Equipment and methods for static gas mass spectrometry

Abstract

Static gas mass spectrometry methods, comprising the following steps: Introducing a sample gas containing two or more isotopes to be analyzed into a static vacuum mass spectrometer at time t0; 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 following t0 up to a time t1, wherein the first period from t0 to t1 is based on an equilibration period for equilibrating the sample gas, the equilibration period being determined by a prior measurement of an isotopic ratio of the sample gas against the time following sample gas injection; and Operating the electron impact ionization source with a second electron energy at least equal to the ionization potential of the sample gas for a second period after time t1; where the isotope ratio mass analysis measurements are carried out during the second period, but not during the first period.

Description

Field of the invention

[0001] The invention relates to the field of mass spectrometry. The invention particularly relates to static gas mass spectrometry, for example, isotope ratio measurements. The invention can be used for the analysis of stable isotopes of noble gases (also referred to as rare or inert gases), namely helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), as well as other gases, such as CO2 or N2. The invention provides both a method and an apparatus. background

[0002] Noble gas mass spectrometry is important for radiometric dating or isotope geochemistry, for example, argon-argon dating and helium or xenon isotope analysis. Noble gas mass spectrometry typically uses a static gas mass spectrometer, in which a gaseous sample containing a noble gas or gases of interest is introduced into a mass spectrometer and then left in the spectrometer without pumping during mass analysis. A typical feature of static mass spectrometers is therefore that they remain evacuated during analysis. Static mass spectrometers are used when a high degree of sensitivity is required. The analysis is typically performed to detect the presence of minute amounts of noble gases (He, Ne, Ar, Kr, Xe), although static mass spectrometers can also be capable of analyzing other gases, such as CO2 or N2.Examples of such instruments are the Helix™ and Argus™ instruments from Thermo Scientific™.

[0003] Specifically, in noble gas isotope ratio mass spectrometry, the sample gas is prepared, typically from a solid sample in a sample preparation gas line, which may be connected, for example, to a heating device such as a high-temperature furnace or a laser heater, to release small amounts of the sample gas trapped in a solid sample, such as a small crystal or mineral(s), by sample heating. The sample gas comprises one or more noble gases to be analyzed. In other cases, the noble gas can also be obtained directly from a gas sample and / or purified, for example, from a gas sample taken from air.

[0004] One or more traps, such as cold traps and / or chemical scavengers, such as getter pumps, are used in the preparation line for sample gas purification. The traps and / or scavengers act to remove active gases from the sample gas, leaving the noble gases for analysis. During sample preparation, typically small amounts of noble gases (for example, approximately 1 µL at standard pressure or less) are released from the solid sample and purified with the chemical scavengers and cold traps before the gas is introduced into the evacuated mass spectrometer by opening an inlet valve to the mass spectrometer. The gas can be drawn from a vacuum of approximately 10 -4 mbar in the sample preparation line to a high vacuum or ultra-high vacuum in the mass spectrometer.

[0005] The point in time at which the gas is introduced into the mass spectrometer is defined as "time zero" according to the prior art. Before introducing the gas into the evacuated mass spectrometer, the spectrometer's vacuum pumps are disconnected from the spectrometer so that no gas is pumped out of the mass spectrometer during the analysis period. Thus, the analysis is performed under static vacuum conditions. Static vacuum conditions require a sealed mass spectrometer (preferably sealed to high or ultra-high vacuum) with clean internal surfaces and very low outgassing rates (usually as a result of a bake-out procedure).

[0006] Ionization of the sample gas in the static gas mass spectrometer is typically achieved by electron impact ionization in an ionization volume of a Nier-type ion source. The ionized noble gas species are extracted from the ionization volume by electric fields and accelerated into the mass analyzer, which is typically a magnetic sector mass analyzer but could alternatively be another type, such as a quadrupole mass analyzer or a time-of-flight mass analyzer. A multicollector, for example, comprising a plurality of Faraday cups and / or electron multipliers (usually a combination of the two types), is typically used to detect the ions, particularly with the preferred magnetic sector mass analyzer.

[0007] The invention is applicable, inter alia, to static gas mass spectrometers of the general type described above, and the features described above are hereby incorporated as features with which the invention may be used.

[0008] In the analysis, isotopic abundances and typically one or more isotopic abundance ratios are measured and the measured data are then extrapolated back to "time zero", the time when the gas sample was first introduced into the mass analyzer, to account for consumption and isotopic fractionation due to ionization of the gas during the measurement.

