GB2590600A

GB2590600A - TOF mass spectrometry

Info

Publication number: GB2590600A
Application number: GB1916445.8A
Authority: GB
Inventors: Kozlov Boris
Original assignee: Micromass UK Ltd
Current assignee: Micromass UK Ltd
Priority date: 2019-11-12
Filing date: 2019-11-12
Publication date: 2021-07-07
Anticipated expiration: 2039-11-12
Also published as: GB201916445D0; GB2590600B

Abstract

A TOF mass analyser is disclosed comprising: an ion detector 14; an ion accelerator 13 for repeatedly pulsing packets of ions into a time of flight region such that ions in each packet separate according to mass to charge ratio as they pass to the ion detector 14, wherein the period with which the packets of ions are pulsed is shorter than the time of flight of ions to the detector; and an ion gate 16 arranged in the flight path of the ions through the time of flight region, wherein the ion gate 16 is configured to open and close at times after each pulse of the ion accelerator 13 such that only ions having a first range of mass to charge ratios are transmitted to the ion detector 14 whereas the other ions are blocked by the ion gate. The TOF analyser is preferably a multi-reflecting time of flight (MRTOF) mass analyser comprising two planar ion mirrors 12.

Description

TOF MASS SPECTROMETRY

CROSS-REFERENCE TO RELATED APPLICATION None

FIELD OF THE INVENTION

The present invention relates generally to mass spectrometers and in particular to time of flight mass spectrometers, such as Multi-Reflecting Time-of-Flight (MRTOF) mass spectrometers (MRTOF), and methods of their use.

BACKGROUND

Multi-Reflecting Time of Flight (MRTOF) mass analysers are known, such as those in Verenchikov et al, Multiplexing in Multi-Reflecting TOF MS, Journal of Applied Solution Chemistry and Modelling, 2017, 6, 1-22. MRTOF mass analysers generally have a relatively low duty cycle as the proportion of ions that are mass analysed is relatively low.

This poor duty cycle occurs in instruments that accumulate ions before pulsing them into the time of flight region, where the accumulation time of ions is substantially shorter than their flight time through the time of flight region. This poor duty cycle also occurs in instruments where a continuous beam of ions is periodically pushed into the time of flight region without first being accumulated.

It is known to improve the duty cycle of such instruments by using Encoded Frequent Pulsing (EFP). In such techniques the packets of ions may be pulsed into the time of flight region before waiting for all of the ions from the preceding packet to fully traverse the time of flight region. This is enabled by pulsing the ions into the time of flight region with a known pulse sequence having non-uniform intervals between the pulses, i.e. encoding the ion pulses. The detected ion signals that relate to the different pulses of ions can then be decoded using knowledge of the pulse sequence. However, relatively low ion signals such as background-level ion signals cannot be seen in EFP decoded spectra, particularly in intensively populated spectra.

SUMMARY

The present invention provides a time-of-flight mass analyser comprising: an ion detector; an ion accelerator for repeatedly pulsing packets of ions into a time of flight region and towards the ion detector such that ions in each packet separate according to mass to charge ratio as they pass to the ion detector, wherein the period with which the packets of ions are pulsed is shorter than the time of flight of ions to the detector; and an ion gate arranged in the flight path of the ions through the time of flight region, wherein the ion gate -2 -is configured to open and close at times after each pulse of the ion accelerator such that only ions having a first range of mass to charge ratios are transmitted to the ion detector whereas the other ions are blocked by the ion gate.

By arranging the ion gate within the flight path of the ions through the time of flight region, the ion gate is able to mass selectively transmit ions that are pulsed by the ion accelerator. This provides the mass selection at a relatively downstream stage of the instrument, thereby avoiding undesired fragment or product ions (of parent ions that had a mass to charge ratio desired to be mass selectively transmitted by the ion gate) from reaching the detector. Such fragment or product ions may be inadvertently generated by fragmentation, charge exchange or ion adduct processes etc. Also, arranging the gate in the time of flight region enables the instrument to be relatively compact. The location of the gate also enables it to be used for additional purposes, such as the simultaneous suppression of multiple selected mass ranges, e.g. to supress abundant peaks so as to extend the life of the detector.

The first range of mass to charge ratios is a subset of the range of mass to charge ratios of the ions that are pulsed out of the ion accelerator in each pulse. The first range of mass to charge ratios may be a predetermined (i.e. preselected) range of mass to charge ratios. This first range may be predetermined (selected) by performing an experimental run in the mass analyser with the ion gate open the whole time (or omitted). The mass spectral data may then be analysed to determine a range of mass to charge ratios in which ions of interest occur, and this range may be set as said first range. Alternatively, the first range of mass to charge ratios of the ions may be simulated/determined based on known ToF calibration (i.e. the relationship between m/z and time of flight) and a known ratio of the effective distance from the ion accelerator to the gate to the effective distance from ion accelerator to the detector.

