Inductively coupled plasma source mass spectrometry for silicon measurement

Abstract

A method for measuring a sample comprising silicon by mass spectrometry is implemented from an inductively coupled plasma-tandem mass spectrometer, or ICP-MS/MS. The measurement method comprises a step of measuring by mass spectrometry by a reactive gas. The reactive gas comprising nitrous oxide.

Claims

1. A method for measuring a sample comprising silicon by mass spectrometry, the method being implemented from an inductively coupled plasma-tandem mass spectrometer, or ICP-MS/MS, the method comprising a step of measuring by mass spectrometry by means of a reactive gas, wherein the reactive gas comprises nitrous oxide.

2. The method according to claim 1, wherein the reactive gas consists of nitrous oxide.

3. The method according to claim 1, wherein the mass spectrometric measurement step allows determining a value relating to an amount of a silicon-28 isotope.

4. The method according to claim 3, wherein the value relating to an amount of the silicon-28 isotope is a molar proportion of the silicon-28 isotope relative to all silicon atoms.

5. The method according to claim 1, wherein the sample has a molar proportion of the silicon-28 isotope relative to all silicon atoms which is greater than 99%.

6. The method according to claim 1, wherein during the mass spectrometric measurement step, the reactive gas is introduced into a reaction cell of the mass spectrometer at a flow rate comprised between 0.03 and 0.28 mL.min.sup.-1 and preferably comprised between 0.06 and 0.15 mL.min.sup.-1.

7. A use of a reactive gas comprising nitrous oxide for a mass spectrometric measurement of a sample comprising silicon from an inductively coupled plasma-tandem mass spectrometer, or ICP-MS/MS.

8. The use of a reactive gas according to claim 7, wherein the reactive gas consists of nitrous oxide.

9. The use of a reactive gas according to claim 7, wherein the sample has a molar proportion of the silicon-28 isotope relative to all silicon atoms which is greater than 99%.

10. The use of a reactive gas according to claim 8, wherein the sample has a molar proportion of the silicon-28 isotope relative to all silicon atoms which is greater than 99%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be better understood on reading the description of exemplary embodiments, given for purely illustrative and non-limiting purposes, with reference to the appended drawings in which:

[0026] FIG. 1 schematically illustrates the different stages of a mass spectrometer as used in the scope of implementation of the invention;

[0027] FIG. 2A graphically illustrates the reactivity profile for the silicon-28 isotope depending on the dioxygen flow rate introduced into a reaction cell of the spectrometer as a reactive gas for a reference silicon sample;

[0028] FIG. 2B graphically illustrates the reactivity profile for the silicon-28 isotope depending on the nitrous oxide flow rate introduced into the reaction cell of the spectrometer as a reactive gas for this same reference silicon sample;

[0029] FIG. 3 graphically illustrates the sensitivity obtained for dioxygen and nitrous oxide, respectively, for the different silicon isotopes;

[0030] Identical, similar or equivalent portions of the different figures bear the same reference numerals so as to facilitate the passage from one figure to the other.

[0031] The different portions represented in the figures are not necessarily represented according to a uniform scale, to make the figures more readable.

[0032] The different possibilities (variants and embodiments) should be understood as not being mutually exclusive and can be combined with each other.

DESCRIPTION OF EMBODIMENTS

[0033] As described below in connection with FIG. 1, the inventors have discovered that the inductively coupled plasma-tandem mass spectrometer 1, or ICP-MS/MS when it is implemented with nitrous oxide as a reactive gas, allows obtaining a measurement of a sample comprising silicon with an improved sensitivity relative to measurements obtained according to the prior art.

[0034] As a reminder, an inductively coupled plasma-tandem mass spectrometer 1 comprises, as illustrated in FIG. 1 the following elements: [0035] a plasma generator system 10 capable of atomising and ionising the species of the sample E into a plasma, [0036] ion optics 20 in order to focus the ions created during the ionisation of the atoms of the sample, [0037] a first electromagnetic filter 31, such as a quadrupole analyser, in order to carry out a first filtering among the ions of the plasma after these have been focused by the ion optics 20, the first filter allowing selecting the ions corresponding to the target atom and the interferents thereof, [0038] a reaction cell 40 in order to cause the ions selected during the filtering performed by the first quadrupole filter 30 to react with a reactive gas and thus produce ionised molecules which form part of the product molecules formed by reaction of the target atom or molecule ionised with the reactive gas, [0039] a second electromagnetic filter 32, such as a quadrupole analyser, in order to carry out a second filtering from the ionised molecules, the second filter allowing selecting only the product molecules, which therefore comprise the target atom, formed in the reaction cell, [0040] a detector capable of intercepting the product molecules after the second filtering carried out by the second filter and providing a signal relating to said interceptions.

