Patent Application
20210134581
This application claims priority from and the benefit of United Kingdom patent application No. 1810573.4 filed on 28 Jun. 2018. The entire content of this application is incorporated herein by reference.
The invention relates to the area of time of flight mass spectrometers, multi-reflecting and multi-turn mass spectrometers, and is particularly concerned with improved duty cycle of pulsed converters.
Time-of-flight mass spectrometers (TOF MS) are widely used in combination with continuous ion sources, like Electron Impact (EI) ion sources, Electrospray Ionisation (ESI) ion sources, Inductively Coupled Plasma (ICP) ion sources and gaseous Matrix Assisted Laser Desorption and Ionization (MALDI) ion sources. To convert an ion beam from an intrinsically continuous ion source into pulsed ion packets there are employed orthogonal accelerators (OA), radiofrequency (RF) ion guides with axial ion ejection, and RF ion traps with radial pulsed ejection.
Initially, the OA method has been introduced by Bendix corporation as described in G. J. O'Halloran et.al, Report ASD-TDR-62-644, The Bendix Corporation, Research Laboratory Division, Southfield, Mich., 1964. Dodonov et.al in SU1681340 and WO9103071 reintroduced and improved the OA injection method by using an ion mirror to compensate for multiple inherent OA aberrations. The beam propagates in the drift Z-direction through a storage gap between plate electrodes. Periodically, an electrical pulse is applied between plates. A portion of the continuous ion beam, located in the storage gap, is accelerated in an orthogonal X-direction, thus forming ribbon-shaped ion packets. Due to conservation of initial Z-velocity, the ion packets drift slowly in the Z-direction, thus traveling within the
TOF MS along an inclined mean ion trajectory, get reflected by ion mirror, and finally reach a detector.
For improving duty cycle of pulsed conversion there were proposed varios radio-frequency ion traps with either axial ion ejection as in U.S. Pat. Nos. 6,020,586 and 6,872,938, or radial ion ejection as in U.S. Pat. No. 6,545,268, 8,373,120, and 8,017,909. Ions are admitted into a radio-frequency ion guide for radial confinement with RF fields. Ions are locked axially by various types of DC plugs, get dampened in gas collisions at gas pressures of about 1 to 10 mTorr, and are ejected by pulsed electric field, either axially or radially. Radial traps reach nearly 100% duty cycle of pulsed conversion, but they are strongly affected by space charge effects. The space charge capacity of RF traps is limited by the useful trap length, which in turn is limited by the geometrical arrangement within MPTOF analyzers (described below), necessary for the ion packet to bypass the trap after the ion mirror reflection.
In the last two decades, the resolution of TOF MS has been substantially improved by using multi-pass TOFMS (MPTOF) instruments. These instruments either have ion mirrors for multiple ion reflections(i.e. a multi-reflecting TOF (MRTOF)), such as described in SU1725289, U.S. Pat. Nos.6,107,625, 6,570,152, GB2403063, and U.S.6,717,132, or have electrostatic sectors for multiple ion turns (i.e. a multi-turn TOF (MTTOF)) such as described in U.S. Pat. Nos. 7,504,620, 7,755,036, and M. Toyoda, et.al, J. Mass Spectrom. 38 (2003) 1125, each of which is incorporated herein by reference. The term “pass” is a generalized term covering ion mirror reflections in an MRTOF and ion turns in an MTTOF. In other words, an MP-TOF covers both MRTOF and MTTOF instruments. The resolving power of an MP-TOF instrument grows with increasing numbers of passes N. However, arranging a conventional OA in an MP-TOF instrument, as in U.S. Pat Nos 6,717,132 and 7,504,620, limits the efficiency of pulsed conversion of the OA, elsewhere called the duty cycle. To avoid spectral overlaps, the duty cycle of an MP-TOF with an OA is limited to being DC<1/N for heaviest ions, and realistically DC<1/2N, accounting for the rims of the OA and of the detector, and further drops as the square root of the specific ion mass μ=m/z for lighter ions (see eq.3 below).
WO2016174462 proposes increasing the OA length and duty cycle by displacing the OA from the central path of the MR-TOF analyser and arranging ion oscillations around the symmetry plane of isochronous trajectory. However, operation off the isochronous plane strongly affects the resolution and the spatial ion focusing of the MRTOF analyzer. Co-pending application WO2019/030475proposes shifting the accelerator from the MPTOF symmetry plane and pulsed deflection of ion packets back onto the symmetry plane. However, the solution poses limits onto the admitted mass range.
The present invention provides a time-of-flight mass analyser comprising: at least one ion mirror or electrostatic sector for reflecting or turning ions, respectively; an orthogonal accelerator having electrodes for receiving ions and orthogonally pulsing packets of the ions into the ion mirror or electrostatic sector such that the ions are reflected or turned, respectively, in a first dimension (x-direction) as they drift in a drift direction (z-direction); and an ion detector; wherein the electrodes of the orthogonal accelerator define slits or comprise meshes for allowing ions that have been reflected by the ion mirror, or turned by the electrostatic sector, to pass back into and through the orthogonal accelerator as they travel towards the detector.
