Systems and methods for detection and quantification of selenium and silicon in samples

Abstract

The present disclosure provides methods and systems for improved detection and/or quantification of selenium (Se) and/or silicon (Si) in samples. In certain embodiment, the methods and systems feature the use of carbon dioxide (CO.sub.2) as a reaction gas in a reaction cell chamber, such as a dynamic reaction cell (DRC), of an inductively coupled plasma mass spectrometer (ICP-MS). It is found that the use of CO.sub.2 as a reaction gas effectively eliminates (or substantially reduces) interfering ionic species for the analytes Se and Si, particularly in samples with complex matrices, and/or in samples with low levels of analyte, thereby enabling more accurate detection of analyte at lower detection limits and in samples having complex matrices.

Claims

1. A method for producing a stream of ions for detection and/or quantification of selenium (Se) in a sample, the method comprising: introducing a sample to an ionization source, thereby producing an ionized sample stream comprising a plurality of ionic species, said plurality of ionic species comprising: (i) one or more analyte ionic species, said one or more analyte ionic species being an ionized form of one or more species of interest present in the sample, said one or more species of interest comprising selenium, and said one or more analyte ionic species comprising Se.sup.+; and (ii) one or more interferer ionic species, said one or more interferer ionic species comprising Ar.sub.2.sup.+ and having nominal m/z substantially equivalent to that of Se.sup.+; admitting the ionized sample stream into a chamber to thereby contact the ionized sample stream with a reaction gas stream comprising CO.sub.2 having a flow rate, thereby reacting the CO.sub.2 with at least one of the one or more interferer ionic species and producing one or more products that are not interferer ionic species; following contact of the ionized sample stream with the reaction gas stream comprising CO.sub.2, directing the resulting product stream to a mass analyzer and detector for detection and/or quantification of selenium in the sample; and detecting and/or quantifying selenium in the sample; wherein the flow rate of the reaction gas stream comprising CO.sub.2 is determined by calculating a background equivalent concentration of Se.sup.+ for each of a plurality of flow rates of the reaction gas stream comprising CO.sub.2 and selecting the flow rate based on the calculated background equivalent concentrations of Se+.

2. The method of claim 1, wherein the ionization source comprises argon.

3. The method of claim 1, wherein the introducing step comprises introducing the sample as a nebulized mist of liquid into the ionization source.

4. The method of claim 1, wherein the sample is a drinking water sample.

5. The method of claim 1, wherein the sample is an environmental sample.

6. The method of claim 5, wherein the environmental sample is a soil digest.

7. The method of claim 5, wherein the environmental sample is seawater and the one or more species of interest comprises .sup.78Se.

8. The method of claim 1, wherein the sample is a biological sample.

9. The method of claim 1, wherein the sample comprises a product consumable by a human.

10. The method of claim 1, wherein the contacting step is conducted with a reaction gas stream having a minimum CO.sub.2 flow rate of 0.1 mL/min and an ionization source gas flow of no greater than 30 L/min.

11. The method of claim 10, wherein the contacting step is conducted with an ionized sample stream resulting from a liquid sample uptake rate of at least 20 L/min.

12. The method of claim 10, wherein the contacting step is conducted with an ionized sample stream resulting from a liquid sample uptake rate no greater than 5 mL/min.

13. The method of claim 1, wherein the one or more species of interest comprises .sup.80Se.

14. The method of claim 1, wherein the one or more species of interest comprises .sup.78Se.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, aspects, features, and advantages of the present disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a plot demonstrating removal of interfering ion .sup.78Ar.sub.2.sup.+ {e.g., .sup.40Ar.sup.38Ar.sup.+} for the analyte .sup.78Se.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), according to an illustrative embodiment of the invention.

(3) FIG. 2 is a plot demonstrating removal of interfering ions .sup.40Ar.sub.2.sup.+ {e.g., .sup.40Ar.sup.40Ar.sup.+} and .sup.64Zn.sup.16O.sup.+ for the analyte .sup.80Se.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), according to an illustrative embodiment of the invention.

(4) FIG. 3 is a plot demonstrating removal of interfering ions .sup.14N.sub.2.sup.+ and .sup.12C.sup.16O.sup.+ for the analyte .sup.28Si.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), according to an illustrative embodiment of the invention.

(5) FIG. 4 is a block diagram representing an example multi-mode ICP-MS system for performing a method for producing a stream of ions for detection and/or quantification of silicon (Si) and/or selenium (Se) in a sample, according to an illustrative embodiment of the invention.

