Ion detector

Abstract

A method of mass spectrometry, comprising: providing a mass spectrometer having an ion detector; allocating a first time period for mass analysing ions in a manner such that only ions having a first restricted range of mass to charge ratios are able to reach the ion detector; allocating a second, subsequent time period for mass analysing ions in a manner such that only ions having a second, different restricted range of mass to charge ratios are able to reach the ion detector; controlling the mass spectrometer, when the first time period begins, to only transmit ions having said first range of mass to charge ratios to the ion detector; and accumulating the ion signal generated by the ion detector during the first time period so as to determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the pre-defined value during the first time period, the mass spectrometer responds by attenuating ions for the remainder of the first time period so as to reduce or stop ions being received at the ion detector during the remainder of the first time period.

Description

[0001] 175226-02v1 (M-4775)

[0002] ION DETECTOR

[0003] CROSS-REFERENCE TO RELATED APPLICATION

[0004] This application claims priority from and the benefit of United Kingdom patent application No. 2417396.5 filed on 27 November 2024, the entire contents of which are incorporated herein by reference.

[0005] FIELD OF THE INVENTION

[0006] The present invention relates generally to a method of mass spectrometry and a mass spectrometer for performing the method.

[0007] BACKGROUND

[0008] Mass spectrometers comprise an ion detector for detecting ions that are being analysed thereby. The ion detector is configured to receive ions at an input surface and generate electrons in response thereto. These electrons are then collected so as to provide a detector signal. However, ion detectors generate a relatively low level baseline signal even when ions are not arriving at the detector. As such, ion detectors tend to only determine that ions are striking the detector when the detector signal rises above a threshold intensity level.

[0009] If relatively high ion currents are received at the ion detector then this can increase the intensity of the baseline signal and cause it to rise above the threshold intensity level. In such circumstances, even if the ion current to the detector stops, the ion detector may still determine that ions are being detected until the baseline signal decays to an intensity that is below the threshold intensity level.

[0010] SUMMARY

[0011] A first aspect of the present invention provides a method of mass spectrometry comprising: providing a mass spectrometer having an ion detector; allocating a first time period for mass analysing ions in a manner such that only ions having a first restricted range of mass to charge ratios are able to reach the ion detector; allocating a second, subsequent time period for mass analysing ions in a manner such that only ions having a second, different restricted range of mass to charge ratios are able to reach the ion detector; controlling the mass spectrometer, when the first time period begins, to only transmit ions having said first range of mass to charge ratios to the ion detector; and accumulating the ion signal generated by the ion detector during the first time period so as to determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the pre-defined value during the first time period, the mass spectrometer responds by attenuating ions for the remainder of the first time period so as to reduce or stop ions being received at the ion detector during the remainder of the first time period.

[0012] As the present invention reduces or stops ions arriving at the ion detector once the accumulated ion signal rises above the pre-defined value, this reduces the rate at which the detector baseline signal rises, or stops the baseline signal rising and allows it to drop. This helps ensure that the detector baseline signal is below a threshold level at which ions are considered to have been detected by the start of the second time period. As such, the present invention reduces the likelihood of the ion detector erroneously determining that ions have been detected in the second time period due to the baseline signal being high, as a result of ions striking the detector during the first time period.

[0013] The method may be performed in real-time.

[0014] As described above, the accumulated ion signal is compared to the pre-defined value to determine if the attenuating step is to be performed. The accumulated ion signal may be converted into a value representing the quantity of ions detected in order to make this comparison, or the area of the accumulated signal may be used in the comparison.

[0015] The value of the pre-defined value may be selected based on the polarity of the ions being mass analysed, the detector gain value used, or the duration of each of the first and second time periods.

[0016] The method may comprise controlling the mass spectrometer, when the second time period begins, to only be capable of transmitting ions having said second range of mass to charge ratios to the ion detector.

[0017] The mass spectrometer may be configured to automatically perform all of the steps described above.

[0018] The first and second ranges of mass to charge ratios are preferably nonoverlapping ranges. The first range of mass to charge ratios may be a range such that it includes only ions having a single mass to charge ratio, e.g. a single Dalton value, or only a fraction of a Dalton. Similarly, the second range of mass to charge ratios may be a range such that it includes only ions having a single mass to charge ratio, e.g. a single Dalton value, or only a fraction of a Dalton.

