Targeted mass analysis

Abstract

A mass spectrometer comprises: an ion source that generates ions having an initial range of mass-to-charge ratios; an auxiliary ion detector, downstream from the ion source that receives a plurality of first ion samples derived from the ions generated by the ion source and determines a respective ion current measurement for each of the plurality of first ion samples; a mass analyzer, downstream from the ion source that receives a second ion sample derived from the ions generated by the ion source and to generate mass spectral data by mass analysis of the second ion sample; and an output stage that establishes an abundance measurement associated with at least some of the ions generated by the ion source based on the ion current measurements determined by the auxiliary ion detector.

Claims

1. A mass spectrometer, comprising: an ion source, arranged to generate ions having an initial range of mass-to-charge ratios; an auxiliary ion detector, located downstream from the ion source and arranged to receive a plurality of first ion samples having a reduced range of mass-to-charge ratios that is narrower than the initial range derived from the ions generated by the ion source and to determine a respective ion current measurement for each of the plurality of first ion samples; a mass analyser, located downstream from the ion source and arranged to receive a second ion sample derived from the ions generated by the ion source and to generate mass spectral data by mass analysis of the second ion sample, wherein a resolution of the mass spectral data is high enough to mass resolve the ion current measurement determined by the auxiliary ion detector within the reduced range of mass-to-charge ratios; and a processor configured to establish an abundance measurement associated with at least some of the ions generated by the ion source based on a combination of the mass spectral data generated by the mass analyser and the ion current measurements determined by the auxiliary ion detector, wherein the mass spectral data is used to mass resolve the ion current measurements within the reduced range of mass-to-charge ratios.

2. The mass spectrometer of claim 1, wherein the auxiliary ion detector is configured to provide the plurality of ion current measurements over a time period, wherein the mass analyser is arranged to generate a single set of mass spectral data over the time period.

3. The mass spectrometer of claim 1, wherein the auxiliary ion detector is configured to have an average frequency of ion current measurement which is higher than the average frequency of mass analysis of the mass analyser.

4. The mass spectrometer of claim 3, wherein the auxiliary ion detector is configured to determine the plurality of ion current measurements with a time interval therebetween and wherein the mass analyser is configured to perform mass analysis of the second ion sample over a time duration that is longer than the time interval between the plurality of ion current measurements.

5. The mass spectrometer of claim 1, further comprising: a mass filter, arranged upstream from the auxiliary ion detector and configured to receive ions generated by the ion source and to transmit ions having a reduced range of mass-to-charge ratios, the reduced range being narrower than the initial range; and wherein the first and second ion samples are derived from the ions transmitted by the mass filter.

6. The mass spectrometer of claim 1, wherein the mass analyser comprises one of: a time-of-flight type; an orbital trapping type; an electrostatic trap; and a Fourier Transform Ion Cyclotron Resonance, FT-ICR, type.

7. The mass spectrometer of claim 1, wherein the processor is configured to provide the abundance measurement associated with at least some of the ions generated by the ion source, by adjusting the mass spectral data generated by the mass analyser on the basis of the ion current measurements determined by the auxiliary ion detector.

8. The mass spectrometer of claim 1, wherein the first and second ion samples are samples of the same set of ions, the auxiliary ion detector being configured to determine a plurality of total ion current measurements for the set of ions, such that the processor is configured, for each of the plurality of total ion current measurements, to establish a plurality of abundance measurements for the set of ions, each abundance measurement being associated with a portion of the mass spectral data.

9. The mass spectrometer of claim 8, wherein each abundance measurement is established by adjusting the respective portion of the mass spectral data based on at least one of the total ion current measurements.

10. The mass spectrometer of claim 1, wherein the mass analyser is arranged to generate a plurality of sets of mass spectral data over a measurement time period and wherein the auxiliary ion detector is configured to determine a plurality of ion current measurements for each set of mass spectral data that is generated, the processor being configured thereby to establish a plurality of abundance measurements, each abundance measurements relating to a respective set of mass spectral data.

11. The mass spectrometer of claim 1, wherein at least one of the plurality of first ion samples has the same range of mass-to-charge ratios as the second ion sample.

