Patent Application
20230324336
The present invention relates to methods of analysing ions, and in particular to time-of-flight (ToF) mass analysers and to ion mobility analysers.
In time-of-flight (ToF) analysers and ion mobility analysers, ions are passed through a drift region of the analyser and are eventually detected by a detector. A physicochemical property of an ion, such as its mass to charge ratio (m/z) or ion mobility, is determined from the ion's drift time through the drift region.
It can often be desirable to increase the resolution of an analyser, both for improved separation of analyte ions and for accurate determination of their physicochemical properties such as mass. An instrument's resolution is limited by (amongst other things) the total length of the ion flight path through the analyser.
Several “cyclic” analysis techniques exist, whereby ions are made to make plural repeated cycles along an ion path within the analyser. Increasing the number of cycles N increases the length of the ion flight path that ions take within the analyser, thereby increasing the resolution of the analyser.
However, during multiple cycles N through the analyser, lighter faster moving ions can overtake (e.g., lap) heavier slower moving ions. This complicates the resulting spectra and can make it difficult to accurately determine the physicochemical property of all of the detected ions.
It is believed that there remains scope for improvements to methods of operating ion analysers.
A first aspect provides a method of operating an analytical instrument that comprises an ion analyser configured to analyse ions by determining drift times of ions along an ion path, the ion path comprising at least a first segment, and a cyclic segment, wherein the ion path is configured such that ions make a single pass of the first segment and make one or more passes of the cyclic segment; the method comprising:
Embodiments relate to methods of operating a cyclic ion analyser. The analyser is configured to analyse ions by determining (e.g., measuring) the drift times of the ions along an ion path, where the ions can make multiple passes through a cyclic segment of the ion path before being detected. In cyclic analysers, ions that have very different physicochemical properties (e.g., mass to charge ratio (m/z) or ion mobility) can have similar drift times through the analyser, e.g., due to faster moving ions overtaking (e.g., lapping) slower moving ions in the cyclic segment of the ion path. This can complicate the resulting spectra and can make it difficult to accurately determine physicochemical properties of the detected ions.
Embodiments provide methods of disambiguating the spectra produced by cyclic ion analysers. As will be described in more detail below, by comparing two sets of ion data that have been obtained using different analyser settings, the number of passes N through the cyclic segment of the ion path taken by ions contributing to an ion peak can be determined, thereby allowing the physicochemical property of those ions to be unambiguously assigned to the ion peak.
The analytical instrument may be a mass spectrometer, an ion mobility spectrometer, or a combination of the two (e.g., a mass spectrometer which includes an ion mobility separator). The instrument may comprise an ion source. Ions may be generated from a sample in the ion source. The ions may be passed from the ion source to the analyser via one or more ion optical devices arranged between the ion source and the analyser.
The one or more ion optical devices may comprise any suitable arrangement of one or more ion guides, one or more lenses, one or more gates, and the like. The one or more ion optical devices may include one or more transfer ions guides for transferring ions, and/or one or more mass selector or filters for mass selecting ions, and/or one or more ion cooling ion guides for cooling ions, and/or one or more collision or reaction cells for fragmenting or reacting ions, and so on. One or more of each ion guide may comprise a multipole ion guide such as a quadrupole ion guide, hexapole ion guide, etc., a segmented multipole ion guide, a stacked ring type ion guide, and the like.
The ion analyser is configured to analyse ions by determining drift times of ions along an ion path. Thus, the ion analyser may comprise an ion injector arranged at the start of the ion path, and an ion detector arranged at the end of the ion path. The ion injector may be configured to receive ions from the ion source via the one or more ion optical devices. The ion injector may be configured to inject (received) ions into the ion path (e.g., by accelerating ions along the ion path), whereupon ions travel along the ion path to the detector. The ion injector can be in any suitable form, such as for example an ion trap, or one or more (e.g., orthogonal) acceleration electrodes. Upon reaching the detector, the ions may be detected by the detector, and e.g., their arrival time may be recorded by the detector. A physicochemical property of the ions, such as their mass to charge ratio and/or ion mobility, may then be determined from the measured drift time.
The ion analyser is a cyclic analyser. Thus, the ion path includes a cyclic segment, wherein ions can make plural (repeated) passes of the cyclic segment when travelling along the ion path (from the ion injector to the detector). The ion path also includes at least one first (non-cyclic) segment, wherein ions make only a single pass of the first segment when travelling along the ion path (from the ion injector to the detector). The first segment may be directly adjacent to (i.e., may directly adjoin) the cyclic segment of the ion path. The first segment may be upstream of or downstream of the cyclic segment.
The ion path may optionally comprise a second (non-cyclic) segment, wherein ions make only a single pass of the second segment when travelling along the ion path (from the ion injector to the detector). The second segment may be directly adjacent to (i.e., may directly adjoin) the cyclic segment of the ion path. The second segment may be upstream of or downstream of the cyclic segment, e.g., such that the ion path comprises a first (non-cyclic) segment, a cyclic segment arranged downstream of the first segment, and a second (non-cyclic) segment arranged downstream of the cyclic segment.
Thus, when travelling along the ion path (from the ion injector to the detector), ions may make a single pass of the first segment, followed by one or more (e.g., plural) passes of the cyclic segment, optionally followed by a single pass of the second segment, before being detected by the detector.
The ion analyser may be a time-of-flight (ToF) mass analyser configured to determine the mass to charge ratio (m/z) of ions from their drift times, or an ion mobility analyser configured to determine the ion mobility of ions from their drift times.
In embodiments, the analyser is a closed-loop multi-reflection ion trap mass analyser. Thus, the analyser may comprise two ion mirrors spaced apart and opposing each other in a first direction X, an ion injector for injecting ions into a space between the ion mirrors, and a detector for detecting ions after they have completed a plurality of reflections between the ion mirrors. The two ion mirrors may together form an ion trap. The two ion mirrors may be configured such that ions trapped in the ion trap will oscillate between the ion mirrors (in the first direction X), e.g., indefinitely until they are released for detection. Ion admission and extraction into the ion trap may be controlled by applying suitable voltage(s) to a deflector arranged in the region between the mirrors.
In these embodiments, the ion path may be configured such that ions make a single pass of a first segment of the ion path between the injector and the deflector, then make multiple passes of a cyclic segment of the ion path between the ion mirrors, and then make a single pass of a second segment of the ion path between the deflector and the detector.