[0019] The ion detector may be configured to receive the ions at an input surface and generate electrons in response thereto. The detector may comprise an electron multiplier for amplifying the number of electrons generated and it is configured to collect the amplified number of electrons so as to provide the detector signal. The detector may comprise a photoemissive device between the input surface and the electron multiplier for converting the ions, or electrons generated at the input surface by ion strikes, into photons. In such detectors the detector is configured such that the photons that are generated strike an electrode and generate electrons that are directed to the electron multiplier.

[0020] The ion detector may comprise a photomultiplier tube.

[0021] The detector produces a baseline signal when no ions are being received at the detector, and said ion signal may be a portion of the detector signal that is above a preselected threshold intensity. The step of attenuating ions for the remainder of the first time period may be performed such that the baseline signal drops below the threshold intensity by the start of the second time period, or such that the baseline signal remains below the threshold intensity until the start of the second time period. The accumulated ion signal is representative of the number of ions received at the ion detector. For example, the accumulated ion signal may be the area of the ion signal obtained between the start of the first time period and the time during the first time period that the mass spectrometer stops ions being received at the ion detector.

[0022] If the accumulated ion signal does not rise above the pre-defined value during the first time period then said attenuating step may not be performed during the first time period. In this event, the mass spectrometer is controlled such that it is capable of transmitting ions having said first range of mass to charge ratios to the ion detector for the entire duration of the first time period.

[0023] An interscan delay period may be provided between the first and second time periods during which voltages that are applied to ion-optics of the mass spectrometer are changed such that only ions having said second range of mass to charge ratios are able to reach the ion detector during the second time period.

[0024] Ions may be prevented from reaching the ion detector during the inter-scan delay period.

[0025] The interscan delay period may be immediately after the first time period, and the second time period may be immediately after the interscan delay period.

[0026] The step of only transmitting ions having said first range of mass to charge ratios to the ion detector may be performed by applying a combination of RF and DC voltages to electrodes of a mass filter such that only these ions are transmitted to the detector; and the RF and / or DC voltage applied to said electrodes maybe changed during the interscan delay such that only ions having said second range of mass to charge ratios are able to reach the ion detector during the second time period.

[0027] The mass filter may be a quadrupole mass filter.

[0028] The mass spectrometer may monitor a first multiple reaction monitoring (MRM) transition during the first time period and monitors a second, different MRM transition during the second time period.

[0029] The first and second MRM transitions may be MRM transitions for different precursor ion species of interest, or they may be MRM transitions for the same precursor ion species of interest but to different fragment ion species of interest.

[0030] The mass spectrometer may be a tandem mass spectrometer comprising: a first mass filter for mass filtering precursor ions; a fragmentation or reaction device downstream of the first mass filter for fragmenting or reacting precursor ions transmitted by the first mass filter so as to produce fragment or other product ions; and a second mass filter for mass filtering said fragment or other product ions. The method may comprise controlling the first and second mass filters during at least a portion of the first time period so that the first mass filter is only capable of transmitting a first pre-selected range of mass to charge ratios corresponding to a first precursor ion species of interest, and the second mass filter is only capable of transmitting a second pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the first precursor ion species of interest.

[0031] When the ion detector detects ions during the first time period it is detecting said fragment or other product ion species, and it then determines that the first precursor ion species of interest is present in the sample being analysed. The spectrometer may calculate the amount or concentration of the analyte giving rise to the first precursor ion species of interest from the ion signal.

[0032] The method may comprise either: (i) controlling the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting said first pre-selected range of mass to charge ratios and the second mass filter is only capable of transmitting a third pre-selected range of mass to charge ratios corresponding to a different fragment or other product ion species of said first precursor ion species of interest; or (ii) controlling the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting a further pre-selected range of mass to charge ratios corresponding to a second, different precursor ion species of interest, and the second mass filter is only capable of transmitting a fourth pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the second precursor ion species of interest.

[0033] If the ion detector detects ions during the second time period, for option (i), then it is detecting said different fragment or other product ion species, and it then determines that the first precursor ion species of interest is present in the sample being analysed. The spectrometer may calculate the amount or concentration of the analyte giving rise to the first precursor ion species of interest from the ion signal.

[0034] If the ion detector detects ions during the second time period, for option (ii), then it is detecting said fragment or other product ion species of the second precursor ion species of interest, and it then determines that the second precursor ion species of interest is present in the sample being analysed. The spectrometer may calculate the amount or concentration of the analyte giving rise to the second precursor ion species of interest from the ion signal.

[0035] The step of attenuating ions may be performed by varying the RF and / or DC voltage applied to said electrodes of a mass filter within the mass spectrometer. For example, the RF and / or DC voltage applied to the mass filter may be varied so as to force substantially all ions to become unstable in the mass filter.