12. The mass spectrometer of claim 1, wherein the ion source is configured to receive a plurality of samples over time and, for each received sample, to generate respective ions, the processor being configured to establish at least one abundance measurement for each of the plurality of samples.

13. A mass spectrometer, comprising: an ion source, arranged to generate ions having an initial range of mass-to-charge ratios; an auxiliary ion detector, located downstream from the ion source and arranged to receive a plurality of first ion samples derived from the ions generated by the ion source and to determine a respective ion current measurement for each of the plurality of first ion samples; a mass analyser, located downstream from the ion source and arranged to receive a second ion sample derived from the ions generated by the ion source and to generate mass spectral data by mass analysis of the second ion sample; and a processor configured to establish an abundance measurement associated with at least some of the ions generated by the ion source based on a combination of the mass spectral data generated by the mass analyser and the ion current measurements determined by the auxiliary ion detector, wherein the plurality of ion current measurements and the mass spectral data relate to ions generated over the same time period and wherein the processor is configured to use the plurality of ion current measurements to deconvolute the mass spectral data over the time period.

14. The mass spectrometer of claim 1, wherein the mass analyser is further configured to adjust the abundance of ions in the second ion sample on the basis of the ion current determined for the first ion sample.

15. The mass spectrometer of claim 1, further comprising: a mass filter; an ion storage device; and a controller, configured to control the mass filter to select ions of a first range of mass-to-charge ratios, to control the auxiliary ion detector to determine an ion current for the ions of the first range of mass-to-charge ratios, to control the ion storage device to accumulate ions of the first range of mass-to-charge ratios in the ion storage device and to repeat selection, determining and accumulating until a threshold quantity of ions of the first range of mass-to-charge ratios are stored in the ion storage device, the controller being further configured to control the mass analyser to mass analyse the ions stored in the ion storage device.

16. The mass spectrometer of claim 15, wherein the controller is further configured to control the mass filter to select ions of a second range of mass-to-charge ratios, to control the auxiliary ion detector to determine an ion current for the ions of the second range of mass-to-charge ratios, to control the ion storage device to accumulate ions of the second range of mass-to-charge ratios in the ion storage device and to repeat selection, determining and accumulating until a threshold quantity of ions of the second range of mass-to-charge ratios are stored in the ion storage device, wherein the controller is configured to control the mass analyser to mass analyse the ions stored in the ion storage device when the ion storage device stores the threshold quantity of ions of the first range of mass-to-charge ratios and the threshold quantity of ions of the second range of mass-to-charge ratios.

17. The mass spectrometer of claim 1, further comprising: a collision cell, downstream from the ion source; and a controller, configured to control the auxiliary ion detector to determine an ion current for a first portion of the ions generated by the ion source, to control the mass analyser to mass analyse the first portion of the ions generated by the ion source and to control the collision cell to fragment a second portion of the ions generated by the ion source so as to generate fragment ions and to control the mass analyser to mass analyse the fragment ions.

18. The mass spectrometer of claim 1, wherein the sensitivity of the auxiliary ion detector is greater than the sensitivity of the mass analyser.

19. The mass spectrometer of claim 1, wherein the ion source generates elemental ions.

20. The mass spectrometer of claim 19, wherein the ion source comprises an inductively coupled plasma torch.

21. The mass spectrometer of claim 1, wherein the processor is further configured to resolve the ion current measurements using the mass spectral data.

22. The mass spectrometer of claim 21, wherein processor is further configured to resolve the ion current measurements using the mass spectral data to remove contributions from interferences.

23. The mass spectrometer of claim 21, wherein the processor is further configured to adjust the ion current measurements according to the share of the current due to an element of interest determined from the mass spectral data.

24. The mass spectrometer of claim 21, wherein the spectrometer is an inductively coupled plasma mass spectrometer.

25. The mass spectrometer of claim 1, wherein the processor is further configured to control the addition of reaction gas to a reaction cell upstream of the auxiliary detector to remove molecular interferences from the ion current measurement using the mass spectral data.

26. The mass spectrometer of claim 13, wherein the processor is configured to provide a plurality of abundance measurements for each of the plurality of samples, each abundance measurement being associated with a portion of the mass spectral data for the respective sample.