The mass analyser is configured such that after the ions have been reflected or turned they pass back into, through and out of the orthogonal accelerator (e.g. into a first side and out of a second, opposite side of the orthogonal accelerator), without the ions hitting the electrodes of the orthogonal accelerator. As such, the orthogonal accelerator can be relatively long in the drift direction, so as to provide a relatively high duty cycle instrument, without the orthogonal accelerator blocking the path of the ions to the detector (or without having to cause the ions to advance a relatively long distance in the drift direction for each ion reflection or turn such that the ions do not impact on the orthogonal accelerator).
The drift direction (z-direction) is perpendicular to the first dimension (x-direction).
The slits described herein may be gridless slits, i.e. they do not include meshes therein.
In the embodiments in which the ions are reflected by the at least one mirror, the mean trajectory of the reflected ions may be in a plane defined by the first dimension and the drift direction (z-direction). The orthogonal accelerator and the slits are arranged in this plane such that the ions pass through the slits.
The mass analyser may be either: (i) a multi-reflecting time-of-flight mass analyser having the orthogonal accelerator arranged between two ion mirrors, and arranged and configured so that the ions are reflected multiple times between the ion mirrors and pass through the orthogonal accelerator, via the slits or meshes, multiple times as the ions travel from the orthogonal accelerator to the detector; or (ii) a multi-turn time-of-flight mass analyser having the orthogonal accelerator arranged between electrostatic sectors of a plurality of electrostatic sectors that turn the ions a plurality of times such that the ions pass through the orthogonal accelerator multiple times, via the slits or meshes, as they travel from the orthogonal accelerator to the detector.
In the embodiments in which the mass analyser is a multi-reflecting time-of-flight mass analyser, the ions may pass into a first side of the orthogonal accelerator and out of a second opposite side of the orthogonal accelerator for at least some of the times, or each and every time, that the ions pass from one mirror to another. Similarly, in the embodiments in which the mass analyser is a multi-turn time-of-flight mass analyser, the ions may pass into a first side of the orthogonal accelerator and out of a second opposite side of the orthogonal accelerator for at least some of the times, or each and every time, that the ions complete one complete turn (i.e. are turned 360 degrees by the sectors).
The electrodes of the orthogonal accelerator and their slits or meshes may extend in the drift direction (z-direction) from an upstream end of the orthogonal accelerator to a point proximate or downstream of the detector.
The electrodes of the orthogonal accelerator may define said slits; and at least one slit, or each slit, may be provided as an aperture through an electrode of the orthogonal accelerator that is elongated in the drift direction, such that electrode material completely surrounds the perimeter of the slit; and/or at least one slit, or each slit, may be defined between electrode portions that are elongated in the drift direction and spaced apart in a direction perpendicular to the first dimension and drift direction. The electrode portions may not be joined together at one or both of their longitudinal ends (in the z-direction). For example, the electrode portions may be two spaced apart wires or rods.
The downstream ends of the orthogonal accelerator electrodes may be spaced apart from the detector, in the drift direction (z-direction); and the electrodes of the orthogonal accelerator may define said slits; wherein each slit is defined between elongated electrode portions that are not joined together at their downstream ends.
The downstream ends are downstream in the drift direction (z-direction).
For example, each slit may be defined by a c-shaped electrode that is open at one longitudinal end, or between two separate elongated electrode portions that are not joined together at both of their longitudinal ends (e.g. two spaced apart wires or rods).
Ions may be reflected by a mirror or between ion mirrors, or turned by one or more of the electrostatic sectors, such that the ions pass through the gap one or more times as they pass from the downstream end (in the z-direction) of the orthogonal accelerator to the detector.
The mass analyser may comprise: one or more voltage supply for applying one or more voltage pulse to the electrodes of the orthogonal accelerator for performing said step of orthogonally pulsing the packets of the ions; and control circuitry configured to control the one or more voltage supply so as to only apply said one or more voltage pulse to the electrodes for orthogonally pulsing a packet of ions out of the orthogonal accelerator when ions that have previously been pulsed out of the orthogonal accelerator are not passing back through the orthogonal accelerator.
The orthogonal accelerator may comprise an ion guide portion having electrodes arranged to receive ions, and one or more voltage supply configured to apply potentials to these electrodes for confining ions in at least one dimension (X- or Y-dimension) orthogonal to the drift direction.
The orthogonal accelerator may comprise: an ion guide portion having electrodes arranged to receive ions travelling along a first axis (Z-direction), including a plurality of DC electrodes spaced along the first axis; and 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 axis they experience an ion confining force, generated by the DC potentials, in at least one dimension (X- or Y-dimension) orthogonal to the first axis.
The mass analyser may comprise focusing electrodes that are arranged and configured to control the motion of ions along the drift direction (z-direction) so as to spatially focus or compress each of the ion packets so that it is smaller, in the drift direction, at the detector than when pulsed out of the orthogonal accelerator.
The focusing electrodes may be configured to impart ions located at different positions, in the drift direction, within the ion packet with different velocities in the drift direction so as to perform the spatial focusing or compression.
The focusing electrodes may comprise a plurality of electrodes configured to generate an electric field region through which ions travel in use that has equipotential field lines that curve and/or diverge as a function of position along the drift direction so as to focus ions in the drift direction.
The focusing electrodes may comprise a plurality of electrodes configured to control the velocities of the ions such that ions within the orthogonal accelerator when it is pulsed have velocities, in the drift direction, that decrease as a function of distance in the drift direction towards the detector.