The ion accelerator may be an orthogonal accelerator for receiving ions along a first axis and accelerating them in a direction orthogonal to the first axis.

The ion accelerator may be configured such that the period with which it pulses packets of ions into the time of flight region is shorter than the flight time from the ion accelerator to the detector of at least some of said ions that are transmitted to the detector when the ion gate is open; and wherein the ion gate is configured to open and close at times after each pulse of the ion accelerator such that ions from different ion accelerator pulses do not temporally overlap at the ion detector.

As such, the ion gate desirably does not pulse packets of ions into the time of flight region using an Encoded Frequency Pulsing technique.

The ion gate has control circuitry configured to perform its various functions described herein.

The ion accelerator may be configured to repeatedly pulse packets of ions into the time of flight region during a single experimental run, and the mass analyser may be configured to sum the ion signals detected at the detector during the single experimental run. -3 -

The ion gate may comprise one or more electrodes arranged in the time of flight region, a voltage supply for applying one or more voltages to the one or more electrodes, and control circuitry for controlling the voltages applied to the electrodes. The control circuitry may be configured such that at least a first voltage is applied to the one or more electrodes, by the voltage source, in the mode in which the ion guide is closed such that all ions that it receives are prevented from passing through the ion gate. The control circuitry may be configured such that at least a second, different voltage is applied to the one or more electrodes, by the voltage source, in the mode in which the ion guide is open such that all ions that it receives are allowed to pass through the ion gate.

The one or more electrodes of the ion gate may be arranged in various forms. For example, the ion gate may be an apertured electrode. A voltage (e.g. a ground voltage) may be applied to the apertured electrode the gate is open such that ions can pass therethrough, whereas a different voltage may be applied to the apertured electrode the gate is closed such that ions cannot pass therethrough. Alternatively, a plurality of electrodes may be arranged circumferentially around the ion path and a voltage may be applied to at least some of these electrodes in the closed mode so as to block the passage of ions, whereas the voltage(s) applied to these electrodes in the open mode do not block the passage of the ions. Alternatively, the ion gate may comprise opposing electrodes (e.g. plate electrodes) and the voltages applied to these electrodes may be varied so as to switch between the open and closed modes. For example, in a closed mode a potential difference may be applied between the electrodes such that ions impact on one of the electrodes and do not pass through the ion gate, whereas in an open mode the voltages applied to the electrodes are such that the ions can pass between the electrodes and be onwardly transmitted to the detector.

The ion gate may be a Bradbury-Nilsen ion gate (e.g. a bipolar grid with a switchable voltage applied between elements of opposite sign) or any other type of ion gate, but desirably has good spatial and temporal resolutions.

The mass analyser may be a Multi-Reflecting Time-of-Flight (MRTOF) mass analyser comprising two ion mirrors arranged and configured such that the ions are accelerated into one of the ion mirrors by the ion accelerator and are reflected by each of the mirrors, and between the mirrors, a plurality of times before reaching the detector. The ion gate may be arranged in the time of flight region at a location so as to receive ions after their first reflection by the ion mirrors and before the second reflection by the ion mirrors.

The ion gate may be located in the time of flight region at a location such that ions only pass through the ion gate a single time before reaching the ion detector.

The ion gate may be located in a time-focusing plane of the mass analyser.

The detector is located in a different time-focusing plane of the mass analyser.

As will be appreciated, a time-focussing plane is a plane where ions of the same mass-to-charge ratio but different energies arrive simultaneously.

The mass analyser may be configured such that the flight time of an ion from the ion accelerator to the detector divided by the flight time of that ion from the ion accelerator -4 -to the ion gate is Ng, and the number of ion accelerator pulses occurring whilst ions from a given ion accelerator pulse are still in flight to the detector is Np, wherein Np.s Ng.

The period with which the packets of ions are pulsed by the ion accelerator (i.e. the inverse of the pulse frequency) is shorter than the time of flight of ions with highest m/z value to the detector, and may be longer than 1/Ng times the time of flight of ions to the detector.

The ion gate may be configured such that each time it opens, it open for a duration corresponding to 1/(Ng*Np) of the flight time to the detector of the minimal m/z transmitted by the ion gate; and/or the ion gate may be configured to open so as to transmit only ions having a range of mass to charge ratios from mb to me, where mb/(me-mb) 2 Ng.

The ion accelerator and ion gate may be configured such that the period with which the ion accelerator pulses packets of ions into the time of flight region is substantially the same as the period with which the ion gate opens.

For the avoidance of doubt, the period with which the ion accelerator pulses packets of ions into the time of flight region is the inverse of the pulse frequency of the ion accelerator, and the period with which the ion gate opens is the inverse of the opening frequency of the ion gate.

The ion accelerator may be configured to vary the period at which it pulses packets of ions into the time of flight region in order to vary the range of mass to charge ratios that are transmitted by the ion gate.

