Accurate Isotope Ratio Measurements Research Articles

Interpretation of butyltin mass spectra using isotope pattern reconstruction for the accurate measurement of isotope ratios from molecular clusters

The fragmentation patterns of butyltin compounds (mono-, di-, and tributyltin) in an electron impact ion source were studied using an isotope pattern reconstruction algorithm with emphasis on isotope ratio measurements from molecular clusters. For this purpose, standards of natural tin isotope abundance and a (119)Sn-enriched mixture of the three compounds were both ethylated and propylated using sodium tetraalkylborates. The corresponding mass spectra of the various tetraalkyltin compounds prepared were obtained by GC/MS after their extraction with hexane. The results showed that pure interference-free molecular clusters were obtained only for certain R(3)Sn(+) ions where no isobaric overlap with R(2)SnH(+) ions occurred (e.g. BuEt(2)Sn(+) overlaps with Bu(2)SnH(+)). These ions are ideal candidates for accurate Sn isotope ratio measurements, while isotope pattern perturbing interferences are observed for other molecular fragments down to Sn(.)(+). Isotope pattern reconstruction algorithm thus can be used as an analytical tool to ensure the absence of molecular interferences--a requirement for accurate isotope ratio measurements from molecular clusters. The relevance of these studies for the determination of butyltin compounds in environmental samples by isotope dilution GC/MS is also discussed.

Isotope ratios on transient signals with GC–MC–ICP-MS

ICP-MS instruments with multiple ion collectors (MC–ICP-MS) provide highly precise and accurate isotope ratio measurements. These instruments are easily coupled to chromatography, laser ablation or flow injection devices because of the ICP source, and thus attract general analytical interest for measurement of highly precise element isotope ratios on transient signals. Some few papers have been published today where MC–ICP-MS was employed for isotope ratios on transient signals, and in general, results are encouraging. Isotope ratio precision obtained is usually at least 10-fold superior compared to measurements with ICP-QMS. Nevertheless, most authors describe a drift of isotope ratios during the transient peak. This behaviour was either interpreted as a problem in the sample introduction system, or was related to instrumental mass bias drift, but has to date not been completely evaluated or explained. In this paper, we systematically investigate the drift observed on lead and mercury isotopes during short transient signals obtained from GC coupled to different MC–ICP-MS systems. We can clearly show that neither changes in instrumental mass bias, nor chromatographic fractionation effects are the source of the observed isotope ratio drift. An influence of analyte concentration on the observed drift was as well excluded as a source for this drift, because the slope on isotope ratios show the same values for different concentrations evaluated. In contrast, the peak width was found to influence the extent to which the isotope ratio drifts during peak elution. Thus, the relative intensity change per time seems to be an important factor for the measurements of transient signals with MC–ICP-MS.

Determination of uranium isotopic ratios in biological samples using laser ablation inductively coupled plasma double focusing sector field mass spectrometry with cooled ablation chamber

An analytical procedure has been proposed for the determination of precise uranium isotope ratios in a thin uranium layer on a biological surface by laser ablation inductively coupled plasma sector field mass spectrometry (LA-ICP-MS). A cooled laser ablation chamber using a Peltier element was developed in order to analyze element distribution in thin cross-sections of frozen tissues with a lateral resolution in the μm range. In order to study the figures of merit of LA-ICP-MS with the cooled laser ablation chamber, one drop (20 μL, U concentration 200 ng mL −1), each of the certified isotope reference materials NIST U350 and NIST U930, the uranium isotopic standard CCLU-500 and also a drop of uranium with a natural isotopic pattern was deposited and analyzed on the biological surface (flower leaf). The precision and accuracy of isotope ratio measurements are significantly improved using cooled laser ablation chamber in comparison to non-cooled chamber. The precision of the measurements of isotope ratios in the range of 2.0–1.6% for 234U/ 238U, 1.3–0.4% for 235U/ 238U and 2.1–1.0% for 236U/ 238U in selected uranium isotopic standards reference material were determined by microlocal analysis (diameter of laser ablation crater: 15, 25 and 50 μm) using LA-ICP-MS with a cooled laser ablation chamber. The accuracy of 234U/ 238U, 235U/ 238U and 236U/ 238U isotope ratio measurements varied in the range of 4.2–1.1%, 2.4–0.5% and 4.8–1.1%, respectively, and were dependent on the diameter of the laser beam used.

