Mass spectrometer compensating ion beams fluctuations

Abstract

A mass spectrometer comprises an interface for receiving an ion beam from an ion source, a mass analyzer unit for selecting from the received ion beam, in two or more time periods, ions having different ranges of mass-to-charge ratios, a first detection unit for detecting, in each of said time period, ions within a selected range and producing first detection signals representative of quantities of detected ions having respective mass-to-charge ratios, and a second detection unit arranged between the interface and the mass analyzer unit for producing a second detection signal representative of a total intensity of the ion beam received from the ion source as a function of time. The mass spectrometer further comprises a processing unit for normalizing the first detection signals by using the second detection signal, which processing unit may output a ratio of normalized first detection signals.

Claims

1. A mass spectrometer comprising: a mass analyzer for selecting from an ion beam, in two or more time periods, ions having different ranges of mass-to-charge ratios; a first detector configured to detect, in each of said time periods, ions within a respective selected range of mass-to-charge ratios and producing first detection signals representative of quantities of detected ions having the respective ranges of mass-to-charge ratios; a second detector configured to produce a second detection signal representative of a total intensity of the ion beam as a function of time; and a processing circuit configured to: cause the mass analyzer to operate in a first mode where the mass analyzer selects for ions having a first selected range of mass-to-charge ratios; cause the mass analyzer to operate in a second mode where the mass analyzer selects for ions having a second selected range of mass-to-charge ratios; and normalize the first detection signals by using the second detection signal; and obtaining a normalized intensity ratio by dividing: a first normalized first signal corresponding with a first time period where the mass analyzer was operating in the first mode; by a second normalized first signal corresponding with a second, different time period where the mass analyzer was operating in the second mode.

2. The mass spectrometer according to claim 1, wherein the processing circuit is further configured for causing the mass analyzer to cycle between at least the first mode of operation and the second mode of operation, and for producing a ratio of normalized first detection signals for a plurality of such cycles.

3. The mass spectrometer according to claim 1, wherein the processing circuit is configured for normalizing the first detection signals by dividing each first detection signal by the second detection signal at a corresponding time period.

4. The mass spectrometer according to claim 1, wherein the first detector comprises a single detector.

5. The mass spectrometer according to claim 1, wherein the mass analyzer is configured for continuously selecting ions in consecutive time periods.

6. The mass spectrometer according to claim 1, wherein the second detector comprises a detection element arranged upstream of the mass analyzer.

7. The mass spectrometer according to claim 6, wherein the detection element comprises a skimmer, an entrance slit, an aperture or an ion lens.

8. The mass spectrometer according to claim 6, wherein the second detector comprises a detection circuit for deriving the second detection signal from an electrical current generated in the detection element by ions from the ion beam.

9. The mass spectrometer according to claim 1, further comprising an ion source for producing the ion beam.

10. The mass spectrometer according to claim 9, wherein the ion source comprises a plasma source.

11. The mass spectrometer according to claim 10, further comprising ion optics for removing plasma gas ions, which ion optics are arranged upstream of a detection element of the second detector.

12. The mass spectrometer according to claim 10, further comprising a pre-mass filter for removing plasma gas ions, which pre-mass filter is arranged upstream of the detection element.

13. The mass spectrometer according to claim 9, wherein the ion source comprises a thermal ionization source or an electron impact source.

14. A method of operating a mass spectrometer comprising: receiving an ion beam from an ion source; selecting from the received ion beam, in two or more time periods, ions having different ranges of mass-to-charge ratios, the two or more time periods comprising: a first time period where ions having a first selected range of mass-to-charge ratios are selected; and a second, different time period where ions having a second, different selected range of mass-to-charge ratios are selected; detecting, in each of said time periods, ions within a respective selected range of mass-to-charge ratios and producing first detection signals representative of quantities of detected ions having respective ranges of mass-to-charge ratios; detecting, in each of said time periods, a total intensity of the ion beam so as to produce a second detection signal; normalizing the first detection signals by using the second detection signal; and obtaining a normalized intensity ratio by dividing: a first normalized first signal corresponding with the first time period where ions having the first selected range of mass-to-charge ratios was selected; by a second normalized first signal corresponding with the second, different time period where ions having the second, different selected range of mass-to-charge ratios was selected.