In particular embodiments, the analyser is a multi-reflection time-of-flight (MR-ToF) analyser, e.g., which may be configured to operation in a so-called “zoom” mode of operation. Thus, the analyser may comprise two ion mirrors spaced apart and opposing each other in a first direction X, each mirror elongated generally along a drift direction Y between a first end and a second end, the drift direction Y being orthogonal to the first direction X, an ion injector for injecting ions into a space between the ion mirrors, the ion injector located in proximity with the first end of the ion mirrors, and a detector for detecting ions after they have completed a plurality of reflections between the ion mirrors, the detector located in proximity with the first end of the ion mirrors.
The analyser may be configured to analyse ions by:
The analyser may further comprise a deflector or lens located in proximity with the first end of the ion mirrors. The analyser may be configured to analyse ions by:
The deflector or lens may be located approximately equidistant (in the X direction) between the first and second ion mirrors. The deflector or lens may be arranged along the ion path after the first ion mirror reflection (in the first ion mirror) that the ion beam experiences after being injected from the injector, but before its second ion mirror reflection (in the second ion mirror). Correspondingly, the deflector or lens may be arranged along the ion path before the final ion mirror reflection (in the second ion mirror) that the ion beam experiences before arriving at the detector, but after its penultimate ion mirror reflection (in the first ion mirror).
The multi-reflection time-of-flight (MR-ToF) mass analyser can comprise any suitable type of MR-ToF. For example, the analyser can comprise an MR-ToF having a set of periodic lenses configured to keep the ion beam focused along its flight path, e.g. as described in the article A. Verenchikov, et al., Journal of Applied Solution Chemistry and Modelling, 2017, 6, 1-22, the entire contents of which is hereby incorporated by reference.
However, in particular embodiments, the analyser is a tilted-mirror type multi-reflection time-of-flight mass analyser, e.g., of the type described in U.S. Pat. No. 9,136,101, the contents of which are incorporated herein by reference. Thus, the ion mirrors may be a non-constant distance from each other in the X direction along at least a portion of their lengths in the drift direction Y. The drift direction velocity of ions towards the second end of the ion mirrors may be opposed by an electric field resulting from the non-constant distance of the two mirrors from each other. This electric field may cause the ions to reverse their drift direction velocity in proximity with the second end of the ion mirrors and drift back along the drift direction towards the deflector.
Alternatively, the analyser may be a single focussing lens type multi-reflection time-of-flight mass analyser, e.g., of the type described in UK patent No. 2,580,089, the contents of which are incorporated herein by reference. Thus, the deflector may be a first deflector, and the analyser may comprise a second deflector located in proximity with the second end of the ion mirrors. The second deflector may be configured to cause the ions to reverse their drift direction velocity in proximity with the second end of the ion mirrors and drift back along the drift direction towards the deflector. To do this, a suitable voltage may be applied to the second deflector, e.g., in the manner described in UK patent No. 2,580,089.
In embodiments, the deflector may comprise one or more trapezoid shaped or prism-like electrodes arranged adjacent to the ion beam. This deflector design has a suitably wide acceptance, such that an ion beam that is spread out relatively broadly in the drift direction can be properly received and deflected by the deflector. The deflector may comprise a first trapezoid shaped or prism-like electrode arranged above the ion beam and a second trapezoid shaped or prism-like electrode arranged below the ion beam. The electrode(s) may be angled with respect to the ion beam, such that when suitable (DC) voltage(s) is (are) applied to the electrode(s), the resulting electric field induces a deflection in the ion beam. Suitable deflection voltages are of the order of ±a few volts, ±tens of volts, or ±hundreds of volts.
The deflector should be (and in embodiments is) configured such that it can cause the ion beam to be deflected by a desired (selected) angle. The angle by which the ion beam is deflected by the deflector may be adjustable, e.g., by adjusting the magnitude of a (DC) voltage(s) applied to the deflector. The deflector may be configured such that it can deflect the ion beam by any desired angle.
In embodiments, the method comprises injecting ions from the ion injector into the space between the ion mirrors. The ions may then be reflected in the first ion mirror and may then travel to the deflector. When the ions reach the deflector, the deflector may be configured so as not to deflect the ion beam (or so as to deflect the ion beam by a suitably small angle), e.g., so as not to substantially change the drift direction velocity of the ions, such that the ions continue beyond the deflector and are reflected in the second ion mirror. This may comprise, for example, not applying or removing a voltage from the deflector (or applying a suitably small voltage to the deflector). The ions are then caused to complete a first cycle in which the ions follow a zigzag ion path having plural (K) reflections between the ion mirrors in the direction X whilst: (a) drifting along the drift direction Y from the deflector towards the second end of the ion mirrors, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors, and (c) drifting back along the drift direction Y to the deflector.
After the ions have completed this first cycle, the deflector may be used to reverse the drift direction velocity of the ions such that the ions are caused to complete a further cycle in which the ions follow a zigzag ion path having plural (K) reflections between the ion mirrors in the direction X whilst: (a) drifting along the drift direction Y from the deflector towards the second end of the ion mirrors, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors, and (c) drifting back along the drift direction Y to the deflector. To do this, the deflector may be configured such that the ion beam is deflected, e.g., such that the drift direction velocity of the ions is reversed. This may comprise applying a suitable voltage(s) to the deflector, e.g., during a time period in which it is expected that the ions will arrive back at the deflector. Suitable deflection voltages to reverse the drift direction of the ions are of the order of hundreds of volts.
The step of using the deflector to reverse the drift direction velocity of the ions may be repeated one or more times. Thus, the method may comprise causing the ions to complete plural (N) cycles within the analyser, where in each cycle the ions follow a zigzag ion path having plural (K) reflections between the ion mirrors in the direction X whilst: (a) drifting along the drift direction Y from the deflector towards the second end of the ion mirrors, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors, and (c) drifting back along the drift direction Y to the deflector. The first cycle may be initiated by injecting the ions into the space between the ion mirrors, and after the ions have completed the first cycle, each further cycle may be initiated by using the deflector to reverse the drift direction velocity of the ions.