[0041] It will be noted that the term “the target atom or molecule” means, herein, and in the rest of this document, the isotope or the molecule comprising the isotope which is the target of the measurement by spectrometry, that is to say which is the object of the quantification in the sample to be measured. In the present example concerning the silicon isotopes, said target atom corresponds in turn to each of the silicon isotopes as described below in connection with FIG. 3.

[0042] Similarly, as indicated above, the term “product molecule” means herein and in the rest of this document, the product molecule obtained during the reaction between the ionised target atom or molecule and the reactive gas. Within the scope of the present invention, namely the measurement of the natural silicon isotopes .sup.28Si, .sup.29Si and .sup.30Si by the use of nitrous oxide N.sub.2O, the product molecules are respectively .sup.28SiO.sub.2.sup.+, .sup.29SiO.sub.2.sup.+ and .sup.30SiO.sub.2.sup.+. It should be noted that for the use of dioxygen O.sub.2, the product molecules are identical.

[0043] During the implementation of a mass spectrometric measurement with an ICP-MS/MS mass spectrometer, the choice of a reactive gas must be adapted to the atoms which will be targeted for the measurement. Thus in the context of silicon isotopes, the reactive gases [2, 3, 4, 5] used in the prior art, whether in the context of ICP-MS/MS mass spectrometry measurement or ICP-QMS mass spectrometry measurement, comprise either ammonia NH.sub.3, or methane CH.sub.4, or dioxygen O.sub.2. More specifically, concerning the ICP-MS/MS mass spectrometry [4, 5], it is dioxygen O.sub.2 which is generally used for the reactive gas. As a result, in this document, the inventors have chosen to compare the results obtained within the scope of the invention, that is to say when the reactive gas is based on nitrous oxide N.sub.2O, with those obtained for a reactive gas based on dioxygen O.sub.2.

[0044] The inventors have discovered, based on the reaction enthalpy calculation (known under the reference ΔH.sub.r), that the reaction of the silicon cations Si+ with nitrous oxide N.sub.2O is significantly exothermic whereas the reaction of silicon cations Si.sup.+ with dioxygen O.sub.2 is endothermic. As a result, the inventors have estimated that the use of a reactive gas based on nitrous oxide N.sub.2O should allow obtaining an optimised reactivity and therefore obtaining an ICP-MS/MS mass spectrometry measurement which is particularly sensitive.

[0045] Indeed, here, in parallel with the reaction enthalpy calculated for the latter, are the reactions likely to be obtained for the silicon isotopes in cationic form in the reaction cell 40 with respectively dioxygen and nitrous oxide N.sub.2O:

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[0046] The reactions with nitrous oxide are therefore favourable and must have a particularly significant yield relative to the reactions with dioxygen. As a result, the inventors have considered that nitrous oxide should allow, in the context of measurement of silicon and the different isotopes thereof by ICP-MS/MS mass spectrometry, reaching a significantly improved sensitivity relative to that obtained with dioxygen.

[0047] In order to verify this, the inventors carried out sensitivity measurements in ICP-MS/MS mass spectrometry from the same silicon sample as a source of silicon-28 isotope .sup.28Si for respectively reactive gases based on dioxygen (oxygen flow rate D.sub.O2 comprised between 0 and 0.78 mL.min.sup.-1), corresponding to the measurements of the prior art, and for reactive gases based on nitrous oxide (nitrous oxide flow rate D.sub.N2O comprised between 0 and 0.28 mL.min.sup.-1). FIG. 2A shows the intensity I(Si.sup.28) of the signal in counts per second obtained for this sample from reactive gas based on dioxygen O.sub.2, this depending on the dioxygen flow rate D.sub.O2. It can be seen on this graph that the maximum is obtained about the 0.19 mL.min.sup.-1 of dioxygen for which about 20,000 counts are obtained.

[0048] FIG. 2B shows the intensity of the signal in counts per second obtained for this sample from reactive gas based on nitrous oxide N.sub.2O this depending on the nitrous oxide flow rate D.sub.N2O. It is considered that for nitrous oxide, the maximum is obtained around 0.09 mL.min.sup.-1 of nitrous oxide for which approximately 132,000 counts are obtained, i.e. more than 6 times the value obtained from dioxygen. It should also be noted that this maximum is obtained for a nitrous oxide N.sub.2O flow rate which is less than that of dioxygen O.sub.2, confirming that nitrous oxide N.sub.2O has a reaction efficiency which is more significant than that of dioxygen O.sub.2.