[0036] The step of attenuating ions may be performed by applying one or more voltage to at least one electrode of the mass spectrometer so as to arrange an electrical potential barrier in the path of the ions or so as to deflect the ions such that the ions are prevented from reaching the detector. For example, an ion gate may be used to block the ion path to the detector. Alternatively, or additionally, the potential on an Einzel lens or other ion- optical element may be changed so as to form a potential barrier that blocks the ions.

[0037] The step of deflecting ions may comprise redirecting the ions or defocussing the ion beam such that ions do not reach the detector. For example, a voltage may be applied to an ion steering lens or ion deflector electrode so as to divert the ions such that they do not reach the detector.

[0038] Ions may be deflected so as to impact on a surface that neutralises the ions during the voltage transition period, e.g. onto an electrode.

[0039] As described above, the method comprises accumulating the ion signal during the first time period and triggering the attenuating step when the accumulated ion signal is determined to rise above the pre-defined value. However, a corresponding process may be performed during the second time period so as to avoid problems occurring during a subsequent third time period.

[0040] Accordingly, the method may comprise controlling the mass spectrometer, when the second time period begins, to only transmit ions having said second range of mass to charge ratios to the ion detector; and accumulating the ion signal generated by the ion detector during the second time period so as to determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the predefined value during the second time period, the mass spectrometer responds by attenuating ions for the remainder of the second time period so as to reduce or stop ions being received at the ion detector during the remainder of the second time period.

[0041] The pre-defined value associated with the second time period may be the same or different to the pre-defined value associated with the first time period.

[0042] The method may comprise allocating a third time period, that is subsequent to the second time period, for mass analysing ions in a manner such that only ions having a third restricted range of mass to charge ratios are able to reach the ion detector. The third range may be different to both the first and second ranges.

[0043] The method may comprise controlling the mass spectrometer, when the third time period begins, to only be capable of transmitting ions having said third range of mass to charge ratios to the ion detector.

[0044] The step of attenuating ions for the remainder of the second time period may be performed such that the baseline signal drops below the threshold intensity by the start of the third time period, or such that the baseline signal remains below the threshold intensity until the start of the third time period.

[0045] If the accumulated ion signal associated with the second time period does not rise above the pre-defined value during the second time period then said attenuating step is not performed during the second time period.

[0046] There is an interscan delay period between the second and third time periods during which voltages that are applied to ion-optics of the mass spectrometer are changed such that only ions having said third range of mass to charge ratios are able to reach the ion detector during the third time period.

[0047] Ions may be prevented from reaching the ion detector during this inter-scan delay period.

[0048] This interscan delay period is immediately between the second and third time periods.

[0049] The present invention also provides a mass spectrometer set up and configured to perform any one of the methods described herein.

[0050] Accordingly, the present invention provides a mass spectrometer comprising: an ion detector; an ion attenuation device for attenuating ions travelling to the ion detector; and control circuitry configured to: allocate a first time period for operating the mass spectrometer in a mode in which only ions having a first restricted range of mass to charge ratios are able to reach the ion detector; allocate a second, subsequent time period for operating the mass spectrometer in a mode in which only ions having a second, different restricted range of mass to charge ratios are able to reach the ion detector; control the mass spectrometer, when the first time period begins, to only transmit ions having said first range of mass to charge ratios to the ion detector; accumulate the ion signal generated by the ion detector during the first time period; and determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the predefined value during the first time period, the control circuitry is configured to control the ion attenuation device to attenuate ions for the remainder of the first time period so as to reduce or stop ions being received at the ion detector during the remainder of the first time period.

[0051] The mass spectrometer may be a tandem mass spectrometer comprising: a first mass filter for mass filtering precursor ions; a fragmentation or reaction device downstream of the first mass filter for fragmenting or reacting precursor ions transmitted by the first mass filter so as to produce fragment or other product ions; and a second mass filter for mass filtering said fragment or other product ions. The control circuitry may be configured to control the first and second mass filters during at least a portion of the first time period so that the first mass filter is only capable of transmitting a first pre-selected range of mass to charge ratios corresponding to a first precursor ion species of interest, and the second mass filter is only capable of transmitting a second pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the first precursor ion species of interest.

[0052] The control circuitry may be configured to either: (i) control the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting said first pre-selected range of mass to charge ratios and the second mass filter is only capable of transmitting a third pre-selected range of mass to charge ratios corresponding to a different fragment or other product ion species of said first precursor ion species of interest; or (ii) control the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting a further pre-selected range of mass to charge ratios corresponding to a second, different precursor ion species of interest, and the second mass filter is only capable of transmitting a fourth pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the second precursor ion species of interest.