[0009] It is important to record all measured isotope ratios starting at "time zero" in order to derive the precise isotope ratio from the measured data. However, there are problems with the current measurement methods and equipment. The ionizing electron beam in the ionization volume of the electron impact ion source is generated from a hot filament by thermionic electron emission. The ion source conditions must be kept stable over time to avoid any distortion of the measured isotope ratios. For example, changes in the filament temperature during sample measurement would lead to uncontrolled isotope fractionation and affect the accuracy and precision of the measurement. Changes in the filament current during the measurement could influence the space charge conditions in the ionization volume and thus impair the mass discrimination of the ion source.Furthermore, there is an initial equilibration time or period, starting at time zero, until the various isotopes have spatially distributed evenly throughout the mass spectrometer volume. Due to the increased viscosity, this equilibration time can be longest for the heavier noble gases such as xenon, which can take several minutes (e.g., up to 10 minutes) before all isotopic species of the noble gas sample have been fully equilibrated from the sample preparation line into the mass spectrometer volume. For example, equilibration can take approximately 3 minutes for argon or 6 to 7 minutes for xenon. The equilibration time depends on the characteristics of both the instrument and the gas.

[0010] Due to the equilibration time, the measured isotope abundance over time can show a behavior from time zero, t0, as schematically shown by curve 4 (solid line) in Fig. 1. There is typically a rapid increase 6 in the measured isotopic abundance as the isotopes fill the spectrometer after their introduction, followed by a decrease, for example a linear or essentially linear decrease 8 after a time t eq, while the isotopes are gradually consumed. The ionization source itself causes a fractionation of the isotopes and therefore a change in the isotopic ratio over time. Space charge effects and the different kinetics of the lighter compared to the heavier isotopes lead to slightly different transfer and ionization probabilities. Due to preferential ionization of one isotope over another, the isotopic composition of the gas in the evacuated system changes over time and thus the measured isotopic ratio changes over time. To calculate the actual isotopic composition of the sample, it is important to measure all isotopes immediately from the time of sample introduction.Therefore, an extrapolation 7 of the stable, equilibrated, and decaying part of the isotope intensity measurement back to time zero (time of gas introduction) is required to calculate an isotopic ratio of the gas at time zero. A curve fit is performed to extrapolate the isotopic intensity to time zero. According to the state of the art, isotope measurements typically only start after the equilibration phase, i.e., in the stable phase with linear behavior, even if the gas has been subjected to ionization since it was first introduced into the spectrometer. For example, measurements for an argon sample, as shown schematically in . Fig. 2, typically not before 200 seconds have passed since the introduction of the gas and linear behavior has been observed. Typically, the intensity, i.e., abundance, of the isotopes, not the ratio, is measured over time. A best fit of the measured ion beam intensity is made and extrapolated back to time zero for each isotope. An isotope ratio is then calculated at time zero from the ratio of the time zero intensities of the two isotopes.

[0011] As mentioned, the period before the observed linear decrease in isotopic abundance begins corresponds to the equilibration time and is most pronounced for heavier noble gases, e.g., argon to xenon. In state-of-the-art systems, to keep the ion source conditions stable, ionization of the sample gas begins from the moment the sample is introduced into the mass spectrometer vacuum. However, this means that the isotopes are consumed in an uncontrolled and unknown manner, so that the isotopic abundance ratios are perturbed by the time the gas is equilibrated (the initial isotopic ratio measured during the equilibration time would be inconsistent with the measured isotopic ratios during the equilibrated period). Furthermore, the ionization and consumption of the gas during the equilibration phase are another source of isotopic fractionation.This major limitation is currently the reason for one of the largest uncertainties in noble gas isotope ratio mass spectrometry.

[0012] The invention aims to address this problem, among others.

[0013] US 3,493,742 A discloses a time-of-flight mass spectrometer that is automatically controlled by voltage drivers, which in turn are controlled by a staircase voltage from a frequency divider according to desired conditions.

[0014] US 4 023 038 A describes a system that uses multiple upper excitation levels in an isotope-selective ionization technique to improve ionization efficiency.

[0015] WO 2007 / 115423 A1 describes a device for the online determination of the isotopic composition of non-exchangeable stable hydrogen in a substance sample. The device comprises an equilibration chamber with a sample compartment, a detection device with a pyrolysis chamber and a continuous-flow mass spectrometer connected thereto, means for loading the sample compartment with a substance sample, means for the controlled supply of a rinsing fluid and a hydrogen exchange fluid into the sample compartment if it contains a substance sample, and means for transferring a substance sample from the sample compartment to the detection device under conditions that are impermeable to the medium. Summary

[0016] The invention involves lowering the electron energy (i.e., electron impact energy) of the electron impact ionization source or maintaining the energy at a low level during an initial equilibration period of the sample gas into the mass spectrometer. The reduced electron energy ensures that no sample gas is ionized during the initial sample equilibration phase. Once the initial equilibration phase is complete, the energy of the ionizing electron beam can be increased so that the sample gas is ionized. The electron energy can be lowered or maintained below the ionization potential of the sample gas and subsequently increased to or above the ionization potential of the sample gas.