The method may comprise causing the ions to travel from the deflector to the detector for detection. That is, after the ions have completed the desired (plural) number (N) of cycles within the analyser, the ions may be allowed to travel from the deflector to the detector for detection. To do this, the deflector may be configured so as not to deflect the ion beam (or so as to deflect the ion beam by a suitably small angle), e.g., so as not to substantially change the drift direction velocity of the ions, such that the ions continue beyond the deflector, are reflected in the second ion mirror, and continue on to the detector. This may comprise, for example, not applying or removing a voltage(s) from the deflector (or applying a suitably small voltage to the deflector) such that the ions are caused to exit the deflector in a direction towards the detector. The ions may be reflected in one of the ion mirrors before travelling to the detector.
Upon reaching the detector, the ions may be detected by the detector, e.g., their arrival time may be recorded by the detector. The time-of-flight and/or mass to charge ratio of the ions may then be determined, optionally combined with time-of-flight and/or mass to charge ratio information of other ions, and e.g., a mass spectrum may be produced. It should be noted that not all of the ions that were injected into the analyser may be detected by the detector, e.g., due to inevitable losses at various points between the injector and the detector and/or detector inefficiencies. Thus, as used herein the term “the ions” should be understood as meaning “some, most or all of the ions”.
In these embodiments, the ion path may be configured such that ions make a single pass of a first segment of the ion path between the injector and the deflector or lens, then make multiple passes of a cyclic segment of the ion path between the first and second ends of the ion mirrors, and then make a single pass of a second segment of the ion path between the deflector or lens and the detector.
In the method, the analyser is initially operated in a first mode of operation, and ions are analysed when the analyser is being operated in the first mode of operation (by determining drift times of ions along the ion path) so as to obtain a first set of ion data. The analyser is then switched to operate in a second mode of operation, and ions are analysed when the analyser is being operated in a second mode of operation (by determining drift times of ions along the ion path) so as to obtain a second set of ion data.
The first set of ion data can include plural ion peaks. The number N of passes of the cyclic segment of the ion path taken by ions associated with (i.e., that give rise to) some, most or all ion peaks in the first set of ion data may (by itself) be ambiguous. Similarly, the second set of ion data can include plural ion peaks, and the number N of passes of the cyclic segment of the ion path taken by ions associated with (i.e. that give rise to) some, most or all ion peaks in the second set of ion data may (by itself) be ambiguous. The first and second sets of ion data may be obtained by analysing ions derived from the same sample (e.g. by analysing ions generated from adjacent regions of a sample, and/or by analysing ions generated from a sample at close (adjacent) points in time), e.g. such that ion peaks corresponding to some, most or all (significant) ion peaks in the first set of ion data appear in the second set of ion data.
In the first mode of operation (i) a first electric potential is provided along the first segment of the ion path, (ii) a second electric potential is provided along the cyclic segment of the ion path, (iii) the first segment of the ion path has a first path length, and (iv) the cyclic segment of the ion path has a second path length. The first electric potential may be an electric potential that is provided along some, most or all of the first segment of the ion path. Similarly, the second electric potential may be an electric potential provided along some, most or all of the cyclic segment of the ion path. The first path length may be the path length of the entire first segment. The second path length may be the path length taken by ions in a single cycle (a single loop) of the cyclic segment of the ion path.
In the second mode of operation, at least one of (i) the first electric potential, (ii) the second electric potential, (iii) the first path length, or (iv) the second path length, is altered (changed) with respect to the first mode of operation. Thus, the method may comprise switching the analyser from the first mode of operation to the second mode of operation by at least one of: (i) altering the first electric potential, (ii) altering the second electric potential, (iii) altering the first path length, or (iv) altering the second path length. The alteration may be done such that the effect of the alternation on the drift time of ions along the first segment is proportionally different to the effect of the alternation on the drift time of ions along the cyclic segment. Thus, for example, in particular embodiments, only one of (i) the first electric potential, (ii) the second electric potential, (iii) the first path length, and (iv) the second path length, is altered (changed) with respect to the first mode of operation (and the others are not changed between the first and second modes of operation).
In particular embodiments, where the analyser is a multi-reflecting time-of-flight (MR-ToF) mass analyser (as described above), the method comprises altering the second path length in the second mode of operation by altering the number K of reflections that ions make between the ion mirrors when following the zigzag ion path. This may be done by altering the angle by which the ion beam is deflected by the deflector, i.e., by altering the voltage applied to the deflector. Suitable deflection voltage shifts to alter the angle of the beam in this manner are of the order of a few volts or tens of volts.
Thus, in the first mode of operation, the analyser may be configured such that in each cycle ions make a first number K1 of reflections between the ion mirrors in the direction X whilst: (a) drifting along the drift direction Y from the deflector towards the second end of the ion mirrors, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors, and (c) drifting back along the drift direction Y to the deflector. In the second mode of operation, the analyser may be configured such that in each cycle ions make a second different number K2 of reflections between the ion mirrors in the direction X whilst: (a) drifting along the drift direction Y from the deflector towards the second end of the ion mirrors, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors, and (c) drifting back along the drift direction Y to the deflector. The first and second numbers may differ by a small integer amount, such as by one, i.e., |K1−K2|=1.
In alternative embodiments, the method comprises altering some, most or all of the first electric potential in the second mode of operation. Thus, in the first mode of operation, the analyser may be configured such that a first electric potential distribution is provided along the first segment of the ion path, and in the second mode of operation, the analyser may be configured such that a different electric potential distribution is provided along the first segment of the ion path.
The first electric potential distribution and the different electric potential distribution may differ such that the electric field experienced by ions passing along the first segment in the first mode of operation is different to the electric field experienced by ions passing along the first segment in the second mode of operation. This difference may cause the time of flight of ions (having a particular m/z) along the first segment in the first mode of operation to be different to the time of flight of ions (having the same particular m/z) along the first segment in the second mode of operation. This time-of-flight difference may be dependent on (e.g., proportional to) the mass to charge ratio (m/z) of the ions. Thus, altering the first electric potential between the first and second modes of operation may result in a mass to charge ratio dependent time-of-flight shift of ions passing along the first segment between the first and second modes of operation.
The first electric potential can be altered between the two modes of operation in any suitable manner. For example, the instrument may comprise a flight tube arranged along at least part of the first segment of the ion path, and the method may comprise altering the first electric potential in the second mode of operation by altering a voltage applied to the flight tube (relative to a voltage applied to the flight tube in the first mode of operation).