The ion gate may be configured to vary the delay time that it is opened after each pulse of the ion accelerator, and/or the duration that it is open for, in order to vary the range of mass to charge ratios that are transmitted by the ion gate.

The ion accelerator may be configured to vary the period at which it pulses packets of ions into the time of flight region, optionally proportionally with the variation of the time that the ion gate is opened after each pulse of the ion accelerator, and/or proportionally with the variation in the duration of time that the ion gate is open for.

The ion gate may be configured to open at timings relative to the pulses of the ion accelerator such that ions from an nth ion accelerator pulse that are transmitted by the ion gate are recorded at the ion detector centred at a point in time that is substantially midway between the timings of the nth and (n+1)th ion accelerator pulses.

The mass analyser may be configured to identify the portion of the ion signal that is detected between the timings of each pair of adjacent ion accelerator pulses, and to sum these ion signal portions together.

The ion accelerator may be configured to maintain the period at which it pulses packets of ions into the time of flight region constant, and the ion gate may be configured to maintain the frequency that it is opened constant.

The mass analyser may be configured to identify portions of the ion signal that are detected in a plurality of time windows, wherein each time window has a duration equal to or less than the period with which the ion accelerator pulses, wherein the start time of each -5 -window is between the timings of each pair of adjacent ion accelerator pulses, and the mass analyser is configured to sum these ion signal portions together.

The mass analyser may be configured to vary the start times of the windows relative to the pulses of the ion accelerator in order to sum different ranges of mass to charge ratios.

The present invention also relates to a mass spectrometer comprising the mass analyser as described herein.

The present invention also relates to a method of mass spectrometry comprising: providing a time-of-flight mass analyser as described herein above; repeatedly pulsing packets of ions into the time of flight region and towards the ion detector with the ion accelerator, such that ions in each packet separate according to mass to charge ratio as they pass to the ion detector; and opening and closing the ion gate at times after each pulse of the ion accelerator such that only ions having a first range of mass to charge ratios are transmitted to the ion detector whereas the other ions are blocked by the ion gate.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a schematic of a known MRTOF mass analyser; Fig. 2 shows an MRTOF mass analyser according to an embodiment of the present invention; Figs. 3A-3B show examples of mass spectra obtained in a normal MRTOF mode and in a zoomed MRTOF mode, respectively; and Fig. 4 shows an example of a mass spectrum obtained in another embodiment of a zoomed MRTOF mode.

DETAILED DESCRIPTION

Fig. 1 illustrates a known MRTOF mass analyser. The instrument comprises two two-dimensional ion mirrors 12 extended along a drift dimension (Z-dimension) for reflecting ions, an accelerator 13 for injecting ions into the device, and a detector 14 for detecting the ions that impact thereon at the end of their folded flight path through the mass analyser. For clarity, throughout this entire text we use a planar MR-TOF described in the standard Cartesian coordinate system. That is, the X-axis corresponds to the main dimension of time-of-flight, i.e. the direction of ion reflections between the ion mirrors, the Z-axis corresponds to the drift direction of the ions, and the vertical Y-axis is orthogonal to both the X and Z axes. However, the invention described herein may be used in a MRTOF having planar or non-planar geometries, curved Z-Y-X-axes, multi-storey mirrors, and any ToF (e.g. having low duty cycle).

In use, ions are accelerated by accelerator 13 towards one of the ions mirrors 12 at an inclination angle a to the X-axis. The ions therefore have a velocity in the X-dimension -6 -and also a drift velocity in the Z-dimension. The ions enter into a first of the ion mirrors 12 and are reflected back towards a second of the ion mirrors 12. The ions then enter the second mirror 12 and are reflected back to the first ion mirror 12. The first ion mirror then reflects the ions back to the second ion mirror 12. This continues and the ions are continually reflected between the two ion mirrors 12 as they drift along the device in the Z-dimension until the ions impact upon detector 14. The ions therefore follow a substantially sinusoidal mean trajectory within the X-Z plane.

Fig. 2 shows an MRTOF mass analyser according to an embodiment of the present invention. The instrument comprises two two-dimensional ion mirrors 12 extended along a drift dimension (Z-dimension) for reflecting ions, an orthogonal accelerator 13 for injecting ions into the ion mirrors, an ion gate 16 for selectively transmitting ions therethrough, deflector electrodes 18 for deflecting ions, and a detector 14 for detecting the ions. In use, ions travelling into the MRTOF mass analyser are orthogonally accelerated by orthogonal accelerator 13 towards one of the ions mirrors 12 such that the mean ion trajectory has an inclination angle a to the X-axis. The ions therefore have a main velocity component in the X-dimension and also a drift velocity component in the Z-dimension. The ions enter into a first of the ion mirrors 12 and are reflected back towards a second of the ion mirrors 12. The ions pass through the ion gate 16 on their way to the second ion mirror. The function of the ion gate 16 will be described in more detail below. The ions then enter the second mirror 12 and are reflected back to the first ion mirror 12. The first ion mirror then reflects the ions back to the second ion mirror 12. This continues and the ions are continually reflected between the two ion mirrors 12 as they drift along the device in the Z-dimension until the ions reach the deflector electrodes 18. A voltage difference is maintained between the deflection electrodes 18 so as to reflect the ions in the Z-dimension back towards the orthogonal accelerator 13. The ions continue to be reflected between the two ion mirrors 12 as they drift back along the device in the Z-dimension until the ions impact upon the detector 14. The ions therefore follow a substantially sinusoidal mean trajectory within the X-Z plane.