(6) FIG. 5 is a flowchart illustrating an example method for producing a stream of ions for detection and/or quantification of silicon (Si) and/or selenium (Se) in a sample, according to an illustrative embodiment of the invention.

(7) The features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

(8) It is contemplated that systems, devices, methods, and processes of the claimed invention encompass variations and adaptations developed using information from the embodiments described herein. Adaptation and/or modification of the systems, devices, methods, and processes described herein may be performed by those of ordinary skill in the relevant art.

(9) Throughout the description, where articles, devices, and systems are described as having, including, or comprising specific components, or where processes and methods are described as having, including, or comprising specific steps, it is contemplated that, additionally, there are articles, devices, and systems of the present invention that consist essentially of, or consist of, the recited components, and that there are processes and methods according to the present invention that consist essentially of, or consist of, the recited processing steps.

(10) It should be understood that the order of steps or order for performing certain action is immaterial so long as the invention remains operable. Moreover, two or more steps or actions may be conducted simultaneously.

(11) The mention herein of any publication, for example, in the Background section, is not an admission that the publication serves as prior art with respect to any of the claims presented herein. The Background section is presented for purposes of clarity and is not meant as a description of prior art with respect to any claim.

(12) Methods and systems are described herein that feature the use of carbon dioxide (CO.sub.2) as a reaction gas in a reaction cell chamber, such as a dynamic reaction cell (DRC), of an inductively coupled plasma mass spectrometer (ICP-MS). It is found that the use of CO.sub.2 as a reaction gas effectively eliminates (or substantially reduces) interfering ionic species for the analytes selenium (Se) and silicon (Si), particularly in samples with complex matrices, and/or in samples with low levels of analyte, thereby enabling more accurate detection of these analytes at lower detection limits and/or in samples having complex matrices.

(13) While the creation of ionization sources described herein is demonstrated with an inductively coupled plasma (ICP) mass spectrometer system, other ionization sources could be used as well. For example, in some embodiments, electron ionization, chemical ionization, ion-attachment ionization, gas discharge ion sources, desorption ionization sources, spray ionization (e.g., electrospray ionization), and/or ambient ionization sources can be used. In some embodiments, in addition to ICP, other gas discharge ion sources include, but are not limited to, microwave induced plasma, glow discharge, spark ionization, and closed drift ion sources.

(14) Thus, methods and systems are described herein for producing a stream of ions for detection and/or quantification of selenium (Se) and/or silicon (Si) in a sample. The resultant beam may be analyzed, for example, via mass spectrometer (MS), for example, linear quadrupole MS, quadrupole ion trap MS, ion cyclotron resonance MS, time-of-flight MS, magnetic and/or electric sector MS, and quadrupole ion trap time-of-flight MS. Combined use of a mass spectrometer (MS) with other tools for speciation analysis is also contemplated, for example, use of a mass spectrometer (MS) with gas chromatography (GC), high-performance liquid chromatography (HPLC) and/or field flow fractionation (FFF).

(15) Selenium Detection and Quantification

(16) Where argon is used as carrier gas to maintain the plasma in ICP-MS, the major isotopes of selenium, .sup.78Se (23.8% abundant) and .sup.80Se (49.6% abundant), have argon-based polyatomic interferences, Ar.sub.2.sup.+. Furthermore, for environmental samples with complex matrices, a currently used reaction gas, methane (CH.sub.4), can result in new interferences forming.

(17) By contrast, it is found that carbon dioxide (CO.sub.2), when used as a reaction gas, reacts rapidly with the primary interferences without creating new interferences. Carbon dioxide (CO.sub.2) is non-reactive with (or negligibly reactive with) Se.sup.+ (rate constant k is less than 510.sup.13 cm.sup.3 molecule.sup.1 s.sup.1), and reacts rapidly with the main interferences in Se detection, as shown in reaction Equation 1:
Ar.sub.2.sup.+CO.sub.2.fwdarw.CO.sub.2.sup.++2Ar k10.sup.9 cm.sup.3 molecule.sup.1 s.sup.1(1)
CO.sub.2 Flow Rate Optimization: Removal of Ar.sub.2.sup.+ for Detection of Se

(18) FIG. 1 is a plot 100 demonstrating removal of interfering ion .sup.78Ar.sub.2.sup.+{e.g., .sup.40Ar.sup.38Ar.sup.+} for the analyte .sup.78Se.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), specifically, the NexION 300D ICP-MS, manufactured by PerkinElmer, Inc. of Waltham, Mass. The instrument conditions for this experiment and other experiments described herein (unless otherwise indicated) were RF Power at 1600 W, use of a glass concentric nebulizer, use of a glass cyclonic spray chamber, and use of nickel cones.