[0017] According to one aspect of the invention, a method of static gas mass spectrometry is provided, comprising the following steps: Introducing a sample gas containing two or more isotopes to be analyzed into a static vacuum mass spectrometer at a time t0 ; 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 following t0 until a time t1, wherein the first period from t0 to t1 is based on an equilibration period for equilibrating the sample gas, the equilibration period being determined by a previous measurement of an isotope ratio of the sample gas against the time following sample gas introduction; and Operating the electron impact ionization source with a second electron energy at least equal to the ionization potential of the sample gas for a second period of time after time t1; wherein the isotope ratio mass analysis measurements are taken during the second period but not during the first period. The second period generally begins immediately after time t1.

[0018] The method comprises the step of mass analysis of two or more isotopes (for example, to determine their isotopic ratio). The mass analysis generally begins after the first period, since ionization of the sample gas is prevented during the first period. Thus, the mass analysis preferably begins with the second period. The mass analysis is performed during the second period.

[0019] The invention also provides a static gas mass spectrometer for carrying out the method.

[0020] In another aspect, the invention provides a static gas mass spectrometer comprising: an electron impact ionization source for receiving a sample gas to be ionized, which contains two or more isotopes, a controller for regulating the electron impact ionization source, a mass analyzer, in particular a magnetic sector mass analyzer, but alternatively a quadrupole or TOF mass analyzer, for analyzing the mass of the generated ions, an ion detector, in particular a multi-collector (especially when a magnetic sector mass analyzer is used), but alternatively a single collector, for detecting ions that have been analyzed for their mass, and at least one pump for generating a vacuum in the mass spectrometer (ie a vacuum in the ion source and / or in the mass analyzer and / or ion detector, preferably in all of them), which pump can be separated from the mass spectrometer before a sample gas is taken up from the ionization source, in order to thereby provide a static vacuum in the spectrometer, wherein the mass spectrometer is adapted to carry out the method according to the invention.

[0021] The ion source generates ions from a sample gas supplied to the ion source, in particular when the electron energy of the electron impact ionization source is at least as high as the ionization potential of the gas. The mass analyzer then analyzes the ions generated in the ion source, and the ion detector detects the ions analyzed for their mass. The ion source can be configured as an electron impact ionization source that can be operated with a first electron energy below the ionization potential of the sample gas for a first period following the sample gas introduction into the ion source t0 up to a time t1; and that can be operated with a second electron energy at least equal to the ionization potential of the sample gas for a second period after time t1. The electron energy can be kept at a low level from a time before the gas is introduced (i.e.it is already low when the gas is introduced).

[0022] The vacuum in the mass spectrometer is preferably at least a high vacuum (for example with a pressure of 1×10 -7 mbar or less, or a pressure of 1×10 -8 mbar or less). The vacuum can be an ultra-high vacuum (for example, with a pressure of 1×10 -9 mbar or less).

[0023] A controller is preferably provided to control the electron energy of the ion source. The controller is preferably used to control the electron energy according to the method of the invention. The controller may comprise a computer, which can, for example, execute firmware or software to control the electron energy. Preferably, the controller comprises a computer that controls the electron energy (including the duration for which the energy is applied) based on the gas type input. The controller may comprise electronics for varying an electron extraction voltage of the ion source and thus the electron energy. The controller's computer preferably controls the electronics, for example, based on an input of data defining the gas type, e.g., the noble gas species, and / or the first and second time periods (such as the start and end of the time periods).The input data can be the time t0 of gas introduction and the time t1, for example, for increasing the electron energy and starting the mass analysis. The input data can come from user input or software.

[0024] The method is preferably a method for determining at least one isotope ratio of the sample gas, ie an isotope ratio mass spectrometry (IRMS) method.

[0025] The sample gas is preferably a noble gas, for example, He, Ne, Ar, Kr, Xe, Rn, especially Ar, Kr, Xe, which have significant equilibration times. However, the sample gas may be another gas that can be isotopically analyzed in the mass analyzer, such as CO2 or N2. Other gases may be present with the sample gas to be analyzed, but preferably the sample gas to be analyzed (e.g., noble gas) is pure of other gases.

[0026] The first period from t0 to t1 is preferably a period that allows the isotopes of the sample gas to equilibrate (i.e., reach equilibrium) in the mass spectrometer. Accordingly, the first period from t0 to t1 is a period that corresponds to a period of time that isotopes of the sample gas need to equilibrate in the mass spectrometer. Thus, time t1 preferably occurs after the equilibration of the isotopes of the sample gas. Equilibration refers to the spatial (geometric) equilibration of the sample gas isotopes within the vacuum space of the mass spectrometer. The period from t0 to t1 is preferably at least equal to the time required for the sample gas to equilibrate. The equilibration time depends on the gas type, in particular its viscosity. Heavier gases tend to have a higher viscosity than lighter ones and consequently require longer equilibration times.The period from t0 to t1 is typically not significantly longer than the time required for the sample gas to equilibrate. The equilibration period from t0 to t1 can be 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, depending on the masses of the isotopes. In some embodiments, the sample gas may be introduced into the ion source and spectrometer at an electron impact ionization energy above the ionization potential of the gas, and the ion (isotope) intensities may be tracked over time to determine the time at which the intensities follow a constant fitting model with a generally stable negative slope (decreasing intensity).When the sample is introduced, the measured ion abundance intensity initially increases (positive slope) and then reaches a maximum (zero slope) before beginning a negative slope with a stable or uniform signal decay. Stability can be assumed to have been achieved due to sufficient gas equilibration when the fitting function closely matches the decay curve. Once this equilibration time has been determined, it can be used as the initial waiting time (first period) for actual sample measurements, when the reduced electron energy is applied before the electron energy is adjusted to the ionization mode.