[0053] The ion attenuation device may be a mass filter and the control circuitry may be configured to control the ion attenuation device to attenuate ions by varying an RF and / or DC voltage applied to electrodes of the mass filter.

[0054] Alternatively, the ion attenuation device may be at least one other electrode and the control circuitry is configured to control the ion attenuation device to attenuate ions by applying one or more voltage to the at least one electrode so as to arrange an electrical potential barrier in the path of the ions or so as to deflect the ions such that the ions are prevented from reaching the detector.

[0055] BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0057] Fig. 1 schematically illustrates an ion detector that comprises a photomultiplier tube;

[0058] Fig. 2 illustrates how the baseline signal of an ion detector may varying relative to the ion counting threshold when monitoring consecutive MRM transitions according to a conventional technique;

[0059] Fig. 3 illustrates how the baseline signal of an ion detector may varying relative to the ion counting threshold when monitoring consecutive MRM transitions according to an embodiment of the present invention;

[0060] Figs. 4A-4D show chromatograms for four MRM transitions obtained according to a conventional technique; and

[0061] Figs. 5A-5D show chromatograms for the same four MRM transitions obtained according to an embodiment of the present invention.

[0062] DETAILED DESCRIPTION

[0063] Mass spectrometers comprise an ion detector for detecting ions that are being analysed thereby. The ion detector is configured to receive ions at an input surface and generate electrons in response thereto. An electron multiplier, such as a photomultiplier tube, is typically used to amplify the number of electrons generated when an ion strikes the input surface of the detector. These electrons are then collected so as to provide a detector signal. However, ion detectors generate a relatively low level signal even when ions are not arriving at the detector, which is known as the detector baseline. As such, ion detectors tend to only determine that ions are striking the detector when the detector signal rises above a threshold intensity level, which is referred to below as the ion counting threshold.

[0064] Fig. 1 schematically illustrates an ion detector that comprises a photomultiplier tube. In such ion detectors, the ions 2 strike an input surface 4 of the ion detector, such as a dynode, which generates electrons 6 in response thereto. The electrons are directed to impact a photoemissive screen 8, such as a phosphor screen that generates photons 10 in response thereto. The photons then impact on photocathode electrode 12, which emits primary electrons 14 in response thereto. The primary electrons are directed to impact the first dynode in a dynode chain 16, which produces multiple secondary electrons in response to each primary electron that it receives. Those secondary electrons are then directed onto the next dynode in the dynode chain, which produces multiple electrons in response to each electron it receives. This process is repeated down the electrode chain and the resulting avalanche of electrons 18 is collected at an anode 20, so as to produce an electrical signal at the detector, i.e. a detector signal.

[0065] However, when relatively high ion currents are received at the input surface of the ion detector, the final stages of the photomultiplier tube continue to emit electrons even after the ion current ceases to impact on the ion detector. The ion detector therefore has a relatively high baseline signal under such circumstances, where the baseline signal begins to decay when the ion current to the detector is stopped or sufficiently reduced. The time it takes the baseline signal to decay back to an acceptable level depends on the magnitude of the ion current that the detector had been receiving and the duration this ion current. If the intensity of the baseline signal becomes higher than the ion counting threshold then the ion detector will determine that ions are being received at the ion detector regardless of whether or not they are. This problem may also occur in other types of ion detector and not just those comprising photomultiplier tubes.

[0066] The problem described above is particularly problematic, for example, in multiple reaction monitoring (MRM), single ion recording (SIR) or selected reaction monitoring (SRM) experiments. As is well known in the art, such experiments are performed using a tandem quadrupole mass spectrometer having an upstream quadrupole mass filter for selectively transmitting precursor ion species, a fragmentation cell for fragmenting the precursor ion species, and a downstream quadrupole mass filter for transmitting the resulting fragment ions. The mass spectrometer is controlled so as to monitor certain precursor-fragment transitions. This is performed by controlling the upstream mass filter so that it is only capable of transmitting a mass to charge ratio for a precursor ion species of interest and controlling the downstream mass filter so that it is only capable of transmitting a mass to charge ratio corresponding to a fragment ion species of that precursor ion species of interest. Precursor ions are then supplied to the upstream mass filter and an ion detector monitors for any ions that are transmitted by the downstream mass filter. If an ion is detected at the detector then the occurrence of the precursor-fragment transition being monitored for is confirmed. The downstream mass filter may be controlled so that it is capable of transmitting mass to charge ratios corresponding to different fragment ion species of the precursor ion species of interest at different times, so as to monitor for multiple precursor-fragment transitions for the same precursor ion species. Typically, precursor-fragment transitions are monitored for multiple different precursor ion species. These different precursor ion species tend to arrive at the upstream mass filter during partially overlapping time periods, e.g. due to the analytical sample being separated upstream of the mass filter by a liquid chromatography. As such, the mass spectrometer may repeatedly perform a cycle, during each of which the mass spectrometer sequentially monitors MRM transitions for different precursor ion species, e.g. as described further below in relation to Figs. 4A-4D and 5A-5D.