(19) A matrixin this example, a 1 weight percent (wt. %) nitric acid (HNO.sub.3) solution in waterwas aspirated, and an intensity reading was obtained for the .sup.78Se.sup.+ analyte at each of a plurality of flow rates of carbon dioxide (CO.sub.2) into the DRC, shown in the plot of FIG. 1. The resulting curve 102 is labeled Matrix=1%/HNO.sub.3 in FIG. 1 (logarithmic plot). As the carbon dioxide (CO.sub.2) flow rate increases, the measured intensity generally decreases.

(20) Next, a solution containing the matrix (1 wt. % HNO.sub.3 solution), with 10 parts-per-billion (ppb) selenium (Se) spike, was aspirated, and an intensity reading was obtained for the analyte .sup.78Se.sup.+ at each of a plurality of flow rates of carbon dioxide (CO.sub.2) injected into the DRC. The resulting curve 104 is labeled Matrix+10 ppb Se in FIG. 1 (in the same logarithmic plot).

(21) From the Matrix curve 102 and Matrix+10 ppb curve 104, a background equivalent concentration (BEC) of the analyte was calculated for each flow rate of carbon dioxide (CO.sub.2) injected into the DRC, and the resulting BEC curve 106 was plotted. The BEC is a function of the analyte contamination in the matrix and the incomplete reaction and/or removal of the interfering ionic species. The optimum flow of carbon dioxide (CO.sub.2) may be achieved and/or determined where the BEC is minimized. In this example, as shown in FIG. 1, the BEC of the analyte .sup.78Se.sup.+ ranged from 25-40 parts-per-trillion (ppt). The plots in FIG. 1 demonstrate the effective removal of interfering species .sup.78Ar.sub.2.sup.+{e.g., .sup.40Ar.sup.38Ar.sup.+} for the analyte .sup.78Se.sup.+.

(22) CO.sup.2 Flow Rate Optimization: Removal of Ar.sub.2.sup.+ and Zn.sup.+ for Detection of Se

(23) FIG. 2 is a plot 200 demonstrating the removal of interfering ions .sup.40Ar.sub.2.sup.+ {e.g., .sup.40Ar.sup.40Ar} and .sup.64Zn.sup.16O.sup.+ for the analyte .sup.80Se.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), again, the NexION 300D ICP-MS. A matrixin this example, a 1 part-per-million (ppm) zinc (Zn) solution in water (H.sub.2O)was aspirated, and an intensity reading was obtained for the .sup.80Se.sup.+ analyte at a plurality of flow rates of carbon dioxide (CO.sub.2) into the DRC, shown in the plot of FIG. 2. The resulting curve 202 is labeled Matrix=1 ppm Zn in FIG. 2 (in a logarithmic plot). As the carbon dioxide (CO.sub.2) flow rate increases, the measured intensity is seen to generally decrease.

(24) Next, a solution containing the matrix (1 ppm Zn solution in water), with a 2 ppb selenium (Se) spike, was aspirated, and an intensity reading was obtained for the .sup.80Se.sup.+ analyte at each of a plurality of flow rates of carbon dioxide (CO.sub.2) injected into the DRC. The resulting curve 204 is labeled Matrix+2 ppb Se in FIG. 2.

(25) From the Matrix curve 202 and Matrix+2 ppb Se curve 204, a background equivalent concentration (BEC) of the analyte was calculated for each flow rate of CO.sub.2 into the DRC, and the resulting BEC curve 206 was plotted. Background equivalent concentration is a function of analyte contamination in the matrix and incomplete reaction/removal of the interfering ionic species. The optimum flow of carbon dioxide (CO.sub.2) may be achieved and/or determined where the BEC is minimized. As shown in FIG. 2, the BEC ranged from 60-120 parts-per-trillion (ppt). The plots in FIG. 2 demonstrate effective removal of interfering species .sup.40Ar.sub.2.sup.+ and .sup.64Zn.sup.16O.sup.+ for the analyte .sup.80Se.sup.+.

(26) Spike Recovery Tests Using CO.sub.2Se Detection in a Drinking Water Standard Reference Material (SRM) Matrix

(27) In another series of experiments for the detection of selenium (Se) and removal of interfering species using carbon dioxide (CO.sub.2) as a reaction gas in a DRC of the ICP-MS, the following conditions were used: auxiliary flow of 1.2 L/min; plasma flow of 15 L/min; injector diameter 2.0 mm; spray chamber temperature at room temperature; no oxygen (O.sub.2) flow (into the spray chamber); CO.sub.2 flow of 0.6 or 1.2 mL/min (higher flow rate was found useful to reduce ZnO.sup.+ at a mass-to-charge (m/z) ratio of 80 in matrices with high zinc (Zn) content); RPq (the q parameter from the Mathieu equation) of 0.80; and a sample uptake of 250 L/min.