[0027] The equilibration period is determined by a prior measurement of an isotope ratio (IR) of the sample gas over time (IR vs. time plot) after sample gas introduction, i.e., from time t0. In such embodiments, the time at which the isotope ratio begins to decrease uniformly, e.g., in a linear or substantially linear manner, can be defined as the end of the equilibration period and thus used to establish t1. A corresponding equilibration time can therefore be known from previous measurements for each gas species, e.g., noble gas, and used to establish t1. The isotope ratio measurements by the mass spectrometer can be taken from t1, after the electron energy of the ion source has been increased above the ionization potential.

[0028] The electron energy of the electron impact ion source can be controlled by varying the extraction voltage applied to a heated filament of the ion source. Therefore, the extraction voltage is preferably variable. The extraction voltage can be applied using an extraction voltage electrode.

[0029] The first electron energy of the ion source below the ionization potential of the sample gas can be at least 2 eV, or at least 4 eV, or at least 6 eV, or at least 8 eV, or at least 10 eV, or at least 12 eV lower than the ionization potential. For example, the first electron energy below the ionization potential can be 10 eV or approximately 10 eV (which can be compared to the ionization potentials of noble gases: He (24.6 eV), Ne (21.6 eV), Ar (15.8 eV), Kr (14 eV), and Xe (12.1 eV)). Preferably, the first electron energy is below the ionization potential of a constituent of the sample gas whose mass is to be analyzed. The second electron energy of the ion source is at least as high as, and preferably higher than, the ionization potential of the sample gas.The second electron energy may be at least 2 times, or at least 3 times, or at least 4 times, or at least 5 times the ionization potential. The second electron energy of the ion source above the ionization potential of the sample gas may be at least 10 eV, or at least 20 eV, or at least 30 eV, or at least 40 eV, or at least 50 eV, or at least 60 eV, or at least 70 eV higher than the ionization potential. For example, the second electron energy may be 80 eV or approximately 80 eV (which can be compared to the aforementioned ionization potentials of the noble gases). The second electron energy may, with general preference, be at least 2 times, or at least 3 times, or at least 4 times, or at least 5 times, or at least 6 times, or at least 7 times, or at least 8 times, or at least 9 times, or at least 10 times the first electron energy.These ranges can be combined. For example, the second electron energy can be at least 20 eV higher than the ionization potential of the sample gas and at least 2 times the first electron energy. The second electron energy can be at least 30 eV higher than the ionization potential of the sample gas and at least 3 times the first electron energy. The second electron energy can be at least 40 eV higher than the ionization potential of the sample gas and at least 4 times the first electron energy. The second electron energy can be at least 50 eV higher than the ionization potential of the sample gas and at least 5 times the first electron energy. The electron emission current from the filament (ionization current) is preferably on the order of several hundred µA (e.g., 100-500 µA or 200-400 µA).

[0030] The filament heating current (typically several amperes, for example, in the range of 2 A to 5 A) for the filament of the electron impact ionization source can be kept the same or substantially the same during the first period, before t1 (when the first electron energy is applied), and during the second period after time t1 (when the second electron energy is applied). A constant filament current can ensure that a substantially constant filament temperature and ion source temperature are maintained. However, in some embodiments, changing the electron energy can have a small temperature-changing effect on the filament. When the electron energy is reduced, the electrons are not accelerated as far from the filament, and the filament can become hotter. To compensate for this, it is preferable to adjust the filament heating current.In this case, the filament current can be adjustable and changed to maintain a substantially constant filament temperature and therefore ion source temperature. Thus, when the electron impact energy changes, the filament heating current is preferably also changed to keep the temperature within the ion source region stable. Therefore, the invention preferably comprises simultaneously changing electron energies and filament heating currents to keep the filament temperature substantially constant. The invention thus aims to achieve both a consistently constant ion source temperature and a reduced electron energy during the equilibration phase. A preferred feature of the invention is therefore the control of a filament heating current of a filament of the electron impact ionization source to keep the temperature of the ionization source substantially constant during the first period and the second period.

[0031] A temperature monitor, such as a pyrometer, can be provided in or directly adjacent to (or in the vicinity of) the ionization source to measure the temperature of the filament and provide a feedback signal to control the filament current (for example, by means of a controller) so that a substantially constant filament temperature is maintained throughout, i.e., during the first and second time periods. Preferably, a change in the filament current can be calibrated with a change in electron energy in this way, e.g., by means of a pyrometer.