Accurate measurement of uranium isotope ratios in soil samples using thermal ionization mass spectrometry equipped with a warp energy filter

A chemical and mass-spectrometric procedure for uranium isotopic analysis using a thermal ionisation mass spectrometer equipped with a Wide Aperture Retardation Potential energy filter has been developed and applied to uranium isotopic measurements for various soil samples. Soil samples were digested using a microwave digestor. Uranium was isolated from soil samples by the chemical separation procedure based on the use of anion-exchange resin and UTEVA extraction chromatography column. The isotope ratios were measured for two certified reference materials by using a VG Sector 54-30 thermal ionisation mass spectrometer in dynamic mode with Faraday cup and Daly ion counting system. Replicates of standard reference materials showed excellent analytical agreement with established values supporting the reliability and accuracy of the method. Precision of the 235U/238U ratio was achieved by a correction factor of 0.22% amu as a function of ion-beam intensity with sample loads of around 250 ng of U. The resulting reproducibility for standards and soil samples was better than 0.2% at two standard deviations (SD). Uranium isotopic compositions have been determined in several reference soil samples such as Buffalo river sediment, NIST 2704, river sediment SRM 4350b and ocean sediment NIST-4357 and a Chernobyl soil sample. There was a significant deviation from the natural uranium in comparison with Chernobyl soil samples.

Accurate and precise Pb isotope ratio measurements in environmental samples by MC-ICP-MS

Analytical protocols for accurate and precise Pb isotope ratio determinations in peat, lichen, vegetable, chimney dust, and ore-bearing granites using MC-ICP-MS and their application to environmental studies are presented. Acid dissolution of various matrix types was achieved using high temperature/high pressure microwave and hot plate digestion procedures. The digests were passed through a column packed with EiChrom Sr-resin employing only hydrochloric acid and one column passage. This simplified column chemistry allowed high sample throughput. Typically, internal precisions for approximately 30 ng Pb were below 100 ppm (±2 σ) on all Pb ratios in all matrices. Thallium was employed to correct for mass discrimination effects and the achieved accuracy was below 80 ppm for all ratios. This involved an optimization procedure for the 205 Tl/ 203 Tl ratio using least square fits relative to certified NIST-SRM 981 Pb values. The long-term reproducibility (±2 σ) for the NIST-SRM 981 Pb standard over a 5-month period (35 measurements) was better than 350 ppm for all ratios. Selected ore-bearing granites were measured with TIMS and MC-ICP-MS and showed good correlation (e.g., r=0.999 for 206 Pb/ 207 Pb ratios, slope=0.996, n=13). Mass bias and signal intensities of Tl spiked into natural (after matrix separation) and in synthetic samples did not differ significantly, indicating that any residual components of the complex peat and lichen matrix did not influence mass bias correction. Environmental samples with very different matrices were analyzed during two different studies: (i) lichens, vegetables, and chimney dust around a Cu smelter in the Urals, and (ii) peat samples from an ombrotrophic bog in the Faroe Islands. The presented procedure for sample preparation, mass spectrometry, and data processing tools resulted in accurate and precise Pb isotope data that allowed the reliable differentiation and identification of Pb sources with variations as small as 0.7% for 206 Pb/ 207 Pb .

Measurement of isotope ratios on transient signals by MC-ICP-MS.

Precise and accurate isotope ratio measurements are an important task in many applications such as isotope-dilution mass spectrometry, bioavailability studies, or the determination of isotope variations in geological or nuclear samples. The technique of MC-ICP-MS has attracted much attention because it permits the precise measurement of isotope compositions for a wide range of elements combined with excellent detection limits due to high ionisation efficiencies. However, the results are based mainly on measurements using continuous sample introduction. In the present study the determination of isotope ratios on various transient signals with a time duration of 30 to 60 s has been achieved by coupling high-performance liquid chromatography to a multicollector inductively coupled plasma mass spectrometer. In order to investigate the origin of ratio drifts across the transient signals for this hyphenated technique, measurements with the same standard solutions were also carried out using a flow-injection device for sample introduction. As a result of this application it could be concluded that the main source of the bias in the measured isotope ratios is within the ICP-MS instead of fractionation effects on the chromatographic column material. Preliminary studies on short transient signals of gaseous samples (dry plasma) showed a reverse fractionation effect compared with wet plasma conditions (flow injection and HPLC).