15. The method according to claim 14, wherein normalizing the first detection signals comprises dividing each first detection signal by the second detection signal at a corresponding time period.

16. The method according to claim 14, further comprising dividing a normalized first signal corresponding with a first time period by a normalized first signal corresponding with a second, different time period to obtain a normalized intensity ratio.

17. The method according to claim 14, further comprising continuously selecting ions in consecutive time periods.

18. The method according to claim 14, further comprising: removing plasma gas ions prior to selecting, in two or more time periods, ions having different ranges of mass-to-charge ratios.

19. The method according to claim 14, wherein the two or more time periods further comprising a third, different time period where ions having the first selected range of mass-to-charge ratios are selected, and a fourth, different time period where ions having the second selected range of mass-to-charge ratios are selected, and the method further comprising: obtaining an additional normalized intensity ratio by dividing: a third normalized first signal corresponding with the third time period where ions having the first selected range of mass-to-charge ratios was selected; by a fourth normalized first signal corresponding with the fourth time period where ions having the second selected range of mass-to-charge ratios was selected.

20. A computer program product comprising one or more non-transitory computer-readable media having computer instructions stored therein, the computer program instructions being configured such that, when executed by one or more computing devices, the computer program instructions cause the one or more computing devices to: identify an ion beam from an ion source; select from the ion beam, in two or more time periods, ions having different ranges of mass-to-charge ratios, the two or more time periods comprising: a first time period where ions having a first selected range of mass-to-charge ratios are selected; and a second, different time period where ions having a second, different selected range of mass-to-charge ratios are selected; detect, in each of said time periods, ions within a respective selected range of mass-to-charge ratios and producing first detection signals representative of quantities of detected ions having respective ranges of mass-to-charge ratios; detect, in each of said time periods, a total intensity of the ion beam so as to produce a second detection signal; normalize the first detection signals by using the second detection signal; and, including obtaining a normalized intensity ratio by dividing: a first normalized first signal corresponding with the first time period; by a second normalized first signal corresponding with the second, different time period.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a first exemplary embodiment of a mass spectrometer according to the present invention.

(2) FIG. 2 schematically shows a second exemplary embodiment of a mass spectrometer according to the present invention.

(3) FIGS. 3A-3B schematically show examples of sequentially determined detector signals according to the prior art.

(4) FIGS. 4A-4C schematically show examples of sequentially determined detector signals according to the present invention.

(5) FIG. 5 schematically shows an exemplary embodiment of a method for operating a mass spectrometer according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(6) The present invention is aiming to improve existing mass spectrometers, in particular those for high precision isotope and elemental abundance measurements, so as to cover a larger mass range in applications in which using multiple parallel detectors does not provide a sufficiently large mass-to-charge range. The present invention allows single collector detections and/or measurements to be made while preserving the advantages of multi-collector detections and/or measurements, in particular the elimination of intensity fluctuations on determining the mass-to-charge ratio of two or more ion species.

(7) The exemplary mass spectrometer 10 schematically illustrated in FIG. 1 is shown to comprise an ion source 11, a mass analyzer 12, a first detection unit 13, a second detection unit 15 comprising a detection element 14, and a processing unit 16. In the embodiment of FIG. 1, the detection element 14 constitutes the interface 17 between the ion source 11 and the other parts of the mass spectrometer 10 and may for example be constituted by a sampler cone. In other embodiments, this interface 17 may be constituted by another part, such as a skimmer cone, or an entrance aperture or slit, or by a dedicated detection element which may for example be ring-shaped or disc-shaped.

(8) The ion source 11 can be a conventional ion source, such as an ICP (Inductively Coupled Plasma) source, a glow discharge source, an electron ionization source, a secondary ion ionization source, a thermal ionization source or any other suitable ion source. It is noted that a mass spectrometer may be supplied without an ion source, and that an ion source may be supplied separately, for example for subsequent assembly with the mass spectrometer. In FIG. 1 the ion source 11 is shown as part of the mass spectrometer 10.