Although, the above embodiment has been described as having deflection electrodes 18 for reflecting ions in the Z-dimension, it is contemplated that such electrodes need not be present and the detector 14 may instead be arranged where the deflection electrodes18 are shown, in the same manner as in Fig. 1.

In a first (normal) mode of MRTOF mass analysis, the ion gate 16 may remain open such that it does not block ions and instead transmits all ions that it receives. However, in some applications it is not necessary to obtain mass spectral data for all ions and instead it may be desired to operate the instrument in a second (stroboscopic or zoom) mode so as to obtain high sensitivity mass spectral data for only a selected, relatively narrow range of mass to charge ratios. In the zoom mode, the timings at which the ion gate 16 is opened and closed (relative to the time at which ions are pulsed from the orthogonal accelerator 13) are controlled such that only a selected range of mass to charge ratios in the ion pulse are able to pass through the ion gate 16. More specifically, the ion gate 16 is controlled so as to be open only when ions having mass to charge ratios in the selected range reach the -7 -ion gate 16 such that they pass therethrough, and closed when ions of other mass to charge ratios reach the ion gate 16 such that their passage is blocked by the closed ion gate. This is illustrated in Fig. 2 by the plurality of dots, which represent ions, arranged along the sinusoidal flight path. Many groups of ions are shown upstream of the ion gate 16, whereas the repetitive blocking of ions by the ion gate results in the selective transmission of the same group of ions from the different pulses flying simultaneously through different stages of the folded path downstream of the ion gate 16. The use of the ion gate allows a narrow mass range to be detected with good sensitivity and high dynamic range The following example is given in order to illustrate the technique described herein.

An MRTOF mass analyser may have a TOF MS flight path of 10 m and a duty cycle of around 1%. It may be desired to observe relatively small mass peaks having mass to charge ratios around m/z=1000Th (a time of flight to the detector of 505 ps). The orthogonal accelerator 13 may be pulsed with a pushing period of 10 ps, and the ion gate 16 may be operated so as to selectively transmit only ions having flight times to the detector between in 500-510 ps. This results in ions of m/z = 980 to 1020 Th being recorded by the detector 14 with a -50% duty cycle. Increasing the period of pushing will proportionally increase the relative mass range analysed, but will lose duty cycle correspondingly.

The ion gate 16 may be located near the start of the ion flight path through the MRTOF mass analyser, i.e. downstream of but near the orthogonal accelerator 13. The ion gate 16 may be positioned such that ions only pass through it a single time. The ion gate 16 may be provided between the first and second ion mirror reflections. Preferably, the ion gate is located in one of the time-focusing planes of the MRTOF mass analyser, e.g. at a point of where ions of the same mass-to-charge ratio but different energies arrive simultaneously. As the ion gate 16 can selectively transmit a relatively narrow range of mass to charge ratios from each orthogonal accelerator pulse, and may be positioned relatively close to the orthogonal accelerator 13, the orthogonal accelerator 13 may be pulsed at a relatively high rate without ions from different orthogonal accelerator pulses temporally overlapping at the detector 14.

The ion gate 16 is located at a distance corresponding to 1/Ng of the effective flight path between the orthogonal accelerator 13 and the detector 14, where Ng is the flight time of an ion from the orthogonal accelerator 13 to the detector 14 divided by the flight time of that ion from the orthogonal accelerator 13 to the ion gate 16. For the avoidance of doubt, the effective flight path between the orthogonal accelerator 13 and the detector 14 corresponds to the flight time of the ion between the orthogonal accelerator 13 and the detector 14, multiplied by the constant speed the ion travels at after it had been accelerated (e.g. multiplied by the speed of the ion in a field-free region between the ion mirrors in an MRTOF mass analyser). In order to avoid ions from different pulses of the orthogonal accelerator 13 temporally overlapping at the ion gate 16, the number of orthogonal acceleration pulses, Np, applied whilst any ions from any given accelerator pulse are still in flight to the detector 14 is to be equal to or lower than Ng (i.e. Np s Ng). -8 -

Therefore, the value of Np is related to the longest flight time from the orthogonal accelerator 13 to the detector 14 for ions in any given pulse. In order to determine Np, the flight time of the ion having the highest mass to charge ratio from the orthogonal accelerator 13 to the detector 14 may be determined or estimated. The number of orthogonal acceleration pulses, Np, may then be determined by dividing this flight time by the period of the orthogonal accelerator pulses (wherein the period is the inverse of the frequency of the pulses). This longest flight time may be estimated based on prior knowledge of the sample being analysed, e.g. based on the ions that are expected to be analysed or based on previously obtained mass spectral data for the sample being analysed.