Impact of matrix effects on the accurate measurement of Li isotope ratios by inductively coupled plasma mass spectrometry (MC-ICP-MS) under “cold” plasma conditions

Lithium isotopic measurements by MC-ICP-MS using “cold” plasma conditions are characterised by fewer baseline interferences and improved reproducibility as compared with conventional hot plasma techniques. The 2σ precisions for 1200 W, 800 W and 680 W are conservatively estimated as 1.1‰, 0.7‰ and 0.5‰. However, matrix effects are most significant at the coldest plasma temperatures, with deviations of >0.5‰ δ7Li being observed at 680 W for 100 ng g−1 Li solutions containing ≥500 ng g−1 Na. However, no matrix effects were detected at 800 W for solutions containing up to 5000 ng g−1 Na. Lithium isotopic measurements under all conditions are strongly sensitive to variations in acid concentration, requiring precise matrix matching between samples and standards. Self-induced Li matrix effects are not evident for Li concentrations below 120 ng g−1, however blank corrections are necessary. The isotopic ratios of Li separated from seawater at 680 W (δ7Li = +30.7 ± 0.4‰) and 800 W (+30.4 ± 0.9) are subtly lower than those obtained for 1200 W (δ7Li = +32.0 ± 0.2), although all lie within error given the reported reproducibility and all are within the range of previously published analyses. With minimisation of matrix induced fractionation, the precision and accuracy achievable at 680 W and 800 W equals or surpasses that previously reported for TIMS and conventional hot plasma ICP-MS.

Accurate isotope ratio measurements of ytterbium by multiple collection inductively coupled plasma mass spectrometry applying erbium and hafnium in an improved double external normalization procedure

Four series of experiments had been carried out for correct ytterbium isotope ratios measurements (IRM). (1) For erbium IRM, 167Er/166Er = 0.68260, derived from recently published thermal ionization mass spectrometry (TIMS) measurements, was applied as an internal normalization factor. (2) Ytterbium has no calibrated isotope ratio values. In an erbium–ytterbium–hafnium mixture solution we determined (171Yb/172Yb)Er-corr = 0.653436(4) using the above 167Er/166Er ratio as external normalization factor. In the same mixture we also determined (171Yb/172Yb)Hf-corr = 0.653297(25) using the well-known IUPAC value for the ratio 179Hf/177Hf = 0.73250 as external normalization factor. In these measurements no atomic isobaric interference exists at the corresponding mass numbers. (3) These two 171Yb/172Yb ratios, derived with data at mass intervals 166–167 and ∼178, were interpolated to mass interval 171–172, applying linear and slightly curved (exponential) line interpolations. 171Yb/172Yb = 0.653367 and 0.653344 are the linear and curved line interpolated values, respectively. Then, all ytterbium isotope ratios were measured in pure ytterbium solutions and internally corrected with each of the two 171Yb/172Yb ratios. (4) Using the literature value for 178Hf/177Hf = 1.467290, we obtained 179Hf/177Hf = 0.73249(2), which is in very good agreement with the above mentioned IUPAC value. The exponential correction equation was applied in this work. The mean Er ratio values are in good agreement with literature, except for the 162Er/166Er ratio, which is 2.19% lower than the reported absolute TIMS value. The mean Yb ratio values obtained in this work are in fair agreement with data reported in the last 25 years. We believe that they are the closest to the true values. The precision (2σ) is at least 3–8 times better comparing to the measured TIMS precision.

A quantitative evaluation of spurious results in the infrared spectroscopic measurement of CO2 isotope ratios

The possible generation of spurious results, arising from the application of infrared spectroscopic techniques to the measurement of carbon isotope ratios in breath, due to coincident absorption bands has been re-examined. An earlier investigation, which approached the problem qualitatively, fulfilled its aspirations in providing an unambiguous assurance that 13C16O2/12C16O2 ratios can be confidently measured for isotopic breath tests using instruments based on infrared absorption. Although this conclusion still stands, subsequent quantitative investigation has revealed an important exception that necessitates a strict adherence to sample collection protocol. The results show that concentrations and decay rates of the coincident breath trace compounds acetonitrile and carbon monoxide, found in the breath sample of a heavy smoker, can produce spurious results. Hence, findings from this investigation justify the concern that breath trace compounds present a risk to the accurate measurement of carbon isotope ratios in breath when using broadband, non-dispersive, ground state absorption infrared spectroscopy. It provides recommendations on the length of smoking abstention required to avoid generation of spurious results and also reaffirms, through quantitative argument, the validity of using infrared absorption spectroscopy to measure CO2 isotope ratios in breath.