27. A mass spectrometer, comprising: an ion source, arranged to generate ions having an initial range of mass-to-charge ratios; an auxiliary ion detector, located downstream from the ion source and arranged to receive a plurality of first ion samples derived from the ions generated by the ion source and to determine a respective ion current measurement for each of the plurality of first ion samples; a mass analyser, located downstream from the ion source and arranged to receive a second ion sample derived from the ions generated by the ion source and to generate mass spectral data by mass analysis of the second ion sample; and a processor configured to establish an abundance measurement associated with at least some of the ions generated by the ion source based on a combination of the mass spectral data generated by the mass analyser and the ion current measurements determined by the auxiliary ion detector, wherein the processor is configured to use the plurality of ion current measurements to deconvolute a mass chromatographic peak.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be put into practice in various ways, a number of which will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows schematic diagrams, illustrating different arrangements of components in order to implement respective embodiments of a mass spectrometer in accordance with the present invention;

(3) FIG. 2A schematically depicts a first view of deflection optics for use in the mass spectrometer of FIG. 1;

(4) FIG. 2B schematically depicts a second view of the deflection optics of FIG. 2A;

(5) FIG. 3 illustrates a schematic diagram of a first mass spectrometer implementation in accordance with a first embodiment shown in FIG. 1;

(6) FIG. 4 illustrates a schematic diagram of a second mass spectrometer implementation based on the embodiments shown in FIG. 1;

(7) FIG. 5 illustrates a schematic diagram of a mass spectrometer implementation in accordance with a third embodiment shown in FIG. 1;

(8) FIG. 6 shows example results from a mass spectrometer in accordance with the present invention illustrating the deconvolution of data;

(9) FIG. 7 shows a table of the specified relative amounts of the elements and interfering components in a mixture of a simulated example;

(10) FIG. 8 depicts an overview spectrum showing the mixture of the simulated example as shown in FIG. 7 plus Ar in amount 1;

(11) FIG. 9 shows a magnified portion of FIG. 8 around the m/z 40 region;

(12) FIG. 10 shows a magnified portion of FIG. 8 around the m/z 54 region;

(13) FIG. 11 shows a magnified portion of FIG. 8 around the m/z 56 region;

(14) FIG. 12 shows a magnified portion of FIG. 8 around the m/z 57 region; and

(15) FIG. 13 shows a magnified portion of FIG. 8 around the m/z 58 region.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(16) Referring first to FIG. 1, there are shown schematic diagrams, illustrating different arrangements of components in order to implement respective embodiments of a mass spectrometer. Three embodiments are shown and each embodiment comprises: an ion source 10; a mass filter 20; an optional collision cell 30; a mass analyzer 40; a data acquisition system 50; and an auxiliary ion detector 60. The auxiliary ion detector 60 is typically a Secondary Electron Multiplier (SEM). The data acquisition system 50 may be understood as the output stage of the invention.

(17) In each embodiment, ions are introduced from the ion source 10 through the mass filter 20. At least some of the ions are fragmented by the collision cell 30 and the fragments are analysed in a high-resolution mass analyzer 40 with data acquisition system 50. The additional auxiliary ion detector 60 is located on a side path downstream from the mass filter 20. The location of the auxiliary ion detector 60 varies between the different embodiments. The location of the auxiliary ion detector 60 may be one of the following. a) At a location immediately downstream from the mass filter 20, prior to the collision cell 30. This location allows direct measuring of the total ion current (TIC) of precursor ions. However, this TIC might be significantly different from the total ion current of fragments (if fragmentation is employed). Also, a sophisticated ion optical system may be required to rapidly switch ions from a straight trajectory to a side path leading to the auxiliary detector 60. b) At a location between the collision cell 30 and the high resolution analyzer 40. This location allows direct measuring of the TIC of fragments (if fragmentation is employed) and this may match the output of the data acquisition system 50 better. However, like option a) above, it may also require a sophisticated ion optical system to allow deflection towards the auxiliary ion detector 60. c) At a location downstream from the collision cell 30 and the high-resolution analyzer 40. This location allows direct measurement of the TIC for fragments without the need for an elaborate ion optical system to deflect ions to the auxiliary ion detector 60. Instead, ions could simply be allowed to pass through the entire system when they are not deflected to the analyzer 40.