Alternatively, the method may comprise altering the first electric potential in the second mode of operation by altering a (pulsed) acceleration field provided by the ion injector (e.g., where the ion injector is an ion trap, by altering a (pulsed) extraction field provided within the ion injector). This may be done by altering one or more (pulsed) acceleration voltage(s) applied to one or more electrode(s) of the ion injector. Thus, in the first mode of operation the ion injector may be configured to accelerate ions along the ion path using a first acceleration field (one or more first acceleration voltage(s)), and in the second mode of operation the ion injector may be configured to accelerate ions along the ion path using a second different acceleration field (one or more second different acceleration voltage(s)). Suitable acceleration fields for the ion injector are of the order of hundreds of V/mm, and suitable acceleration field shifts between the first and second modes of operation are of the order of tens of V/mm.
The method comprises comparing the first set of ion data to the second set of ion data, e.g., so as to identify a first ion peak in the first set of ion data that corresponds to a second ion peak in the second set of ion data. The method may comprise identifying plural such pairs of corresponding ions peaks in the first and second sets of ion data. An ion peak may correspond to another ion peak in that the ions that give rise to those ion peaks may have the same value of the physicochemical property (e.g., may be the same species).
Identifying a first ion peak in the first set of ion data that corresponds to a second ion peak in the second set of ion data may comprise identifying ions peaks that have values of the physicochemical property within an expected (e.g., small) range.
Alternatively, identifying a first ion peak in the first set of ion data that corresponds to a second ion peak in the second set of ion data may comprise:
The method comprises determining the number N of passes of the cyclic segment of the ion path taken by ions associated with (i.e., that give rise to) the corresponding first and second ion peaks. This determination may be done on the basis of the comparison of the first set of ion data to the second set of ion data. For example, determining the number N of passes of the cyclic segment of the ion path taken by ions associated with the corresponding first and second ion peaks may comprise measuring a drift time difference between first and second ion peaks, and using the measured drift time difference to estimate the number N of passes of the cyclic segment of the ion path taken by ions associated with the corresponding first and second ion peaks.
The method comprises using the determined number of passes N to determine a value of the physicochemical property of the ions associated with (i.e., that give rise to) the corresponding first and second ion peaks. This process of determining a value of the physicochemical property of the pair of corresponding ion peaks (based on the determined value of N) may be repeated for each identified pair of corresponding ion peaks of interest.
A further aspect provides a non-transitory computer readable storage medium storing computer software code which when executed on a processor performs the method(s) described above.
A further aspect provides a control system for an analytical instrument such as a mass and/or ion mobility spectrometer, the control system configured to cause the analytical instrument to perform the method(s) described above.
A further aspect provides an analytical instrument, such as a mass and/or ion mobility spectrometer, comprising the control system described above.
A further aspect provides an analytical instrument, such as a mass and/or ion mobility spectrometer, comprising:
These aspects and embodiments can, and in embodiments do, include any one or more or each of the optional features described herein.
For example, the ion analyser may be a time-of-flight (ToF) mass analyser, and the physicochemical property may be mass to charge ratio (m/z).
Thus, the analyser may comprise:
The analyser may be configured to analyse ions by:
Alternatively, the analyser may be an ion mobility analyser, and the physicochemical property may be ion mobility.
Various embodiments will now be described in more detail with reference to the accompanying Figures, in which:
The ion source 10 is configured to generate ions from a sample. The ion source 10 can be any suitable continuous or pulsed ion source, such as an electrospray ionisation (ESI) ion source, a MALDI ion source, an atmospheric pressure ionisation (API) ion source, a plasma ion source, an electron ionisation ion source, a chemical ionisation ion source, and so on. In some embodiments, more than one ion source may be provided and used. The ions may be any suitable type of ions to be analysed, e.g., small and large organic molecules, biomolecules, DNA, RNA, proteins, peptides, fragments thereof, and the like.
The ion source 10 may optionally be coupled to a separation device such as a liquid chromatography separation device or a capillary electrophoresis separation device (not shown), such that the sample which is ionised in the ion source 10 comes from the separation device.
The ion transfer stage(s) 20 are arranged downstream of the ion source 10 and may include an atmospheric pressure interface and one or more ion guides, lenses and/or other ion optical devices configured such that some or all of the ions generated by the ion source 10 can be transferred from the ion source 10 to the analyser 30. The ion transfer stage(s) 20 may include any suitable number and configuration of ion optical devices, for example optionally including any one or more of: one or more RF and/or multipole ion guides, one or more ion guides for cooling ions, one or more mass selective ion guides, and so on.
The analyser 30 is arranged downstream of the ion transfer stage(s) 20 and is configured to receive ions from the ion transfer stage(s) 20. The analyser is configured to analyse the ions so as to determine a physicochemical property of the ions, such as their mass to charge ratio, mass, ion mobility and/or collision cross section (CCS). To do this, the analyser 30 is configured to pass ions along an ion path within the analyser 30, and to measure the time taken (the drift time) for ions to pass along the ion path. Thus, the analyser 30 can comprise an ion detector arranged at the end of the ion path, wherein the analyser is configured to record the time of arrival of ions at the detector. The instrument may be configured to determine the physicochemical property of the ions from their measured drift time. The instrument may be configured to produce a spectrum of the analysed ions, such as a mass spectrum or an ion mobility spectrum.
In particular embodiments, the analyser 30 is a time-of-flight (ToF) mass analyser, e.g., configured to determine the mass to charge ratio (m/z) of ions by passing the ions along an ion path within a drift region of the analyser, where the drift region is maintained at high vacuum (e.g., <1×10−5 mbar). Ions may be accelerated into the drift region by an electric field and may be detected by an ion detector arranged at the end of the ion path. The acceleration may cause ions having a relatively low mass to charge ratio to achieve a relatively high velocity and reach the ion detector prior to ions having a relatively high mass to charge ratio. Thus, ions arrive at the ion detector after a time determined by their velocity and the length of the ion path, which enables the mass to charge ratio of the ions to be determined. Each ion or group of ions arriving at the detector may be sampled by the detector, and the signal from the detector may be digitised. A processor may then determine a value indicative of the time of flight and/or mass-to-charge ratio (“m/z”) of the ion or group of ions. Data for multiple ions may be collected and combined to generate a time of flight (“ToF”) spectrum and/or a mass spectrum.
In alternative embodiments, the analyser 30 is an ion mobility analyser, e.g., configured to determine the ion mobility of ions by passing the ions along an ion path within a drift region of the analyser, where a buffer gas is provided in the drift region. Ions may be urged through the buffer gas by an electric field (or ions may be urged through the drift region by a gas flow where an electric field is arranged to oppose the gas flow) and may be detected by an ion detector arranged at the end of the ion path. Ions having a relatively high mobility will reach the ion detector prior to ions having a relatively low mobility. Thus, ions may separate according to their ion mobility, and may arrive at the ion detector after a time determined by their ion mobility. Each ion or group of ions arriving at the detector may be sampled by the detector, and the signal from the detector may be digitised. A processor may then determine a value indicative of the drift time and/or ion mobility of the ion or group of ions. Data for multiple ions may be collected and combined to generate a drift time spectrum and/or an ion mobility spectrum.
It would also be possible for the analyser 30 to comprise an ion mobility separator coupled to a mass analyser, e.g., where a mass analyser is provided at the end of the ion mobility part of the ion path. In these embodiments, any suitable type of mass analyser may be provided, such as for example a time-of-flight mass analyser, or an electrostatic ion trap mass analyser such as an electrostatic orbital trap, and more specifically an Orbitrap™ FT mass analyser as made by Thermo Fisher Scientific.
It should be noted that
As also shown in
In accordance with various embodiments, the analyser 30 is a cyclic analyser. Thus, the ion path within the analyser 30 is made up of at least a first part, and a second cyclic part, wherein the ion path is configured such that ions travelling along the ion path will make only a single pass of the first part and will make one or more (e.g. plural) passes of the second cyclic part before they are detected. This is illustrated schematically by
As shown in
It will be appreciated that cyclic analysers beneficially allow the length of the ion path 32 taken by ions within the analyser 30 (between the injector 31 and the detector 33) to be increased, thereby increasing the resolution of the analyser 30.
The cyclic analyser 30 can comprise any suitable cyclic ion analyser having an ion path 32 configured such that ions can make plural passes of a cyclic segment 32b of the ion path before being detected. Thus, for example, the analyser 30 can be a cyclic time-of-flight (ToF) mass analyser, a cyclic ion mobility analyser, or a cyclic ion mobility separator coupled to a mass analyser.
As shown in
In the embodiment depicted in
In this type of cyclic analyser, the ion flight time can be many milliseconds long, so the resolution can typically reach >100,000, or even >500,000. However, space charge within the limited volume can degrade the analyser performance due to strong coalescence effects.
As shown in
An ion source (injector) 31, which may be in the form of an ion trap, is arranged at one end (the first end) of the analyser. The ion source 31 may be arranged and configured to receive ions from the ion transfer stage(s) 20. Ions may be accumulated in the ion source 31, before being injected into the space between the ion mirrors 34, 35. As shown in
One or more lenses and/or deflectors may be arranged along the ion path, between the ion source 31 and the ion mirror 35 first encountered by the ions. For example, as shown in
The analyser also includes another deflector 36, which is arranged along the ion path, between the ion mirrors 34, 35. As shown in
The analyser also includes a detector 33. The detector 33 may be any suitable ion detector configured to detect ions, and e.g., to record an intensity and time of arrival associated with the arrival of ion(s) at the detector. Suitable detectors include, for example, one or more conversion dynodes, optionally followed by one or more electron multipliers, and the like.
In its “normal” mode of operation, ions are injected from the ion source 31 into the space between the ion mirrors 34, 35, in such a way that the ions adopt a zigzag ion path having plural reflections between the ion mirrors 34, 35 in the X direction, whilst: (a) drifting along the drift direction Y from the deflector 36 towards the opposite (second) end of the ion mirrors 34, 35, (b) reversing drift direction velocity in proximity with the second end of the ion mirrors 34, 35, and then (c) drifting back along the drift direction Y to the deflector 36. The ions can then be caused to travel from the deflector 36 to the detector 33 for detection.
In the analyser of
The analyser depicted in
Further detail of the tilted-mirror type multireflection time-of-flight mass analyser of
In the analyser of
As also shown in
Further detail of the single-lens type multireflection time-of-flight mass analyser of
In the analysers depicted in
In the embodiments depicted in
In embodiments, the multi-reflecting time-of-flight (MR-ToF) mass analyser is operated in a multi-pass “zoom” (cyclic) mode of operation. Ions are made to make multiple cycles within the analyser in the drift direction Y. Increasing the number of cycles N increases the length of the ion path that ions take within the analyser (between the injector and the detector), thereby increasing the resolution of the analyser. In the Verenchikov analyser, this may be done by controlling a voltage onto an entrance lens. For the analysers depicted in
Thus, in a multi-pass “zoom” (cyclic) mode of operation, ions are caused to complete plural (N) cycles within the analyser, where in each cycle the ions drift in the drift direction Y from the deflector 36 (or entrance lens) towards the opposite (second) end of the ion mirrors 34, 35, and then back to the deflector 36 (or entrance lens). In each cycle, the ions also complete plural (K) reflections between the ion mirrors in the X direction. Thus, in each cycle, the ions adopt a zigzag ion path 32b through the space between the ion mirrors 34, 35.
In the analysers depicted in
After the ions have completed this initial cycle, each further cycle is initiated by using the deflector 36 to reverse the drift direction velocity of the ions (in proximity with the first end of the ion mirrors). To do this, an appropriate voltage may be applied to the deflector 36 that causes ions to leave the deflector 36 with a drift direction velocity opposite to the drift direction velocity with which the ions originally entered the deflector 36.
After the ions have completed the desired (plural) number (N) of cycles within the analyser, the ions are allowed to travel from the deflector 36 to the detector 33 for detection. To do this, the voltage may be removed from the deflector 36 (or else an appropriate voltage may be applied to the deflector) such that the ions are caused to exit the deflector 36 in a direction towards the detector 33. The ions may be reflected in (the other) one of the ion mirrors 34 before travelling to (and being detected by) the detector 33.
In the embodiments depicted in
Although
In these embodiments, the cyclic ion mobility analyser or the cyclic ion mobility separator may comprise a closed-loop ion separator, e.g., of the type described in UK Patent Application No. GB 2,562,690, the entire contents of which is incorporated herein by reference. Ions may be caused to separate according to their ion mobility over a fixed integer number of cycles around the ion mobility separator. A gate may be provided which may be closed to allow multi-pass operation. The gate may be opened to allow ions to exit the ion mobility separator after ions have made one or more circuits of the ion mobility separator. Using a cyclic ion mobility separator can allow a higher degree of separation, and so higher ion mobility resolution.
