Inductively coupled plasma mass spectrometry for stable isotope tracer investigations

Inductively coupled plasma mass spectrometry (ICP-MS) has now been developed for application to stable isotope tracer investigations of several minerals/trace elements. Use of this method for such purposes requires an understanding of a number of fundamental issues: analytical chemistry performance of the method of isotopic analysis, relationship of the level of enriched isotope administered to the subject with background level of the isotope already present, the issues of cost, and finally the specific details of the biological issues to be explored.In this paper, a brief discussion of these issues is presented. As an example, the discussion is presented in relation to selected aspects of metabolism of selenium, employing the three stable isotopes⁷⁴Se,⁷⁷Se, and⁸²Se in the rat as the biological model.Analytical performance of hydride generation/ICP-MS is discussed for the required analyses of selenium isotopes. It is shown that for solutions containing 10 ng/ml Se of natural isotopic composition, optimized signal/background ratios greater than 40/1 can be obtained, resulting in worst-case detection limits (ng Se) of 2 (⁷⁴Se), and 0.6 (^77,82Se). The precision and accuracy of isotope ratio measurements for the method used routinely in biological studies is 1%. The accuracy of the method for quantitative isotopic analysis is compared with hydride generation/atomic absorption spectrophotometry (HG/AAS). The following results are given (g Se/g or ml; mean + 1 SD,n = 3–5; first HG/ICP-MS, second HG/AAS): SRM 1577a [bovine liver] 0.697 ± 0.002 versus 0.69 ± 0.01; human blood plasma 0.098 ± 0.001 versus 0.135 ± 0.008; human red cells 0.211 ± 0.002 versus 0.216 ± 0.012; and human urine 0.0473 ± 0.0003 versus 0.0489 ± 0.0003.An experiment is described with the rat to show the feasibility of the method for studies of selenium metabolism. Rats were placed on Se-free diet for eight weeks, given their Se requirements in the drinking water in the form of⁷⁶SeO ₃ ^2– and a single-day (day 3) replacement of their water with that containing highly enriched⁷⁴SeO ₃ ^2– . Isotopic analysis of carcass and selected organs revealed a high degree of isotopic enrichment with respect to⁷⁴Se during the entire eight weeks of the experiment, indicating the feasibility of this approach for detailed investigations of selenium metabolism in the rat.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

GC–ICP–MS determination of dimethylselenide in human breath after ingestion of <Superscript>77</Superscript>Se-enriched selenite: monitoring of in-vivo methylation of selenium

The amount of volatile dimethylselenide (DMSe) in breath has been monitored after ingestion of sub-toxic amounts of selenium (300 μg ⁷⁷Se, as selenite) by a healthy male volunteer. The breath samples were collected in Tedlar bags every hour in the first 12 h and then at longer intervals for the next 10 days. The samples were subjected to speciation analysis for volatile selenium compounds by use of cryotrapping–cryofocussing–GC–ICP–MS. Simultaneously, all urine was collected and subjected to total selenium determination by use of ICP–MS. By monitoring m/z 82 and 77, background or dietary selenium and selenium from the administered selenite were simultaneously determined in the urine and in the breath—dietary selenium only was measured by monitoring m/z 82 whereas the amount of spiked ⁷⁷Se (99.1% [enriched spike]) and naturally occurring selenium (7.6% [natural abundance]) were measured by monitoring m/z 77. Quantification of DMSe was performed by using DMSe gas samples prepared in Tedlar bags (linear range 10–300 pg, R ²=0.996, detection limit of Se as DMSe was 10 pg Se, or 0.02 ng L⁻¹, when 0.5 L gas was collected). Dimethylselenide was the only selenium species detected in breath samples before and after the ingestion of ⁷⁷Se-enriched selenite. Additional DM⁷⁷Se was identified as early as 15 min after ingestion of the isotopically-labelled selenite. Although the maximum concentration of ⁷⁷Se in DMSe was recorded 90 min after ingestion, the natural isotope ratio for selenium in DMSe (77/82) was not reached after 20 days. The concentration of DMSe correlated with the total Se concentration in the urine during the experiment (R ²=0.80). Furthermore, the sub-toxic dose of 300 μg selenium led to a significant increase of DMSe and renal excretion of background selenium, confirming that selenium ingested as selenite is homeostatically controlled by excretion. The maximum concentration of DMSe resulting from the spiked selenite was 1.4 ng Se L⁻¹ whereas the dietary background level was less than 0.4 ng Se L⁻¹. Overall excretion as DMSe was calculated to be 11.2% from the ingested selenite within the first 10 days whereas urinary excretion accounts for nearly 18.5%.     

