Apparatus and method for static gas mass spectrometry

Abstract

A method of static gas mass spectrometry is provided. The method includes the steps of: introducing a sample gas comprising two or more isotopes to be analyzed into a static vacuum mass spectrometer at a time, t.sub.0; operating an electron impact ionization source of the mass spectrometer with a first electron energy below the ionization potential of the sample gas for a first period of time that is following t.sub.0 until a time t.sub.1; and operating the electron impact ionization source with a second electron energy at least as high as the ionization potential of the sample gas for a second period of time that is after time t.sub.1. The first time period from t.sub.0 to t.sub.1 is a period corresponding to a period taken for the isotopes of the sample gas to equilibrate in the mass spectrometer. A constant ion source temperature is preferably maintained. Also provided is a static gas mass spectrometer.

Claims

1. A method of static gas mass spectrometry comprising the steps of: introducing a sample gas comprising two or more isotopes to be analyzed into a static vacuum mass spectrometer at a time, t.sub.0; operating an electron impact ionization source of the mass spectrometer with a first electron energy below the ionization potential of the sample gas for a first period of time that is following t.sub.0 until a time t.sub.1, wherein the first time period from t.sub.0 to t.sub.1 is set based on a previous determination of an equilibration period taken for the isotopes of the sample gas to equilibrate in the mass spectrometer; and operating the electron impact ionization source with a second electron energy at least as high as the ionization potential of the sample gas for a second period of time that is after time t.sub.1; wherein isotope ratio measurements are taken by the spectrometer during the second period but not during the first period.

2. The method of claim 1 further comprising regulating a filament heating current of a filament of the electron impact ionization source so as to keep the temperature of the ionization source substantially the same during the first period and the second period.

3. The method of claim 1 further comprising the step of mass analyzing the two or more isotopes in the mass spectrometer beginning with the second period of time.

4. The method of claim 3 wherein the step of mass analyzing comprises determining at least one isotope ratio of the sample gas.

5. The method of claim 4 wherein the mass analyzing comprises, for each of two or more isotopes, measuring the intensity of the isotope over time; performing a best fit of each measured isotope intensity with time, extrapolating each best fit to a time zero when the second electron energy is raised at least as high as the ionization potential of the sample gas, and calculating a ratio of the extrapolated time zero isotope intensities of two isotopes to give an isotope ratio of the sample gas.

6. The method of claim 1 wherein the sample gas is a noble gas.

7. The method of claim 1 wherein the first time period from t.sub.0 to t.sub.1 is not significantly longer than a time for the sample gas to equilibrate in the mass spectrometer.

8. The method of claim 1 wherein the first time period is at least 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 5.5, 6, 6.5, 7, 8, 9, 10, 11, 12, 13, 14 or 15 minutes.

9. The method of claim 1 wherein the first electron energy of the ionization source is lower than the ionization potential by at least 2 eV, 4 eV, 6 eV, 8 eV, 10 eV, or 12 eV.

10. The method of claim 1 wherein the first electron energy of the ionization source is about 10 eV.

11. The method of claim 1 wherein the second electron energy of the ionization source is higher than the ionization potential of the sample gas by at least 10 eV, or 20 eV, or 30 eV, or 40 eV, or 50 eV, or 60 eV, or 70 eV.

12. The method of claim 1 wherein the second electron energy of the ionization source is about 80 eV.

13. The method of claim 1 wherein the second electron energy is at least 2×, or 3×, or 4×, or 5×, or 6×, or 7×, or 8×, or 9×, or 10× the first electron energy.

14. A static gas mass spectrometer comprising: an electron impact ionization source for receiving a sample gas comprising two or more isotopes and ionising the sample gas, a controller to control the electron impact ionization source, a mass analyzer for mass analyzing the generated ions, an ion detector for detecting ions that have been mass analyzed, and at least one pump for generating a vacuum in the mass spectrometer, which can be isolated from the mass spectrometer before a sample gas is received by the ionization source, wherein the ionization source is operable with a first electron energy below the ionization potential of the sample gas for a first period of time following a sample gas introduction into the ion source at time t.sub.0 until a time t.sub.1; and operable with a second electron energy at least as high as the ionization potential of the sample gas for a second period of time that is after time t.sub.1, wherein the controller controls the electron energy of the ionization source and sets the first time period from t.sub.0 to t.sub.1 based on a previous determination of an equilibration period taken for the isotopes of the sample gas to equilibrate in the mass spectrometer.