In these embodiments, the ion path may be configured such that ions make a single pass of a first segment of the ion path (before the closed-loop ion separator), then make multiple passes of a second cyclic segment of the ion path (within the closed-loop ion separator), and then make a single pass of a third segment of the ion path (after the closed-loop ion separator).
A common benefit between the various types of cyclic analysers (in which ions are made to make plural N repeated cycles along an ion path within the analyser) is that increasing the number of cycles N increases the length of the ion path that ions take within the analyser, thereby increasing the resolution of the analyser.
However, a common problem is that during multiple cycles N through the analyser, faster moving (e.g., lighter) ions can overtake (e.g., lap) slower moving (e.g., heavier) ions. This complicates the resulting spectra and can make it difficult to accurately determine the desired physicochemical property (e.g., m/z or ion mobility) of all the detected ions, because the number of cycles N taken by each ion peak in a spectrum becomes ambiguous.
Thus, embodiments provide methods of disambiguating a spectrum produced by a cyclic ion analyser. By comparing two sets of ion data that have been obtained under different analyser settings, the number of passes N through the cyclic segment 32b of the ion path 32 taken by ions contributing to an ion peak can be determined, thereby allowing the physicochemical property of those ions to be unambiguously determined and assigned to the ion peak.
Although parts of the following discussion are described in terms of the MR-ToF analysers of
The ion path depicted in
The first switching between Modes 1 and 2 should happen not earlier than the heaviest ion (m/z)2 passes the deflector 36 for the first time, and not later than the lightest ion (m/z)1 makes a0+K oscillations, where K is the number of oscillations per loop (between subsequent passages of the deflector 36) and a0 represents a portion of an oscillation before the ion source 31 and the first passage of the deflector 36. Otherwise, the lightest ions will not be set to the next loop properly. This gives the double inequality:
a
0
T
2
≤t
12≤(a0+K)T1 (a)
where T1 and T2 are the times of oscillation for lightest and the heaviest ions correspondingly. In the embodiments of
The second switching from Mode 2 to Mode 3 should happen not earlier than the heaviest ion makes a0+(N−1)K oscillations, where N is the intended number of loops. Otherwise, the heaviest ion will exit the loop before all loops are made. On the other hand, the second switching should be not later than the lightest ion makes a0+NK oscillations, otherwise this ion will stay in the analyser for the next, unwanted, loop. This double inequality reads:
(a0+NK−T2≤t23≤(a0+NK)T1 (b)
Both inequalities (a) and (b) impose upper bounds for the ratio of T2 and T1 under which for a pair t12 and t23 exists; and the bound from (b) is stronger (lower) than that from (a) for any N>1:
As the time of flight is proportional to the square root of m/z, this inequality translates directly to the maximum unambiguous mass range (UMR) as:
To realize the full UMR, the switching time t23 must be:
t
23=(a0+NK)T1=(a0NK−K)T2
The first switching time leaves some freedom to define. It may be assumed, for example, its minimal possible value may be adopted t12=a0T2, which allows for electronic ripples before the lightest ion comes to the deflector for the next time.
Table 1 shows simulations of a mass analyser with a 1.25 m effective oscillation distance and twenty oscillations per loop. The resolution is calculated in terms of peak full-width-half-maximum. The collapse in m/z range is rather pronounced as the number of loops is increased.
The m/z range of ions entering the analyser 30 could be limited, e.g., via use of the switchable deflector, mass filter (e.g., quadrupole mass filter), or otherwise, to approximately match their m/z range to the UMR of the zoom method and to thereby remove ambiguity in m/z assignment. This is, however, rather wasteful for ion transmission and more efficient methods may be preferable to maintain sensitivity.
Accordingly, disambiguation of complex spectra in accordance with embodiments is in general preferable, whereby the correct number of drift reflections is assigned to individual ion peaks, and from there the accurate m/z for each ion peak is determined.
One possible approach to disambiguation would be to directly assign the correct number of cycles N for each ion peak based on the resolution of that peak. From the resolution shifts observed in Table 1, this at first appears as a rather appealing approach. Similarly, m/z dependent properties (such as single ion detector response, spacing between different charge states, isotopes or common fragmentation routes such as loss of ammonia or water, and so on) could be used to pre-assign an approximate m/z to individual ion peaks, and thus cycle number. Comparison to a survey scan with no zoom mode is also possible, particularly for MR-ToF analysers with drift separation, where even the survey scan is very highly resolving and mass accurate. However, in practice these approaches are significantly complicated by space charge effects for intense ion peaks, and statistical problems for small ion peaks.
In accordance with embodiments, disambiguation of cyclic analyser spectra (such as ToF mass analyser spectra or ion mobility analyser spectra), is done by varying the time-of-flight separately on the ion path segments which are included (32b) or not included (32a, 32c) in the repeating loop.
As used herein, the “effective ion path” is defined as the time-of-flight times the ion velocity under a nominal acceleration voltage. The effective ion path may be varied either by altering the ion path length directly, or by changing a voltage(s) which changes the time-of-flight.
Referring again to
L=L
0
+L
m
N+L
1
where L0 and L1 correspond to the non-repeated segments 32a, 32c outside of the loop (e.g., in
When the effective ion paths L0, Lm, and L1 are modified proportionally, the measured time-of-flight will change in the same proportion for every ion, regardless of how many loops N the ion makes. However, changing Lm by ΔLm, while leaving L0+L1 unaltered modifies the time-of-flight by Δt in the proportion:
which is resolvable for N as:
In the other case that the sum L0+L1 is changed by ΔL0 and Lm is unchanged, the relative time of flight shift is:
which yields another formula for N:
Therefore, in both cases, the number of loops N can be determined based on the measured time shift Δt for an ion peak detected in the moment t after injection. With the number of oscillations N known, the time of flight t can be converted into the mass-to-charge ratio (or ion mobility) using a normal conversion.
Thus, in embodiments, a first set of ion data is obtained when operating the analytical instrument in a first mode of operation, and a second set of ion data is obtained when operating the analytical instrument in a second, different mode of operation. The first and second sets of ion data may be obtained by analysing ions derived from the same sample (e.g. by analysing ions generated from adjacent regions of a sample, and/or by analysing ions generated from a sample at close (adjacent) points in time), e.g. such that ion peaks corresponding to some, most or all (significant) ion peaks in the first set of ion data appear in the second set of ion data.