Comparative performance study of different sample introduction techniques for rapid and precise selenium isotope ratio determination using multi-collector inductively coupled plasma mass spectrometry (MC-ICP/MS)

The analytical performance of five sample introduction systems, a cross flow nebulizer spray chamber, two different solvent desolvation systems, a multi-mode sample introduction system (MSIS), and a hydride generation (LI2) system were compared for the determination of Se isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP/MS). The optimal operating parameters for obtaining the highest Se signal-to-noise (S/N) ratios and isotope ratio precision for each sample introduction were determined. The hydride generation (LI2) system was identified as the most suitable sample introduction method yielding maximum sensitivity and precision for Se isotope ratio measurement. It provided five times higher S/N ratios for all Se isotopes compared to the MSIS, 20 times the S/N ratios of both desolvation units, and 100 times the S/N ratios produced by the conventional spray chamber sample introduction method. The internal precision achieved for the ⁷⁸Se/⁸²Se ratio at 100 ng mL⁻¹ Se with the spray chamber, two desolvation, MSIS, and the LI2 systems coupled to MC-ICP/MS was 150, 125, 114, 13, and 7 ppm, respectively. Instrument mass bias factors (K) were calculated using an exponential law correction function. Among the five studied sample introduction systems the LI2 showed the lowest mass bias of −0.0265 and the desolvation system showed the largest bias with −0.0321. Figure Illustration of the multi-mode sample introduction system for Se isotope ratiomeasurements     

Comparing the precision of selenium isotope ratio measurements using collision cell and sector field inductively coupled plasma mass spectrometry

The performance of two different types of inductively coupled plasma mass spectrometry (ICP-MS) instruments for resolving spectral overlaps on Se isotopes was compared by means of selenium isotopic ratio measurements. Examined were a quadrupole-based, hexapole collisions cell CC-ICP-MS and a double-focusing sector field SF-ICP-MS. Due to the importance of precise and accurate isotope ratio determination in environmental, clinical and nutritional studies, a thorough investigation of the critical instrumental parameters of each technique was performed. A hydride generation system was coupled with SF-ICP-MS to maintain high signal-to-noise ratios (S/N) at high mass resolution. However, 80Se+ was not completely separated from the argon dimer (40)Ar2+ at m/z=80, even in high-resolution mode. The same hydride generation system was coupled to a collision cell instrument and it was found that argon dimers are significantly reduced using a mixture of H2 and He gas with the cell. A lower mass bias of 2.5% per amu was determined for measured Se isotopes using the SF-ICP-MS instrument compared 3.6% observed for the CC-ICP-MS instrument. Under optimized conditions, the precision for Se isotope ratio measurements of both instruments was evaluated and compared measuring NIST-3149 Se standard solution. On average, the uncertainty determined by repeated measurements over the span of three individual measuring sessions in a period of 3 weeks ranged from 0.06% to 0.15% and 0.09% to 0.30% R.S.D. for the various isotope ratios using the CC-ICP-MS and SF-ICP-MS instrument, respectively. The detection limits (3) for total Se were determined by measuring 82Se and found to be 1.7 and 4.0 ng L(-1) for the CC-ICP-MS and SF-ICP-MS, respectively.     

