ADDITION OF REACTIVE SPECIES TO ICP SOURCE IN A MASS SPECTROMETER

Abstract

Disclosed is a method of inductively coupled plasma mass spectrometry (ICP-MS), comprising steps of introducing at least one sample comprising at least one sample species, and at least one reactive species, into an inductively coupled plasma source, such that at least one molecular adduct ion of the at least one reactive species and the at least one sample species is formed; transferring the at least one molecular adduct ion into a collision cell that is arranged between the inductively coupled plasma source and at least one mass analyzer, transferring the at least one molecular adduct ion, or a product thereof, into the at least one mass analyzer, and analyzing the mass of the at least one molecular adduct ion, or the product thereof, in the at least one mass analyzer. Also disclosed is a mass spectrometer that is adapted to perform the method.

Claims

1. A method of inductively coupled plasma mass spectrometry (ICP-MS), comprising: a. providing at least one inductively coupled plasma source; b. introducing at least one sample comprising at least one sample species, and at least one reactive species, into the plasma source, such that at least one molecular adduct ion of the at least one reactive species and the at least one sample species is formed; c. transferring the at least one molecular adduct ion into a collision cell that is arranged between the inductively coupled plasma source and at least one mass analyzer, d. transferring the at least one molecular adduct ion, or a product thereof, into the at least one mass analyzer, and e. analyzing the mass of the at least one molecular adduct ion, or the product thereof, in the at least one mass analyzer.

2. The method of claim 1, wherein the product is a fragment ion or a further molecular adduct ion.

3. The method of claim 1, wherein at least one interfering sample ion having the same mass as the molecular adduction ion is formed in the inductively coupled plasma source, and wherein the method further comprises fragmenting the at least one interfering sample ion but not the molecular adduct ion in the collision cell.

4. The method of claim 1, further comprising a. introducing at least one gas into the collision cell; b. forming at least one product ion in the collision cell from the at least one molecular adduct ion and the at least one gas; c. transferring the at least one product ion into the at least one mass analyzer; and d. analyzing mass of the at least one product ion in the at least one mass analyzer.

5. The method of claim 4, wherein the product ion is formed in the collision cell by fragmenting the molecular adduct ion by the introduction of at least one collision gas in the collision cell, to generate at least one fragment ion that represents the product ion, and/or by reacting the molecular adduct ion by the introduction of at least one reactive gas in the collision cell, to generate at least one further molecular adduct ion from the molecular adduct ion and the reactive gas, that represents the product ion.

6. The method of claim 1, further comprising transferring the at least one molecular adduct ion that is formed in the inductively coupled plasma source through at least one mass filter that is provided between the inductively coupled plasma source and the collision cell, and that is configured to only transmit ions with a mass-to-charge ratio in a range that includes the mass-to-charge ratio of the at least one molecular adduct ion.

7. The method of claim 6, wherein the mass filter is configured to transmit ion species with a mass-to-charge ratio in a range that includes the mass-to-charge ratio of the molecular adduct ion.

8. The method of claim 6, wherein the mass filter is configured to not transmit ions with mass-to-charge ratio of molecular adduct and/or fragment ions that are produced in the collision cell.

9. The method of claim 6, wherein the mass-to-charge ratios transmitted by the mass filter has a width not greater than 24 amu.

10. The method of claim 6, wherein the mass filter is configured to only transmit ion species with substantially the mass-to-charge ratios of the molecular adduct ions formed in the inductively coupled plasma source.

11. The method of claim 1, wherein the sample and/or the reactive species are provided in a gas that is introduced into the plasma source.

12. The method of claim 1, wherein the sample species is an elemental species.

13. The method of claim 1, wherein the at least one reactive species is provided as at least one reactive gas that is introduced into the plasma source.

14. The method of claim 1, wherein the sample contains a plurality of interfering isotopes having the same nominal mass and wherein a molecular adduct ion is formed in the plasma source from one of the interfering isotopes at a higher rate than from the other interfering isotope(s).

15. The method of claim 1, wherein the mass analyzer is a sector analyzer, optionally having a multicollector, and wherein analyzing the mass comprises determining an isotope composition.

16. The method of claim 1, wherein the sample is introduced into the plasma source as an aerosol in a carrier gas.

17. The method of claim 16, wherein the reactive species is introduced into the aerosol.

18. The method of claim 1, wherein the sample is introduced into the plasma source by laser ablation.

19. The method of claim 1, wherein the reactive species is selected from H.sub.2, N.sub.2, O.sub.2, NH.sub.3, SO.sub.2, CS.sub.2, N.sub.2O, SF.sub.6, Ne, Kr, CO.sub.2.