The ion gate 16 is controlled so as to generally remain closed and only be opened for short durations each time, wherein each duration that the gate is opened is to be restricted to.s 1/(Np*Ng) of the flight time between the orthogonal accelerator 13 and detector 14 for the fastest ions of interest (i.e. the lowest m/z ions transmitted by the ion gate in the zoom mode), if we want to avoid overlapping of the selected ion trains.

Each gated train of ions will have leading and trailing portions with impaired abundance due to factors such as the spatial spread of ion packs, the spatial length of the ion gate, and the rising and falling edges of the gating pulse applied to the ion gate. In order to avoid ions from different trains temporally overlapping at the detector, the spatial (and time) resolution at the ion gate is to be better than 1/(Np*Ng) of the full flight length (or the full time of flight) between the orthogonal accelerator and the detector. If these conditions are implemented, a mass range of Am m * 2/Np is detected, where m is the lowest mass to charge ratio of the mass range that is to be recorded. The detected ion signal is approximately Np times higher in intensity than would be seen without the ion gate (as the signal is accumulated for the Np pulses) and is without interference.

As described above, the ion gate 16 may be positioned at a distance downstream of the orthogonal accelerator corresponding to 1/Ng of the effective flight path between the orthogonal accelerator and the detector (so that the flight path from the orthogonal accelerator to the ion gate is Ng times shorter than the flight path to the detector). The ion gate may remain closed other than for relatively narrow durations when a gating pulse is applied to the ion gate. The start times at which the gating pulses are applied to the ion gate may, in some embodiments, coincide with the times at which ions are pulsed out of the orthogonal accelerator (if Np=Ng). However, this need not be the case.

If the selected mass range to be transmitted through the ion gate has lowest mass to charge ratio mb and has a flight time tb between the orthogonal accelerator and the detector, then mb = a*tb2, where a is a constant. The last mass to charge ratio in the range that is transmitted by the ion gate when it is open, me, has a time of flight te between the orthogonal accelerator and the detector, where te = tb + tb/Np = tb(1+1/Np). As me = age2, where a is a constant, the value of the last mass to charge ratio in the transmitted range is me = a*tb2(1+2/Np+1/Np2). The times of flight of the ions having mass to charge ratios mb and me to the ion gate are tgb=tb/Ng and tge=te/Ng. -9 -

If Ng>Np, the ion flight distance from the orthogonal accelerator to the ion gate is less than 1/Np of the distance from the orthogonal accelerator to the detector. Then, the duration of time between the orthogonal accelerator push and ion gate pulse is tb/Ng, which is less than tb/Np (gate pulse starts before the next orthogonal accelerator push pulse).

This may induce pick-up noise in the middle of acquired mass range. Furthermore, the resolving power of the ion gate decreases with decreasing distance of the ion gate from the orthogonal accelerator. However, if the period at which the accelerator pushes occur is made longer (smaller Np), this helps to avoid overlaps with ions having very long flight time.

On the other hand, increasing the distance of the ion gate from the orthogonal accelerator to be significantly above 1/Np of the distance from the orthogonal accelerator to the detector may also not be desirable. This is because whilst the ion gate is transmitting the selected ions from any given pulse, low mass to charge ratio ions from the next pulse of the orthogonal accelerator may reach and pass through the ion gate. In contrast, if the distance between the orthogonal accelerator and ion gate is relatively low then the selected ions from a given pulse can be transmitted by the ion gate before the low mass to charge ions from the subsequent pulse arrive at the ion gate.

Therefore, desirably, the time of flight of ions having a mass to charge ratio mb to the ion gate is to = tb/Ng; the time of flight of ions having mass to charge ratio me to the ion gate is t" = te/Ng = tb(1+1/Np)/Ng, and the time difference tje-to = tb/(NpNg).

In a first embodiment, Np=Ng, the period of the pushing pulses is chosen to be equal to 1/Ng of the time of flight of the ion having mb (i.e. having the minimal mass of the selected range). To change the selected mass range to be transmitted by the ion gate and detected, it may be desired to scale the pulsing pattern by changing the period of the orthogonal accelerator pushing pulses proportionally with both the length of the ion gate pulses and the delay time between the orthogonal accelerator pulse and the ion gate pulse, so that the selected ions of interest from an nth orthogonal accelerator pulse will be recorded at the detector centred at a point in time that is substantially midway between the nth and (n+l)th orthogonal accelerator pulses (independently of the variation of the period).