15. The static gas mass spectrometer of claim 14 wherein the first time period from t.sub.0 to t.sub.1 is not significantly longer than a time for the sample gas to equilibrate in the mass spectrometer.

16. The static gas mass spectrometer of claim 14 wherein the vacuum is an ultra high vacuum, the mass analyzer is a magnetic sector mass analyzer and the ion detector is a multicollector.

17. The static gas mass spectrometer of claim 14 further comprising a temperature monitor to measure the temperature of a filament of the electron impact ionization source and provide a feedback signal to control a filament current supplied to the filament so as to maintain substantially constant filament temperature during the first and second periods.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 shows schematically a prior art measured isotope abundance over time from time zero.

[0035] FIG. 2 shows schematically a prior art measurement of isotope abundance over time for an argon sample.

[0036] FIG. 3 shows schematically a configuration of a static mass spectrometer according to an embodiment.

[0037] FIG. 4 shows schematically an arrangement of a static mass spectrometer according to another embodiment.

[0038] FIG. 5 shows schematically an arrangement of an electron impact ionisation source according to a further embodiment.

[0039] FIG. 6 shows schematically a measured isotope abundance according to an embodiment measuring from time t.sub.1 after an isotope equilibration phase.

[0040] FIG. 7 shows schematically a measured isotope ratio according to an embodiment measuring from time t.sub.1 after an isotope equilibration phase.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] In order to enable a more detailed understanding of the invention, embodiments will now be described by way of example and with reference to the accompanying drawings.

[0042] Referring to FIG. 3, there is shown schematically a typical configuration of a static mass spectrometer 200, which can be used in the invention, comprising: a sample preparation region 205; a transfer region 230; an ion source region 240; and a mass analyzer 250. The sample preparation region 205 comprises a chamber 210 (such as a furnace or laser irradiated chamber) and an optional preparation bench 220. Between each of the furnace 210, the sample preparation bench 220, the transfer region 230 and the ion source region 240, valves 215 are provided.

[0043] The admission to the static mass spectrometer 200 is indirect via an intermediate chamber. A normal application is the determination of the isotope ratios of various isotopes of a noble gas that is trapped in a sample, such as a piece of rock or similar.

[0044] In current instruments, the sample, typically a piece of rock, is put into a chamber (such as furnace 210) and then heated, possibly with a laser. This treatment releases trapped gases, which comprise the desired analytes. The released gases are transferred to the sample preparation bench 220, where they may be manipulated in various ways. For example, they may be partially or wholly transferred to storage volumes (“pipettes”) and then they may be partially released, giving a smaller amount of sample at a lower pressure.

[0045] In other cases the gas can be a gas sample directly, for instance an air gas sample.

[0046] The gas is then transferred to the transfer region 230, which may act as a cleaning unit. In older devices, the sample gas was collected on a cold finger. The gases could then be thawed to “distil” the gases, releasing them one after another. More modern devices comprise a general type of “trap” installed, typically comprising chemical getters, and optionally cold traps, to remove unwanted substances (this usually means everything but noble gases). The vacuum pump or pumps (not shown) to the mass spectrometer (i.e. to the ion source and mass analyser) are closed off by valves (not shown) before the sample is released into the chambers (240, 250).

[0047] From here, the sample gas is equilibrated with the ion source region 240 where the gas is subsequently ionized following equilibration (by electron ionization) and the generated ions are subsequently analyzed in the mass analyzer 250.

[0048] In the ion source 240, the gas to be analyzed is typically ionized by means of electron bombardment (electron impact ionization). Due to the statistical distribution in the mass spectrometer of the gas to be analyzed, there are only a small number of molecules in the region of the ion source. This therefore results in only a small ion stream and hence a requirement for high detection sensitivity.

[0049] Typical pressures in the ion source region 240 and mass analyzer 250 are 10.sup.−9 to 10.sup.−10 mbar before the sample is admitted and subsequently, 10.sup.−6 to 10.sup.−7 mbar (or 10.sup.−7 to 10.sup.−9 mbar), depending on the sample amount (which cannot always be predicted). The gas to be analyzed spreads throughout the ion source region 240 and the mass analyser 250, with a small number of molecules entering the ion source. In the mass analyser 250, the ions generated from the ion source travel along a flight path, such as along flight tube 255, before being detected in detector region 260.

[0050] The strong vacuum and the removal of “undesired” gases from the sample are important in order to improve the signal to noise ratio (that is the ion count from the sample gas against the ion count from other gases, such as remaining from a previous measurement or from other “interferences”, such as isobaric ions, like hydrocarbons).