(9) The mass analyzer 12 can be a conventional mass analyzer, such as a quadrupole mass analyzer or a sector field mass analyzer (e.g. a magnetic sector and/or electric sector mass analyzer), which allows a continuous mass filtering of ions. The first detection unit 13 can be a conventional detection unit comprising a single ion detector, such as a Faraday cup. In some embodiments, the first detection unit 13 may comprise two or more detectors (e.g. Faraday cup and Secondary Electron Multiplier—SEM), which may be optimized for different mass-to-charge ratios. The first detection unit 13 is configured for producing first detection signals representative of quantities of detected ions. As these ions have been filtered by the mass analyzer 12, the detected ions will have a mass-to-charge ratio, or a range of mass-to-charge ratios corresponding with the ratio or range selected by the mass analyzer. The first detection signals 1 are output to the processing unit 16.

(10) As illustrated in FIG. 1, an original ion beam 20 produced by the ion source 11 can pass through the detection element 14 to the mass analyzer 12 which filters the ion beam. As a consequence, a filtered ion beam 22 consisting of ions having a limited range of mass-to-charge values leaves the mass analyzer 12 and reaches the first detection unit 13, where the ions are detected. The direction D in which the ions travel, from the ion source 11 to the first detection unit 13, causes the first detection unit 13 to be located downstream of the mass analyzer 12 and, conversely, causes the mass analyzer 12 to be located upstream of the detection unit 13.

(11) The detection element 14 may be constituted by a suitable object having at least one through opening for passing the ion beam 20. The detection element 14 may comprise a sampler cone, a skimmer cone, ion optics, or an object specifically designed for this purpose, such as a ring-shaped object or a set of plates arranged in parallel with the ion beam 20. The detection element 14 is electrically connected to a detection circuit of the second detection unit 15. The detection element 14 can be electrically conductive so as to allow a current to flow from the detection element 14 to the second detection unit 15 (or vice versa). This current is caused by a portion of ions from the ion beam 20 hitting the detection element 14. In an embodiment, ions in a peripheral portion of the ion beam hit the detection element 14. If the detection element 14 is constituted by a skimmer cone, for example, between 10% and 20% of the ions of beam 20 may hit the detection element 14 and thus contribute to the current supplied to the second detection unit 15. The actual percentage can depend on the width and focus of the ion beam, and on the diameter and/or position of the opening in the detection element.

(12) The second detection unit 15 can comprise a detection circuit for deriving the second detection signal from an electrical current generated in the detection element 14 by the portion of the ions from the ion beam. This second detection signal 2, which represents the intensity of the ion beam, is also output to the processing unit 16.

(13) The processing unit 16 can comprise one or more microprocessors, a memory and suitable I/O (Input/Output) circuits. The memory can contain instructions which allow the microprocessor(s) to carry out a method according to the invention. More in particular, the (at least one) microprocessor can normalize the first detection signals 1 by using the second detection signal 2 and can output normalized first detection signals 3. The microprocessor of the processing unit 16 may normalize the first detection signals by dividing each first detection signal by the second detection signal at a corresponding time period. The normalization process will later be explained in more detail with reference to FIGS. 4A-4C.

(14) The exemplary mass spectrometer 10 illustrated in FIG. 2 is shown to also comprise an ion source 11, a mass analyzer 12, a first detection unit 13, a detection element 14, a second detection unit 15 and a processing unit 16. In addition, the mass spectrometer of FIG. 2 comprises a pre-filter (which may also be referred to as pre-mass filter or mass pre-filter) 18. In the embodiment of FIG. 2, the interface 17 comprises an element separate from the detection element 14. The interface 17 of FIG. 2 typically comprises an aperture and may be constituted by a sampling cone or a skimmer cone, for example, in which case the detection element 14 may be constituted by ion optics, an entrance slit, or by a dedicated detection element, such as a detection ring or detection tube, preferably made of metal. The original ion beam 20 passes through the pre-filter 18 to become the pre-filtered ion beam 21, which in turn passes through the mass analyzer 12 to become the filtered ion beam 22 consisting of ions having a limited range of mass-to-charge values. This filtered ion beam 22 is detected by the first detection unit 13.

(15) The mass pre-filter 18 may comprise a quadrupole filter, a Wien filter, a collision-reaction cell, ion optics or any other suitable filter. In particular when a plasma ion source is used, as in the case of ICP-MS (Inductively Coupled Plasma Mass Spectrometry), the pre-filter 18 may serve to remove matrix (e.g. plasma gas) ions, such as argon ions, from the ion beam. Advantageously, this enables the ion beam that is detected by the second detector and used as a measure of total ion beam intensity to comprise mostly or substantially ions from the sample and not, for example, from the plasma gas.