[0032] The reduced electron energy in the first period from t0 to t1 can ensure that no sample gas is ionized during the initial sample equilibration phase. Once the initial equilibration phase is complete, the energy of the ionizing electron beam is increased to a typical level for the ionization of sample gases in a static gas mass spectrometer, e.g., at least 50 eV, or at least 60 eV, or at least 70 eV, such as approximately 80 eV for noble gases. The second, higher energy level can ensure high ionization yields within the ion source.

[0033] It can be seen that the invention therefore offers improvements related to sample introduction and sample measurement in noble gas isotope ratio mass spectrometry running under static vacuum conditions. Advantageously, the invention addresses the problem of sample gas consumption and / or isotope fractionation during the initial equilibration phase after sample gas introduction into the static mass spectrometer, which previously compromised the accuracy of isotope ratio measurements. Consequently, the invention can eliminate a significant uncertainty in the calculation of isotope ratios from the measured data set.While isotope fractionation and gas consumption can be corrected during the equilibration time, a further advantage of the invention is that it allows for a significant reduction in the volume of the gas conditioning system and, importantly, increases the effective sensitivity without the usual concerns about gas conductivity. The conditioning volume could therefore be made very small, as the length of the equilibration time is no longer such a significant problem.

[0034] The isotope ratio mass analysis measurements are taken during the second period, rather than the first period, preferably once the gas isotopes in the ion source have equilibrated and preferably once the electron energy is increased above the gas ionization potential. The isotope ratio mass analysis measurements preferably begin at or after time t1. Preferably, for each of two or more isotopes, the intensity, i.e., abundance, of the isotope is measured over time. A best fit, such as a linear interpolation, of each measured isotope intensity with time may be made and extrapolated back to time zero, i.e., the time at which the second electron energy is increased to at least the ionization potential of the sample gas.In the present invention, time zero thus means the time at which the electron energy is set to the ionization mode, i.e., t1. Time zero is therefore the time at which ionization begins to consume sample gas or change the isotopic abundances of the sample gas, which in the prior art occurred when the gas was introduced into the spectrometer (t0), but which in the invention occurs at t1, when ionization begins. The ratio of the extrapolated time zero isotopic intensities determines the isotopic ratio value of the sample gas.In a variation of this method, an isotopic ratio of two isotopes could be calculated for each time point at which the individual isotopic abundances are measured; this could provide a variety of isotopic ratios over time, which can be fitted by a best-fit line extrapolated to time zero, when ionization begins (in this case, t1), to determine the (exact) isotopic ratio. The invention enables a workflow for introducing a sample gas into a static gas mass spectrometer, particularly for noble gases. The ion source temperature conditions can be kept stable throughout, while a reduced electron beam energy below the initial ionization energy of the sample gas is applied during the initial equilibration time of the sample into the mass spectrometer.Sample gas consumption during the initial equilibration phase is thus avoided and no longer represents a limitation for isotope ratio measurements with higher precision, especially for noble gases. Short description of the drawings Fig. Figure 1 shows schematically an isotope abundance measured according to the previous state of the art over time from a point in time zero. Fig. Figure 2 shows a schematic of a state-of-the-art measurement of isotopic abundance over time for an argon sample. Fig. 3 schematically shows a configuration of a static mass spectrometer according to an embodiment. Fig. 4 shows schematically an arrangement of a static mass spectrometer according to another embodiment. Fig. 5 shows schematically an arrangement of an electron impact ionization source according to another embodiment. Fig. Figure 6 schematically shows a measured isotope abundance according to one embodiment, wherein measurements are taken from time t1 after an isotope equilibration phase. Fig. Figure 7 schematically shows a measured isotope ratio according to one embodiment, wherein measurements are taken from time t1 after an isotope equilibration phase. Description of the preferred embodiments

[0035] 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.

[0036] With reference to Fig. Figure 3 schematically illustrates a typical configuration of a static mass spectrometer 200 that can be used in the invention, comprising: a sample preparation area 205, a transfer area 230, an ion source area 240, and a mass analyzer 250. The sample preparation area 205 includes a chamber 210 (such as a furnace or a laser irradiated chamber) and an optional preparation bench 220. Valves 215 are provided between each of the furnace 210, sample preparation bench 220, transfer area 230, and ion source area 240.

[0037] The inlet into the static mass spectrometer 200 is indirect, via an intermediate chamber. A typical application is the determination of the isotopic ratios of various isotopes of a noble gas enclosed in a sample, such as a piece of rock or the like.

[0038] In current instruments, the sample, typically a piece of rock, is placed in a chamber (such as furnace 210) and then heated, possibly with a laser. This treatment releases trapped gases containing the desired analytes. The released gases are transferred to the sample preparation bench 220, where they can be processed in various ways. For example, they can be partially or completely transferred into storage volumes ("pipettes") and then partially released, yielding a smaller sample volume at a lower pressure.