Use of enriched <Superscript>74</Superscript>Se and <Superscript>77</Superscript>Se in combination with isotope pattern deconvolution to differentiate and determine endogenous and supplemented selenium in lactating rats

A quantitative methodology has been developed to differentiate between endogenous and supplemented selenium in lactating rats using two enriched selenium isotopes. Lactating rats were fed for 2 weeks with formula milk containing one enriched Se isotope, ⁷⁷Se, as the metabolic tracer. The isotopic composition of selenium in serum and urine samples was then measured by collision cell ICP-MS after the addition of a solution containing another enriched isotope, ⁷⁴Se, as quantitation tracer, before analysis. Isotope pattern deconvolution allowed the transformation of measured Se isotopic abundances into concentrations of natural abundance (endogenous) selenium and enriched ⁷⁷Se (supplemented) present in the samples. The proposed methodology was validated using serum and urine reference materials spiked with both ⁷⁷Se and ⁷⁴Se. The obtained results are discussed in terms of selenium exchange and half-life in lactating rats (11–12 days) and selenium levels in serum in comparison with non-supplemented rats and control rats after maternal feeding.     

Incorporation of selenium into selenoprotein P and extracellular glutathione peroxidase: HPLC-ICPMS data with enriched selenite

The metabolic turnover of selenoprotein P (Sel P) and extracellular glutathione peroxidase (eGPx) in plasma was examined by HPLC-ICPMS, with the use of enriched Se, [82Se]selenite. [82Se]selenite was injected intravenously into rats at a dose of 25 micrograms 82Se kg-1 body weight, and the concentrations of labeled 82Se and naturally occurring 77Se in the serum, liver and kidneys were determined in samples obtained 1, 3, 6, 9, 12, 18 and 24 h after the injection. The distributions of both exogenous (labeled) 82Se and endogenous (naturally occurring) 77Se in serum, and supernatant fractions of the liver and kidneys were determined on a gel filtration column by HPLC-ICPMS. This dose was shown not to affect the constitutive levels of cellular GPx (cGPx), eGPx and Sel P. The labeled Se in Sel P increased soon after the injection, peaked at 6-9 h and then decreased, whereas that in eGPx continued increasing after 6 h post-injection and then throughout the remaining observation period in the bloodstream. These observations demonstrated the rapid and efficient incorporation of Se into Sel P in the liver and excretion into the plasma followed by the slow and steady incorporation of Se into eGPx in the kidneys and excretion into the plasma, with a minimal response of cGPx to selenite injection.     

Isotope dilution quantification of ultratrace gamma-glutamyl-Se-methylselenocysteine species using HPLC with enhanced ICP–MS detection by ultrasonic nebulisation or carbon-loaded plasma

A method for the accurate determination of ultratrace selenium species of relevance to cancer research, such as gamma-glutamyl-Se-methylselenocysteine (γ-glutamyl-SeMC), using species-specific double isotope dilution analysis (IDA) with HPLC–ICP–MS is reported for the first time. The ⁷⁷Se-enriched γ-glutamyl-SeMC spike was produced in-house by collecting the fraction at the retention time of the γ-glutamyl-SeMC peak from a chromatographed aqueous extract of ⁷⁷Se-enriched yeast, pooling the collected fractions and freeze-drying the homogenate. The Se content of this spike was characterised using reverse isotope dilution mass spectrometry (IDMS) and the isotopic composition of this spike was checked prior to quantification of the natural abundance dipeptide species in garlic using speciated IDA. The extraction of the γ-glutamyl-SeMC species in water was performed in a sonication bath for 2 h after adding an appropriate quantity of ⁷⁷Se-enriched γ-glutamyl-SeMC to 50 mg of garlic to give optimal ⁷⁸Se/⁷⁷Se and ⁸²Se/⁷⁷Se ratios of 1.5 and 0.6, respectively. The effect of ultrasonic nebulisation, in comparison with the loading of the ICP with carbon (through the addition of methane gas on-line), on the detection of Se associated with γ-glutamyl-SeMC using collision/reaction cell ICP–MS with hydrogen as collision gas was investigated. Sensitivity enhancements of approximately fourfold and twofold were achieved using USN and methane mixed plasma, respectively, in comparison with conventional nebulisation and conventional Ar ICP–MS. However, an approximately twofold improvement in the detection limit was achieved using both approaches (42 ng kg⁻¹ for ⁷⁸Se using peak height measurements). The use of species-specific IDMS enabled quantification of the dipeptide species at ng g⁻¹ levels (603 ng g⁻¹ Se) in the complex food matrix with a relative standard deviation (RSD, n = 4) of 4.5%, which was approximately half that obtained using standard addition as a confirmatory technique. Furthermore, good agreement was found between the γ-glutamyl-SeMC species concentrations obtained using both calibration methods.     