(28) First, to demonstrate detection of selenium (Se) in an environmental sample, with elimination of interfering ion species, a drinking water SRM matrix was used for spike recovery tests. A spike recovery test can be carried out to determine levels of analyte in a sample that can be analyzed without significant matrix suppression. Calibrations were performed (external) in a 1 wt. % nitric acid (HNO.sub.3) solution in water, with 2, 5, and 10 g/L Se. Results of the detection of .sup.78Se and .sup.80Se in the drinking water SRM using the two different flow rates (0.6 or 1.2 mL/min) of CO.sub.2 are shown in Table 1 and Table 2, respectively.

(29) TABLE-US-00001 TABLE 1 Detection of .sup.78Se and .sup.80Se in Drinking Water SRM using CO.sub.2 flow rate of 0.60 mL/min Certified .sup.78Se % .sup.80Se % Sample ID (g/L) (g/L) Recovery (g/L) Recovery Trace Metals 10 10.1 101 10.0 100 in Drinking Water (TMDW)

(30) TABLE-US-00002 TABLE 2 Detection of .sup.80Se in Drinking Water SRM using CO.sub.2 flow rate of 1.20 mL/min Certified .sup.80Se % Sample ID (g/L) (g/L) Recovery Trace Metals in 10 10.4 104 Drinking Water (TMDW)

(31) As shown in Table 1 and Table 2 above, good recoveries for both selenium (Se) isotopes, .sup.78Se and .sup.80Se, were achieved at both carbon dioxide (CO.sub.2) flow rates, 0.60 mL/min and 1.20 mL/min.

(32) Spike Recovery Tests Using CO.sub.2Se Detection in Soil-Digest SRM Matrices

(33) Additional experiments were conducted to detect selenium (Se), with elimination of interfering ion species, in soil digest SRM matrices (including river sediment, soil solution, and estuarian soil). Calibrations were performed (external) in a 1 wt. % nitric acid (HNO.sub.3) solution in water, with 2, 5, and 10 g/L Se. Results of the detection of .sup.78Se and .sup.80Se in the soil digest SRM using two different flow rates (0.6 or 1.2 mL/min) of CO.sub.2 are shown in Table 3 and Table 4, respectively.

(34) TABLE-US-00003 TABLE 3 Detection of .sup.78Se and .sup.80Se in Soil Sample using CO.sub.2 flow rate of 0.60 mL/min Certified .sup.78Se % .sup.80Se % Sample (g/L) (g/L) Recovery (g/L) Recovery River Sediment-A 20 20.1 101 20.8 104 Soil Solution-A 10 10.5 105 9.51 95 Estuarian Soil 50 48.1 96 48.4 97

(35) TABLE-US-00004 TABLE 4 Detection of .sup.80Se in Soil Sample using CO.sub.2 flow rate of 1.20 mL/min Certified .sup.80Se % Sample (g/L) (g/L) Recovery River Sediment-A 20 20.3 102 Soil Solution-A 10 9.07 91 Estuarine Soil 50 47.5 95

(36) As shown in Table 3 and Table 4, good recoveries for both Se isotopes, .sup.78Se and .sup.80Se, were achieved at both CO.sub.2 flow rates, for all three soil digest matrices.

(37) Spike Recovery Tests Using CO.sub.2Se Detection in Spiked and Non-Spiked Interferents Check Standard A (ICS-A) Matrices

(38) Next, experiments were conducted to detect selenium (Se), with elimination of interfering ion species, in a check standard, Interferents Check Standard A (ICS-A), spiked with either 0, 1, or 5 g/L Se. Calibrations were performed (external) in a 1 wt. % nitric acid (HNO.sub.3) solution in water, with 2, 5, and 10 g/L Se. Results of the detection of .sup.78Se and .sup.80Se in the spiked and non-spiked Interferents A Check Solutions using the two different flow rates of CO.sub.2 (0.6 and 1.2 mL/min) are shown in Table 5 and Table 6, respectively.

