Calculation methods for direct internal mass fractionation correction of spiked isotopic ratios from multi-collector mass spectrometric measurements

It is difficult to do internal mass fractionation corrections for isotope dilution analysis by thermal ionization mass spectrometry (TIMS) or multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), especially for MC-ICP-MS. In this study, calculation methods for direct internal fractionation correction of spiked isotope analysis by TIMS or MC-ICP-MS cycle by cycle for elements having at least two internal reference isotopic ratios are presented. For TIMS, direct internal mass fractionation correction calculation methods, based on both power and exponential laws, are derived; whereas for MC-ICP-MS, due to larger mass fractionation effects, only exponential law is considered. These calculation strategies can be applied for both static and multi-dynamic measurements. For multi-dynamic measurements, the isotope fractionation effect, gain and cup efficiency effects of different collectors, as well as ion beam fluctuation effects are all simultaneously eliminated. The calculation methods were verified by Sr isotopic analyses of spiked NBS987 standard solutions by TIMS and Hf isotopic analyses of spiked geological reference materials by MC-ICP-MS. In addition, precise and accurate calibrations of isotopic ratios of the spikes, based on the calculation methods, are discussed.     

Accuracy and long-term reproducibility of lead isotopic measurements by multiple-collector inductively coupled plasma mass spectrometry using an external method for correction of mass discrimination

The precision of isotopic measurements of Pb by thermal ionization mass spectrometry (TIMS) is limited by the fact that this element does not possess an invariant isotope ratio that can be used for the correction of mass fractionation by internal normalization. Multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) can overcome this limitation, because with plasma ionization, elements with overlapping mass ranges are thought to display identical mass discrimination. With respect to Pb, this can be exploited by the addition of Tl to the sample solutions; the mass discrimination factor obtained for Tl can then be used for the correction of the measured Pb isotope ratios. In this article we present the results of a detailed study that investigates the accuracy and precision of such an external correction technique for mass discrimination based upon the results of multiple analyses of a mixed standard solution of NIST SRM-981 Pb and SRM-997 Tl. Our data indicate that normalization of the Pb isotope ratios to the certified isotopic composition of SRM-997 Tl produces Pb isotopic results that are significantly lower than recently published reference values by TIMS. This systematic offset can be eliminated by renormalization of the Pb data to a different Tl isotopic composition to obtain an empirically determined mass discrimination factor for Pb that generates accurate results. It is furthermore shown that a linear law is least suited for the correction of mass discrimination, whereas a power or exponential law function provide significantly more accurate and precise results. In detail, it appears that a power law may provide the most appropriate correction procedure, because the corrected Pb isotope ratios display less residual correlations with mass discrimination compared to the exponentially corrected data. Using an exponential or power law correction our results, obtained over a period of over seven months, display a precision (2σ) of better than 60 parts per million (ppm) for ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb and of better than 350 ppm for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb. This represents a significant improvement compared to conventional TIMS techniques and demonstrates the potential of MC-ICPMS for routine, high-precision measurements of Pb isotopic compositions.     

A new variable dispersion double-focusing plasma mass spectrometer with performance illustrated for Pb isotopes

A new generation multiple collector plasma source mass spectrometer (PSMS) produced by Nu Instruments Ltd is evaluated. The instrument has a double-focusing Nier–Johnson analyser with laminated magnet and a novel variable dispersion ion optical arrangement, enabling all masses to be located in the centre of the Faraday collectors of a fixed static array. mThe performance of the instrument has been assessed through the analysis of NBS-981 Pb using a Tl doping technique with Faraday collector efficiencies and amplifier gains determined independently. A second method of analysis involves comparison of interspersed standard and sample Pb measurements with effective gains for each collector determined from the standards. In both cases the repetition of Pb isotope measurement is competetive with the thermal ionization mass spectrometry (TIMS) double-spike method.     

Comparison of thermal ionization mass spectrometry and Multiple Collector Inductively Coupled Plasma Mass Spectrometry for cesium isotope ratio measurements

In the nuclear domain, precise and accurate isotopic composition determination of elements in spent nuclear fuels is mandatory to validate neutron calculation codes and for nuclear waste disposal. The present study presents the results obtained on Cs isotope ratio by mass spectrometric measurements. Natural cesium is monoisotopic (¹³³Cs) whereas cesium in spent fuels has 4 isotopes (¹³³Cs, ¹³⁴Cs, ¹³⁵Cs, and ¹³⁷Cs). As no standard reference material is available to evaluate the accuracy of Cs isotopic measurements, a comparison of cesium isotopic composition in spent nuclear fuels has been performed between Thermal Ionization Mass Spectrometry (TIMS) and a new method involving Multiple Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) measurements. For TIMS measurements, isotopic fractionation has been evaluated by studying the behavior of cesium isotope ratios (¹³³Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs) during the analyses. For MC-ICPMS measurements, the mass bias effects have been corrected with an external mass bias correction using elements (Eu and Sb) close to cesium masses. The results obtained by the two techniques show good agreement: relative difference on ¹³³Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs ratios for two nuclear samples, analyzed after chemical separation, ranges from 0.2% to 0.5% depending on the choice of reference value for mass bias correction by MC-ICPMS. Finally the quantification of the ¹³⁵Cs/²³⁸U ratio by the isotope dilution technique is presented in the case of a MOx (mixed oxide) spent fuel sample. Evaluation of the global uncertainties shows that this ratio could be defined at an uncertainty of 0.5% (k = 2). The intercomparison between two independent mass spectrometric techniques is fundamental for the evaluation of uncertainty when no isotopic standard is available.     

Investigation on elemental and isotopic fractionation during 196 nm femtosecond laser ablation multiple collector inductively coupled plasma mass spectrometry

Despite the large number of successful applications of laser ablation, elemental and isotopic fractionation coupled to inductively coupled plasma mass spectrometry (ICP-MS) remain as the main limitations for many applications of this technique in the fields of analytical chemistry and Earth Sciences. A substantial effort has been made to control such fractionations, which are well-established features of nanosecond laser ablation systems. Technological advancements made over the past decade now allow the ablation of solids by femtosecond laser pulses in the deep ultraviolet (UV) region at wavelengths less than 200 nm. Here the use of femtosecond laser ablation and its effects on elemental and isotopic fractionation is investigated. The Pb/U system is used to illustrate elemental fractionation and stable Fe isotopes are used to illustrate isotopic fractionation. No elemental fractionation is observed beyond the precision of the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) measurements. Without a matrix match between standard and sample, elemental fractionation is absent even when using different laser ablation protocols for standardization and samples (spot versus raster). Furthermore, we found that laser ablation-induced isotope ratio drifts, commonly observed during nanosecond laser ablation, are undetectable during ultraviolet femtosecond laser ablation. So far the precision obtained for Fe isotope ratio determinations is 0.1‰ (2 standard deviation) for the ⁵⁶Fe/⁵⁴Fe ratio. This is close to that obtainable by solution multiple-collector inductively coupled plasma mass spectrometry. The accuracy of the results appears to be independent of the matrix used for standardization. The resulting smaller particle sizes reduce fractionation processes. