H by an empirical formula: E3/E2=[0.75+0.05*exp((4X3/H)−1)], that is E3/E2 grows with deeper X3/H penetration. The penetration depth X3/H is limited to 1.7.
In some exceptional cases, where the penetration depth X3/H is small, E3 can be somewhat smaller that E2; in this case the proper sign of the field strength non-linearity at the ion turning point is provided by penetration of the field E4 from the next (4-th) segment. Thus, in the most general case the ratio of field strengths E3/E2 is E3/E2>0.8 and is linked to the X3 distance by the relation E3/E2=A*[0.75+0.05*exp((4X3/H)−1)], where 0.5<A<2 to provide for a controlled non-linearity of the axial field distribution, demonstrated to enhance the energy acceptance of the ion mirror.
The above presented graphs and empirical rules tell that in all simulated cases novel ion mirrors reproduce a similar structure of ion reflecting field, characterized by a weak though controlled field non-linearity 0.01<(E0−E2)/E2<0.04 at the ion turning point X=0. This non-linearity is achieved by a field penetration from adjacent field segments with E1 and E3 fields, where steps in field strength E1/E2 and E3/E2 appear linked with the depth of field penetration X2/H and X3/H for improving the ion mirror energy acceptance.
Referring to
Similar to
It must be understood that the range of sweet spot parameters presented in
Two reflecting segments: Referring to
Referring to
Novel ion mirror embodiments: Referring to
Uniform fields between electrodes within each segment are supported by resistive chains 134, say, using commercially available resistors with 0.1%-1% precision and 10 ppm/C thermal coefficients. Potentials 135, denoted as U0, U1 . . . and UD are then applied to “knot” electrodes (inter-segment electrodes) 133 only. The power supply U2 may be omitted and the ratio of the field strengths E1 and E2 adjusted by additional shunt resistors Rs with at least better than 1% precision. Diagram 136 shows potential distributions: 138—at the electrodes, and 139—at the mirror axis. It is of practical importance that minor variations of individual electrode thickness or voltages are expected to be smoothed and compensated by potential tuning. To provide a reasonably uniform field at least within the E2 segment, the electrode period P in this segment shall be at least 5 times finer than the window height H: P≤H/5. Since the optimal window height H is about 1/40 to 1/50 of cap-cap distance Lcc≅2D in MRTOF, design 130 requires making physically narrow electrodes. Say, for Lcc=50 cm the above requirement converts into P<2 mm, while electrodes 131 shall be yet thinner to allow for insulating gaps. Thus, making and assembly methods shall provide for mechanical stability and straightness of electrodes 131.
Thin electrodes designs: Referring to
In embodiment 140, the straightness of electrodes 131 is sustained with slots in the substrate 142, where the substrate may be either plastic, ceramic, glass, Teflon, or epoxy (say, G-10) material. A pair of opposite substrates 142 may be aligned by pins or shoulder screws in thick electrodes, such as the cap 131C electrode and the thick entrance electrode 132.
In embodiment 143, straightness of electrodes 131 is sustained by precise insulating spacers 144 at electrodes clamping with screws (e.g. made of plastic threaded rods or metal screws with PTFE sleeve). Spacers 144 may be either ring spacers or insulating sheets, both made of either plastic, PTFE, PCB, or ceramic. Electrode side shift is controlled by assembly with technological jigs and electrode displacement is prevented by tight clamping. Note that the design 143 is least preferred for accumulating inaccuracies in stack assembly and for being susceptible to electrode bend if spacers' surfaces are not highly parallel.
In embodiment 145, straightness of electrodes 131 is ensured by: (a) making initially flat electrodes (e.g., EDM made or stamped and then improved with thermal relief in stack); (b) aligning electrodes 131 with a side technological fixtures (not shown jig); and then (c) fixing electrodes 131 to the substrate 147 with connecting features 146. Preferred substrate 146 is PCB with metal coated vias. Other insulating substrates are usable, including plastic, ceramic, PTFE, glass and quartz. Preferred methods of attachment are epoxy gluing or soldering. When soldering, the preferred material for electrodes 131 is nickel 400 material, so as nickel or silver coated stainless steel. When gluing, the preferred electrode material is stainless steel. Electrodes 131 are preferably EDM machined or stamped with multiple connecting pins. Alternatively, electrodes 131 may be attached by brazing or spot welding to metal coated vias or pins in ceramic PCB. Yet alternatively, electrodes may be attached by rivets or connected by side clamps to plastic or PCB substrates.
In embodiment 148, electrodes 131 are made of carbon filled epoxy protrusion, optionally coated by metal for reducing chips and dust. The material provides an exceptional initial straightness, not achievable with metal rods. Electrodes 131 are aligned by technological jigs on each support plate 147 (PCB, plastic, ceramic, PTFE, or glass) for gluing or soldering via standoffs 146. Epoxy based PCB (like FR-4) are preferred for matching and low thermal coefficients TCE=4-5 ppm/C.
In case of using PCB supports 147, dividing chains may employ surface mount (SMD) resistors or a resistive strip generated with resistive inks, in particular developed for ceramic substrates.