The plurality of electrodes may comprise an ion guide or ion trap upstream of the orthogonal accelerator and one or more electrodes configured to pulse ions out of the ion guide or ion trap such that the ions arrive at the orthogonal accelerator at different times and with velocities in the drift direction that increase as a function of the time at which they arrive at the orthogonal accelerator.
The mass analyser may comprise circuitry that synchronises the pulsing of ions out of the ion guide or ion trap with the pulsing of ion packets out of the orthogonal accelerator, wherein the circuitry is configured to provide a time delay between the pulsing of ions out of the ion guide or ion trap and the pulsing of ion packets out of the orthogonal accelerator, wherein the time delay is set based on a predetermined range of mass to charge ratios of interest to be mass analysed.
The plurality of electrodes may comprise electrodes arranged within the orthogonal accelerator to generate an axial potential distribution along the drift direction that slows ions by different amounts depending on their location, in the drift direction, within the orthogonal accelerator.
The mass analyser may be configured such that the length of the orthogonal accelerator from which ions are pulsed (Lz) is longer, in the drift direction, than half of the distance (Az) that the ion packet advances for each mirror reflection or sector turn in the first dimension.
For an MRTOF mass analyser, the distance Az may be determined along the axis that is half-way between the mirrors (i.e. half way in the x-direction). The distance Az may be determined based on the positions of the centre (in the z-direction) of the ion packet before and after each reflection. The distance Az may be the mean distance Az for all of the mirror reflections. Similarly, for an MTTOF mass analyser, the distance Az may be determined along the axis that is half-way between opposing sectors (i.e. half way in the x-direction). The distance Az may be determined based on the positions of the centre (in the z-direction) of the ion packet before and after each 180 degree turn . The distance Az may be the mean distance Az for all of the mirror reflections.
The ratio Lz/Az may be selected from the group of: (i) 0.5<Lz/Az <1; (ii) 1<Lz/Az <2; (iii) 2 Lz/Az <5; (iv) 5<Lz/Az <10; (v) 10<Lz/Az <20; and (vi) 20<Lz/Az <50; or the length of the region of the orthogonal accelerator from which ions are pulsed (Lz) may be longer, in the drift direction, than x % of the distance, in the drift direction, between the entrance to the orthogonal accelerator and the midpoint of the detector, wherein x is: ≥10, ≥15, ≥20, ≥25, ≥30, ≥35, ≥40, ≥45, or ≥50.
The present invention also provides a mass spectrometer comprising: an ion source; and a mass analyser as described herein.
The present invention also provides a method of mass spectrometry comprising: providing a mass analyser as described herein; receiving ions in said orthogonal accelerator; pulsing ions from said orthogonal accelerator into said ion mirror or sector; reflecting or turning the ions with the ion mirror or electrostatic sector, respectively, so that the ions pass back into and through the orthogonal accelerator via the slits defined by the electrodes or the meshes in the orthogonal accelerator; and receiving ions at said detector.
An improved orthogonal accelerator is proposed for multi-pass time-of-flight mass spectrometers (MPTOF). The orthogonal accelerator is elongated in the drift Z-direction and placed on the MPTOF surface of isochronous ion motion in the orthogonal Y-direction, being a symmetry plane in MRTOF. The electrodes of orthogonal accelerator are made transparent, for example using slits in all electrodes, including the push plate. As described elsewhere herein, each slit may be formed by a slot in an electrode, or between elongated electrode portions (such as between wire or rod electrode portions, or by between the electrodes on different PCBs. Less preferably, the electrodes of the orthogonal accelerator may be made transparent by using mesh electrodes through which the ions can pass. Thus, ions may pass through the switched off accelerator after at least one reflection or turn in the MPTOF analyzer. To ease the detector bypassing and to avoid spectral confusion, ion packets may be isochronously focused in the drift Z-direction onto detector, either by isochronous trans-axial or Fresnel lens and wedge, or by arranging spatial and temporal correlation within a continuous ion beam.
To retain the ion beam within a relatively long orthogonal accelerator before it is pulsed, the ion beam may be confined with an RF quadrupolar field or within a spatially alternated DC quadrupolar field.
A long orthogonal accelerator may improve the duty cycle and space charge capacity of an MPTOF by an order of magnitude, without introducing additional analyzer aberrations and need not set limits onto the admitted mass range.
The approach may be adopted for MPTOF with radially ejecting RF ion traps. RF traps are elongated for larger space charge capacity. The trap is placed on the plane of isochronous ion motion in the MPTOF and is made of electrodes with slits, so that ions may pass through the switched off trap after at least one turn or reflection. Ion packets may be spatially focused by isochronous lens to fit the detector size after multiple passes in MPTOF.
According to the one aspect of the invention, there is provided a multi-pass MPTOF (multi-reflecting or multi-turn) time-of-flight mass spectrometer comprising:
Optionally, said means for ion beam spatial confinement may comprise at least one mean of the group: (i) side plates connected to radiofrequency (RF) signal; (ii) side plates connected to an attracting DC potential; (iii) segmented side plates connected to spatially alternated DC potentials; (iv) segmented DC dipoles connected to spatially alternated dipolar DC potentials.
Optionally, said isochronous means for ion packet focusing in the Z-direction may comprise at least one means of the group: (i) a set of trans-axial lens and wedges; (ii) a Freznel lens and wedge arranged in multi-segmented deflector; and (iii) means for spatial or temporal variations of ion beam energy for arranging negative correlation between energy and position in Z-direction.