(39) TABLE-US-00005 TABLE 5 Detection of .sup.78Se and .sup.80Se in ICS-A using CO.sub.2 flow rate of 0.60 mL/min .sup.78Se % .sup.80Se % Sample (g/L) Recovery (g/L) Recovery Interferents A-10x 0.68 0.33 Interferents A-10x + 1 g/L 1.21 53 1.34 101 Se Interferents A-10x + 5 g/L 4.84 83 4.96 93 Se

(40) TABLE-US-00006 TABLE 6 Detection of .sup.80Se in ICS-A using CO.sub.2 flow-rate of 1.20 mL/min .sup.80Se % Sample (g/L) Recovery Interferents A-10x 0.06 Interferents A-10x + 1 g/L Se 1.15 109 Interferents A-10x + 5 g/L Se 4.83 95

(41) As shown in Table 5 and Table 6, good recoveries for .sup.80Se were seen at both CO.sub.2 flow rates.

(42) Thus, the use of carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS) is demonstrated to eliminate interfering ion species, thereby enabling accurate quantification of levels of Se in environmental samples.

(43) Silicon (Si) Detection and Quantification

(44) For detection and quantification of silicon (Si) in samples via ICP-MS, prior use of ammonia (NH.sub.3) as a reaction gas in a DRC have proven ineffective for detection of silicon (Si) in organic matrices, where interfering species such as CO.sup.+ are dominant. In the petrochemical industry, there is a strong desire to measure Si in naphtha, for example, which are a class of organic compounds typically analyzed at about ten times (10) dilution in xylene or other suitable solvent. The major isotope of silicon (mass-to-charge (m/z) ratio of 28, at 92.2% abundance) suffers from polyatomic interferences, N.sub.2.sup.+ and CO.sup.+. In organic solvents such as xylene, the CO.sup.+ signal is much higher than normal due to excess carbon.

(45) It is found that when carbon dioxide (CO.sub.2) is used as a reaction gas, it reacts rapidly with the primary interferences (CO.sup.+ and N.sub.2.sup.+) without creating new interferences. Carbon dioxide (CO.sub.2) is non-reactive with Si.sup.+, and reacts rapidly with the main interferences in silicon (Si) detection, as shown in reaction Equations 2 and 3 as follows:
CO.sup.++CO.sub.2.fwdarw.CO.sub.2.sup.++CO k10.sup.9 cm.sup.3 molecule.sup.1 s.sup.1(2)
N.sub.2.sup.++CO.sub.2.fwdarw.CO.sub.2.sup.++N.sub.2 k10.sup.10 cm.sup.3 molecule.sup.1 s.sup.1(3)

(46) Experiments described herein demonstrate that the use of carbon dioxide (CO.sub.2) as a reaction gas enables measurement of silicon (Si) at levels as low as 10 g/L in organic solvents.

(47) CO.sub.2 Flow Rate Optimization: Removal of N.sub.2.sup.+ and CO.sup.+ for Detection of Si

(48) FIG. 3 is a plot 300 demonstrating the removal of interfering ions .sup.14N.sub.2.sup.+ and .sup.12C.sup.16O.sup.+ for the analyte .sup.28Si.sup.+ using carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS), specifically, the NexION 300D ICP-MS, manufactured by PerkinElmer, Inc. of Waltham, Mass. The instrument conditions for this experiment and others described herein (unless noted otherwise) were RF Power at 1600 W, use of a glass concentric nebulizer, use of a glass cyclonic spray chamber, and use of nickel cones.

(49) A matrixin this case, PGMEA (propylene glycol monomethyl ether acetate), an organic solvent used in the semiconductor industrywas aspirated, and an intensity reading was obtained for the .sup.28Si.sup.+ analyte at each of a plurality of flow rates of carbon dioxide (CO.sub.2) injected into the DRC, shown in the plot of FIG. 3. The resulting curve 302 is labeled Matrix=PGMEA in FIG. 3 (logarithmic plot). As the carbon dioxide (CO.sub.2) flow rate increases, the measured intensity is seen to decrease.

(50) Next, a solution containing the matrix (PGMEA), with a 50 parts-per-billion (ppb) silicon (Si) spike, was aspirated, and an intensity reading was obtained for the .sup.28Si.sup.+ analyte at each of a plurality of flow rates of carbon dioxide (CO.sub.2) into the DRC. The resulting curve 304 is labeled Matrix+50 ppb Si in FIG. 3. From the Matrix curve 302 and Matrix+50 ppb Si curve 304, a background equivalent concentration (BEC) of the analyte was calculated for each flow rate of carbon dioxide (CO.sub.2) into the DRC, and the resulting BEC curve 306 was plotted. BEC is a function of analyte contamination in the matrix and incomplete reaction/removal of the interfering ionic species. The optimum flow of carbon dioxide (CO.sub.2) may be achieved and/or determined where the BEC is minimized. Here, the BEC was about 30 parts-per-billion (ppb). The plots in FIG. 3 demonstrate the effective removal of interfering species .sup.14N.sub.2.sup.+ and .sup.12C.sup.16O.sup.+ for the analyte .sup.28Si.sup.+. The sample has significant silicon (Si) contamination, resulting in the high BEC; nevertheless, the signal at mass-to-charge (m/z) ratio 28 is reduced significantly with carbon dioxide (CO.sub.2) as reaction gas, allowing the silicon (Si) spike to be seen.