This is shown in the example of Figs. 3A-3B. The noise of pulse pick-ups from the orthogonal accelerator will then be beside the ion signal of interest. The beginning of the recorded mass range will be defined by the period T between orthogonal accelerator pushes, according to the known ratio between mass to charge ratio and flight time and such that: rib = a(NgT)2 me = Mb(1+2/Ng) Fig. 3A shows an example of the mass spectrum (detector signal) obtained in a normal MRTOF mode in which the ion gate 16 does not restrict the ions, and Fig. 3B shows an example of the mass spectrum (detector signal) obtained in a zoomed mode in which the ion gate 16 selectively transmits only ions in a pre-selected range of mass to -10 -charge ratios. Fig. 3A shows the mass spectrum obtained after a first pulse 20 of the orthogonal accelerator 13, when the spectrometer is operating in a normal mode, i.e. when the ion gate 16 is permanently open (or there is no ion gate). The straight vertical lines represent the mass peaks detected and the smaller vertically oriented ovals 22 represent the pulse timings of subsequent orthogonal accelerator pulses. Portion 24 of the mass spectrum illustrates the portion of the mass spectrum that is of interest and which is therefore desired to be analysed in a zoom mode, i.e. these are the mass to charge ratios that are desired to be selectively transmitted by the ion gate 16 in the zoom mode.

Fig. 3B shows the mass spectrum obtained as a result of multiple pulses of the orthogonal accelerator when the spectrometer is operating in the zoom mode, i.e. when the ion gate 16 is selectively opened at times so as to allow only the mass to charge ratios of interest to pass through it and to the detector 14. The straight vertical lines represent the mass peaks detected and the smaller, vertically oriented ovals 22 represent the pulse timings of the orthogonal accelerator pulses. Each pair of larger, vertically oriented ovals represents the start and end times of an ion gate pulse 26 that allows ions to pass through the ion gate 16. As can be seen, the spectral data detected at timings between each pair of adjacent orthogonal accelerator pulses 22 corresponds to the portion 24 of the mass spectrum obtained in the normal mode that is of interest On Fig. 3A). The repeated part of the spectrum in Fig. 38 may be summed so as to obtain a spectrum for the ions of interest with high sensitivity. As it can be understood, the desired mass range in this case is essentially contained between adjacent pulses of the orthogonal accelerator 22, enabling the data accumulation to be synchronized with the starts of the orthogonal accelerator pulses 22. The flight time, equal to NpT (where T is the period of accelerator pulses), is to be accounted for mass calibration (added to the time inside the recorded interval).

In a second embodiment the ion gating window can be scanned in frames of (constant) accelerator pulse period, having a duration corresponding to -1/Ng of the time of flight of selected ions having the highest expected m/z. In contrast to the first embodiment described above, it may be advantageous for various reasons to maintain the orthogonal accelerator pulse frequency constant whilst varying the mass range that is selectively transmitted by the ion gate 16 and detected. Correspondingly, the gate pulse frequency may also remain constant, but the delay between the orthogonal accelerator and ion gate pulses (and optionally the duration of the gate pulse) is varied in order to vary the mass range that is selectively transmitted by the ion gate 16 and detected. As described previously, if it is desired to avoid temporal overlapping, at the detector 14, of ions trains from consecutive orthogonal accelerator pulses, the duration of gate delay from the orthogonal accelerator pulse should not substantially exceed the period of the orthogonal accelerator pulses. If there are ions arriving at the gate with a delay exceeding the period of the orthogonal accelerator pulses, they will arrive at the gate at the same time as fast ions pushed by the next orthogonal accelerator pulse. This may complicate identification of the mass to charge ratios of the ions. Therefore, the period T of the orthogonal accelerator pulses may be restricted by the flight time of the maximum mass to charge ratio and by the ratio Th t,"," /Ng As was described in relation to Figs. 3A-3B, it may be desired to sum a repeated part of the spectrum so as to obtain a spectrum for the ions of interest with high sensitivity. However, the repeated parts of the spectrum that are desired to be summed may be parts other than the parts that are centred half-way between adjacent orthogonal accelerator pulses. For example, sometimes it may be desired to see a mass range of the ions that is shifted from this centred point, such as a mass range centred about the timing of the orthogonal accelerator pulse. In these cases, it is desired to calculate the windows/portions of the signal that are summed, e.g. as described below in relation to Fig. 4.

Fig. 4 shows a mass spectrum obtained as a result of multiple pulses of the orthogonal accelerator 13 when the spectrometer is operating in the zoom mode, i.e. when the ion gate 16 is selectively opened at times so as to allow only the mass to charge ratios of interest to pass through it and to the detector 14 and so that there is no overlapping of spectral data from different orthogonal accelerator pulses. The straight vertical lines represent the mass peaks detected and the vertically oriented ovals 22 represent the pulse timings of the orthogonal accelerator pulses. The ion gate pulses are not shown, but they do not coincide with the edges of each window 28 to be summed. Rather, when it is desired for the ion gate 16 to transmit relatively low mass to charge ratios to the detector 14, each gate pulses has a relatively small delay from the preceding accelerator pulse 22.