Optionally, said spatial-temporal correlation may be arranged with at least one means of the group: (i) pulsed acceleration of continuous ion beam in the Z-direction either within electrostatic channel or within a radio frequency RF ion guide, located upstream of said orthogonal accelerator; (ii) a time-variable floated elevator within an electrostatic channel or an RF ion guide, located upstream of said pulsed converter; (iii) a Z-dependent deceleration of ion beam within said orthogonal accelerator.
Optionally, said drift space of said multi-pass analyzer may be set at ground and wherein electrodes of said orthogonal accelerator may be energized by pulsed voltages to extract said ion packets.
According to the another aspect of the invention, there is provided a method of time-of-flight mass spectrometry comprising the following steps:
Optionally, said step of ion beam spatial confinement may comprise at least one step of the group: (i) radial ion confinement by radiofrequency (RF) quadrupolar field; (ii) ion confinement in the X-direction by quadrupolar DC field; (iii) radial ion confinement within periodic DC field of annular ion guide; and (iv) radial ion confinement within quadrupolar and spatially periodic DC field.
Optionally, said step of isochronous ion packet focusing in the Z-direction may comprise at least one step of the group: (i) ion focusing in electrostatic field of trans-axial lens and wedges; (ii) ion focusing by a Freznel lens and wedge arranged in multi-segmented deflector; and (iii) arranging negative correlation between ion energy and position in Z-direction within said ion storage gap.
Optionally, said spatial-temporal correlation may be arranged with at least one step of the group: (i) pulsed acceleration of continuous ion beam in the Z-direction either within electrostatic channel or within a radio frequency RF ion guide, located upstream of said orthogonal accelerator; (ii) a time-variable floating of an elevator within an electrostatic channel or an RF ion guide, located upstream of said pulsed converter; and (iii) a Z-dependent deceleration of ion beam at said step of ion beam spatial confinement.
Optionally, said drift space of said multi-pass analyzer may be set at ground and wherein electrodes of said orthogonal accelerator may be energized by pulsed voltages to extract said ion packets.
Optionally, the ratio Lz/Az of said of ion packet length and of an ion advance per single pass (reflection or turn) may be one of the group: (i) 0.5<Lz/Az≤1; (ii) 1<Lz/Az≤2; (iii) 2<Lz/Az≤5; (iv) 5 <Lz/Az≤10; (v) 10<Lz/Az≤20; and (vi) 20<Lz/Az≤50.
According to another aspect of the invention, there is provided a multi-pass MPTOF (multi-reflecting or multi-turn) time-of-flight mass spectrometer comprising:
(MPTOF), built of parallel ion mirrors or electrostatic sectors, separated by a drift space and substantially elongated in the Z-direction to form an electrostatic field in an orthogonal XY-plane; 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 s-surface—either symmetry s-XY plane of said ion mirrors or curved s-surface of electrostatic sectors; wherein said s-surface is aligned with the symmetry plane of said pulsed converter and of said z-focusing means; and
Optionally, said pulsed converter may be tilted to the Z-axis for angle α/2 and said means for Z-spatial focusing comprise means for ion ray steering, so that steering of ion trajectories at inclination angle α within said analyzer may be arranged isochronously.
Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:
Gaseous ion sources (like ESI, APCI, APPI, gaseous MALDI or ICP) comprise gas-filled radio-frequency (RF) ion guides (not shown) for dampening of the ion beams, followed by a lens 12 to form a substantially parallel continuous ion beam 13. Typical ion beam parameters are: lmm diameter, 1 degree angular divergence at a specific ion energy (energy per charge) Uz of from 10 to 50V at typical axial energy spread of 1eV.
The beam 13 propagates in the Z-direction through storage gap 14, which is a field-free region between plate electrodes. Periodically, an electrical pulse is applied between the plates defining the storage gap 14. A portion of the continuous ion beam 13, in the storage gap 14, is accelerated in the X-direction by a pulsed field and is accelerated to specific energy Ux, thus forming a ribbon-shaped ion packet 16 travelling along the mean ion trajectory 17. Since the ion packets preserve the z-velocity of the continuous ion beam 13, the trajectories 17 are inclined at an angle α to the X-dimension, typically being several degrees, where:
a=(Uz/Ux)0.5 (eq.1)
Ion packets 16 are reflected by ion mirrors 18 in the X-direction, and continue to slowly drift in the Z-direction, and arrive on detector 19 after multiple N ion mirror reflections during a jigsaw shaped ion trajectory 17. To obtain higher resolving power,
MRTOF analyzers are designed for longer flight paths and for larger numbers of reflections N>>1 (e.g. N=10). To avoid spectral overlaps on the detector 19 (i.e. confusion between various numbers of reflections), the useful length of each ion packet Lz becomes limited to:
L
z
<D
z
/N (eq. 2)
where Dz is the distance in the Z-dimension from the most upstream point of the OA 15 that ions are ejected from to the mid-point of the detector 19 at which ions are detected.