(16) The other units of the mass spectrometer 10 of FIG. 2 may be similar to those of the mass spectrometer of FIG. 1.

(17) The invention will further be explained with reference to FIGS. 3A-3B and FIGS. 4A-4C. As mentioned above, it can be advantageous to detect multiple different ion types substantially simultaneously using multiple parallel detectors, each detector being arranged for detecting a particular ion type or limited ion type range. In this so-called multi-collector approach, any fluctuations in the ion beam intensity will appear at all detectors substantially simultaneously and will therefore be cancelled out when calculating relative ion counts. Due to physical limitations, however, the multi-collector approach only allows a limited (approx. 20%) range of mass-to-charge ratios. This is clearly insufficient for determining the relative abundances of argon and xenon, for example, where a mass-to-charge ratio of approx. 370% is required.

(18) FIG. 3A schematically shows detected intensities I of individual ions species (or limited mass-to-charge ranges), detected by a single detector, as a function of time t. Detections take place in subsequent time periods T1, T2, etc. In time periods T1, T3 and T5, the (first) intensity I1 of a first ion species is detected, while in time periods T2, T4 and T6, the (second) intensity I2 of a second ion species is detected. Due to fluctuations in the ion beam, the detected intensities are not constant.

(19) While FIGS. 3A-3B show ion intensities processed in accordance with the prior art, FIGS. 4A-4C show ion intensities processed in accordance with the invention.

(20) The calculated ion ratios are schematically illustrated in FIG. 3B. These ratios may be calculated, for example, by dividing the average value of the first intensity I1 during the first time period T1 by the average value of the second intensity I2 during the second time period T2, resulting in an ion ratio for the combined time period T1+T2, shown in FIG. 4B at time t=(T1+T2)/2. Instead of an average value of the intensity during a time period, a median value could be used, or the intensity value in the middle of the respective time period. Similarly, ion ratios for the combined time periods T3+T4, T5+T6 etc. can be determined. Additionally, intermediate ion ratios for the combined time periods T2+T3, T4+T5 etc. can be determined in a similar manner. As can be seen in the example of FIG. 3B, these calculated ratios vary over time, thus making the ratios less reliable.

(21) The present invention offers a solution to this problem by detecting the intensity of the total ion beam and using this detected total intensity to determine the individual ion intensities and ion ratios. This is schematically illustrated in FIGS. 4A-4C.

(22) In FIG. 4A, the first ion intensity I1 and second ion intensity I2 are shown at time periods T1, T2 etc., as in FIG. 3A. It is noted that, as in FIG. 3A, the intensities I1, I2 etc. are functions of time and may therefore be written as I1(t), I2(t), etc. In accordance with the invention, FIG. 4A also shows a total ion intensity IT, which may be represented by the second detection signal (2 in FIGS. 1 and 2). The total ion intensity IT is also a function of time and may therefore be written as IT(t).

(23) In the example of FIG. 4A, the total ion intensity IT, which may correspond with the intensity of the ion beam (20 in FIGS. 1 & 2) before it enters the mass analyzer (12 in FIGS. 1 & 2), is not constant over time but fluctuates. As a result, the first and second detected ion intensities I1 and I2, which may be represented by the first detection signals (1 in FIGS. 1 and 2), vary over time. However, in accordance with the present invention, the fluctuations of the detected ion intensities are compensated. This can be achieved by normalizing the first detection signals representing the detected ion intensities by using the second detection signal representing the total ion beam intensity. In particular, normalizing the first detection signals may be carried out by dividing each first detection signal by the second detection signal at a corresponding time period.

(24) In the present example, the corresponding time period is the same time period: the first detected ion intensity I1 in time period T1 is divided by the total ion intensity IT in time period T1. Similarly, the second detected ion intensity I2 in time period T2 is divided by the total ion intensity IT in time period T2. As mentioned before, the first and second ion intensities I1 and I2, as well as the total ion intensity IT, may be determined by averaging the respective intensity during the corresponding time period, calculating the mean during the time period, by determining the value in the middle of the time period (so, in the case of T1, at t=T1/2), or in another way. The results are depicted in FIG. 4B.