In contrast, when it is desired for the ion gate 16 to transmit higher mass to charge ratios to the detector 14, each gate pulse has a larger delay from the preceding accelerator pulse 22 approaching the next accelerator pulse accelerator pulse.

Repetitive parts of the spectrum 28 may be substantially centred between adjacent orthogonal accelerator pulses 22 (for ions of low mass having flight times i*T/Ng, where i is any integer less than or equal Ng, and T is pushing period). However, in the general case, lower m/z ions at the beginning of the selected range arrive at the detector 14 at a time before the timing of one of any given accelerator pulse 22, and ions of higher m/z at the end of the selected range arrive after this accelerator pulse. The top left corner of Fig. 4 shows a sum of the signal between adjacent acceleration pulses 22. The large oval 30 separates the signal due to the beginning of the selected mass range from one orthogonal accelerator pulse (to the right of the mark 30) and the end of the selected mass range from a different orthogonal accelerator pulse (to the left of the mark 30). As shown in Fig. 4, there are repetitive parts of the spectrum 24 defined by pushing pulses 22 that would be normally summed by the acquisition system, and there are parts desired to be summed that may be defined by a plurality of windows 28, where the start and end of each window 28 is shifted relative to accelerator pulses 22 in accordance with the position of the gate pulses relative to the accelerator pushes 22 (as described above). The window 28 may have a duration corresponding to, or smaller than, the period T of the orthogonal accelerator pulses 22. The mass spectral data in the windows 28 may then be summed so as to produce the summed spectrum 30 shown at the bottom of Fig. 4. The boundaries between the windows 28 should be wide enough to exclude overlapping and provide reliable assignment of the entities.

-12 -For the application of periodic frequent pulsing of the orthogonal accelerator with period T, and desired observation of ions of interest having a mass range starting with m/z = mb, the ion gate selectively transmits a mass to charge ratio range between masses mb and me so that their respective flight times between the orthogonal accelerator and detector tb and te satisfy te tb+T.

The analyte molecules and/or ions derived therefrom may be separated according to one or more physico-chemical property in a preliminary stage of separation before the analyte ions enter the TOF mass analyser. For example, the analyte may be separated by liquid or gas chromotography, or by capillary electrophoresis. Additionally, or alternatively, the analyte (once ionised to form analyte ions) may be separated by ion mobility and/or mass to charge ratio upstream of the TOF mass analyser. The techniques described herein can be used in a synchronous mode with the preliminary stages of separation. For example, the timings of the orthogonal accelerator pulses 22 and gate pulses 26 can be synchronised with the operation of the preliminary stage of separation such that the ions of interest that are selectively passed by the ion gate 16 (in the zoom mode) have the desired physico-chemical property. For example, the ions may be separated by ion mobility in the preliminary stage of separation and the ion gate 16 may be controlled in synchronism with the ion mobility separator such that only ions having a selected charge state (e.g. doubly charged ions) are transmitted by the ion gate to the detector 14. This is possible as ions having a given charge state tend to follow a trend line correlating mobility with mass to charge ratio.

The techniques described herein may be used to extend the range of applications of MRTOF mass spectrometry, increase limits of detection, increase sensitivity of detection, and increase the dynamic range of the mass analyser in the selected mass range.

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

For example, although embodiments have been described in which the mass analyser is an MRTOF mass analyser, it is contemplated that the mass analyser may be a TOF analyser having a single ion mirror.

Although the mass analyser has been described as comprising an orthogonal accelerator, it is contemplated that alternatively the TOF mass analyser may be any type of ion accelerator such as an in-line TOF mass analyser that has an accelerator that receives ions and pulses them into the flight region in the same direction that they were received. Alternatively, ions may be generated inside the ion accelerator, e.g. by using a laser beam or electron beam etc. to ionise analyte located in the ion accelerator. The ion accelerator may then pulse the ions into the flight region of the mass analyser.

Although embodiments have been described wherein an ion gate selectively transmits ions of the selected mass range, it is alternatively contemplated that the mass range can be selected by an alternative mass spectrometer stage upstream of the TOF -13 -mass analyser. In such embodiments, fragmentation of the selected ions may be avoided. This may be difficult to implement in practice, for example, as some complex ions have long life times before they spontaneously decompose into fragment ions. However, after acceleration inside the MRTOF mass analyser such fragment ions may have a different trajectory to the ions of interest such that they are not detected, or their ion signal may be excluded from being recorded by known methods based on their lower energy or widened arrival time signal.