For the realistic values of Dz=300mm and N=10, the ion packet length Lz is therefore under 30 mm. In practice, the packet length is actually about twice as small, accounting for the OA and detector rims. This in turn limits the conversion efficiency of continuous ion beam 13 into pulsed packets 16, denoted as the duty cycle DC of the orthogonal accelerator 15:
DC=sqrt(μ/μ*)Lz/Dz, <sqrt(μ/μ*)/2N (eq.3)
Here μ=m/z denotes the specific mass, i.e. mass to charge ratio, and μ* defines the heaviest specific mass in the beam 13. Assuming N=10 and the smallest μ/μ*=0.01, the duty cycle for heaviest ions is under 10% and for the lightest ions in the beam is under 1% (and realistically under 0.5%). Thus, the OA-MRTOF instrument has a low duty cycle.
The duty cycle limit occurs due to the ion trajectory arrangement within the s-XZ symmetry plane of mirrors 18 and OA 15. The alignment of the ion trajectory within the s-XZ XZ plane is forced to keep the isochronous properties of the ion mirrors and gridless OA, reaching up to third order full isochronicity as described in WO2014142897. The prior art MRTOF 10 has been designed with recognition of the symmetry requirements. The duty cycle is sacrificed in exchange for higher resolving power of OA-MRTOF.
Similarly to the instrument in
The laminated sectors 28 provide three dimensional electrostatic fields for ion packet confinement in the drift Z-direction along the mean spiral trajectory 27. The fields of the four electrostatic sectors 28 also provide for isochronous ion oscillations along the figure-of-eight shaped central curved ion trajectory 27 in the XY-plane, also denoted as s. These sector analyzers are known to provide so-called triple focusing, i.e. first-order focusing with respect to energy spread around a mean ion energy and with respect to angular and spatial spread of ion packets around the mean ion trajectory. The sector MTTOF isochronicity has been recently improved with electrostatic sectors of non-equal radii, as described in WO2017042665.
The ion trajectory in the MTTOF 20 is locked to the fixed spiral trajectory 27 (s), which forces the sequential arrangement of the OA 15, sectors 28 and of the detector 19, thus limiting the duty cycle of the OA to being under 1/N, where N is the number of full turns. In addition, to arrange the spatial ion confinement within laminated sectors 28 in the Z-direction, the length Lz of ion packets 26 in the Z-dimension shall be at least twice as small as the width in the Z-dimension of each laminated channel in the sectors, and hence, the duty cycle of the MTTOF 20 is limited as described by eq.3 above. Embodiments of the present invention provide a method and apparatus for improving the duty cycle of orthogonal accelerators (OA) for multi-pass MPTOF analysers, i.e. for both multi-reflecting OA-MRTOF and multi-turn OA-MTTOF analysers.
Contrary to the prior art, the electrodes 31 of the OA 30 are made transparent for the ions being reflected back from an ion mirror 41,42. The accelerating field of the OA 30 is pulsed during ion extraction from the OA and is switched off afterwards, so that the ions reflected by the mirrors and returning through the OA are not defocussed. In operation, a continuous or quasi-continuous ion source 11 generates ions. A substantially parallel ion beam 13 passes through ion optics 12 and enters OA 30 substantially along the Z-direction. Optionally, the beam may be spatially confined in at least the X-direction (optionally also the Y-direction) with confinement means 32 within the z-elongated storage gap of OA 30. An Lz long portion of continuous beam 13 is converted into a pulsed ion packet 35 by an orthogonal pulsed acceleration field of OA 30, energized by pulse generator 34. Ejected ion packets 35 move at some inclination angle α to the X-direction, which is controlled by the Uz specific energy of the incoming ion beam 13 and acceleration voltage Ux gained at pulsed acceleration in the OA (see eq.1 above). Each ion packet 35 is reflected between ion mirrors 41 and 42 in the X-direction within the s-XZ symmetry plane for a large number of reflections (say, N=between 6 and 20) and while drifting in the Z-direction towards the detector 44 because the ions retain the Kz component of ion energy from the ion source 11.
Similarly to
(A) To improve the duty cycle of the OA 30, the length Lz of the ion packets 35 ejected from the OA 30 may be made longer than half the distance in the Z-direction that the ion packet advances per single mirror reflection 2Lz>Az=Dz/N. The distance Az in the z-direction may be determined along the axis that is half-way (in the x-direction) between the mirrors, and based on the positions of the centre (in the z-direction) of the ion packet. Ultimately, the Lz length may be comparable to a notable portion (say, ½) of the total drift length Dz (e.g. where Dz is the distance in the Z-dimension from the most upstream point of the OA 30 that ions are ejected from to the mid-point of the detector 44 at which ions are detected), even if using a large number of mirror reflections. Optionally, the ratio Lz/Az may be one of the group: (i) 0.5<Lz/Az≤1; (ii) 1<Lz/Az≤2; (iii) 2<Lz/Az≤5; (iv) 5<Lz/Az≤10; (v) 10<Lz/Az≤20; and (vi) 20<Lz/Az≤50.