[0051] Referring now to FIG. 4, there is shown schematically an arrangement with further details of a static mass spectrometer in accordance with the invention. The overall arrangement of the static mass spectrometer of FIG. 4 does not differ significantly from that shown in FIG. 3. The static mass spectrometer 1 comprises: an electron impact ion source 30; a flight tube 110; a magnetic sector mass analyser 130; a detector housing 140; a multicollector detector arrangement 150; and electronics 160. A vacuum pump 180 is coupled to the ion source assembly 30 via an automatic valve 170. A sample preparation region and gas transfer region, as described with reference to FIG. 3, is not shown in this drawing, but would typically be included. Additionally a further vacuum pump (not shown) is connected to the detector housing 140, with a valve (also not shown).

[0052] The detector arrangement 150 is shown as a multicollector device, comprising a plurality of collectors for detection of ions. This could comprise at least one Faraday cup, at least one ion counter, or a combination thereof, such as described in WO-2012/007559, which is commonly assigned. Three collectors are shown in FIG. 4, but a preferred embodiment has five collectors and embodiments with more collectors are envisaged as well. The electronics 160 may comprise electronics and/or a computer of a detection system for data acquisition, storage and/or processing. Moreover, the electronics 160 comprises a controller, which further comprises ion source control, valve control, pump control, etc.

[0053] Turning next to the ion source, FIG. 5 shows schematically the arrangement of the electron impact ionization source 30 of FIG. 4 for use in the invention. The electron impact ion source 30 is a Nier type. The neutral sample gas is admitted into the ionisation chamber 35, which is generally held at high voltage (e.g. 3-5 kV). Electrons are produced by thermionic emission from a filament 40 that is heated by passing a heating current through it and the electrons are accelerated by an extraction voltage applied to trap electrode 50 (e.g. 10-100V). Magnets 45 cause the electrons to follow a helical path across the chamber. The electrons, provided they have sufficient energy, ionise the gas and the gas ions are extracted by the high voltage as applied to the repeller 55 and chamber 35. The ion beam is generated through the extraction slit 60 and can be steered and/or focussed by focus electrodes 65. The ion source 30 is controlled by a controller that is part of electronics 160 as shown schematically by line 165 in FIG. 4.

[0054] In use, once the sample of noble gas is introduced into the ion source 30 from the gas preparation and transfer region at a time t.sub.0, there follows an initial equilibration time of the isotopes of the gas into the mass spectrometer. The controller of the electronics 160 controls the electron energy voltage, i.e. extraction voltage to electrode 50, to lower the electron impact energy from the usual ˜80 electron volts typically used to ionise noble gases, for example, xenon, down to 10 electron volts. This lower level of electron energy is below the ionization potential of the noble gas that is to be analyzed. The reduced electron energy ensures that no sample gas is ionized during the initial sample equilibration phase. The energy may be kept at the low level (first energy) at all times except during the ionization and mass analysis period (second period), so that it is already at the low level from a time before the gas is introduced to the spectrometer. After a first time period from t.sub.0, at a time t.sub.1 the controller increases the electron energy so that the ionizing electron beam energy is reset to the usual high level, such as ˜80 eV, to ensure high ionization yields of noble gas inside the ion source.

[0055] By inputting to the controller, which can comprise a computer, the type of noble gas to be detected in the mass analysis the controller can select and set both the period of the equilibration time (the first period from t.sub.0 to t.sub.1) and optionally the first (lower) and second (higher) electron energies. The duration of the first period for each sample gas species to be set by the controller can be determined from a previous measurement of the isotope intensity with time from which the time for equilibration can be found. The first (lower) and second (higher) electron energies in some embodiments can be set to values that are applicable for all noble gas species from He to Xe and thus do not need to be set specifically for each gas species. These could be, for example, 12 eV or lower, or 10 eV or lower for the first electron energy and at least 50 eV, 60 eV, 70 eV or 80 eV for the second electron energy.

[0056] All the time the filament heating current is kept constant, i.e. the same during the sample measurement (mass analysis phase) as during the equilibration phase. Only the electron impact energy is changed. The ion source conditions should be kept stable over time in order to avoid distortion of the measured isotope ratios. Any change in filament temperature during sample measurement may result in uncontrolled isotope fractionation and affect the accuracy and precision of the measurement. To better ensure that the filament and hence in source temperature remains substantially constant, for example in view of changes to the electron energy, a pyrometer 70, can be provided adjacent the filament 40 to monitor the temperature of the filament and provide a feedback signal to the controller 160 to control the filament current so as to maintain substantially constant filament temperature. A change of filament current with change of electron energy can be calibrated this way.