[0039] In other cases, the gas can also be a direct gas sample, for example a gas sample taken from the air.

[0040] The gas is then transferred to the transfer area 230, which can act as a purification unit. In older devices, the sample gas was collected on a cold finger. The gases could then be thawed to "distill" the gases, releasing them sequentially. More modern devices include a general type of installed "trap," typically chemical scavengers and optionally cold traps to remove unwanted substances (this usually means everything except noble gases). The vacuum pump or pumps (not shown) to the mass spectrometer (i.e., the ion source and mass analyzer) are shut off by valves (not shown) before the sample is released into the chambers (240, 250).

[0041] From here, the gas is equilibrated with the ion source region 240, where the gas is subsequently ionized after equilibration (by electron ionization), and the generated ions are then analyzed in the mass analyzer 250.

[0042] In the ion source 240, the gas is typically ionized by electron bombardment (electron impact ionization). Due to the statistical distribution of the gas to be analyzed in the mass spectrometer, there are only a small number of molecules in the region of the ion source. This results in only a small ion current and thus the need for high detection sensitivity.

[0043] Typical pressures in the ion source range 240 and mass analyzer 250 are 10 -9 up to 10 -10 mbar before the sample is admitted, and then 10 -6 up to 10 -7 mbar (or 10 -7 up to 10 -9mbar), depending on the sample volume (which cannot always be predicted). The gas to be analyzed spreads throughout the ion source region 240 and the mass analyzer 250, with a small number of molecules also entering the ion source. In the mass analyzer 250, the generated ions travel from the ion source along a flight path, such as a flight tube 255, before being detected in the detector region 260.

[0044] The deep vacuum and the removal of "unwanted" gases from the sample are important to improve the signal-to-noise ratio (that is, the number of ions from the sample versus the number of ions from other gases remaining from a previous measurement or other "noise", such as isobaric ions like hydrocarbons).

[0045] With reference now to Fig. Figure 4 shows a schematic arrangement with further details of a static mass spectrometer according to the invention. The overall arrangement of the static mass spectrometer of Fig. 4 does not differ significantly from that in Fig. 3. The static mass spectrometer 1 comprises: an electron impact ion source 30; a flight tube 110; a magnetic sector mass analyzer 130; a detector housing 140; a multicollector detector assembly 150; and electronics 160. A vacuum pump 180 is coupled to the ion source assembly 30 via an automatic valve 170. A sample preparation area and a gas transfer area, as described with reference to Fig. 3 is not shown in this drawing, but would typically be included. Additionally, another vacuum pump (not shown) is connected to the detector housing 140 via a valve (also not shown).

[0046] The detector assembly 150 is depicted as a multi-collector device comprising a plurality of collectors for ion detection. This could be at least one Faraday cup, at least one ion counter, or a combination of these, as described, for example, in WO 2012 / 007559 A2, which is commonly assigned. Three collectors are shown in Fig. 4, but a preferred embodiment has five collectors, and embodiments with more collectors are also contemplated. Electronics 160 may include electronics and / or a computer of a detection system for data acquisition, storage, and / or processing. Furthermore, electronics 160 may include a controller that further includes ion source control, valve control, pump control, etc.

[0047] With subsequent reference to the ion source, Fig. 5 schematically shows the arrangement of the electron impact ionization source 30 of Fig. 4 for use in the invention. The electron impact ion source 30 is of the Nier type. A neutral sample gas is admitted to the ionization chamber 35, which is generally maintained at a high voltage (e.g., 3-5 kV). Electrons are generated by thermionic emission from a filament 40 heated by passing a heating current through it, and the electrons are accelerated by an extraction voltage (e.g., 10-100 V) applied to the collecting electrode 50. Magnets 45 cause the electrons to follow a spiral path through the chamber. Provided they have sufficient energy, the electrons ionize the gas, and the gas ions are extracted by the high voltage applied to the repeller 55 and chamber 35. The ion beam is generated by the extraction slit 60 and can be steered and / or focused by the focusing electrodes 65.The ion source 30 is controlled by a controller which is part of the electronics 160, as schematically shown by line 165 in . Fig. 4 shown.

[0048] In use, once the noble gas sample is introduced into the ion source 30 from the gas preparation and transfer section at a time t0, an initial equilibration period of the gas isotopes in the mass spectrometer follows. 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 ionize noble gases, for example, xenon, to 10 electron volts. This lower electron energy level is below the ionization potential of the noble gas being analyzed. The reduced electron energy ensures that no sample gas is ionized during the initial sample equilibration phase.The energy can be kept low at all times except during the ionization and mass analysis period (second period), so that it is already at a low level before the gas is introduced into the spectrometer. After an initial period starting at t0, the controller increases the electron energy at time t1, so that the energy of the ionizing electron beam is reset to the usual high level, such as ~80 eV, to ensure a high ionization yield of the noble gas within the ion source.