(B) Means 32 may be arranged for spatial confinement of the ion beam so as to prevent the natural expansion of ion beam 13 within the OA 30 and to allow substantial (potentially indefinite) elongation of the OA without ionic losses and without ion beam spread, as detailed below in
(C) To avoid ion losses on the detector 44, so as to avoid spectral overlaps and spectral confusion (contrary to prior art open traps, described in WO2011107836), the ion packets 35 may be spatially focused in the Z-direction by a trans-axial lens 33 that may be within the OA 30 (or immediately downstream of the OA), or by a Fresnel lens, or by spatial space-velocity correlation of the continuous ion beam 13 within the OA, e.g. as described in co-pending applicationWO2019/030475. The trans-axial lens 33 may comprise focusing electrodes configured to generate an electric field region through which ions travel in use that has equipotential field lines that curve and/or diverge as a function of position along the drift Z-direction so as to focus ions in the drift Z-direction. As the result of ion packet z-focusing, the inclination angle α of ion trajectories with respect to the x-direction become dependent on z-position, as illustrated by ion packet vectors 36 having inclination angles α1 and α2. The spatial focusing (in the z-direction) leads to ion packet confinement towards the detector 44. The Z-focusing may be arranged isochronous, i.e. with compensation of T|Z and T|ZZ time aberrations per Z-width of ion packets, which otherwise would occur if using a conventional Einzel lens.
(D) The ion mirrors 41,42 return the long ion packets 35 towards the OA 30, as shown by trajectories A′ and B′. To avoid interferences of ion rays A′ and B′ with the long OA 30, OA electrodes 31 are made transparent to the reflected ions, say, with elongated slits. In other words, the mass analyser is configured such that the ions pass back through the OA30 between exiting one mirror and entering the other ion mirror (for at least some of the passes between the ion mirrors), and without the ions hitting the electrodes 31 of the OA 30. Although not shown in
Examples of the slits are well seen in both the XZ and XY views of
To pulse each ion packet out of the OA 30, high voltage (e.g. 3 to 10 kV) pulses, produced by high voltage generators such as those built of Behlke switches, are applied to the OA electrodes 31, as shown in
A numerical example for the preferred embodiment 40 is presented below, where the main parameters are as shown in the Table 1 below.
In this example, the distance between the mirror caps of the mirrors in the x-direction direction is Dx=1 m and Dz=300 mm (accounting for the useful Z-width of ion mirrors, which is not affected by 3D fringing fields at the Z-edges). The acceleration voltage for acceleration ions in the x-direction into the ion mirror is Ux=10 kV. By setting the ion beam specific energy to Uz=10V, the average inclination angle α is set to α˜30 mrad (by eq.1), i.e.
the ion packet advance Az per ion mirror reflection is Az=30 mm, and the number of ion mirror reflections is N=Dz/Az=10 (the approximate total flight path L=Dx*N=10 m). If using a conventional OA-MRTOF 10, and accounting for rims of the OA and detector, the ion packet length Lz shall be limited under Dz/2N=15 mm and the duty cycle for the heaviest ,u mass component would be limited to under DC=1/2N=5%, as defined by equation (3). With the improvements of the embodiment 40, the ion packet length can be increased, say, to Lz =150 mm, thus improving the OA duty cycle for the heaviest μ to DC=50%, i.e. by an order of magnitude.
Accounting for eq.3, the duty cycle DC of any OA instrument drops for lighter (smaller μ=m/z) ions. As an example, even if the DC=50% for an upper mass (say μ=2500), the duty cycle is still limited to DC=10% for μ=100 ions. The duty cycle for lighter ions can be further improved if using an ion guide (e.g. RF ion guide) of the ion source 11 (or between the source and OA 30) in a so-called “Pulsar” mode. Ions may be intermittently stored within the ion guide and released therefrom in a pulsed manner that is synchronized with the OA pulses, e.g. by operating an ion gate between the ion guide and OA (as indicated by pulse symbol at the exit aperture of the ion guide) such that the ions stored in the ion guide are pulsed by the OA 30. The propagation time of light ions within the OA (estimated as 50us for μ=100 at Kz=10eV and Lz=150mm) appears smaller then the time delay for extraction of heavy ions from the “Pulsar” RF ion guide, known to be about 20-30us for μ=1000 ions. Thus, using long OA 30 allows analysis of wide mass range at enhanced sensitivity.
Using a long OA 30 substantially extends the mass range able to be mass analysed to match M/m (i.e. the ratio of the heaviest mass ion to the lightest mass ion) for ions simultaneously transmitted from the RF ion guide, i.e. the Pulsar mode does not limit the mass range. Contrary to Pulsar OA-TOF instruments, the “Pulsar” gain is substantially higher for OA-MRTOF at substantially longer flight times and flight paths (say, tens and hundreds of meters). Indeed, ions may be stored in the RF ion guide between rare OA pulses, while ion packets ejected from the ion guide may be admitted into the OA with nearly unity duty cycle and a wide mass range.
Referring to
In operation, orthogonal accelerator 30 receives (e.g. continuous) ion beam 13 within a Z-elongated storage gap, wherein means 32 may be provided to confine the ion beam at least in the X-direction (and optionally the y-direction), as detailed in
The stadium shaped ion trajectory s-surface is arranged between electrostatic sectors 51 and 52, separated by grounded field-free regions 53. The sectors XY-field and ion packet energy in the X-direction may be adjusted for isochronous ion packet motion within the trajectory surface S. The inclination angle α is controlled by the ion beam 13 energy and by Z-focusing means 32 only. The drift length Dz and the injection inclination angle α are chosen to allow for multiple (say N=10) full ion turns, before ions hit the detector 54. To improve the duty cycle of OA 30, the length Lz of the ion packets 35 may be made comparable (say ½) of the total drift length Dz. At a large number of ion turns (say N=10) the ion packet length Lz appears much longer than the ion packet advance Az per single turn.