Effect of alkali metals on the accuracy of isotope ratio measurement of uranium by ICP-MS

The effect of alkali metals on the accuracy of uranium isotope ratio measurements by inductively-coupled plasma sector field mass spectrometry (ICP-SFMS) was investigated. The isotope ratio measurements were performed on the isotopic reference materials, NBS SRM U-015 and U-350, in solutions containing various amounts of any one of Na, K and Rb. The mass bias of the instrument was corrected using an isotopic standard solution without alkali metals. The observed 235U/238U isotope ratios for SRM U-015 began to increase at around 1 μg g−1 for K and Rb, and at around 10 μg g−1 for Na. The deviations from the certified value at the concentration of 300 μg g−1 were evaluated to be 0.8%, 1.9% and 2.5% for Na, K and Rb, respectively. For SRM U-350, the 234U/238U, 235U/238U and 236U/238U isotope ratios were measured in various K concentrations. The deviation per unit mass at 300 μg g−1 K was evaluated as 0.56%.

State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS : Plenary Lecture

Download options Please wait... Article information DOI https://doi.org/10.1039/B203028B Article type Review Article Submitted 26 Mar 2002 Accepted 17 Jul 2002 First published 06 Aug 2002 Download Citation J. Anal. At. Spectrom., 2002,17, 1172-1185 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions State-of-the-art and progress in precise and accurate isotope ratio measurements by ICP-MS and LA-ICP-MS J. S. Becker, J. Anal. At. Spectrom., 2002, 17, 1172 DOI: 10.1039/B203028B To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Johanna Sabine Becker

Isotope ratio measurement by hexapole ICP-MS: mass bias effect, precision and accuracy

Isotope ratios were measured from the low mass (25Mg/26Mg) to the high mass (203Tl/205Tl) region, using an ELAN 5000 and a Platform Hex-ICP-MS, to investigate mass bias effects on the two instruments. For the ELAN 5000, the percentage mass discrimination per atomic mass unit (MD%) varies from 5% for 25Mg/26Mg to 0.7% for 203Tl/205Tl, whereas for the Platform it ranges from 10% to 0.11%, respectively. The Platform has higher mass bias at low mass and lower mass bias at the high mass region compared to the ELAN 5000. The difference in mass bias between the two instruments is attributed to the different ion transferring lens system, i.e., electrostatic lens system in the ELAN 5000 and hexapole collision cell in the Platform. For the Platform Hex-ICP-MS, the addition and variation of collision/reaction gases (He and H2) does not introduce any additional mass bias. The precision and accuracy of isotope ratio measurement by the Platform Hex-ICP-MS were assessed using the NBS982 Pb isotope reference material and an elemental Sn standard spiked with 117Sn-enriched spike. The overall precision was found to be 0.1–0.6% relative standard deviation.

Comment: Variations in the isotope composition of mercury in a freshwater sediment sequence and food web