Internal correction of spectral interferences and mass bias for selenium metabolism studies using enriched stable isotopes in combination with multiple linear regression

The analytical methodology for the in vivo study of selenium metabolism using two enriched selenium isotopes has been modified, allowing for the internal correction of spectral interferences and mass bias both for total selenium and speciation analysis. The method is based on the combination of an already described dual-isotope procedure with a new data treatment strategy based on multiple linear regression. A metabolic enriched isotope (⁷⁷Se) is given orally to the test subject and a second isotope (⁷⁴Se) is employed for quantification. In our approach, all possible polyatomic interferences occurring in the measurement of the isotope composition of selenium by collision cell quadrupole ICP-MS are taken into account and their relative contribution calculated by multiple linear regression after minimisation of the residuals. As a result, all spectral interferences and mass bias are corrected internally allowing the fast and independent quantification of natural abundance selenium (^natSe) and enriched ⁷⁷Se. In this sense, the calculation of the tracer/tracee ratio in each sample is straightforward. The method has been applied to study the time-related tissue incorporation of ⁷⁷Se in male Wistar rats while maintaining the ^natSe steady-state conditions. Additionally, metabolically relevant information such as selenoprotein synthesis and selenium elimination in urine could be studied using the proposed methodology. In this case, serum proteins were separated by affinity chromatography while reverse phase was employed for urine metabolites. In both cases, ⁷⁴Se was used as a post-column isotope dilution spike. The application of multiple linear regression to the whole chromatogram allowed us to calculate the contribution of bromine hydride, selenium hydride, argon polyatomics and mass bias on the observed selenium isotope patterns. By minimising the square sum of residuals for the whole chromatogram, internal correction of spectral interferences and mass bias could be accomplished. As a result, the tracer/tracee ratio could be calculated for each selenium-containing species and a time relationship for synthesis and degradation established. Both selenite and selenized yeast labelled with ⁷⁷Se were employed for comparative purposes.     

Negative Thermionen-Massenspektrometrie von Selen

Summary Negative thermal ionization is used to determine the selenium isotope ratios in a double-filament ion source. A thin film of barium hydroxide on the rhenium ionization filament is applied to increase the Se^– thermal ion current. The produced Se^– ion beam is by a factor of about four higher when selenious acid instead of barium selenite or sodium selenate is used. A strong dependence of the ion current on the temperature of the ionization filament is found showing the maximum ion intensity at temperatures of 970–1000 C. The different selenium isotope ratios of samples with natural isotopic abundance can be determined with relative standard deviations of 0.3–0.6%. This reproducibility is a good basis to improve the accuracy of the selenium atomic weight in the future by a calibrated measurement. An enriched ⁸²Se spike is used to analyse selenium traces in aquatic systems with isotope dilution mass spectrometry down to the pg/g level. In the concentration range of 4–23 ng/g the selenium content is determined with relative standard deviations of 0.1–5%. The results agree well with those obtained with a hydride generation atomic absorption system. It is shown that the described method of isotope dilution mass spectrometry analyses the sum of the inorganic species selenate, selenite and selenide, but not volatile organic selenium compounds.