Similarly to embodiment 40 in
It is desired to prevent the ion beam expanding (before being pulsed) in the field free storage gap of the OA. Even with ion beam dampening in an RF ion guide upstream of the OA, the ion beam emittance is still finite (about lmm*deg at 10eV), and the ion beam would naturally diverge by several mm within 100mm along OA. This would compromise the combination of time and energy spreads of ion packets, affecting MPTOF resolution.
Referring to
Embodiment 61 employs a rectilinear RF ion guide similar to U.S. Pat. No.5,763,878. RF signals are applied to electrodes 62 so as to generate a quadrupolar RF field, radially confining ion beam 13 as it travels in the z-direction along the OA 30. Embodiment 61 has drawbacks: (i) RF confinement is mass dependent; (ii) the RF field must be turned off before the acceleration pulse within microseconds, where an RF signal decay is incomplete; (iii) pulses applied to electrodes are known to excite a resonant generator of the RF signal; and (iv) the initial ion position and initial velocity are mass and RF-phase dependent, which affect resolution, mass accuracy and angular losses in TOF analyzers.
Embodiment 63 employs a rectilinear electrostatic quadrupolar lens, formed by applying negative DC potentials to electrodes 64, as proposed in RU2013149761. A weak electrostatic quadrupolar field focuses and confines the ion beam in the critical TOF X-direction, while defocusing the ion beam in the non-critical transverse Y-direction. The method allows lossless ion beam transfer for up to Lz<50mm.
Embodiment 65 employs the spatially alternated electrostatic DC quadrupolar field along the Z-axis by alternating the polarity on DC electrodes 66, as in co-pending application WO2019/030475. The embodiment provides for indefinite ion beam confinement in both X and Y directions, although a variable central potential along the Z-axis may have a negative effect on ion beam packet focusing in the Z-direction.
Embodiment 67 provides for ion beam spatial confinement by spatial alternation of electrostatic quadrupolar field, now achieved without spatial modulation of the center-line potential U(z). The field is formed by an array of alternating DC dipoles 68, as described in co-pending application WO2019/030475. Optionally, the average potential (DC1+DC2)/2 is slightly negative to form a combination of the alternated quadrupolar field with a weak static quadrupolar field, thus providing somewhat stronger compression of the ion beam 13 in the X-direction Vs Y-direction. Relative to RF confinement 61, the electrostatic confinement 67 provides multiple advantages: (i) it is mass independent; (ii) it does not require resonant RF circuits and can be readily switched off; (iii) the strength and shape of the transverse confining field can be readily varied along the guide length; (iv) it can provide axial gradient of the guide potential without constructing complex RF circuits.
Ion packet Z-focusing may be provided as detailed in co-pending application WO2019/030475, such as by the following means:
(A) A trans-axial (TA) lens may be incorporated into the exit lens of a gridless OA (focusing in the Y-direction). For reaching isochronicity, the TA-lens may be compensated by a slight curvature of the accelerating field around the continuous ion beam. Such compensating curvature of the accelerating field may be achieved for example by TA curvature of the next extracting electrode or by an even yet slighter TA-curvature of the first push electrode. The TA-lens and the TA-compensator may be arranged for at least the Z-length of the extracted ion packet and they may be turned off by removing the pulsed OA voltages during the ion packet return passage through the OA. Optionally, the TA-lens may be combined with a TA-wedge for compensating non-intended misalignments of the OA and of the analyzer;
(B) A Fresnel lens, achieved with a multi-segmented deflector, which is energized with a gradient step voltage between thin deflecting plates may be provided. The Fresnel lens also allows arranging wedge fields with a constant bias applied to all deflecting segments, this way serving as a compensator for mechanical misalignments. The Fresnel lens may be arranged at least for the z-length of extracted ion packets and may be switched off together with the OA to allow a non-distorted return ion passage;
(C) Z-focusing may be provided by spatial-temporal correlations within a continuous ion beam, which is described below. Ion packet auto-focusing may be obtained by controlling and correlating the axial velocity Vz of the continuous ion beam with its z-position within the OA. In this case the OA does not need means for spatial Z-focusing 33.
Co-pending application WO2019/030475 describes two general methods of Z-auto-focusing, which may be used in embodiments of the present invention, and which are as follows:
(A) To focus (or compress) a range of μ=m/z ions in the z-direction by the time they reach the detector, the z-directional speed of the ions may be caused to be different as a function of their z-directional position within the OA. Ions arranged within the OA at locations progressively further away from the detector (in the z-direction) may be given progressively higher z-directional speeds towards the detector such that the ion packet is compressed in the z-direction by the time that it reaches the detector. For example, one may arrange a negative correlation Vz(z) within the storage gap of the OA 30 according to Vz(z)/Vz0=1−z/Dz, where Dz is the distance from beginning of the OA to the detector, Vz(z) is the axial velocity for depending on ions' z-position within the OA, Vz0=Vz(z=0), where Z=0 is the beginning (upstream end) of the OA.
(B) To focus (or compress) ions in a wide mass range (e.g. for all ,u), the z-dependent specific energy per charge U(z) may be caused to be different as a function of their z-directional position within the instrument. Ions arranged at locations progressively further away from the detector (in the z-direction) may be given progressively higher specific energy per charge U(z) in a direction towards the detector such that the ion packet is compressed in the z-direction by the time that it reaches the detector. For example, the z-dependent specific energy per charge U(z) may satisfy: U(z)/Uz0=(1−z/Dz)2, where Uz0=U(z=0).