(18) As noted above, the first and second embodiments (marked a and b respectively) may require deflection optics for diverting ions towards the auxiliary ion detector 60. Referring next to FIG. 2, there is shown a schematic depiction of deflection optics for use in embodiments of the mass spectrometer shown in FIG. 1. A first view of the deflection optics is shown in FIG. 2A. A second view of the deflection optics of FIG. 2A is shown in FIG. 2B. This shows a cross-section through the line marked A-A. Where the same elements as shown in FIG. 1 are shown in FIG. 2, identical reference numerals have been used. The mass filter 20 has an exit aperture 21. The mass filter 20 is a quadrupole device with rods 22, 23, 24 and 25. The auxiliary ion detector comprises a SEM 61 and a conversion dynode 62.

(19) As ions exit the quadrupole mass filter 20 through the aperture 21, they are transported by RF-only quadrupole ion guide rods 22-25 towards the collision cell 30 and/or mass analyzer 40 (not shown in this drawing). Preferably, the RF frequency of the potential applied to the rods 22-25 is between 2 and 5 MHz. Moreover, the rod outer diameter is preferably smaller than the gap between the rods.

(20) For deflection towards the SEM 61, RF is rapidly switched off and rods 22 and 23 receive DC of the same polarity as the ion polarity (for example, +300V for positive ions). Rods 24 and 25 receive DC of the opposite polarity as ion polarity (for example, −300V for positive ions). This diverts ions to the SEM 61 which is biased at a high DC voltage of the opposite polarity than the ion polarity (for example, −2000V). Examples of appropriate switching electronics may be found in U.S. Pat. No. 7,498,571.

(21) When molecular ions are to be detected, it is preferable to use post-acceleration. For example, this may be achieved by deflecting ions in the direction opposite to that of the SEM 61 (this is the upwards direction in FIG. 2) to the conversion dynode 62. Then, the DC field can be utilised for transporting resulting secondary ions or electrons towards the SEM 61 as known in the art.

(22) Referring next to FIG. 3, there is illustrated a schematic diagram of a first mass spectrometer implementation in accordance with the first embodiment shown in FIG. 1. The embodiment shown in FIG. 1a) may be especially suitable for a situation when no collision cell 30 is required. For example, this may be the case in an instrument combining an Inductively-Coupled Plasma (ICP) source with a quadrupole mass filter 20 and a mass analyzer that is based on orbital trapping or TOF technology. Such an embodiment is shown in FIG. 3.

(23) This implementation comprises: ICP torch 11; cone 12; skimmer 13; ion optics 14; collision cell 15; curved trap (C-trap) 41; orbital trapping mass analyzer 42; and ion optics 43. Control ion optics 70 are also provided downstream from the mass filter 20.

(24) The mass filter 20 is a quadrupole device that isolates ions in a narrow range of mass-to-charge ratios. These are transmitted through the control ion optics 70 to the C-trap 41. Intermittently (for example, every 20 ms), these ions are deflected to the auxiliary ion detector (not shown) by the control ion optics 70, for accurate quantitation. The auxiliary ion detector may be located within the control ion optics 70, for instance in accordance with the design shown in FIG. 2. Any other means of selecting ions could be used alternatively or in addition, for example a drift tube, differential ion mobility filter, time-of-flight filter, magnetic sector or ion trap of any type.

(25) The C-trap 41 accumulates ions over a prolonged period of time. This can therefore be used to store ions from multiple windows of mass-to-charge ratios (as selected by mass filter 20). These ions are ejected from the C-trap 41 through the ion optics 43 into the orbital trapping analyzer 42 for analysis. The analysis cycle of the orbital trapping analyzer 42 is relatively long in comparison with other periods, for instance, 100-300 ms. Thus, the ions are accumulated in a C-trap 41 until the orbital trapping analyzer 42 is ready for detection in each cycle.

(26) Data obtained using this approach can be used to resolve interferences within the mass range or mass ranges of interest. The measured ion current is adjusted according to the share of element of interest obtained by means of the high-resolution mass spectrum. For example, if the TIC over a mass range of 10 amu is measured to be 10.sup.9±1% ions/second, whilst the orbital trapping mass analyzer measures that 20%±1% of this TIC comes from molecular interferences, then elements of interest represent 80%±1% and their correct intensity is 8×10.sup.8±1.4%. In other words, the use of a quadrupole mass analyzer alone would have given a measurement that is 20% inaccurate, though misleadingly precise. The presence of the high-resolution mass analyzer allows an improvement in accuracy, although there may be a slight deterioration of precision as a result.