---

---

Herrn Prof. Dr. W. Fresenius zum 75. Geburtstag gewidmet     

Determination of copper, molybdenum and selenium in biological reference materials by inductively coupled plasma mass spectrometry

Summary In a contribution to the elemental characterization of 10 new reference materials, Bovine Muscle Powder (136), Corn Starch (162), Hard Red Spring Wheat Flour (165), Soft Winter Wheat Flour (166), Whole Milk Powder (183), Wheat Gluten (184), Corn Bran (186), Durum Wheat Flour (187), Whole Egg Powder (188) and Microcrystalline Cellulose (189), the total concentrations of Cu, Mo and Se were determined by the application of an analytical method based on isotope dilution inductively coupled plasma mass spectrometry. Cu and Mo contents were quantified by measurement of ⁶⁵Cu/⁶³Cu and ⁹⁷Mo/¹⁰⁰Mo isotopic ratios following spiking with ⁶⁵Cu and ⁹⁷Mo and digestion with nitric acid. Selenium was separated as hydrogen selenide from the matrix using sodium borohydride after spiking with ⁸²Se and acid digestion-dry ashing and quantified by measurement of the ⁸²Se/⁷⁸Se isotopic ratio. Comparison of these results with those from a variety of other methods and assessment of the procedures using certified reference materials indicated that the determinations of Cu, Mo and Se were performed without analytical bias.     

Evaluation and correction of isotope ratio inaccuracy on inductively coupled plasma time-of-flight mass spectrometry

Isotope ratio measurements are found to have systematic bias when using the analog detection mode on an inductively coupled plasma time-of-flight (TOF) mass spectrometer. This bias is dependent upon the value of the ratio, the intensity of the signal, and the gain of the electron multiplier tube. The error should not appear if ion counting is employed instead of analog detection, although analog detection with time-of-flight has other distinct advantages. The cause of this isotope ratio inaccuracy is rooted in disproportionate recording of the analog signal because of the need to filter out noise by blocking analog signals below a threshold voltage. This attenuates smaller signals to a greater degree than larger signals. This variable “detection efficiency” causes a larger systematic error in the isotopic ratio as the isotopic abundances become more disparate. Ratios close to unity are generally accurate within the precision of the measurement. The use of an increased gain on the detector leads to improved ratio accuracy, but at the cost of decreased detector lifetime. This research presents a method of analyzing solutions using natural, known isotopic ratios to produce an efficiency correction curve. The average error of several isotope ratios for a 500 ng/mL solution of various elements with ratios between 3.4 and 10 was found to be 6.5% without correction, 3.0% with increased detector gain, 1.1% with efficiency correction and 0.6% with both increased gain and efficiency correction.     

Determination of Cd,Hg, Pb and Se in sediments slurries by isotopic dilution calibration ICP-MS after chemical vapor generation using an on-line system or retention in an electrothermal vaporizer treated with iridium

A method for the determination of Cd, Hg, Pb and Se in sediments reference materials by slurry sampling chemical vapor generation (CVG) using isotopic dilution (ID) calibration and detection by inductively coupled plasma mass spectrometry (ICP-MS) is proposed. Two different systems were used for the investigation: an on-line flow injection system (FI-CVG-ICP-MS) and an off-line system with in situ trapping electrothermal vaporization (CVG-ETV-ICP-MS). About 100 mg of the reference material, ground to a particle size ≤50 μm, was mixed with acid solutions (aqua regia, HF and HCl) in an ultrasonic bath. The enriched isotopes ¹¹¹Cd, ¹⁹⁸Hg, ²⁰⁶Pb and ⁷⁷Se were then added to the slurry in an adequate amount in order to produce an altered isotopic ratio close to 1. For the on-line system, a standing time for the slurry of 12 h before measurement was required, while for the batch system, no standing time is needed to obtain accurate results. The conditions for the formation of the analyte vapor were optimized for the evaluated systems. The following altered isotope ratios were measured: ¹¹¹Cd/¹¹⁴Cd, ¹⁹⁸Hg/¹⁹⁹Hg, ²⁰⁶Pb/²⁰⁸Pb e ⁷⁷Se/⁸²Se. The obtained detection limits in the on-line system, in μg g⁻¹, were: Cd: 0.15; Hg: 0.09; Pb: 6.0 and Se: 0.03. Similar detection limits were obtained with the system that uses the ETV: 0.21 for Hg, 6.0 for Pb and 0.06 μg g⁻¹ for Se. No signal for Cd was obtained in this system. One estuarine, two marine and two river certified sediments were analyzed to check the accuracy. The obtained values by both systems were generally in agreement with the certified concentrations, according to the t-test for a confidence level of 95%, demonstrating that isotope equilibration was attained in the slurries submitted to a chemical vapor generation procedure and detection by ICP-MS. The relative standard deviations were lower than 10%, adequate for slurry analysis. The almost quantitative analytes extractions to the aqueous phase of the slurry must favor equilibration of the added enriched isotope with the isotope in the sample, allowing the use of isotopic dilution calibration for slurry analysis.     