(51) Spike Recovery Tests Using CO.sub.2Si Detection in Naphtha Samples

(52) In another series of experiments for the detection of silicon (Si) and removal of interfering species using carbon dioxide (CO.sub.2) as a reaction gas in a DRC of the ICP-MS, the following conditions were used: auxiliary flow of 2.0 L/min; plasma flow of 20 L/min; injector diameter of 0.85 mm; spray chamber temperature at 20 C.; O.sub.2 flow (into the spray chamber) of 40 mL/min; CO.sub.2 flow of 0.5 mL/min; RPq of 0.50; and sample uptake of 190 L/min (Viton+PTFE tubing).

(53) Naphtha samples were used (Stoddard Solvent, Ligroin, and Petroleum Ether), each diluted ten times (10) in xylene. Calibrations were performed (external) in xylene, with 10, 20, 30, and 40 g/L Si. Results of the detection of .sup.28Si using 0.5 mL/min flow rate of CO.sub.2 are shown in Table 7 below (units in g/L).

(54) TABLE-US-00007 TABLE 7 Detection of .sup.28Si in naphtha using CO.sub.2 flow-rate of 0.5 mL/min Sample +20 g/L Si % Recovery Stoddard Solvent 4.09 21.3 86 Ligroin 6.50 26.1 98 Petroleum Ether 5.72 27.5 109

(55) Readings below 10 parts-per-billion (ppb) were achieved, and good spike recoveries were seen for all matrices.

(56) Thus, the use of carbon dioxide (CO.sub.2) as a reaction gas in a dynamic reaction cell (DRC) of an inductively coupled plasma mass spectrometer (ICP-MS) is demonstrated to eliminate interfering ion species, thereby enabling accurate quantification of levels of Si in organic solvents.

(57) ICP-MS System

(58) FIG. 4 is a block diagram of an example multi-mode inductively coupled plasma mass spectrometry (ICP-MS) system 400 for producing a stream of ions for detection and/or quantification of silicon (Si) and/or selenium (Se) in a sample, according to embodiments described herein.

(59) In FIG. 4, the ICP-MS system 402 includes a sample introduction system to receive an analyte sample 404. The analyte sample 404 is preferably a liquid or dispensed in a liquid, though, in some embodiments, the analyte sample is a solid. In some embodiments, the analyte sample 404 is introduced, for example, by a peristaltic pump 406 or through self-aspiration to a nebulizer 408 to transform the analyte sample into an aerosol of fine droplets 410. Examples of the nebulizer 408 may include, but are not limited to, concentric, cross-flow, Babington, V-Groove, HEN (high-efficiency), and MCN (micro-concentric) nebulizers. The fine droplets 410 generated by the nebulizer 408 may be passed through a spray chamber 412 to allow only fine droplets 414 that are below certain sizes to enter a plasma 416, typically composed of argon, generated by an ICP torch 418 and RF-coil 420. In some embodiments, examples of the spray chamber 412 include, but are not limited to, Scott or Cyclonic chambers. The plasma gas (e.g., argon) may be introduced by a gas regulator 422 that is coupled to a plasma gas source 424. In some implementations, the ICP torch 418 may comprise a series of concentric quartz tubes that are enveloped by the RF-coil 420. In some embodiments, the RF coil 420 is coupled to and energetically supplied by an RF-generator 426.

(60) Upon entering the plasma 414, the fine droplets 414 are dried and heated until the fine droplets 414 turn into a gas. As the atoms of the heated gas 414 continue to travel through the plasma 416, they absorb energy from the plasma 416 and form singly charged ions. The singly charged ions 424 exit the plasma 416 and are directed, as an ion beam 424 to an ion optics assembly 428.

(61) The ion optics assembly 428 provides an interface to the plasma 416. In some implementations, the ion optics assembly 428 includes a series of inverted cones having an orifice to allow the passage of the ion beam 424 while maintaining a high-vacuum environment within a vacuum chamber 430. The vacuum environment reduces the chances of ions of the ion beam 424 from inadvertently colliding with gas molecules between the ion optic assembly 428 and the detector 432. In some implementations, the vacuum chamber 430 is coupled to one or more vacuum pumps 433 such as, for example, a turbo-molecular pump and a mechanical roughing pump that operate together to provide the high-vacuum environment. In some implementations, the vacuum pump 433, and/or another pump, may be employed to evacuate the interface region of the ion optic assembly 428.