Claims

-14 -Claims 1. A time-of-flight mass analyser comprising: an ion detector; an ion accelerator for repeatedly pulsing packets of ions into a time of flight region and towards the ion detector such that ions in each packet separate according to mass to charge ratio as they pass to the ion detector, wherein the period with which the packets of ions are pulsed is shorter than the time of flight of ions to the detector; and an ion gate arranged in the flight path of the ions through the time of flight region, wherein the ion gate is configured to open and close at times after each pulse of the ion accelerator such that only ions having a first range of mass to charge ratios are transmitted to the ion detector whereas the other ions are blocked by the ion gate.
2. The mass analyser of claim 1, wherein the ion accelerator is configured such that the period with which it pulses packets of ions into the time of flight region is shorter than the flight time from the ion accelerator to the detector of at least some of said ions that are transmitted to the detector when the ion gate is open; and wherein the ion gate is configured to open and close at times after each pulse of the ion accelerator such that ions from different ion accelerator pulses do not temporally overlap at the ion detector.
3. The mass analyser of claim 1 or 2, wherein the ion accelerator is configured to repeatedly pulse packets of ions into the time of flight region during a single experimental run, and the mass analyser is configured to sum the ion signals detected at the detector during the single experimental run.
4. The mass analyser of claim 1, 2 or 3, wherein the mass analyser is a Multi-Reflecting Time-of-Flight (MRTOF) mass analyser comprising two ion mirrors arranged and configured such that the ions are accelerated into one of the ion mirrors by the ion accelerator and are reflected by each of the mirrors, and between the mirrors, a plurality of times before reaching the detector.
5. The mass analyser of claim 4, wherein the ion gate is arranged in the time of flight region at a location so as to receive ions after their first reflection by the ion mirrors and before the second reflection by the ion mirrors.
6. The mass analyser of any preceding claim, wherein the ion gate is located in the time of flight region at a location such that ions only pass through the ion gate a single time before reaching the ion detector.
7. The mass analyser of any preceding claim, wherein the ion gate is located in a time-focusing plane of the mass analyser.
8. The mass analyser of any preceding claim, configured such that the flight time of an ion from the ion accelerator to the detector divided by the flight time of that ion from the ion accelerator to the ion gate is Ng, and the number of ion accelerator pulses occurring whilst ions from a given ion accelerator pulse are still in flight to the detector is Np, wherein Np Ng.
9. The mass analyser of claim 8, wherein: (i) the ion gate is configured such that each time it opens, it open for a duration corresponding to 1/(Ng*Np) of the flight time to the detector of the minimal m/z transmitted by the ion gate; and/or (ii) the ion gate is configured to open so as to transmit only ions having a range of mass to charge ratios from mb to me, where mb/(me-mb) a 2 Ng.
10. The mass analyser of any preceding claim, wherein the ion accelerator and ion gate are configured such that the period with which the ion accelerator pulses packets of ions into the time of flight region is substantially the same as the period with which the ion gate 20 opens.
11. The mass analyser of any preceding claim, wherein the ion accelerator is configured to vary the period at which it pulses packets of ions into the time of flight region in order to vary the range of mass to charge ratios that are transmitted by the ion gate.
12. The mass analyser of any preceding claim, wherein the ion gate is configured to vary the delay time that it is opened after each pulse of the ion accelerator, and/or the duration that it is open for, in order to vary the range of mass to charge ratios that are transmitted by the ion gate.
13. The mass analyser of claim 12, wherein the ion accelerator is configured to vary the period at which it pulses packets of ions into the time of flight region, optionally proportionally with the variation of the time that the ion gate is opened after each pulse of the ion accelerator, and/or proportionally with the variation in the duration of time that the ion gate is open for.
14. The mass analyser of claim 12 or 13, wherein the ion gate is configured to open at timings relative to the pulses of the ion accelerator such that ions from an nth ion accelerator pulse that are transmitted by the ion gate are recorded at the ion detector centred at a point in time that is substantially midway between the timings of the nth and (n+1)th ion accelerator pulses.
-16 - 15. The mass analyser of any preceding claim, wherein the mass analyser is configured to identify the portion of the ion signal that is detected between the timings of each pair of adjacent ion accelerator pulses, and to sum these ion signal portions together.
16. The mass analyser of claim 12, wherein the ion accelerator is configured to maintain the period at which it pulses packets of ions into the time of flight region constant, and the ion gate is configured to maintain the frequency that it is opened constant.
17. The mass analyser of any preceding claim, wherein the mass analyser is configured to identify portions of the ion signal that are detected in a plurality of time windows, wherein each time window has a duration equal to or less than the period with which the ion accelerator pulses, wherein the start time of each window is between the timings of each pair of adjacent ion accelerator pulses, and the mass analyser is configured to sum these ion signal portions together.
18. The mass analyser of claim 17, configured to vary the start times of the windows relative to the pulses of the ion accelerator in order to sum different ranges of mass to charge ratios.
19. A method of mass spectrometry comprising: providing a time-of-flight mass analyser as claimed in any preceding claim; repeatedly pulsing packets of ions into the time of flight region and towards the ion detector with the ion accelerator, such that ions in each packet separate according to mass to charge ratio as they pass to the ion detector; and opening and closing the ion gate at times after each pulse of the ion accelerator such that only ions having a first range of mass to charge ratios are transmitted to the ion detector whereas the other ions are blocked by the ion gate.