Discussion 2311 In a recent paper, Jackson (2001) presented data on variations of the Hg isotope composition in sediments and associated food webs in Lake Ontario. The author claimed that the results demonstrated fractionation of Hg isotopes along a sediment depth profile and in a simple food chain. He went on to correlate sequentially extracted metal concentrations with Hg ratios in the sediment profile and identified zones of anthropogenically derived mercury. In this comment, we question the main conclusion of the paper, which is that this investigation demonstrates a systematic isotope fractionation of Hg in a sediment profile or in a food chain. The paper is lacking simple statistical evaluation of the data and neglects basic rules of stable isotope analysis. If the data were valid, it would demonstrate that more recent, presumably anthropogenic sources of mercury to lakes have a different isotopic composition than historically deposited Hg. This would be a great tool to elucidate natural mercury cycling and very useful to regulators developing policies on the controls of mercury emissions. In addition, if there were isotope discrimination in food chains, this would enable us to better understand mercury bioaccumulation in food webs, using similar strategies that have been applied in the study of other elements (e.g., C, N, and S, Peterson and Fry 1987). We do agree, however, with his conclusion that more precise and sensitive mass spectrometric instrumentation should be used in any future work. Only high-precision instrumentation, such as a multicollector ICP/MS, will provide the ability to determine potential variations in isotope ratios in the environment; these will almost certainly be smaller than the variations reported in this paper. It is well known in the field of precise and accurate isotope ratio measurements by ICP/MS that the instrument used in this study (quadrupole ICP/MS) is incapable of measuring isotope ratios with the precision necessary to observe the level of discrimination reported in the paper (Becker and Dietze 2000; Heumann et al. 1998). With respect to the data on isotope fractionation in sediments, the author based most of his conclusion on one selected pair of Hg isotopes (199Hg/201Hg) that apparently showed a trend with depth in the sediment (the heavier Hg isotope is enriched in deeper segments, see fig. 1 in Jackson 2001). The conclusion was that more recently deposited mercury had a different isotopic ratio than mercury deposited in historic times. However, the author failed to include the uncertainty of his measurements in the interpretation. As the standard deviations of the reported Hg ratio determinations are given in Jackson’s table 5, we were able to reconstruct part of his fig. 1 to included error bars (Fig. 1, this paper). We show means ± 2 standard deviations (SD) to illustrate that there are no significant differences between ratios measured in surface sediments and in deeper sections of the core. We further believe that mean ratios have to be compared rather than median values because isotope ratios in different samples can only be compared considering the external reproducibility of replicate measurements. A more serious and fundamental criticism of the paper by Jackson is that the author seems to have selectively chosen to examine a single pair of Hg isotopes (199Hg/201Hg). The 199Hg/201Hg ratio is in fact the only pair of Hg isotopes that apparently shows such a trend and even then, only if a statistical evaluation is neglected. None of the other 15 possible pairs of Hg isotopes confirms the postulated trend. Indeed, some pairs of isotopes even suggest the opposite trend, that is, the lighter isotope is enriched with depth. If fractionation processes

Isotope ratio measurements using gas chromatography inductively coupled plasma mass spectrometry for the assessment of organolead sources

The measurement of isotope ratios (within individual species), for a range of organolead compounds, was carried out by coupling a capillary gas chromatograph (HP 6890) to an inductively coupled plasma mass spectrometer (HP 4500). Optimum conditions for the separation and detection of the derivatised lead organometallic compounds were studied focusing on the precise and accurate measurement of isotope ratios on each chromatographic peak. Three lead isotopes were monitored, 206, 207 and 208 using 66 ms integration time per isotope. Under optimum conditions adequate chromatographic peak profiles could be measured. Based on peak area ratios, isotope ratios were determined for five butylated lead compounds (trimethyllead, dimethyllead, triethyllead, diethyllead and inorganic lead) and mixed methyl-ethyl tetraalkyllead compounds from leaded gasoline. Mass bias was corrected using both ethylated and butylated NIST 981 isotopic standard (Common Lead) and a certified enriched 204Pb solution. No isotope fractionation effects were observed during derivatisation of the different organometallic compounds. Applications of this methodology include the differentiation between Pb sources of the different standards used, the measurement of Pb isotope ratios in the organolead compounds present in airborne particulate matter of Oviedo to ascertain their origin and the comparison with isotope ratios in the BCR CRM 605 urban dust reference material. Significant differences between lead isotope ratios in the different species of organolead compounds and in total lead (after acid digestion of the samples) were observed. These differences can be used to trace back sources of lead contamination, by resorting to the speciation data, in cases where the isotope ratios of total lead in the samples are inconclusive in identifying Pb contamination sources.