[0067] If one transition is being monitored during which a relatively high ion current is received at the ion detector, this may cause the detector baseline to rise to such an extent that it is above the ion counting threshold of the detector at the time the mass spectrometer switches to monitor the next transition. As such, the ion detector will determine that ions are being detected in said next transition, regardless of whether or not they actually are. This may cause ion peaks to show up for transitions in which ions are not actually being received at the ion detector, which may lead to the spectrometer incorrectly determining that there is an analyte in the sample being analysed that gives rise to said next transition. Even if ions are actually received at the detector when monitoring said next transition, the above problem may affect the accuracy of the ion signal and hence the amount or concentration determined for the precursor ion species being monitored. This problem is known as detector crosstalk or detector ringing and can be particularly problematic for techniques that require the ability to detect only a trace level of an analyte of interest and which may also experience high ion currents at the detector in the same experimental run, such as in toxicology.

[0068] Fig. 2 illustrates the above-described problem with reference to an MRM experiment, although it will be appreciated that this problem also applies to other types of experiments. Fig. 2 illustrates the ion current 22 arriving at the ion detector over a first time period 24 during which a first MRM transition is being monitored, a second time period 26 during which a second, different MRM transition is being monitored, and an inter-scan delay 28 between the two MRM transitions. Fig. 2 also shows the ion counting threshold 30 of the ion detector. During the first time period 24 the mass filters of the mass spectrometer are controlled so as to monitor for a first MRM transition. As can be seen from the ion current 22, during the first MRM transition a relatively high ion current is received at the ion detector. This causes the baseline signal 32 of the ion detector to gradually rise throughout the first time period 24, and to rise significantly above the ion counting threshold 30.

[0069] During the interscan delay 28, the voltages that are applied to the mass filters are changed such that the mass spectrometer is able to monitor for the second MRM transition during the subsequent second time period 26. It can be seen from Fig. 2 that when the voltages applied to the mass filters are changed during the interscan delay period 28 the ion current 22 to the ion detector rapidly drops until no ions reach the detector. In other words, the second MRM transition does not occur for the ions being analysed. Accordingly, the ion current to the detector continues to be zero during the second time period 26 in which the second MRM transition is being monitored.

[0070] At the start of the interscan delay 28, the ion current to the detector begins to drop to zero for the reasons described above. This causes the baseline signal 32 to also begin to reduce, i.e. decay. However, the interscan delay 28 is relatively short and so the baseline signal 32 remains above the ion counting threshold 30 at the time that the second time period 26 starts. As such, the mass spectrometer determines that ions are being detected by the ion detector when monitoring for the second MRM transition, even though they are not.

[0071] In order to mitigate this problem, it is conventionally necessary to use a relatively high ion counting threshold 30, a relatively low ion detector gain, or add a time delay between transitions. However, these solutions are not straightforward and can negatively impact the sensitivity and / or linear dynamic range of the mass spectrometer.

[0072] It has been recognised that it is not always necessary to transmit ions to the ion detector for the entire time period allocated to monitoring a given transition, and that once a sufficient number of ions have reached the detector for that transition then further ions may be prevented from reaching the detector (or partially attenuated) during the time period allocated to that transition. Not only does this prevent the baseline signal from continuing to rise, but it also allows the baseline signal to begin to drop before the end of time period allocated to the transition. This technique may be used in a variety of different types of experiment, although an example is described below in relation to monitoring MRM transitions. For example, a desired number of ions may be allowed to reach the ion detector during the time period allocated to monitor any given MRM transition, and then further ions are prevented from reaching the ion detector for the remainder of that MRM transition (or partially attenuated). The number of ions allowed to reach the ion detector may be the number sufficient to generate a reproducible signal that may reliably be used to quantify the amount or concentration of analyte giving rise to the ion signal during the MRM transition, since detecting further ions during this period would not have a significant benefit. In other words, when a relatively high ion current is transmitted to the ion detector during an MRM transition, it is unnecessary to monitor the ion signal for the entire dwell time of that transition, as the ion count at the ion detector reaches statistical accuracy relatively quickly, making further ion counting redundant.