Improving boron isotope ratio measurement precision with quadrupole inductively coupled plasma-mass spectrometry

A method was developed to improve the precision of inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) for the determination of boron isotope ratios (¹¹B/¹⁰B) in various environmental materials including seawater. This approach is based on the common analyte internal standardization (CAIS) chemometric algorithm. The sample solution obtained after digestion is spiked with lithium, and both ⁷Li/⁶Li and ¹¹B/¹⁰B values are measured using long-counting periods (20 min). The CAIS algorithm corrects the measured ¹¹B/¹⁰B values for (a) statistical fluctuations resulting from short-term noise; (b) drift in ¹¹B-to-¹⁰B ratio as a result of long-term deviation in instrumental parameters likely to occur during long counting times; (c) change in ¹¹B-to-¹⁰B ratio caused by variation in matrix elements concentrations; and (d) drift in mass bias correction factor. Comparing boron isotopic ratios in seawater measured by conventional and the new isotope ratio methods validates the procedure. A synthetic isotopic mixture of boron SRM 951 and enriched ¹⁰B SRM 952 also was examined. The CAIS method provided a measured boron isotopic ratio precision of 0.05% R.S.D. while eliminating 5.1% matrix concentration error and 0.25% instrumental drift error.     

Old commercialized magnetic particles new trick: Intrinsic internal standard

Magnetic particles（MPs） are the most widely used commercialized engineering particles,which gained great success in various biological applications.Inspired by their intrinsic Fe isotope composition,we discovered a commercialized MPs-internal standard’s novel function to realize the accurate quantification of biomolecules.The bioassay of carcinoembryonic antigen（CEA） was chosen as a modal system.The Fe isotope in MPs and Au isotope in report probes were simultaneously and sensitively detected by...     

Development of an ICP-IDMS method for accurate routine analyses of toxic heavy metals in polyolefins and comparison with results by TI-IDMS

An inductively coupled plasma isotope dilution mass spectrometric (ICP-IDMS) method was developed as a suitable method - with respect to its sensitivity, precision, accuracy, and time-consumption - for the analysis of toxic heavy metal traces (Pb, Cd, Cr, and Hg) in polyolefins. Results for Pb, Cd, and Cr were compared with those obtained by thermal ionization isotope dilution mass spectrometry (TI-IDMS), which was used as a reference method. Because of its high first ionization potential and its high volatility mercury could not be determined by TI-IDMS. A multi-element spike solution, containing isotopically enriched 206Pb, 116Cd, 53Cr, and 201Hg, was used for the isotope dilution step. Decomposition of the polyolefin samples was carried out with concentrated HNO3 at temperatures of about 300 degrees C in a high pressure asher (HPA). This procedure decomposes polyolefins completely and allows isotopic equilibration between sample and spike isotopes. Detection limits of 16 ng/g, 5 ng/g, 164 ng/g, and 9 ng/g were obtained for Pb, Cd, Cr, and Hg by ICP-IDMS using only sample weights of 0.25 g. In different commercially available polyethylene samples heavy metal concentrations in the range of < 5 ng/g to 4 x 10(3) ng/g were analyzed. Both mass spectrometric methods were applied within the EU project "Polymeric Elemental Reference Material (PERM)" for the certification of two polyethylene reference materials. The ICP-IDMS results agreed very well with those of TI-IDMS which demonstrates the accuracy of the ICP-IDMS method also suitable for routine analyses.     