(62) In some embodiments, the ICP-MS system 402 includes a quadrupole ion deflector (QID) 434, to allow only ions of a specified mass range to pass into the cell 440 and prevent (or substantially reduce) the passage of non-ionized materials, such as neutrals and photons. The QID 434 is configured to filter the non-ionized materials that may cause measurement drifts or degrade the detection limits of the analyte ions of interest. Non-ionized material may be erroneously counted as ions by the detectors 432. In some implementations, the QID 434 includes a number of rods, which may be a magnetic or an electromagnetic source, configured to turn the direction of the ion beam 436 received from the ion optic assembly 428 to disaggregate (i.e., filter) the ionized portion of the beam 438 (which includes the analyte ions) from the non-ionized portion of the beam (e.g., neutrals, photons, and other non-ionized particles). Alternatively, in certain implementations, an autolens assembly may be employed to provide such mass pre-filtering functions.

(63) In some embodiments, the ICP-MS system 402 includes one or more collision and/or reaction cells. In some implementations, the collision or reaction cell may be integrated as a universal cell 440, and may be operated as either a reaction cell chamber or a collision cell chamber, depending on the selected mode of operation of the ICP-MS. The universal cell 440 may couple to one or more gas sources 441 that provide(s) pressurized gas 443 (for example, carbon dioxide (CO.sub.2)) to the chamber to react with interferer ionic species (such as .sup.78Ar.sub.2.sup.+, .sup.40Ar.sub.2.sup.+, .sup.64Zn.sup.16O.sup.+, .sup.14N.sub.2.sup.+, and .sup.12C.sup.16O.sup.+) in the ion stream 438. The universal cell 440 may optionally include an energy barrier, which may be energized, such as during the operation of the ICP-MS system 402 in collision mode, to further distinguish high-energy analyte ions (ions of interest) from interferent lower-energy ions. The universal cell 440 may include a quadrupole rod set within its interior spacing. The quadrupole rod set may be linked to a voltage source to receive an RF voltage suitable for creating a quadrupolar field.

(64) Thus, in certain embodiments, the reaction cell (or, in this case, universal cell) 440 includes a pressurized chamber into which the ionized sample stream 438 is admitted to contact the carbon dioxide (CO.sub.2), thereby reacting the carbon dioxide (CO.sub.2) with at least one of the one or more interferer ionic species and producing one or more products that are not interferer ionic species. The ion stream 438 includes the analyte ionic species, such as Se.sup.+ (e.g., .sup.80Se.sup.+, .sup.78Se.sup.+, among others) and/or Si.sup.+ (e.g., .sup.28Si.sup.+, among others). The ion stream 438 also includes interferer ionic species (for example, .sup.78Ar.sub.2.sup.+, .sup.64Ar.sub.2.sup.+, .sup.6Zn.sup.16O.sup.+, .sup.14N.sub.2.sup.+, and .sup.12C.sup.16O.sup.+) for the particular analyte ionic species. In the universal cell 440, the carbon dioxide (CO.sub.2) quickly reacts with the interferer ionic species, while remaining non-reactive (or negligibly reactive) with the analyte ionic species. The resulting reaction produces byproduct ions (for example, CO.sub.2.sup.+), as shown above in Equations 1-3. The byproduct ions no longer have the same or substantially the same m/z ratio as the analyte ions, and conventional mass filtering can be applied to eliminate the product interferer ions without disruption of the flow of analyte ions. For example, the stream can be subjected to a band pass mass filter to transmit only the analyte ions to the mass analysis stage. Use of a reaction cell to eliminate interferer ions is described further in U.S. Pat. Nos. 6,140,638; 6,627,912; and 8,426,804. In certain embodiments, the quadrupolar field generated by the quadrupole cell rod provides radial confinement of ions being transmitted along its length from the entrance end toward the exit end of the cell 440, allowing passage of the analyte ionic species out of the cell and restricting passage of byproduct ions out of the cell.