20. An inductively coupled plasma mass spectrometer (ICP-MS), comprising: a. at least one sample introduction device; b. an inductively coupled plasma source; c. at least one mass filter, d. at least one collision cell, and e. at least one mass analyzer; wherein the at least one mass filter is arranged between the inductively coupled plasma source and the collision cell, and wherein the spectrometer further comprises at least one sample introduction system for delivering at least one reactive species into the inductively coupled plasma source, whereby the reactive species forms at least one molecular adduct ion with at least one ion generated from a sample in the inductively coupled plasma source, wherein the sample introduction system comprises at least one reactive gas inlet fluidly connected to the inductively couple plasma source and/or the sample introduction device.

21. The mass spectrometer of claim 20, wherein the mass filter is configured to transmit ion species with a mass-to-charge ratio in a range that includes the mass-to-charge ratio of the molecular adduct ion that is formed in the inductively coupled plasma source but does not include the mass-to-charge ratio of product ions that are formed in the collision cell.

22. The mass spectrometer of claim 20, wherein the mass filter is configured to only transmit ion species with substantially the mass-to-charge ratios of the molecular adduct ions formed in the inductively coupled plasma source.

23. The mass spectrometer of claim 20, wherein the sample introduction device comprises a nebulizer or a laser ablation source.

24. The mass spectrometer of claim 20, further comprising a dual-function electrostatic lens, for selectively transmitting and reflecting ions, wherein the electrostatic lens is preferably arranged between the mass filter and the collision cell.

25. The mass spectrometer of claim 20, wherein the mass analyzer is a sector field analyzer, optionally comprising a multicollector for isotope ratio measurements.

26. The mass spectrometer of claim 20, further comprising at least one controller configured to operate the spectrometer such that the at least one molecular adduct ion formed in the inductively coupled plasma source is transmitted by the mass filter to the collision cell, whereby a product ion is formed in the collision cell from the at least one molecular adduct ion; wherein the product ion has a mass-to-charge ratio that is not transmitted by the mass filter; and wherein the product ion is mass analyzed in the mass analyzer.

27. The mass spectrometer of claim 26, wherein the product ion comprises a fragment ion of the at least one molecular adduct ion formed in the inductively coupled plasma source.

28. The mass spectrometer of claim 26, wherein the product ion comprises at least one further molecular adduct ion of the molecular adduction ion formed in the inductively coupled plasma source and a reactive gas that is introduced into the collision cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] FIG. 1 shows an Inductively Coupled Plasma (ICP) source in accordance with the invention, indicating two alternate configurations for introduction of reaction gas into the ICP source

[0083] FIG. 2 shows a sample introduction system that consists of a nebulizer and a spray chamber, for introducing an aerosol into the ICP source. Two alternate configurations for introducing a reactive species into the sample introduction system are indicated.

[0084] FIG. 3 shows a mass spectrometer in accordance with the invention, the mass spectrometer consisting of a sample introduction system, an ICP source, a mass filter, a collision cell and a mass analyzer. As indicated, the sample introduction system and/or the ICP source can have a connection for introduction of a reaction gas, as further illustrated in FIG. 1 and FIG. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

[0085] In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0086] In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

[0087] It should be appreciated that the invention is applicable for mass analysis of materials in general, such as gases, liquids, solids, particles and aerosols. In general, therefore, the sample that is being analyzed in the system will be variable.

[0088] An Inductively Coupled Plasma (ICP) source 10 in accordance with the invention is shown in FIG. 1. The ICP source exemplified contains three concentric tubes 11, 12, 13 which are typically made from quartz, and a load coil 21. Plasma gas can be introduced through the sample inlet 14, an auxiliary gas inlet 17 via an auxiliary gas line 15 and/or a cooling gas inlet 20 via a cooling gas line 18. The plasma is then sampled through a cone 22 before entering downstream ion optics of the mass spectrometer (not shown).

[0089] A sample is introduced through the sample inlet 14, typically in a plasma gas such as Argon. The sample can be an aerosol that is generated by a means of a nebulizer and a spray chamber, as further illustrated in FIG. 2. The reactive species gas can be introduced into the ICP source through the sample inlet 14 together with the sample, either in a gaseous form in mixture with the plasma gas, or in liquid form in the sample aerosol, when present. Alternatively, or additionally, the reactive species can be introduced as a reactive gas via inlets 16, 19 on the auxiliary gas inlet line 17 and/or the cooling gas inlet line 20, respectively.