[0049] By inputting the noble gas type to be detected during mass analysis into the controller, which may comprise a computer, the controller can select and adjust both the equilibration time period (the first period from t0 to t1) and, optionally, the first (lower) and second (higher) electron energies. The duration of the first period to be set by the controller for each sample gas species can be determined from a previous measurement of the isotope intensity over time, from which the equilibration time can be found. The first (lower) and second (higher) electron energies can, in some embodiments, be set to values ​​that are applicable to 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 less, or 10 eV or less for the first electron energy and at least 50 eV, 60 eV, 70 eV, or 80 eV for the second electron energy.

[0050] The filament heating current is kept constant throughout, meaning it remains the same during 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 to avoid distortion of the measured isotope ratios. Any change in the filament temperature during sample measurement can lead to uncontrolled isotope fractionation and affect the accuracy and precision of the measurement.To better ensure that the filament temperature, and consequently the temperature in the source, remains substantially constant, for example, in the face of changes in electron energy, a pyrometer 70 may be provided adjacent to 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 that a substantially constant filament temperature is maintained. A change in the filament current with a change in electron energy can be calibrated in this way.

[0051] From the above, it is clear that the invention addresses the problem of sample consumption during the equilibration phase of sample gas introduction into the static gas mass spectrometer, which currently represents a significant limitation for high-precision isotope ratio measurements of heavier noble gases. According to the invention, during the first equilibration phase, the electron energy is kept below the first ionization potential of the gas, but all other ion source parameters remain essentially unchanged compared to the subsequent phase after the first equilibration phase. After the first equilibration phase is over, the electron energy is increased to achieve the required high ionization yield (while all other ion source parameters remain essentially unchanged as explained).The reduced electron energy avoids ionization of the sample gas during the initial equilibration phase and thus avoids consuming sample gas, which would result in the preferential consumption of some isotopes over others. Such a workflow can help avoid a bias in the measured isotope ratios that traditionally occurs during the initial equilibration phase. The ion source temperature conditions can be kept stable throughout, while only the energy of the ionizing electron beam is reduced to below the initial ionization energy of the gas during the initial equilibration time of the sample into the mass spectrometer.

[0052] With reference to Fig. 6, which schematically illustrates a measured isotopic abundance 28 of a noble gas according to an embodiment of the invention, the isotope ratio mass analysis measurements begin at or after time t1 (i.e., when the second period begins), once the gas isotopes have equilibrated in the ion source and once the electron energy is increased above the gas ionization potential. The graph of Fig. Figure 6 shows the intensity, i.e., abundance, of an isotope over time. Two or more different isotopes are measured together. The curves of the different isotopes are slightly different due to changing mass measurement error and changing gas composition resulting from slightly different ionization probabilities of the different isotopes. A best fit, such as linear interpolation, of the measured ion beam intensity is made and extrapolated back to time zero for each isotope. An isotopic ratio is then calculated from the ratio of the time zero intensities of the two isotopes. A ratio of any two isotopes, usually of the same element, can be obtained in this way.The problem with the prior art is that during the equilibration time immediately following gas introduction into the ion source and spectrometer, the sample gas is already consumed in an uncontrolled manner, and heavier and lighter isotopes are ionized in an uncontrolled manner. Moreover, this uncontrolled fractionation and ionization during the initial equilibration time window leads to a change in the isotopic composition of the residual gas, which represents a major limitation for high-precision isotope ratio measurements of gases. Time zero extrapolation back to the time at which the gas is introduced, according to the prior art, cannot compensate for this. In the invention, since no gas ionization or ionization takes place before time t1 in the equilibration phase,no gas consumption has occurred, the time zero for the present invention is in fact t1, and the isotope ratio calculated from the measurements or fit curves of the isotope intensities at that time zero will represent a more accurate measurement than the prior art method in which ionization occurs from the moment the gas is introduced into the ion source.

[0053] In the present invention, time zero means the time at which the electron energy is set to the ionization mode, i.e., t1. Time zero is the time at which ionization begins to consume or change the isotopic abundances of the sample gas, which in the prior art was when the gas was introduced into the spectrometer (t0), but which in the invention is t1 when ionization begins. In other words, the isotopic abundance of the corresponding isotopes 25 at time t1 can be used to calculate the accurate isotopic ratio of the gas. A single measurement at time t1, however, could be prone to error. In practice, due to limitations in measurement accuracy, it is better that several measurements are taken from time t1 and plotted against time so that a best fit line can be drawn through them.Subsequently, the value of the fitted line at time zero of t1 can provide the intensity for calculating the isotope ratio.

[0054] It is understood that in most embodiments, the individual isotope abundances are each measured and fitted using an extrapolated best-fit line, from which an (exact) isotope ratio is calculated from the extrapolated line at time zero (in this case, t1). In other embodiments, however, the isotope ratio could instead be calculated for each time point at which the individual isotope abundances are measured, thereby providing a plurality of isotope ratios over time, which can be fitted using a best-fit line extrapolated to time zero (in this case, t1) to determine the (exact) isotope ratio. This is Fig. 7 shown schematically.