(25) FIG. 4B shows the normalized first intensities I1/IT and normalized second intensities I2/IT respectively. For each time period T1, T2, etc., a normalized intensity I1/IT or I2/IT respectively has been determined. More specifically, a normalized intensity I1(T1)/IT(T1) is determined for the first time period T1, a further normalized intensity I2(T2)/IT(T2) is determined for the second time period T2, a still further normalized intensity I1(T3)/IT(T3) is determined for the third time period T3, etc. Then the ratio of these normalized intensities can be determined for each pair of adjacent time periods to provide a normalized ratio I1′/I2′ for each of those pairs of time periods, where I1′=I1/IT and I2′=I2/IT. More specifically, the normalized ratio for the first pair of time periods, T1 and T2, is I1′(T1)/I2′(T2). Similarly, the normalized ratio for the second pair of time periods, T2 and T3, is I2′(T2)/I1′(T3). Thus, for each pair of adjacent time periods a common normalized ratio may be determined.

(26) In FIG. 4C this normalized ratio I1′/I2′ is represented for each pair of adjacent time periods at their border. As can be seen, this ratio is substantially constant over all time periods T1, T2, etc. Thus, the effect of fluctuations in the total ion beam intensity, as represented by the signal IT in FIG. 4A, on the ratio has been eliminated.

(27) It is noted that in the example described above with reference to FIGS. 4A-4C, the ions are detected continuously. That is, the time periods T1, T2, T3, . . . etc. are contiguous time periods. Although contiguous time periods are advantageous as they minimize the total measurement time, they are not essential. In some embodiments, no detection could take place during a time period. In addition, the time periods may have equal durations, as illustrated in FIGS. 4A-4C, or have different durations. The duration of a time period may be, for example, be 10 ns or 1000 ms, or any suitable value in between.

(28) In the example described above, only two different ion intensities I1 and I2 are determined. It will be understood that the invention can also be applied to more than two different ion types or ion ranges (that is, mass-to-charge ratio ranges). The invention can therefore also be applied when three, four, five, six or more different ion intensities I1, I2, I3, etc. are determined.

(29) An exemplary embodiment of a method in accordance with the invention is schematically illustrated in FIG. 5. The method 5 starts with initialization step 50. In step 51, an ion beam is received from an ion source. In step 52, ions having different ranges of mass-to-charge ratios are selected from the received ion beam, in two or more time periods. In step 53 ions within a selected range are detected in each of said time periods and first detection signals representative of quantities of detected ions having respective mass-to-charge ratios are produced. In step 54, a second detection signal representative of a total intensity of the ion beam received from the ion source as a function of time is produced, which may be done by measuring the total ion beam intensity. As can be seen, step 54 may be carried out in parallel with steps 52 and 53.

(30) In step 55, the first detection signals are normalized by using the second detection signal. In step 56, normalized first detection signals are output. The method ends in step 57, although the method 5 can be seen as a continuous process which repeats itself.

(31) Normalizing the first detection signals, at 55, may comprise dividing each first detection signal by the second detection signal in a corresponding time period. Normalizing the first detection signals, at 55, may further comprise dividing a normalized first signal corresponding with a first time period by a normalized first signal corresponding with a second, different time period corresponding with another ion intensity, to obtain a normalized intensity ratio. Step 55 may therefore comprise the sub steps of dividing each first detection signal by the second detection signal in a corresponding time period and dividing a normalized first signal corresponding with a first time period by a normalized first signal corresponding with a second, different time period corresponding with another ion intensity.

(32) The method of the invention may further, at 52, comprise continuously selecting ions in consecutive time periods. In some embodiments, however, selecting ions may not take place in consecutive time periods.

(33) The invention uses sequential detection of ion intensities. This does, however, not preclude the use of multiple detectors in the first detection unit. Thus, the first detection unit (13 in FIGS. 1 & 2) may include two, three or more detectors, which may for example each be designed for detecting a specific ion or range of ions. At least one of those detectors is used sequentially and the advantages of the present invention can therefore be obtained. In some embodiments, two or more detectors may be used alternatingly, for example, but this still constitutes sequential use of the detectors.

(34) It will be understood by those skilled in the art that the invention is not limited to the embodiments shown and that many modifications and additions are possible without departing from the scope of the invention as defined in the appending claims.