[0073] The ion detector may be configured to track the accumulated ion signal during the time period allocated to monitoring any given MRM transition, i.e. during the dwell time. The spectrometer is configured to then determine when the accumulated ion signal exceeds a pre-defined value and, in response thereto, to prevent further ions reaching the ion detector for the remaining period allocated to monitoring that MRM transition. For example, the spectrometer may increase the mass resolution of one or more of the mass filters to an extent that substantially prevents ions being transmitted to the ion detector. This is able to prevent ions reaching the detector within, for example, approximately 100 ps of the pre-defined value being reached. The spectrometer may be configured to calculate and report the average intensity of the ion signal obtained during the portion of the time period allocated to monitoring the MRM transition that occurred before the ions were blocked from reaching the detector. In other words, when the spectrometer is calculating the amount or concentration of the analyte that has given rise to the ion signal during the MRM transition, it accounts for the ion signal having been accumulated over a period that is shorter than the time period allocated to monitor that MRM transition, i.e. shorter than the dwell time.

[0074] Fig. 3 illustrates the above-described technique according to an embodiment of the present invention. Fig. 3 illustrates the ion signal 22 arriving at the ion detector over a first time period 24 during which a first MRM transition is being monitored, a second time period 26 during which a second, different MRM transition is being monitored, and an inter-scan delay 28 between the two MRM transitions. Fig. 3 also shows the ion counting threshold 30 of the ion detector. During the first time period the mass filters of the mass spectrometer are controlled so as to monitor for a first MRM transition. As can be seen from the ion signal 22, this initially results in a relatively high ion current being transmitted to the ion detector, and consequently the detector baseline 32 rises accordingly. However, at time 34 during the first time period 24, the spectrometer determines that a sufficient number of ions have been detected by the ion detector for the first MRM transition. The spectrometer then controls its ion-optics so as to prevent further ions from reaching the detector for the remainder of the first time period 24 that has been allocated to monitoring the first MRM transition, i.e. until the start of the interscan delay 28. For example, as mentioned above, the mass resolution of one or more of the mass filters may be increased so as to achieve this. Therefore, at time 34 the ion signal arriving at the detector begins to rapidly decrease to zero. Consequently, at time 34 the baseline signal 32 begins to drop towards the value it would have if no ions are being received at the detector.

[0075] During the interscan delay 28, the voltages that are applied to the mass filters are changed such that the mass spectrometer is able to monitor for the second MRM transition during the subsequent second time period 26. It can be seen from Fig. 3 that no ions are transmitted to the ion detector during the second MRM transition, i.e. there are no analytes being analysed that give rise to the second MRM transition. It can also be seen from Fig. 3 that at the start of the second time period 26 the baseline signal 32 has dropped to being below the ion counting threshold 30. As such, the spectrometer correctly determines that no ions are detected when monitoring the second MRM transition, even though a high ion current 22 arrived at the detector during the preceding MRM transition.

[0076] As mentioned above, the problem of detector crosstalk or detector ringing is particularly problematic for techniques that require the ability to detect only a trace level of an analyte of interest and which may also experience high ion currents at the detector in the same experimental run, such as in toxicology. Methadone is an example of a compound which needs to be measured at trace levels but can also be present at extremely high concentrations in sample, e.g. when analysing a sample from a patient that has taken an overdose of methadone. A toxicology method was performed on a sample using MRM analysis in the conventional manner and also in accordance with an embodiment of the present invention, i.e. using the ion blocking technique described herein for each MRM transition. The results of these methods are shown in Figs. 4A-4D and Figs. 5A-5D.

[0077] Each method involved separating the sample using liquid chromatography and supplying the eluting sample to an electrospray ionisation ion source operating in positive ion mode in order to ionise the eluting analytes. The resulting ions were then mass analysed using a tandem quadrupole mass spectrometer that repeatedly cycled through monitoring several different MRM transitions as the sample elutes from the liquid chromatography device. An ion chromatogram of the detected signal for each of the transitions was then obtained. The MRM transitions monitored included an MRM transition for methadone from m / z=310.3 to m / z=265.2, another MRM transition for methadone from m / z=310.3 to m / z=105.1 , a MRM transition for methamphetamine from m / z=150.1 to m / z=119.1, and another MRM transition for methamphetamine from m / z=150.1 to m / z=91.1.