Determination of selenium in nutritional supplements and shampoos by flow injection-hydride generation-atomic fluorescence spectrometry

A method for the determination of Se in pharmaceutical samples (nutritional supplements and shampoos) is proposed. The method involves two steps: (1) digestion of the samples and reduction of all forms of Se to Se(IV), which is complete in only 10 min by the use of a focused microwave digestor; and (2) continuous derivatisation (hydride formation) and spectrometry detection by atomic fluorescence. The method can be applied over a wide range of concentrations (0.3-1300 ng ml(-1) of Se) with good repeatability (RSD values lower than 4.6%). The method has been applied successfully to a reference material, and two different types of pharmaceuticals (namely, five different nutritional supplements-with Se present as sodium selenite and Se-methionine-and two shampoos, with selenium sulphide), in agreements with the certified and nominal values, respectively. Yields ranged between 86.5 and 104.8%, and good precision (RSD values lower than 4.2%) were obtained in all instances.     

应用多接收器电感耦合等离子体质谱法测定海洋碳酸盐中稳定铅同位素组成

研究了多接收器电感耦合等离子体质谱仪(MC-ICP MS)测定铅同位素比值时,影响测试结果准确度和精密度的主要因素及其优化过程。在优化条件下,10 h内连续30次测定4 ng/mL NIST SRM 981同位素标准溶液铅同位素比值,获得208Pb/206Pb、207Pb/206Pb和206Pb/204Pb相对标准偏差(RSDs)分别为0.005%、0.004%和0.054%。长期监测208Pb/206Pb、207Pb/206Pb和206Pb/204Pb,标准偏差(2SDs)分别为0.000 06、0.000 05和0.006 7。采用NEPTUNE MC-ICP MS法测定了低铅海洋碳酸盐样品中稳定铅的同位素比值,并对南海橙黄滨珊瑚(Porites lutea)和库氏砗磲(Tridacna gigas)进行了分析,得到全流程空白为8～10 pg,重复样误差优于0.1%。经0.50 mol/L HNO3洗脱之后,得到海南珊瑚样品中208Pb/206Pb、207Pb/206Pb的比值分别为2.086 2±0.001 5、0.849 90±0.001 47(n=16);海南砗磲样品中208Pb/206Pb、207Pb/206Pb的比值分别为2.116 9±0.004 2、0.864 81±0.001 62(n=9)。进一步考察了南海海洋碳酸盐中204Pb的同位素比值。分析结果表明,南海海洋碳酸盐中稳定铅同位素比值与中国气溶胶、珠江三角洲大气沉降、黄土及南海海底玄武岩等具有很好的相关性。方法适用于复杂基体高钙低铅的海洋碳酸盐样品中铅同位素比值的分析。     

The determination of arsenic and selenium in standard reference materials using sector field ICP-MS in high resolution mode