(27) Another example may be explained as follows. If it is established that molecular interferences dominate at, for instance, 50% or more, this may become a trigger for adding a reaction gas to the optical reaction cell 15 to react with the molecular interferences. The reaction gas may be helium, hydrogen or their mixture. Conventional reaction cells in single-quadrupole instruments result in a significant loss of ion current (between 3 and 10 fold). This is due to the need to attenuate interferences by many orders of magnitude. The presence of a high-resolution mass analyzer reduces this requirement. It may then be sufficient to provide the analyte signal with the same order of magnitude of intensity as any interferences combined. It may also provide reliable attenuation control, thus allowing reduction in the rate of reaction, gas density and consequently, ion losses.

(28) The embodiments shown in FIGS. 1a) and 1b) are most appropriate for use with high-resolution mass analyzers of orbital trapping, FT-ICR and electrostatic trap type, because they require prolonged storage times. Moreover, the embodiment shown in FIG. 1c) is non-trivial to implement using these type of mass analyzers. However, the careful reduction of trapping potentials during fly-through may allow these mass analyzers to be used with the third embodiment as well.

(29) Referring next to FIG. 4, there is shown a schematic diagram of a second mass spectrometer implementation based on the embodiments shown in FIG. 1. However, unlike the embodiments shown in FIG. 1, the position of the optional collision cell is altered, as will be explained below. Where the same elements are shown as in previous drawings, identical reference numerals have been used. The only component shown in FIG. 4 that is not shown in the previous drawings is the collision cell 31.

(30) Ions generated in the ion source 10 are passed to the mass filter 20 and an auxiliary ion detector (not shown), as part of the control ion optics 70, is used to provide TIC measurements. Some ions transmitted by the mass filter 20 pass straight through the C-trap 41 into the dead-end reaction cell or collision cell 31. This can act as a storage device, but it can also act as a fragmentation cell in some circumstances. Ions stored in the C-trap 41 can selectively be ejected through the ion optics 43 to the orbital trapping mass analyzer 42. The data acquisition system 50 is coupled to the orbital trapping mass analyzer 42 to obtain detection image current output.

(31) This design is a preferred embodiment for a tandem orbital trapping-based mass spectrometer, interfaced to fast separations, such as GC, HPLC or UHPLC. The auxiliary ion detector can be used to provide intermediate points on a chromatogram.

(32) Referring next to FIG. 5, there is illustrated a schematic diagram of a mass spectrometer implementation in accordance with a third embodiment shown in FIG. 1. As before, where the same elements are shown as used in previous drawings, identical reference numerals have been employed. This embodiment is preferred for tandem mass spectrometry based on orthogonal acceleration TOF (oaTOF) mass analyzeranalyzers. Also provided are: lens optics 44; an orthogonal accelerator 45; a detector 46; and at least one ion mirror 47.

(33) The high-resolution oaTOF is interfaced to the collision cell 30 by the lens optics 44. Although an oaTOF mass analyzer is capable of pulsing ion packets with a repetition rate of up to 10-30 kHz, for instance, its low transmission (such as 0.2% to a few percent) requires prolonged addition of spectra in order to acquire sufficient statistics. Typically, such mass analyzers pulse out only a portion of the ion beam, equivalent to several microseconds of flow and the orthogonal accelerator 45 is then refilled with ions until the entire analyzer is free of previously injected ions. This could take up to hundreds of microseconds. Therefore, ions are free to pass through the orthogonal accelerator 45 and to be detected by the detector 60 (preferably with post-acceleration, as described above) until the next pulse. Using this approach, the detector 60 may be used to detect up to 50 to 70% of all ions arriving at the mass analyzer. In other words, it may require five to ten times less time to reach the same statistical precision compared with detector 46.

(34) The design of this third embodiment could be implemented with the instrument of FIG. 4, for example if the detector is located behind the collision cell at the end of the ion path.