(65) Referring back to FIG. 4, in certain embodiments, following contact of the ionized sample stream with the reaction gas stream in the cell 440, the resulting product stream is directed to a mass analyzer and detector for detection and/or quantification of analyte ionic species. As shown in FIG. 4, in some embodiments, the ICP-MS system 402 includes a mass spectrometer such as a quadrupole mass spectrometer 442 to separate singly charged ions from each other by mass. In some embodiments, the quadrupole mass spectrometer 442 restricts the passage of the ions 444 to only one mass-charge (m/z) ratio (e.g., pre-specified m/z ratio) associated with a given ion in the ion beam. In some implementations, time-of-flight or magnetic sector mass spectrometer may be employed. The quadrupole mass spectrometer 442 may couple with an RF generator 446 that provides a RF power at specified voltages and frequencies. The quadrupole mass spectrometer 442 may employ both direct current and alternating current electrical fields to separate the ions.

(66) Subsequent to the quadrupole mass spectrometer 442, the detector 432 receives the mass-filtered ions 444 to produce an electronic signal that corresponds to the number of detected analyte ionic species. The detector 432 may couple to a signal processing and amplification circuitries to process the measured signal. The detector 432 counts the total signal for each mass charge, which may be aggregated to form a mass spectrum. The magnitude of the measured intensity values may be scaled based on a calibration standard such that the outputs are provided on a scale proportional to the concentration of the elements or analyte ions.

(67) In some embodiments, the ICP-MS system 402 includes one or more controllers to operate and monitor the operation of the quadrupole mass filter 442, the ignition of the plasma 416 by the ICP torch 418 and the RF coil 420, the pressure regulation of the vacuum chamber 430, the operation of the universal cell 440, and/or the operation of the quadrupole ion deflector 434, among other functions. The controller 400 may be operatively connected to the various mechanical and electrical components of the ICP-MS system 402.

(68) In some embodiments, the controller 400 includes hardware and/or software capable of executing algorithms, computer programs, and/or computer applications necessary for the operation of the ICP-MS system. For example, the controller 400 may include a processor and a non-transitory computer readable medium having instructions stored thereon, wherein the instructions, when executed by the processor, cause the processor to perform the functions necessary for operation of the ICP-MS system.

(69) FIG. 5 is a flowchart 500 illustrating an example method for producing a stream of ions for detection and/or quantification of silicon (Si) and/or selenium (Se) in a sample, according to an illustrative embodiment of the invention. Step 502 is introducing the sample to an ionization source such as an ionized carrier gas (e.g., a plasma), thereby producing an ionized sample stream comprising a plurality of ionic species. The plurality of ionic species includes: (i) one or more analyte ionic species, where an analyte ionic species is an ionized form of a species of interest in the sample (the analyte); and (ii) one or more interferer ionic species having nominal m/z substantially equivalent (and hence, creating a detection interference with) that of one or more of the analyte species. In this example, the analyte ionic species includes either or both of Se.sup.+ and Si.sup.+, and the interferer ionic species can include one or more of the following: .sup.78Ar.sub.2.sup.+, .sup.40Ar.sub.2.sup.+, .sup.64Zn.sup.16O.sup.+, .sup.14N.sub.2.sup.+, and .sup.12C.sup.16O.sup.+.

(70) Step 504 is admitting the ionized sample stream into a chamber (e.g., a reaction cell, such as a dynamic reaction cell, or other suitable enclosure or channel) to thereby contact the ionized sample stream with a reaction gas stream containing carbon dioxide (CO.sub.2). In certain embodiments, the chamber is pressurized with the reaction gas prior to and/or during introduction of the ionized sample stream into the cell, and the reaction gas stream includes the volume of reaction gas already in the chamber and/or includes a stream of the reaction gas provided to the chamber, e.g., sufficient to maintain a certain pressure and/or concentration of reaction gas. Contact of the interferer ionic species in the ionized sample stream with the carbon dioxide results in a reaction, producing one or more products that are not interferer ionic species, e.g., ionic species such as CO.sub.2.sup.+ and neutral species such as Ar, CO, and N.sub.2. The byproduct ions no longer have the same or substantially the same m/z ratio as the analyte ions, and conventional mass filtering can be applied to eliminate the product interferer ions without disruption of the flow of analyte ions. The byproduct neutral species do not interfere with detection of the analyte ions.

(71) Following contact of the ionized sample stream with the reaction gas stream comprising CO.sub.2, step 506 is directing the resulting product stream to a mass analyzer and detector for detection and/or quantification of the analyte ion(s) in the sample, e.g., Se.sup.+ and/or Si.sup.+. For example, the mass analyzer may be a quadrupole mass spectrometer, such that the detector receives mass-filtered ions to produce an electronic signal that corresponds to the number of detected analyte ionic species. The signal may be analyzed to quantify the detected analyte, e.g., to determine a concentration of the analyte in the sample.

EQUIVALENTS

(72) While the invention has been particularly shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.