[0055] It is understood that changes may be made to the above embodiments of the invention, but these changes still fall within the scope of the invention. Any feature disclosed in the specification may, unless otherwise stated, be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless otherwise stated, each disclosed feature represents an example of a generic series of equivalent or similar features.

[0056] The use of any and all examples or exemplary language provided herein ("for example," "such as," "for example," and the like) is intended only to better illustrate the invention and is not intended to be a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed to indicate any unclaimed element as essential to the practice of the invention.

[0057] As used in this document, including the claims, singular forms of terms used in this document should be construed to include the plural form and vice versa, unless the context indicates otherwise. For example, unless the context indicates otherwise, a singular reference used in this document, including in the claims, such as "a" or "an" means "one or more."

[0058] Throughout the description and claims of this specification, the words "comprise", "including", "having" and "containing" and variations of the words, for example, "comprising" and "comprises", etc., mean "including, without limitation", and are not intended to exclude (and do not exclude) other components.

[0059] All steps described in this specification may be performed in any order or simultaneously, unless otherwise specified or the context requires otherwise.

Claims

[1] Method of static gas mass spectrometry, comprising the following steps: Introducing a sample gas containing two or more isotopes to be analyzed into a static vacuum mass spectrometer at a time t0; 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 following t0 until a time t1, wherein the first period from t0 to t1 is based on an equilibration period for equilibrating the sample gas, the equilibration period being determined by a previous measurement of an isotope ratio of the sample gas against the time following sample gas introduction; and Operating the electron impact ionization source with a second electron energy at least equal to the ionization potential of the sample gas for a second period of time after time t1; wherein the isotope ratio mass analysis measurements are taken 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 to maintain the temperature of the ionization source substantially the same during the first period and the second period. [3] The method of claim 1 or 2, further comprising the step of analyzing the two or more isotopes for mass in the mass spectrometer beginning with the second period. [4] The method of claim 3, wherein the mass analysis comprises, for each of the two or more isotopes, measuring the intensity of the isotope over time, performing a fit calculation of each measured isotopic intensity with time, extrapolating each fit calculation to a time zero at which the second electron energy is increased to at least the level of the ionization potential of the sample gas, and calculating a ratio of the extrapolated time zero isotopic intensities of two isotopes to yield an isotopic ratio of the sample gas. [5] A method according to any one of the preceding claims, wherein the sample gas is a noble gas. [6] A method according to any one of the preceding claims, wherein the first period from t0 to t1 is at least equal to the equilibration period required for the sample gas to equilibrate. [7] A method according to any one of the preceding claims, wherein the first period of time 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. [8] A method according to any one of the preceding claims, wherein the first electron energy of the ionization source is at least 2 eV, 4 eV, 6 eV, 8 eV, 10 eV or 12 eV lower than the ionization potential. [9] A method according to any one of the preceding claims, wherein the first electron energy of the ionization source is 10 eV. [10] A method according to any one of the preceding claims, wherein the second electron energy of the ionization source is at least 10 eV, or 20 eV, or 30 eV, or 40 eV, or 50 eV, or 60 eV, or 70 eV higher than the ionization potential of the sample gas. [11] A method according to any one of the preceding claims, wherein the second electron energy of the ionization source is 80 eV. [12] A method according to any one of the preceding claims, wherein the second electron energy is at least 2 times, or 3 times, or 4 times, or 5 times, or 6 times, or 7 times, or 8 times, or 9 times, or 10 times the first electron energy. [13] A static gas mass spectrometer comprising: Electron impact ionization source for receiving a sample gas containing two or more isotopes and for ionizing the sample gas, Controller for regulating the electron impact ionization source, Mass analyzer for mass analysis of the generated ions, Ion detector for detecting the ions that have been analyzed for their mass, and at least one pump for generating a vacuum in the mass spectrometer, which can be separated from the mass spectrometer before a sample gas is taken from the ionization source, wherein the mass spectrometer is designed to carry out the method according to one of claims 1 to 12. [14] A statistical gas mass spectrometer according to claim 13, wherein the controller regulates the electron energy of the ionization source and sets the first period from t0 to t1 based on the previous determination of an equilibration period required for isotopes of the sample gas to equilibrate in the mass spectrometer. [15] A statistical gas mass spectrometer according to claim 13 or 14, wherein the controller is arranged to regulate the electron energy of the ionization source according to the method according to any one of claims 1 to 12. [16] A static gas mass spectrometer according to any one of claims 13 to 15, wherein the vacuum is an ultra-high vacuum, the mass analyzer is a magnetic sector mass analyzer and the ion detector is a multi-collector. [17] A static gas mass spectrometer according to claims 13 to 16, further comprising a temperature monitor for measuring the temperature of a filament of the electron impact ionization source and providing a feedback signal for regulating a filament current supplied to the filament to maintain a substantially constant filament temperature during the first and second time periods.

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