Referring to
In operation, substantially elongated ion beam 33 may be retained within long OA 30 by spatial confinement means 60, e.g. as described in relation to
In one embodiment, an acceleration pulse 76 is applied to RF ion guide 74 (for example, a segmented quadrupole, or quadrupole with auxiliary electrodes, or an ion tunnel ion guide), thus forming a pulsed axial Z-field. Alternatively, a negative pulse 76 may be applied to gate 75, to follow a Pulsar method such as that described herein above. The pulse 76 amplitude and the length of axial Z-field within the guide 74 are arranged for time-of-flight compression of ion packets in the z-direction by the time the ions reach the detector 44 plane, located at distance Dz. Ions located at the entrance of the axial acceleration Z-field when it is pulsed on will arrive at the OA 30 at a later time then ions located towards the exit of the axial acceleration Z-field when it is pulsed on. However, the ions that were initially located at the entrance of the axial acceleration Z-field will have a larger speed in the z-direction Vz by the time they enter the OA, as compared to ions that were initially located further from the entrance of the axial acceleration Z-field. This produces ion packet compression or bunching (in the z-direction) at the detector 44. The desired negative Z-Vz correlation occurs for a mass range only, where ,u range is controlled by the time delay between pulse 76 and OA pulse 79. This 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 dynamic range of the detector.
In another embodiment, the potential of a field-free elevator is controlled by the time variable floating U(t) 77 of either ion guide 74, or of ion optics downstream of the guide 74. The voltage at which the ion guide 74 or ion optics is floated may be varied with time so as to achieve the above described bunching effect (in the z-direction), though the elevator exit may be set closer to the OA entrance and allows a somewhat wider μ range to be accelerated by the OA.
In yet another embodiment, the beam 33 is slowed down within the confinement means 60 by arranging a Z-dependent axial potential distribution U(z) 78, e.g. by a resistive divider connected to electrodes of the confinement means 60 such that different potentials are applied at different z-locations in the confinement means 60. Then the desired z-focusing of ion packets may be achieved for the entire ionic mass range, i.e. for ions of all μ. This method is particularly attractive when using the RF ion guide in the Pulsar mode, i.e. accumulating and pulse releasing ion packets from the guide 74 in a manner that is synchronized with pulses 79 of the OA.
The OA may be a pulsed converter based on a radiofrequency (RF) ion trap with radial pulsed ejection. The space charge capacity of the trap and of the MRTOF analyzer are then improved by substantial trap elongation.
OA along a tilted axis. The embodiment 90 comprises: a continuous ion source 11 generating a continuous ion beam 13; a multi-reflecting TOF 40, similar to one in
In operation, an ion source 11 generates continuous or quasi-continuous (e.g. time-modulated within RF ion guide of the interface) ion beam 13. Beam 13 enters and get trapped within ion guide 82 by a radial confining RF field and by using electrostatic blocking potentials at one or both z-directional ends of the ion guide 82, for example, produced by a DC or pulsed bias of a segment of the ion guide 82. Ions may be trapped as a ribbon, optionally dampened with a pulsed gas admission. At the ion ejection stage, an ion ribbon is accelerated by pulsed application of voltages by the generator 34 and the ions are urged orthogonally to electrodes 81 so as to travel at an inclination angle α/2 relative to the X-axis. To correct for the tilt of the ion packet time front relative to the ion mirrors (that occurs due to the axis of the ion trap being tilted relative to the z-direction), a wedge shaped field of TA-lens 84 steers ions forward for another α/2 angle, so that ion packets 85 are parallel to the Z-direction. The wedge 84 may also have a lens component for ion packet z-focusing, whose time aberrations may be compensated for by a weaker TA-wedge 83. After pulsed ejection and steering in the wedge field 84, ion packets 85 are aligned parallel to the Z-axis and move at an inclination angle α to the z-direction. On their way back, i.e. after the first ion mirror reflection, the ions pass through the slits of the switched off trap 80. Spatially focused (in the z-direction by lens 84) ion packets eventually reach the detector 44 after multiple mirror reflections, thus, improving MRTOF (or MTTOF in embodiments using sectors rather than mirrors) resolution.
Compared to the embodiment of the OA 30 in
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.
For example, although embodiments have been described in which the mass analyser is an MRTOF or a MTTOF, it is contemplated that the mass analyser may instead only have a single ion mirror or sector that reflects or turns the ions, respectively, onto the detector.
Although the OA electrodes in the specific embodiments have been described as being transparent to the ions by providing them with slits, it is contemplated that the electrodes may instead be provided as mesh electrodes or with mesh portions through which the ions pass.
In the depicted embodiments, the OA electrodes and their slits extend in the drift direction (z-direction) from an upstream end of the orthogonal accelerator to a point proximate or downstream of the detector. However, it is contemplated herein that the OA electrodes and their slits (or meshes) may not extend, in the drift direction (z-direction), all of the way to the detector. Rather, there may be a gap, in the drift direction (z-direction), between the downstream end of the OA electrodes and the detector. In such embodiments, it is preferred that each slit is defined between separate elongated electrode portions (separated in the y-direction) rather than a slot in an electrode.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1810573.4 | Jun 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2019/051839 | 6/28/2019 | WO | 00 |