[0049] Thus, within the scope of the invention, the reactive gas flow rate used for the mass spectrometric measurement is advantageously comprised between 0.03 and 0.28 mL.min.sup.-1 and preferably between 0.06 and 0.15 mL.min.sup.-1.

[0050] Based on this study, the inventors were able to optimise the instrumental parameters in order to maximise the sensitivity for the silicon isotopes .sup.28Si, .sup.29Si and .sup.30Si for the reactive gases based on dioxygen O.sub.2 and based on nitrous oxide N.sub.2O and to estimate the sensitivity and the background equivalent concentration, better known by its acronym BEC. These results are summarised in the following table.

[0051] It will be noted that the present document does not describe such a parameter optimisation. This optimisation is indeed part of the usual practice of the person skilled in the art and being dependent on the apparatus used for the mass spectrometry measurement. The description of such an optimisation therefore has no interest in the present document.

TABLE-US-00002 Reactive gas N.sub.2O O.sub.2 Sensitivity .sup.28SiO.sub.2 (counts ppb-1) 4.0.10.sup.3 4.0.10.sup.2 BEC (ppb Si) 2 3

[0052] It can be seen that nitrous oxide N.sub.2O allows achieving a sensitivity which is almost 10 times greater than that of dioxygen O.sub.2, with a signal to noise ratio reduced by 50%. With a reactive gas based on nitrous oxide N.sub.2O, the inventors have therefore estimated being able to carry out measurements on samples comprising a high proportion in a silicon isotope with an optimised sensitivity. FIG. 3 shows that this improved sensitivity obtained within the framework of such a measurement method applies both for the majority silicon Si isotope, which is the Si-28 isotope .sup.28Si, and for the minority silicon Si isotopes which are the Si-29 isotope .sup.29Si and Si-30 isotope .sup.30Si.

[0053] In order to demonstrate this, the inventors have performed 3 campaigns (indicated as session) of 6 ICP-MS/MS mass spectrometry measurements from a reactive gas based on nitrous oxide N.sub.2O from a silicon sample showing an enrichment in the silicon-28 isotope .sup.28Si. The results of these measurement campaigns are summarised in the following table with, for each of the measurements, an estimate of the proportion of the silicon-28 isotope .sup.28Si (%mol .sup.28Si) and the estimated uncertainty U with a coverage factor k equal to 2 (such a coverage factor corresponds to a confidence level of 95%). On the “mean” and “standard deviation” lines, the mean and the standard deviation obtained for each of the campaigns (therefore encompassing the 6 measurements) and, for the global column, obtained for all of these measurements, are shown respectively.

TABLE-US-00003 Session 1 Session 2 Session 3 Global Replica %mol .sup.28Si U(k=2) %mol .sup.28Si U(k=2) %mol .sup.28Si U(k=2) 1 99.836 0.002 99.843 0.003 99.841 0.003 2 99.839 0.003 99.844 0.003 99.842 0.003 3 99.840 0.002 99.843 0.004 99.842 0.003 4 99.841 0.003 99.844 0.003 99.843 0.003 5 99.842 0.002 99.844 0.003 99.842 0.003 6 99.842 0.003 99.845 0.003 99.841 0.003 average 99.840 99.844 99.842 99.842 standard deviation 0.0024 0.0008 0.0007 0.002

[0054] It can be seen that the values measured during these different campaigns and these different measurements do not differ significantly and that the standard deviation is less than 0.004%. The method according to the invention therefore allows obtaining a measurement which is more sensitive than those of the prior art and is perfectly adapted for the measurement of samples with a high enrichment in the silicon-28 isotope .sup.28Si.

[0055] It will be noted that the present description, if it is focused on the method, of course also covers the use of a reactive gas comprising nitrous oxide in the context of mass spectrometric measurements of samples comprising silicon.

BIBLIOGRAPHIC REFERENCES

[0056] [0056] [1] P. Becker (2003) “Metrologia” volume 40 number 6 pages 366 to 375 [0057] [0057] [2] A. Pramann et al. (2011) “International Journal of Mass Spectrometry” volume 299 number 2-3 pages 78-86, 2011 [0058] [0058] [3] H.T. Liu et al. (2003) “Spectrochimica Acta Part B: Atomic Spectroscopy” volume 58, number 1, pages 153-157, [0059] [0059] [4] F. Aureli et al. (2012) “Journal of Analytical Spectroscopy” volume 27, pages 1540-1548 [0060] [0060] [5] A. Virgilio et al. (2016) “Spectrochimica Acta Part B: Atomic Spectroscopy” Volume 116 pages 31-36