Sector field ICP-MS was used to analyse As and Se in a range of standard reference materials (NIST 1643d Water, NIST 1573a Tomato Leaves, NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine Level 2). A spectral resolution of m/Δm = 7500 enabled ⁷⁵As and ⁷⁷Se to be separated from problematic ArCl interferences. Following microwave acid digestion, solid samples were typically diluted 1 + 99 prior to analysis, while the urine sample was diluted 1 + 9. The water sample was analysed undiluted and diluted 1 + 9. Despite near baseline spectral separation, ⁷⁵As and ⁷⁷Se were still found to be influenced by ArCl at high Cl concentrations, the effect being most pronounced for ⁷⁷Se. When necessary ⁸²Se was also monitored to determine the accuracy of the ⁷⁷Se results. Detection limits (LOD, based on 3σ of 10 replicates) for ⁷⁵As, ⁷⁷Se and ⁸²Se in ultra-pure water, 1% (w/w) HNO₃ and 1% (w/w) HCl were ∼ 0.1, ∼ 0.2 and ∼ 0.5 ng g^–1, respectively. Although signal intensities when using high resolution were ∼ 1% of that found when using low resolution mode (m/Δm = 300), measured As concentrations and certified values were found to agree to within ± 11% for all samples analysed. The concentration of Se in NIST 1566a Oyster Tissue, NIST 2704 Buffalo River Sediment and Bio-Rad Reference Urine were found to be in agreement with certified values to within ± 15– 20%, as measured by ⁷⁷Se. However, closer agreement (± 5%) was found when these samples were analysed using ⁸²Se. The Se concentration in NIST 1643d Water was found to agree to within ± 5% of the certified value (depending on dilution factor). Due to the low concentration of Se in NIST 1573a Tomato Leaves, quantitation was not possible (below LOQ, 10σ). As a consequence of the lower ion transmission when using resolution 7500, analytical precisions were found to be elevated over that normally observed using low resolution mode, typically ± 5–20% (depending on analyte concentration and isotopic abundance). Received: 28 December 1998 / Accepted: 9 February 1999     

Application of the total evaporation technique to chromium isotope ratio measurement by thermal ionization mass spectrometry

The total evaporation technique of thermal ionization mass spectrometry was applied to the isotopic analysis of chromium. High measurement reproducibility of the chromium isotope ratios was verified (2 S.E. < 0.05% (⁵³Cr/⁵²Cr)), while a clear mass fractionation effect was observed by using conventional measurement technique. The chromium isotope ratios analyzed by the total evaporation method were not affected by the sample amount on the rhenium filament (50-500 ng Cr). The isotopic analysis under the coexistence of zinc was also performed, and its effect to the chromium isotope ratios was confirmed.     

Species-specific isotope-ratio measurements of volatile tin and antimony compounds using capillary GC-ICP-time-of-flight MS

The analytical performance of an axial inductively-coupled-plasma time-of-flight mass spectrometer (ICP-TOFMS) as a detector for fast transient chromatographic signals resulting from the coupling to capillary gas chromatography (CGC) was investigated. A cryotrapping GC-ICP-TOFMS method for the determination of volatile metal(loid) compounds (VOMs) in gases was used and the suitability of the TOF mass analyzer for multielemental speciation analysis and multi-isotope ratio determinations was studied in terms of accuracy and precision. Isotope ratios 118Sn/120Sn and 121Sb/123Sb have been determined in in-house gas standard atmospheres in Tedlar bags at two different levels (100 pg and 1 ng) for different elemental species (SnH4, MeSnH3, Me2SnH2, Me3SnH, BuSnH3, SbH3, and MeSbH2). A limitation arising from counting statistics in both detection modes could be shown. A solution containing rhodium (10 ng mL(-1)) and cadmium (40 ng mL(-1)) was introduced simultaneously to the GC outlet. Rhodium acts as a continuous internal standard and Cd is used for mass-bias correction (by measuring the 111Cd/113Cd ratio). The detection system in both pulse counting and analog mode was examined. The best attainable precision was established for Me2SnH2 (analog mode, 12 replicates, 1 ng, RSD 0.34%, accuracy 0.31%) whereas most other species ranged between 0.4 and 0.5% RSD if higher concentrations were used. The limitations of the pulse counting system are clearly seen, with peak heights of more than 2000 counts reaching saturation (for an integration time of 100 ms), which reduces the accuracy of isotope ratio determinations. A dozen VOM could be detected in an aged landfill gas sample; several unidentified Sn compounds were present. Although their isotope ratios are within the confidence value of the standards, it is not yet clear if the acquired precision is good enough to identify isotopic fractionation of metal(loid)s through biovolatilization processes. With the precision achieved, the combination of cryotrapping GC and ICP-TOFMS is a powerful tool for monitoring volatile multi-element species in multi-tracer experiments and isotope dilution methodology.