PCB designs: Referring to
Preferably, PCB plates 152 and side PCB plates 152s are attached to thick supports 132 with aligning pins or shoulder screws, though thick plates may be replaced by metal coated PCB 159 for better thermal match and lower weight. In this case, the overall assembly 150 is fixed by technological jigs and soldered or glued. Preferably, stiffness of boards 152 is improved with PCB ribs 158. Preferably, SMD resistors 134 are soldered on outer PCB surfaces, where connection of conductive stripes 154 to power supplies 135 and to dividing resistors 134 may be arranged either with vias 156, or with edge conductive strips, or with rivet holes, or with side clamps. SMD resistors may be replaced by a distributed resistor, formed by a paste with resistance in MOhm/square range, with the resistive paste being applied between and on top of electrodes 154. Then the dividing chain may be placed on inner box surface without making vias 156. PCB 152 may further comprise conductive lines to connecting pads for convenient connection to vacuum feedthroughs, or may have an intermediate multi-pin connector for connecting assembly 150 by a ribbon cable. PCB 152 may further comprise mounting and aligning features for assembling the overall MRTOF analyzer.
Antistatic PCB features: It is advantageous to provide antistatic properties to the inner PCB surfaces (in box 150) that may be exposed to stray ions. On one hand, it is desired that the antistatic features shall not distort the accuracy of the resistive dividers 134, at least at 1% precision, meaning that the resistance between strips may be above 100 MOhm, which corresponds to approximately 10 GOgm/square minimal surface resistance, accounting about 100:1 length to width ratio of insulating strips. On the other hand, ions scattered from nA beams may produce up to 10 fA/mm2 currents onto the insulating support. To maintain potential distortions well under 0.1V, the antistatic surface resistance may be under 10 TOhm/square. Thus, antistatic coatings do not have to be precise and uniform but could be maintained in a wide range from 1E+10 to 1E+13 Ohm/square. This is 10-100 fold lower relative to standard resistance of FR-4 PCB boards, specified at 1E+14 to 1E+15 Ohm/square.
One solution is to use ceramics substrates having lower own resistance, such as Zr02, Si3N4, BN, AlN, Mullite, Frialite and Sialon. However, ceramics are less attractive as they are higher cost and have a fragile overall construction. More favorable solutions are shown in
Again referring to
PCB embodiment 152-B shows an example of antistatic coating 155 deposited on top of PCB 153 conductive stripes 154. The coating may be then made after PCB manufacturing. Antistatic coating 152 may be formed by exposing epoxy or ceramic PCB to glow discharge with deposition of copper, aluminium, tin, lead, zirconium, or titanium. Alternatively antistatic coating may be produced by depositing conductive particles (say carbon powder) with thin polymer coating. Embodiment 126 shows example of resistive layer (similar to one used in electron tubes and scopes) under conductive stripes 121, which may be preferred for better adhesion on ceramic, quartz and glass substrates.
PCB embodiment 152-C presents a reversed case, where the antistatic coating 155 is deposited on top of PCB 153 before depositing conductive stripes.
Solving antistatic PCB properties opens an opportunity of using economy PCB for making ion mirrors. PCB technology provides an advantage of forming thin and sufficiently parallel electrodes, so as provides a convenient method of making fine resistive dividers by using economy and compact SMD resistors. PCB technology is a perfect match for novel ion mirrors. We can state that novel ion mirrors are designed for PCB technology and PCB technology is the best way of making novel ion mirrors composed of field segments.
Mirror stack: Referring to
In operation, ions from the ion source S are ejected into the OA 161 and travel along the confining means 162 at a moderate energy, say, 20 to 50 eV. Periodically, pulses are applied to (not shown) Push and Pull electrodes of the OA 161, optionally accompanied by switching voltage on the confining means 162. Long ion packets (50-150 mm long) 164 are extracted from the OA, spatially focused by a trans-axial (TA) lens 163 in the Z-direction and enter a field-free space between the ion mirrors 166 at a moderate inclination angle, expected in the order of 3 to 5 degrees. Two stacks of slim PCB ion mirrors 166 are arranged for opposed ion reflections. The opposed stacks are half-period shifted in the Y-direction. Ion packets 168 get side displaced in the Y-direction at every ion mirror reflection, while being spatially focused in the Z-direction by one of the following actions: (i) either by the action of TA-lens 164 alone; (ii) or being assisted by spatial focusing of PCB mirrors with curved strips as in embodiment 152-D of
As a result, the long ion packet 168 does not interfere with the OA after the first ion mirror reflection, even though the ion drift displacement AZ per mirror reflection is much shorter compared to the Z-length of the ion packet 168. Ion packets are spatially focused in the Z-direction (by a TA lens, optionally assisted by curved fields in PCB mirror) at prolonged flight path, corresponding to several ion mirror reflections to focus (in the Z-direction) ion packets when they hit the ion detector 167. Thus, the novel embodiment achieves multi-reflecting TOF separation of long ion packets at fully static operation of MRTOF. Absence of deflecting pulses preserves the full mass range of mass analysis.
The embodiment 160 also illustrates that the ion injection from wider (in Y-direction) OA and into slim ion mirrors 166 may be assisted by using two pair of deflection plates 165 for side ion deflection in the Y-direction at a relatively small angle and moderate time-of-flight aberrations associated with the Y-steering. Large duty cycles of OA in the order of 20-30% are expected at static ion beam operation, and the duty cycle may be further improved to nearly unity if accumulating ions in the RF ion guide and synchronizing pulsed ion ejection with OA 161 pulses.
The stack 166 of slim (in Y-direction) and low cost PCB based TOF and MRTOF analyzers allows various known multiplexing solutions, such as: E-trap with enhanced dynamic range, as described in WO2011086430; using multiple ions sources, or increasing pulsing rate of single ion source, and using multiple channels for MS2 analysis in MS-MS tandems as described in WO2017091501 and WO2017042665.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1806507.8 | Apr 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2019/051118 | 4/23/2019 | WO | 00 |