[0090] The reactive species can also be introduced into a sample introduction system such as a spray chamber assembly 30, as illustrated in FIG. 2. The assembly includes a nebulizer 31, which has a sample inlet 32, and a nebulizer gas inlet 34, which typically will be identical to the plasma gas (such as Argon). An inlet 33 can be provided on the nebulizer gas inlet, and that can be used to provide a reactive gas, in mixture with the nebulizer gas, into the nebulizer.

[0091] The nebulizer delivers a sample spray into the spray chamber 37, which has a drain 36 and an outlet 38 that feeds into the sample inlet 14 of the ICP source 10. The spray chamber can further have a gas inlet 35 that can be used to deliver reaction gas into the spray chamber, where it will form a mixture with the sample aerosol and be delivered into the ICP source through the outlet 38.

[0092] Thus, alternative embodiments for delivering sample gas into the spray chamber assembly are possible. These embodiments can be used alternatively, or they can be used in combination.

[0093] A variety of factors including temperature, nebulizer flow rate, RF power that is applied to the torch, and concentration of the reactive species are expected to influence the rate of formation of molecular adduct ions that result from reaction between a sample species and a reactive species. Thus, depending on the nature of the reactive and sample species, and the desirable adduct ion, the appropriate configuration for introducing the reactive species can be selected, i.e. through one or more such inlets on the spray chamber assembly or the ICP source. Additional parameters, including the rate of introduction of the reactive species, can be optimized so as to maximize the yield of the molecular adduct ion in the ICP source.

[0094] It should be appreciated that it can be advantageous to use more than one inlet for delivering reactive species into the ICP source simultaneously. More than one inlet can be used simultaneously, either for delivering the same reactive species, or alternatively for delivering different reactive species, which subsequently can form distinct molecular ion adducts in the ICP source.

[0095] Turning to FIG. 3, a mass spectrometer in accordance with the invention is shown. The mass spectrometer includes a sample introduction system 25, and an ICP source 10. Reactive species can be introduced through one or more inlets 16, 19, 33, 35, as described in the above with respect to FIG. 1 and FIG. 2. The mass spectrometer further contains a mass filter 40. The mass filter can be configured to selectively transmit molecular adduct ions formed in the inductively coupled plasma source (and optionally ions in a limited mass range about the mass-to-charge ratio of the adduct ions), but not ions that have a smaller or larger mass-to-charge ratio than the selected transmitted range. As a consequence, mass analysis of the molecular adduct ion or a fragment thereof in the downstream mass analyzer can be performed free of isobaric interferences, e.g. after mass shifting reactions or fragmentations in the collision cell as described further below.

[0096] Downstream of the mass analyzer, collision cell 50 is arranged. By the introduction of a collision gas into the collision cell, molecular adduct ions that are formed in the ICP source and that are transferred into the collision cell can be fragmented, so as to generate sample or product ions (e.g. sample ions from which the molecular adduct ions were formed in the source by reaction of the sample ions with the reactive species). Alternatively, further molecular adduction ions of molecular adduct ions that are transmitted into the collision cell can be generated, by reaction with a reaction gas that is provided in the collision cell. The mass of sample ions or further molecular ions thus generated can subsequently be determined in the downstream mass analyzer 60 free from interferences due to the use of the mass filter.

[0097] A distinct advantage of this setup is the possibility to remove isobaric interferences. Thus, the selective transmission by the mass filter of molecular adduct ions having a mass-to-charge ratio that does not include the mass-to-charge ratio of isobaric interferences of the sample ions that may be present or be generated in the ICP source, mass analysis of sample ions (that is formed by fragmentation of molecular adduct ions in the collision cell), or molecular adduct ions, can be performed in the absence of such interferences. The result is a mass spectrum with improved specificity.

[0098] Molecular adduct ions can be formed within ICP source at different rates. For example, rate of oxide formation is highly variable, leading to the possibility to selectively form metal oxides to eliminate isobaric interferences. By way of example, Ti oxides form about 100 times faster than Ca oxides. As a consequence, the invention will find general application for removal of interferences on different molecular and/or elemental background, where the interfering ion(s) and the sample ions have different reaction probabilities.

[0099] The following non-limiting examples provide exemplary descriptions of certain analytical benefits of the present invention.

Example 1

[0100] Introduction of O.sub.2 into the ICP source preferentially leads to the formation of TiO over CaO. The metal oxide that is formed in the ICP source is fragmented in the collision cell, leading to the formation of elemental ions that is mass analyzed in the downstream mass analyzer. When analyzed on a mass spec that has a mass filter upstream of the collision cell, the mass filter is preferably set to only transmit oxides in a mass range that includes TiO. This means that potential interferences on Ti isotope analysis are not transmitted by the mass filter, leading to reduced interference on the mass spectrum.