[0078] Figs. 4A-4D show the chromatograms for the four MRM transitions described above when a conventional MRM method was used that did not block the ions from reaching the ion detector. More specifically, Fig. 4A shows the chromatogram for the MRM transition for methadone from m / z=310.3 to m / z=265.2, Fig. 4B shows the chromatogram for the MRM transition for methadone from m / z=310.3 to m / z=105.1, Fig. 4C shows the chromatogram for the MRM transition for methamphetamine from m / z=150.1 to m / z=119.1, and Fig. 4D shows the chromatogram for the MRM transition for methamphetamine from m / z=150.1 to m / z=91.1. It can be seen from Figs. 4A-4D that ion peaks are obtained in all four chromatograms, implying that both methadone and methamphetamine are present in the sample being analysed.

[0079] Figs. 5A-5D show chromatograms for the same four MRM transitions as in Figs. 4A-4D and for analysis of the same sample, but when an MRM method according to an embodiment of the present invention was used to block ions from reaching the ion detector after the accumulated ion signal rises above the pre-defined value during each MRM transition. It can be seen from Figs. 5A-5B that significant ion peaks are obtained in the two chromatograms for the methadone MRM transitions, indicating that methadone is present in the sample being analysed. However, it can be seen from Figs. 5C-5D that no significant ion peaks were observed in the two chromatograms for the methamphetamine MRM transitions, indicating that methamphetamine is not present in the sample being analysed.

[0080] It is therefore clear from comparing Figs. 4A-4D with Figs. 5A-5D that the embodiments of the present invention prevent cross talk between the MRM transitions and the erroneous detection of analytes in the sample.

[0081] Although the present invention has been described with reference to various 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.

[0082] For example, although Fig. 3 illustrates a method in which only two MRM transitions are monitored for, it will be appreciated that further MRM transitions may be monitored subsequently, with corresponding inter-scan delay periods between consecutive MRM transitions for changing the voltages applied to the mass filters.

[0083] It will also be appreciated that the invention is not limited to monitoring MRM transitions, but that it is also applicable to other mass analysis techniques in which different ranges of mass to charge ratios are consecutively transmitted to the detector over different respective consecutive time periods.

Claims

175226-02v1Claims:

1. A method of mass spectrometry comprising: providing a mass spectrometer having an ion detector; allocating a first time period for mass analysing ions in a manner such that only ions having a first restricted range of mass to charge ratios are able to reach the ion detector; allocating a second, subsequent time period for mass analysing ions in a manner such that only ions having a second, different restricted range of mass to charge ratios are able to reach the ion detector; controlling the mass spectrometer, when the first time period begins, to only transmit ions having said first range of mass to charge ratios to the ion detector; and accumulating the ion signal generated by the ion detector during the first time period so as to determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the pre-defined value during the first time period, the mass spectrometer responds by attenuating ions for the remainder of the first time period so as to reduce or stop ions being received at the ion detector during the remainder of the first time period.

2. The method of claim 1 , comprising controlling the mass spectrometer, when the second time period begins, to only be capable of transmitting ions having said second range of mass to charge ratios to the ion detector.

3. The method of claim 1 or 2, wherein the detector produces a baseline signal when no ions are being received at the detector, and wherein said ion signal is a portion of the detector signal that is above a pre-selected threshold intensity; wherein the step of attenuating ions for the remainder of the first time period is performed such that the baseline signal drops below the threshold intensity by the start of the second time period, or such that the baseline signal remains below the threshold intensity until the start of the second time period.

4. The method of any preceding claim, wherein if the accumulated ion signal does not rise above the pre-defined value during the first time period then said attenuating step is not performed during the first time period.

5. The method of any preceding claim, wherein there is an interscan delay period between the first and second time periods during which voltages that are applied to ionoptics of the mass spectrometer are changed such that only ions having said second range of mass to charge ratios are able to reach the ion detector during the second time period.

6. The method of claim 5, wherein the interscan delay period is immediately after the first time period, and wherein the second time period is immediately after the interscan delay period.

7. The method of claim 5 or 6, wherein the step of only transmitting ions having said first range of mass to charge ratios to the ion detector is performed by applying a combination of RF and DC voltages to electrodes of a mass filter such that only these ions are transmitted to the detector; and wherein the RF and / or DC voltage applied to said electrodes is changed during the interscan delay such that only ions having said second range of mass to charge ratios are able to reach the ion detector during the second time period.