The first mode of operation and the second mode of operation differ in respect of at least one parameter of the analyser 30. In effect, the analyser's ion path 32 is separated into two regions, one with a flight path 32b affected by the number N of passes, and at least one 32a, 32c unaffected. A parameter change is applied between the two modes of operation that disproportionately alters the drift time through one of these segments versus the other. Thus, the proportional change in drift time depends on how many passes N through the cyclic segment 32b an ion undergoes. In embodiments, either the effective ion path in the loop Lm is altered between the two modes of operation, or the effective ion path outside the loop L0+L1 is altered between the two modes of operation.
An effective ion path can be changed either by altering the ion path length directly, or by changing a voltage(s) which changes the time-of-flight. Thus, in the first mode of operation (i) a first electric potential is provided along the first segment 32a, 32c of the ion path, (ii) a second electric potential is provided along the cyclic segment 32b of the ion path, (iii) the first segment of the ion path 32a, 32c has a first path length, and (iv) the cyclic segment of the ion path 32b has a second path length. In the second mode of operation at least one of: (i) the first electric potential, (ii) the second electric potential, (iii) the first path length, or (iv) the second path length; is changed relative to the first mode of operation, e.g. such that one of the effective ion path in the loop Lm and the effective ion path outside the loop L0+L1 is altered relative to the first mode of operation.
This parameter change will induce a time shift Δt for each ion peak between the two sets of ion data. Thus, the first set of ion data is compared to the second set of ion data so as to identify corresponding (matching) ion peaks. For each identified ion peak pair of interest, the time shift Δt for that ion peak pair between the two sets of data is measured. The number of loops N for each peak is then estimated from the measured time shift Δt (e.g., using the equations described above), and N is used to calculate the mass-to-charge ratio (or other physiochemical property) of the ions corresponding to the ion peak.
Various exemplary embodiments for altering either the effective ion path in the loop Lm, or the effective ion path outside the loop L0+L1 are described below. It will be understood, however, that various alternatives are possible, e.g., depending on the particular design of cyclic ion analyser 30.
In a first exemplary embodiment, the path length of the cyclic segment 32b of the ion path is altered between the first and second modes of operation.
In the multi-reflection analyser of
In embodiments, the number of reflections K between the ion mirrors 34, 35 is changed by ±1 between the two modes of operation, and corresponding time shifts Δt are measured for individual ion peaks. As the number of passes N is low (normally below <6), the time shifts Δt may be measured with moderate precision, and the exact number of loops N can be determined by rounding (eq. Na) to the nearest integer.
The effective length of the loop Lm is proportional to K, which leads to a relative change ΔLm/Lm=1/K when the number of oscillations K is increased by one. In this case, the formula (eq. Na) reads:
where a0 is the fraction of an oscillation between injection and the first passage through the switchable deflector 36, and a1 is the fraction of an oscillation after exiting the loop and before impinging on the detector 33. In the analyser shown in
Table 2 shows an example of this disambiguation algorithm applied to a ToF spectrum of the Flexmix calibration mixture. The low m/z ions are set to arrive at the detector after making N=2 loops, each loop containing K1=21 oscillations. Corresponding times of flight are presented in the first column. Some ions with higher m/z (predominantly Ultramark ions), however, make one more loop, N=3, due to their lower propagation velocities. To assign a correct number of loops to each peak, the system was turned to a mode with K2=22 oscillations in each loop, and corresponding times of flight for each of the peaks were detected. These are given in the second column. The formula (eq.N.dk) was applied to estimate the values of N* from the time-of-flight differences, and these values were rounded to the nearest integer. Finally, the m/z ratios were calculated with the formula:
where U0 is the acceleration voltage and CN<<1 is a calibration coefficient which was a-priori experimentally defined for each number of loops N=2 and 3.
In embodiments, the fractional part of N* comes from a limited precision of calibration. Nevertheless, the assignment of the integer N as the rounded value N* is unambiguous. The fewer loops that need to be distinguished from one another, the more reliable the disambiguation procedure becomes.
An advantage of the above-described “zoom” mode in the analyser of
Another approach to determine the number of loops from the times of flight under different numbers of oscillations K per loop is to calculate a list of possible mass to charge ratios for each ion peak (from eq.mz) under different assumptions about the possible number of loops N. This gives several possible values
where N is a candidate number of loops and K=K1, K2. Only for the correct value of N, the candidate values
and
are (approximately) equal (e.g. within a narrow tolerance, like 10 ppm), while incorrect assumptions about N result in substantially different values.
As illustrated in Table 3A and 3B, only one candidate N (3 and 2 correspondingly) give close candidates for m/z under different values of K. These candidate values are assumed to correct, and all other m/z calculated with other assumptions about the number of loops are discarded as being incorrect.
In some embodiments, because of the possibility of some peaks disappearing because of the shift of K, e.g., due to ions being present within the deflector 36 during the voltage switch, it may be beneficial to use more than two values of K, at cost of overall acquisition speed. Disappearance of ion peaks in the spectrum can also or instead be used to provide information for disambiguation, because the m/z values that disappear can be calculated based on the deflector size, switching speed/times, and so on.
In general, embodiments can comprise analysing ions in a third mode of operation (by determining drift times of ions along the ion path) so as to obtain a third set of ion data, wherein in the third mode of operation at least one of: (i) the first electric potential, (ii) the second electric potential, (iii) the first path length, or (iv) the second path length; is changed relative to the first and/or second mode of operation, comparing the third set of ion data to the first and/or second set of ion data, and determining, on the basis of the comparison, the number N of passes of the second part of the ion path taken by ions associated with the corresponding ion peaks.
In a second exemplary embodiment, disambiguation is done by comparing flight times between two spectra where the non-reflecting segment of the flight path has been altered by changing an electric potential of the non-reflecting segment of the flight path.
The time-of-flight shift caused by a voltage v applied to the flight tube 70 is proportional to √{square root over (m/z)}, and does not depend on the number of cycles N. Measuring the time-of-flight spectrum with two (or more) values of v allows independent assessment of m/z. Therefore, the number of cycles N can be assigned to each peak. Accurate values of m/z can then be determined from the perplexed time-of-flight spectrum, and the number of cycles that was assigned for each ion peak of interest.