A comparison between quadrupole, double focusing and multicollector ICP-MS : Part II. Evaluation of total combined uncertainty in the determination of lead in biological matrices by isotope dilution

The performance of three different ICP-MS instruments for the determination of lead in biological reference materials by isotope dilution analysis has been evaluated. A quadrupole-based instrument (HP-4500), a double focusing single collector instrument (Element) and a single focusing multicollector instrument (IsoProbe) were all evaluated for precise and accurate measurement of lead isotope ratios and their application to lead isotope dilution analysis using enriched 204Pb. Detector dead time influenced the measured ratios only for the Element while mass bias was corrected using thallium added to each sample. Accurate lead concentration results were obtained for all three instruments. For the HP-4500 and the Element, the precision of the isotope ratios was observed to be independent of the ratio itself over two orders of magnitude, allowing the use of the random error propagation theory for uncertainty calculations. Finally, an uncertainty budget was also estimated for all measurements, showing that the measurement uncertainty and the concentration of the spike were the main sources of uncertainty. For the HP-4500, the main contribution to the total uncertainty came from the measurements, while for the IsoProbe the main contribution came from the uncertainty in the concentration of the spike. The Element provided results in between the other two instruments.

Laser ablation inductively coupled plasma mass spectrometry for the trace, ultratrace and isotope analysis of long-lived radionuclides in solid samples

The capability of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for determination of long-lived radionuclides in different materials (e.g., in geological samples, high-purity graphite and nonconducting concrete matrix) was investigated. The main problem in the quantification of the analytical results of long-lived radionuclides is that (except for geological samples) no suitable standard reference materials are available. Therefore, synthetic laboratory standards (graphite and concrete matrix doped with long-lived radionuclides, such as 99Tc, 232Th, 233U, 235U, 237Np, 238U) were prepared and used for quantification purposes in LA-ICP-MS. Different calibration procedures—the correction of analytical results with experimentally determined relative sensitivity coefficients (RSCs), the use of calibration curves and solution calibration by coupling LA-ICP-MS with an ultrasonic nebulizer—were applied for the determination of long-lived radionuclides, especially for Th and U in different solid samples. The limits of detection of long-lived radionuclides investigated in concrete matrix are determined in the pg g −1 range (e.g., for 237Np-50 pg g −1 in quadrupole LA-ICP-MS; for 233U-1.3 pg g −1 in double-focusing sector field LA-ICP-MS). Results of isotope ratio measurements of Th and U in synthetic laboratory standards and different solid radioactive waste materials of direct analysis on solid samples using LA-ICP-MS are comparable to measurements using the double-focusing sector field ICP-MS after separation of the analyte, even if no possible interference of atomic ions of analyte and molecular ions are expected. Furthermore, LA-ICP-MS allows precise and accurate isotope ratio measurements of Th and U in solid samples. For example, the isotope ratio 234U/ 238U = 0.000067 in radioactive reactor graphite was determined with a precision of 1.1% relative standard deviation (RSD).

Precise and accurate isotope ratio measurements by ICP-MS.

The precise and accurate determination of isotope ratios by inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) is important for quite different application fields (e.g. for isotope ratio measurements of stable isotopes in nature, especially for the investigation of isotope variation in nature or age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, quality assurance of fuel material, for reprocessing plants, nuclear material accounting and radioactive waste control, for tracer experiments using stable isotopes or long-lived radionuclides in biological or medical studies). Thermal ionization mass spectrometry (TIMS), which used to be the dominant analytical technique for precise isotope ratio measurements, is being increasingly replaced for isotope ratio measurements by ICP-MS due to its excellent sensitivity, precision and good accuracy. Instrumental progress in ICP-MS was achieved by the introduction of the collision cell interface in order to dissociate many disturbing argon-based molecular ions, thermalize the ions and neutralize the disturbing argon ions of plasma gas (Ar+). The application of the collision cell in ICP-QMS results in a higher ion transmission, improved sensitivity and better precision of isotope ratio measurements compared to quadrupole ICP-MS without the collision cell [e.g., for 235U/238U approximately 1 (10 microg x L(-1) uranium) 0.07% relative standard deviation (RSD) vs. 0.2% RSD in short-term measurements (n = 5)]. A significant instrumental improvement for ICP-MS is the multicollector device (MC-ICP-MS) in order to obtain a better precision of isotope ratio measurements (with a precision of up to 0.002%, RSD). CE- and HPLC-ICP-MS are used for the separation of isobaric interferences of long-lived radionuclides and stable isotopes by determination of spallation nuclide abundances in an irradiated tantalum target.

Enhanced sensitivity for Os isotope ratios by magnetic sector ICP-MS with a capacitive decoupling Pt guard electrode.