(35) Referring now to FIG. 6, there are shown example output results from a mass spectrometer in accordance with the present invention used to sample a chromatographic peak. This is used to illustrate the deconvolution of data. The process of deconvolution uses inputs from both the auxiliary ion detector and the mass analyzer. In this example, the high-resolution detector of the mass analyzer is six times slower than the auxiliary ion detector (for example, a SEM). In other words, the auxiliary ion detector samples the peak six times faster. As a result, the high-resolution detector of the mass analyzer under-samples the chromatographic peak. Nevertheless, utilising the measurements from the auxiliary ion detector, deconvolution allows restoration of the peak shape and makes it more suitable for quantitation.

(36) The output of the auxiliary ion detector (showing total ion current) is plotted against time in FIG. 6a). For each of the points marked with a black dot, in FIG. 6a), a mass spectrum (the output of the mass analyzer) is shown in FIG. 6b). Up to three peaks (labelled 1, 2 and 3) are marked in each mass spectrum. The first peak 1 is marked with a thick solid line, the second peak 2 is marked with a thin solid line and the third peak 3 is marked with a thin dotted line. The deconvoluted traces showing the ion currents for first peak 1, second peak 2 and third peak 3 are then shown in FIG. 6c), allowing for better definition of the peak form and area under the peak. The latter may be directly linked to the amount of sample injected. If only the peak intensities from the mass spectrum in FIG. 6b were used, it would lead to different peak shapes and less accurate quantitation.

(37) The quality of deconvolution may be dependent on the quality of chromatographic peak model, reproducibility of the peak shapes and signal-to-noise (S/N) ratios of the underlined peaks. It is anticipated that the majority of practical cases permit integration of the chromatographic peaks. As a result, the accuracy of the quantitative analysis may be significantly improved in view of the introduction of the auxiliary ion detector. It is also anticipated that such deconvolution could be run in real time as peak elutes, thus allowing for data-dependent change of conditions, for example of temporal points of sampling ions by either the auxiliary ion detector or the mass analyzer.

(38) A number of mathematical methods can be used to improve deconvolution. These may include: methods of multi-scale modelling; best-fitting methods with different norms (for instance, L2 or Huber norms) and scale space theory in signal processing (including pyramid representation and edge detection).

(39) Referring now to FIGS. 7 to 13, a simulated example will be described illustrating how the ion current measurements may be deconvoluted or resolved using the mass spectral data, thereby to result in a more accurate abundance measurement from the auxiliary detector for a particular ion species or element of interest. In particular, the example shows how the mass spectral data can be used to remove contributions to the ion current from interferences. If the auxiliary detector were used alone, or if only low resolution mass spectral data were available, the observed ion current measurement may not only represent the ion species of interest but also interfering ion species of the same or similar mass to the ion species of interest. Using one or more of the techniques described herein, the measured ion current obtained from the auxiliary ion detector is adjusted according to the share of the current due to an element of interest determined from the high resolution mass spectral data obtained from the mass analyzer.

(40) The example described simulates the determination of calcium and other major elements in a stainless steel sample. The sample ions may be produced and analysed by an ICP-MS spectrometer, for example as shown in FIG. 3.

(41) Referring to FIG. 7, there is shown a table of the specified relative amounts of the elements and interfering components in the mixture of the simulated example. The amounts indicated are only for the purpose of illustration of the invention, such that they do not represent typical peak intensities for the elements. The system is first studied at high resolution (500 k; such resolution is well within the possible range for an Orbital Trapping mass analyzer such as the Orbitrap™). An overview spectrum is depicted in FIG. 8. The spectrum shows the mixture as entered (see FIG. 7) plus Ar in amount 1.

(42) Zooming into the peaks one by one, it can be seen in FIG. 9, which zooms into the m/z 40 region, that there are two peaks not one and that the ratio Ar:Ca=1:1. However, the quantitation of Ca at mass 40 may be difficult even at 500 k resolution. In real measurements the Ar peak may be orders of magnitude higher than Ca, which means that, besides possible dynamic range issues, Ca may appear as just a small feature in the tail of the Ar peak. The peaks at m/z 42 and 44 (shown in FIG. 8) are undisturbed Ca peaks and this is where Ca should be observed and/or quantified, despite the low relative abundance (2%) of these peaks compared to .sup.40Ca.

(43) Other elemental peaks that can be resolved from interferences using one or more of the technique disclosed herein are shown with reference to FIGS. 8 and 10 to 13. At m/z 50, chromium (.sup.50Cr) has a possible interference from .sup.36Ar.sup.14N. At m/z 52, chromium has a very minor interference from .sup.34Ar.sup.18O, which looks small in this case, but may become significant when trying to determine Cr at trace levels on this isotope. At m/z 53, chromium appears with even smaller interferences of ArN and ArO. At m/z 54, there are small peaks of Cr, Fe and .sup.40Ar.sup.14N (see FIG. 10). At m/z 55 is .sup.40Ar.sup.15N. At m/z 56 is Fe (the main isotope) with interferences of ArO and CaO. As can be clearly seen, while resolving the two interferences from one another requires the full 500 k resolution, separation of both interferences from Fe will be possible at much lower resolution (see FIG. 11). At m/z 57 is Fe with interference of .sup.40Ar.sup.17O (and .sup.40Ca.sup.17O; see FIG. 12) and at m/z 58 is Fe (interference) with Ni and interferences of CaO and ArO (see FIG. 13).

(44) The spectra show that even a common “simple” element like iron in steel is difficult to measure interference-free, without the benefit of one or more of the techniques described herein.

(45) Although specific embodiments have now been described, the skilled person will appreciate that variations and modifications are possible. For example, types of detectors could be used as an auxiliary ion detector other than an SEM, such as an avalanche diode, microchannel and microsphere plates, channeltrons, and similar types of detector. Types of external storage device other than a C-trap 41 could be used, as known in practice.

(46) It should be noted that the auxiliary ion detector (such as SEM) could additionally be used for automatic gain control (AGC), as known in the art. Depending on the ion current, the fill time for the mass analyzer may be adjusted for subsequent accumulation upstream from an orbital trapping mass analyzer, or transmission at the lens optics 44 in the oaTOF embodiment. The combination of AGC and the analytical measurement of TIC in the same detection cycle may be especially advantageous, for example as explained in International Patent Publication No. WO-2012/160001 (having common ownership with the present invention).

(47) Whilst some of the specific implementations described above have used a specific mass analyzer, it may be recognised that another type of mass analyzer can be substituted in some cases. Similarly, it will be understood that a part of the configuration in embodiment can be combined with another part of the configuration in another embodiment. For example, the ICP source and interface configuration of FIG. 3 might be employed with the dead-end collision cell of FIG. 4 or oaTOF mass analyzer of FIG. 5.

(48) The interleaved operation of two detectors (the auxiliary ion detector and the detector of the mass analyzer) could be combined with multiplexed filling of the high-resolution mass analyzer, especially in the case of trapping analyzers. By switching the quadrupole mass filter between different mass windows, the auxiliary ion detector could acquire TIC information for each mass window until sufficient ion statistics are obtained. Typically, this may be up to 1000 or 10000 ion counts or equivalent. Then, ions could be directed to a downstream ion storage device, such as the C-trap 41 and/or dead-end collision cell 31 in the embodiments described above, for sufficient fill time and these could be accumulated with the already stored ions. Subsequently, the next mass window may be selected and the process repeated until the mass analyzer is ready to detect stored ions. Then, the summed (that is, multiplexed) ion population is injected into the mass analyzer and the next cycle starts. Each mass window in the spectrum generated by the mass analyzer can then be related to the corresponding TIC reading from the auxiliary ion detector which could be used for quantitation, removal of interferences or both.

(49) A further possible application for such a mode of operation is in the targeted quantitation of peptides and proteins. In this case, the auxiliary ion detector can measure the TIC of precursor ions with high temporal resolution while the mass analyzer can determine the share of impurities or interferences (in a full MS scan). This can then confirm the presence of a precursor of interest by detection of multiple predicted fragments (in an MS/MS scan mode using fragmentation in a collision cell). Such an approach would allow a coefficient variation of a few percent even when the signal-to-noise ratio of fragments is less than 5 and chromatographic peak width is below 1 second.

(50) Any number of further mass analysis or ion production and processing stages could be added to any item of the schematic diagram shown in FIG. 1. This also includes possible reversal or looping of the ion path, as known in the art.