[0101] The mass filter can be set to only transmit adduct ions that are formed in the ICP source (e.g. oxidized species, nitrogen adducts, etc.) but not the mass of the product ions that are produced in the collision cell. Adduct ions can be broken into smaller mass product ions in the collision cell so that products appear at a smaller mass which was not transmitted by the first mass filter. By doing this it is possible to mass analyze the smaller mass (e.g. elemental) ion on a clean background in the downstream mass analyzer. For example, the aforesaid transmitted TiO oxides can be fragmented to Ti ions in the collision cell and subsequently measured on an interference-free background in the mass analyzer.

Example 2

[0102] In an isotope ratio analysis of Fe on an interfering background of Cr species, the addition of N.sub.2 to ICP source, for example to the nebulizer, leads to formation of FeN and CrN. However, the rate of formation of FeN is much greater than for CrN, which means that the molecular adduct ions formed in the ICP source will be predominantly FeN species. Other interferences can include .sup.40Ar.sup.16O on .sup.56Fe and .sup.46Ar.sup.14N on .sup.54Fe. Mass filter can be set to transmit only masses 63 to 73, i.e. the mass filter does not transmit the interfering .sup.40Ar.sup.16O and .sup.40Ar.sup.14N species and also not unreacted Cr isotopes. The transmitted FeN species is fragmented by adding a collision gas such as He to the collision cell, leading to the formation of elemental Fe isotopes, which are mass analyzed in the downstream mass analyzer.

[0103] Adduct ions that are transmitted by the mass filter can also be converted into further molecular adducts in the collision cell. Thereby, it is possible to mass analyze a larger mass ion on a clean background in the downstream mass analyzer.

Example 3

[0104] The mass filter is controlled to only transmit ions with a mass-to-charge ratio in a range that includes the mass-to-charge ratio of molecular adduct ions formed in the ICP source, for example a mass window of 16 amu centered around .sup.48Ti.sup.16O that has a mass of 64. The transmitted adduct ions are further reacted into larger mass product ions in the collision cell. The further molecular adduct species (i.e. the larger mass product ions) are subsequently transmitted into the mass analyzer, where their mass is analyzed on a clean background. For example, background interference by Ca, V and/or Cr species can be problematic during analysis of .sup.48Ti.sup.16O. To overcome such interference, a further reaction of .sup.48Ti.sup.16O with CO.sub.2 can be done in the collision cell, generating higher molecular weight species that can be mass analyzed in the absence of interferences.

Example 4

[0105] Here, molecular adduct ions derived from the sample that are formed in the ICP source are transmitted through the collision cell, while interfering molecular adduct ions are fragmented in the collision cell, so that the molecular adduct can be mass analyzed in the downstream mass analyzer on a clean background. This mode of operation is also possible in an instrument having a mass filter upstream of the collision cell, for removal of other potentially interfering species, for example higher molecular weight species that could be fragmented in the collision cell, forming fragments that could interfere on the mass spectrum of the sample molecular adduct ion.

[0106] In summary, the present invention provides numerous advantages, including: [0107] a. improved sensitivity of mass analysis, due to elimination of isobaric interfering species; [0108] b. selective mass-shifting within the inductively coupled plasma source, to allow for removal of isobaric species; [0109] c. providing a mass filter upstream of the collision cell so as to only transmit ions with a mass-to-charge ratio that does not include the mass-to-charge ratio of the mass analyzed (product ion) species; [0110] d. selective fragmentation of molecular species transmitted by the mass filter into the collision cell, leading to mass analysis of mass-shifted species having a lower mass than transmitted by the mass filter; [0111] e. selective formation of molecular adducts in the collision cell, leading to mass analysis of mass-shifted species having a greater mass than transmitted by the mass filter.

[0112] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms a, an, and the include plural references unless the context clearly dictates otherwise.

[0113] Throughout the description and claims, the terms comprise, including, having, and contain and their variations should be understood as meaning including but not limited to, and are not intended to exclude other components.

[0114] The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., about 3 shall also cover exactly 3 or substantially constant shall also cover exactly constant).

[0115] The term at least one should be understood as meaning one or more, and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with at least one have the same meaning, both when the feature is referred to as the and the at least one.

[0116] 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 can be made while still falling within scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0117] Use of exemplary language, such as for instance, such as, for example and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.

[0118] All of the features and/or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and/or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.