[0057] It can be seen from above that the invention addresses the problem of sample consumption during the equilibration phase of the sample gas introduction into the static gas mass spectrometer, which is currently a significant limitation for high precision isotope ratio measurements of the heavier noble gases. According to the invention, during the first, equilibration phase the electron energy is maintained below the first ionization potential of the gas but all other ion source parameters are substantially unchanged compared to the subsequent phase after the first, equilibration phase. After the first, equilibration phase has passed, the electron energy is increased to achieve the necessary high ionization yields (all other ion source parameters remaining substantially unchanged as mentioned). The reduced electron energy avoids ionization of the sample gas during the first equilibration phase and thus does not consume any sample gas, which would involve preferentially consuming some isotopes over others. Such a workflow can help to avoid a distortion of the measured isotope ratios that conventionally occurs during the first equilibration phase. The ion source temperature conditions can be kept stable all the time while only reducing the ionizing electron beam energy during the initial equilibration time of the sample into the mass spectrometer below the first ionization energy of the gas.

[0058] Referring to FIG. 6, which shows schematically a measured isotope abundance 28 of a noble gas according to an embodiment of the invention, the isotope ratio mass analysis measurements begin at or after time t.sub.1 (i.e. when the second time period starts) once the gas isotopes have equilibrated in the ion source and once the electron energy is increased above the gas ionization potential. The plot of FIG. 6 shows the intensity, i.e. abundance, of an isotope over time. Two or more different isotopes are measured in all. The curves for the different isotopes are slightly different because of changing mass bias and changing gas composition because of slightly different ionization probabilities of the different isotopes. A best fit, such as linear interpolation for example, of the measured ion beam intensity is performed and extrapolated back to time zero for each isotope. An isotope ratio is then calculated from the ratio of the time zero intensities of two isotopes. A ratio of any two isotopes, usually of the same element, can be obtained in this way. The problem in the prior art is that during the time required for equilibration immediately following gas introduction to the ion source and spectrometer, sample gas already becomes consumed in an uncontrolled way and heavier and lighter isotopes are ionized in an uncontrolled way. Even more, this uncontrolled fractionation and ionization during the initial equilibration time window results in a change of the isotope composition of the remaining gas, which is a fundamental limitation to high precision isotope ratio measurements of gases. The prior art time zero extrapolation back to when the gas is introduced cannot correct for this. With the invention, since no gas ionization or consumption has occurred prior to time t.sub.1 in the equilibration phase, the time zero for the present invention is in fact t.sub.1 and the isotope ratio calculated from the measurements or best fit curves of the isotope intensities at this time zero will be a more accurate measurement than in the prior art method in which ionization occurs from the moment the gas is introduced to the ion source.

[0059] In the present invention, time zero means the time when the electron energy is adjusted to the ionization mode, i.e. t.sub.1. Time zero is the time when ionization starts to consume or change the isotope abundances of the sample gas, which in the prior art is when the gas is introduced into the spectrometer (t.sub.0) but in the invention is t.sub.1 when the ionization begins. In other words, the isotope intensity of the respective isotopes 25 at time t.sub.1 can be used to calculate the accurate isotope ratio of the gas. However, a single measurement at time t.sub.1 could be prone to error. In practice, due to limitations of measurement precision, it is better that several measurements are made from time t.sub.1 onwards and plotted against time so that a best fit line through them can be made. Then the value of the fitted line at the time zero of t.sub.1 can provide the intensity to calculate the isotope ratio.

[0060] It will be appreciated that in most embodiments the individual isotope abundances will each be measured and fitted by a best fit line that is extrapolated from which an (accurate) isotope ratio is calculated from the extrapolated line at time zero (in this case t.sub.1). However, in other embodiments, instead the isotope ratio could be calculated for each time point that the individual isotope abundances are measured, thereby providing a plurality of isotope ratios with time, which can be fitted by a best fit line that is extrapolated to time zero (in this case t.sub.1) to determine the (accurate) isotope ratio. This is shown schematically in FIG. 7.

[0061] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0062] The use of any and all examples, or exemplary language (“for instance”, “such as”, “for example” and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0063] As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as “a” or “an” means “one or more”.

[0064] Throughout the description and claims of this specification, the words “comprise”, “including”, “having” and “contain” and variations of the words, for example “comprising” and “comprises” etc, mean “including but not limited to”, and are not intended to (and do not) exclude other components.

[0065] Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise.

[0066] All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).