In the schematic diagram of
where ε0 is the acceleration voltage and L0 and Lm are effective lengths. Let the second mirror abruptly switch from a reflection mode to a transmission mode in the time moment T2. The number of reflections completed in the second mirror before T2 is:
where the double brackets [[ . . . ]] denote the integer part. The time when the ion is detected is:
As the number of cycles N decreases step-wise with the mass-to-charge ratio μ, the function tD(μ) is non-monotonous. This means that a ToF spectrum tD(μ) is ambiguous, and a peak located at tD may correspond to a number of different mass-to-charge ratios μ.
Intervals of μ which correspond to a particular integer N(μ) are referred to as unambiguous mass intervals. For N cycles, the corresponding unambiguous interval ranges from MN+1 to MN, where:
The unambiguous mass range is, correspondingly:
Consider the short flight tube 80 with length Lt situated between the second mirror and the detector 33 and biased with a voltage v<<ε0. When the voltage is applied, the peak appears shifted by:
As v<<ε0, the peak width is not widened much, and the centroid shift is measurable. This allows a rough estimation of the inverse ion velocity and the mass-to-charge ratio μ as:
The precision is low but is sufficient to determine the number of cycles N for a particular peak. To this end, the estimate μ* is to be substituted into the formula (equation Nμ)N(μ*)=[[N*]], where:
The accurate μ is then determined as:
where N* is rounded down.
Thus, in these embodiments a first set of ion data is obtained when operating the analytical instrument in a first mode of operation, and a second set of ion data is obtained when operating the analytical instrument in a second mode of operation, where in the second mode of operation the electric potential along the first (and/or third) segment 32a, 32c of the ion path is changed relative to the first mode of operation. This change induces a time shift Δt for each ion peak between the two sets of ion data, which is used to estimate the number of cycles N for each peak, and so the mass-to-charge ratio (or other physiochemical property), e.g., in the manner described above.
In the embodiments illustrated by
Another embodiment is to alter the acceleration field provided by the ion injector 31 for accelerating ions along the ion path. Where the ion injector is an ion trap, this may comprise altering the extraction field provided within the ion trap for accelerating ions from the ion trap along the ion path (e.g., where, in this embodiment, at least part of the first segment 32a of the ion path can be considered to be within the ion trap). Suitable extraction fields are of the order of hundreds of V/mm, and suitable extraction field shifts between the first and second modes of operation are of the order of tens of V/mm.
It should also be noted that, if the voltage applied to the flight tube 80 is relatively small, the number of cycles N will be preserved for the vast majority of ion peaks, except ion peaks located close to MN.
These embodiments can also be straightforwardly implemented in cyclic ion mobility spectrometry (which measures time-of-flight through a gas filled drift path). For example, UK Patent Application No. GB 2,562,690 describes an instrument combining a cyclic ion mobility analyser and a short linear drift tube that could straightforwardly be adapted to shift total drift times in a similar manner to that described above.
Although the above exemplary embodiments have been described in terms of changing (between the first and second modes of operation) either (i) the electric potential along the non-cyclic segment 32a, 32c of the ion path, or (ii) the path length of the cyclic segment 32b of the ion path, it will be appreciated that it would instead be possible to change either (iii) the electric potential along the cyclic segment 32b of the ion path (e.g. by including a flight tube along the cyclic segment 32b of the ion path), or (iv) the path length of the non-cyclic segment 32a, 32c of the ion path (e.g. by controlling the number of reflections K ions make between two ion mirrors), i.e. such that one of the effective ion path in the loop Lm and the effective ion path outside the loop L0+L1 is altered relative to the first mode of operation.
A numerical example of the arrangement depicted in
This example assumes no difficulty in matching peaks before and after the shift, which may be reasonable for small shifts and uncongested spectra. In more complex cases, it may be beneficial to have precise calibrations for both shifted and unshifted spectra, and to assign multiple possible m/z values to each ion peak, and to then match peaks together as described above in respect of the first exemplary disambiguation method.
A mass spectrometer incorporating the analyser design of
Ion dispersion was controlled by a pair of lenses and the ions' direction was set by the first prism deflector 38 so that ions passed through to the second prism deflector 36 via a reflection from an ion mirror 35. The second prism deflector 36 was set to −160V, to admit ions to the analyser. After −200 μs this prism deflector was switched to +280V trapping mode, and held there for 800 μs, sufficient for the ions to make a second drift pass. The prism 37 was then switched back to −160V transmission mode, and the trapped ions were extracted to an electron multiplier detector 33.
A similar experiment in accordance with the second exemplary embodiment was performed by varying the ion injector's pulsed extraction field from 330 to 240 V/mm, which caused the high m/z ions to shift by −40 ppm.
It will be appreciated from the above that embodiments provide a method of operating an analytical instrument, such as a time-of-flight mass spectrometer, that comprises an analyser configured to determine flight times of ions along a path which comprises a cyclic segment and a non-cyclic segment. The cyclic segment is configured such that at least some ions make more than one loop in it, and the non-cycling segment is configured such that all ions make only one pass along it. At least one of the cyclic and non-cyclic segments is controlled, e.g., with at least one electrode with a switchable voltage which, when switched, modifies the time-of-flight of an ion in this segment.
The method may comprise determining a first set of times-of-flight of ions upon completion of the cyclic and non-cyclic segments, changing a voltage on at least one of the control electrodes, and then determining a second set of times-of-flight of ions upon completion of the cyclic and non-cyclic segments. The method may comprise determining a number of loops in the cyclic segment made by ions of interest based on time-of-flight differences between the first and the second sets of times-of-flight. The method may comprise then determining a mass-to-charge ratio of at least one ion based on the full flight path that comprises the determined number of loops in the cyclic part.
The cyclic or non-cyclic segment may be controlled by a voltage which modifies, when applied, an ion velocity in at least a section of the path which, in turn, modifies the time-of-flight in the said cyclic or non-cyclic segment. The cyclic segment may be controlled by a voltage which modifies an ion flight length in this segment. The ions may make more than one oscillation within a single loop in the cyclic segment, and the number of such oscillations may be controlled by application of a control voltage.
The relative difference of the times-of-flight in the first set and the second set may substantially depend on the number of loops of an ion trajectory in the cyclic segment, and the number of said loops may be estimated from the said difference for at least one ion. The mass-to-charge ratios may be estimated for a set of candidate number of loops, and the true number of loops may be determined by comparison of the mass-to-charge ratios estimated on the first and the second sets of the times-of-flight.
Although the present invention has been described with reference to various embodiments, it will be understood that various changes may be made without departing from the scope of the invention as set out in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| GB2203184.3 | Mar 2022 | GB | national |