8. The method of any preceding claim, wherein the mass spectrometer monitors a first multiple reaction monitoring (MRM) transition during the first time period and monitors a second, different MRM transition during the second time period.

9. The method of any preceding claim, wherein the mass spectrometer is a tandem mass spectrometer comprising: a first mass filter for mass filtering precursor ions; a fragmentation or reaction device downstream of the first mass filter for fragmenting or reacting precursor ions transmitted by the first mass filter so as to produce fragment or other product ions; and a second mass filter for mass filtering said fragment or other product ions; and wherein the method comprises controlling the first and second mass filters during at least a portion of the first time period so that the first mass filter is only capable of transmitting a first pre-selected range of mass to charge ratios corresponding to a first precursor ion species of interest, and the second mass filter is only capable of transmitting a second pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the first precursor ion species of interest.

10. The method of claim 9, comprising either:(i) controlling the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting said first pre-selected range of mass to charge ratios and the second mass filter is only capable of transmitting a third pre-selected range of mass to charge ratios corresponding to a different fragment or other product ion species of said first precursor ion species of interest; or(ii) controlling the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting a further pre-selected range of mass to charge ratios corresponding to a second, different precursor ion species of interest, and the second mass filter is only capable of transmitting a fourth pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the second precursor ion species of interest.

11. The method of any preceding claim, wherein the step of attenuating ions is performed by varying the RF and / or DC voltage applied to said electrodes of a mass filter within the mass spectrometer.

12. The method of any one of claims 1-10, wherein the step of attenuating ions is performed by applying one or more voltage to at least one electrode of the mass spectrometer so as to arrange an electrical potential barrier in the path of the ions or so as to deflect the ions such that the ions are prevented from reaching the detector.

13. The method of any preceding claim, comprising controlling the mass spectrometer, when the second time period begins, to only transmit ions having said second range of mass to charge ratios to the ion detector; and accumulating the ion signal generated by the ion detector during the second time period so as to determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the pre-defined value during the second time period, the mass spectrometer responds by attenuating ions for the remainder of the second time period so as to reduce or stop ions being received at the ion detector during the remainder of the second time period.

14. A mass spectrometer comprising: an ion detector; an ion attenuation device for attenuating ions travelling to the ion detector; and control circuitry configured to: allocate a first time period for operating the mass spectrometer in a mode in which only ions having a first restricted range of mass to charge ratios are able to reach the ion detector; allocate a second, subsequent time period for operating the mass spectrometer in a mode in which only ions having a second, different restricted range of mass to charge ratios are able to reach the ion detector; control the mass spectrometer, when the first time period begins, to only transmit ions having said first range of mass to charge ratios to the ion detector; accumulate the ion signal generated by the ion detector during the first time period; and determine if the accumulated ion signal rises above a pre-defined value; wherein, if the accumulated ion signal rises above the pre-defined value during the first time period, the control circuitry is configured to control the ion attenuation device to attenuate ions for the remainder of the first time period so as to reduce or stop ions being received at the ion detector during the remainder of the first time period.

15. The mass spectrometer of claim 14 , wherein the mass spectrometer is a tandem mass spectrometer comprising: a first mass filter for mass filtering precursor ions; a fragmentation or reaction device downstream of the first mass filter for fragmenting or reacting precursor ions transmitted by the first mass filter so as to produce fragment or other product ions; and a second mass filter for mass filtering said fragment or other product ions; and wherein the control circuitry is configured to control the first and second mass filters during at least a portion of the first time period so that the first mass filter is only capable of transmitting a first pre-selected range of mass to charge ratios corresponding to a firstprecursor ion species of interest, and the second mass filter is only capable of transmitting a second pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the first precursor ion species of interest.

16. The mass spectrometer of claim 15, wherein the control circuitry is configured to either:(i) control the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting said first pre-selected range of mass to charge ratios and the second mass filter is only capable of transmitting a third pre-selected range of mass to charge ratios corresponding to a different fragment or other product ion species of said first precursor ion species of interest; or(ii) control the first and second mass filters during the second time period so that the first mass filter is only capable of transmitting a further pre-selected range of mass to charge ratios corresponding to a second, different precursor ion species of interest, and the second mass filter is only capable of transmitting a fourth pre-selected range of mass to charge ratios corresponding to a fragment or other product ion species of the second precursor ion species of interest.

17. The mass spectrometer of claim 15 or 16, wherein the ion attenuation device is a mass filter and the control circuitry is configured to control the ion attenuation device to attenuate ions by varying an RF and / or DC voltage applied to electrodes of the mass filter.

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