A magnetic sector ICP-MS with enhanced sensitivity was used to measure Os isotope ratios in solutions of low Os concentration (approximately 1 ng g(-1) or less). Ratios with 192Os as the basis were determined, while the geologically useful 187Os/188Os ratio was also measured. Sample introduction was via the traditional nebuliser-spray chamber method. A capacitive decoupling Pt shield torch was developed "in-house" and was found to increase Os signals by approximately 5 x under "moderate" plasma conditions (1050 W) over that found during normal operation (1250 W). Sensitivity using the guard electrode for 192Os was approximately 250-350,000 counts s(-1) per ng g(-1) Os. For a I ng g(-1) Os solution with no guard electrode, precisions of the order of 0.2-0.3% (189Os/192Os and 190Os/192Os) to approximately 1% or greater (186Os/192Os, 187Os/192Os and 187Os/188Os) were found (values as 1 sigma for n = 10). With the guard electrode in use, ratio precisions were found to improve to 0.2 to 0.8%. The total amount of Os used in the acquisition of this data was approximately 2.5 ng per measurement per replicate. At the higher concentration of 10 ng g(-1), precisions of the order of 0.15-0.3% were measured (for all ratios), irrespective of whether the shield torch was used. Ratio accuracy was confirmed by comparison with independently obtained NTIMS data. For both Os concentrations considered, the improvement in precision offered by the guard electrode (if any) was small in comparison to calculated theoretical values based on Poisson counting statistics, suggesting noise contributions from other sources (such as the sample introduction system, plasma flicker etc). At lower Os concentrations (to 100 pg g(-1)) no appreciable loss of ratio accuracy was observed, although as expected based on counting statistics, poorer precisions of the order of 0.45-3% (1 sigma, n = 5) were noted. Re was found to have a detrimental effect on the precision of Os ratios involving 187Os, indicating that separation of Re and Os samples is a necessary pre-requisite for highly accurate and precise Os isotope ratio measurements.

Magnesium isotope ratio measurements by negative thermal ionisation mass spectrometry using molecular fluoride ions

A new technique for accurate Mg isotope ratio measurements has been developed with MgF2 as sample compound. With the help of a fluorinating agent negatively charged MgF3 molecular ions were formed in the ion source of a thermal ionisation mass spectrometer. An evaporation study has been performed and the results clearly show that Mg is evaporated from the filament as MgF2 molecules. The MgF2 technique has been applied in the certification of a candidate 26Mg-enriched Spike Isotope Reference Material. The result from this new technique has been compared with results obtained for the same material using ICP-MS.

Improved lead isotope ratio measurements in environmental and biological samples with a double focussing magnetic sector inductively coupled plasma mass spectrometer (ICP-MS)

A method to measure lead (Pb) isotope ratios in biological and environmental samples using a single collector double focussing magnetic sector inductively coupled plasma mass spectrometer (ICP-MS) is presented. This method is intercalibrated with multicollector thermal ionization mass spectrometry (TIMS) to assess the suitability of the ICP-MS method for accurately and precisely identifying sources of environmental Pb exposure to humans. Results indicate that the external measurement precision (reproducibility) of this method, as evaluated by repeated analyses of the Pb isotope standard NBS 981 over the course of a year, was <0.1% (1σ) for 206 Pb: 204 Pb, 207 Pb: 206 Pb and 208 Pb: 206 Pb ratios. Accuracy of isotope ratio measurements by ICP-MS was evaluated by comparing analyses of whole blood (biological) and household dust (environmental) samples with the definitive TIMS analyses of the same samples. For whole blood and dust samples, the 206 Pb: 204 Pb, 207 Pb: 206 Pb and 208 Pb: 206 Pb ratios measured by ICP-MS and TIMS generally agreed to within twice the propagated standard deviation (2σ, σ=√σTIMS2+σICP-MS2)of both methods. However, a small but systematic difference between the Pb isotope ratios of blood measured with the two methods was apparent. The source of this difference remains under investigation. These data demonstrate that this ICP-MS method provides improved accuracy and precision of lead isotope ratio measurements compared to previous ICP-MS methods, and is suitable for use in studies to evaluate sources of environmental Pb exposure to children. Further, the increased samplethroughput and reduced cost of analyses using this method are substantial advantages over the more time-consuming and expensive TIMS method. Consequently, this Pb isotope methodology should prove useful as a routine tool in the investigation and mitigation of Pb exposure associated hazards.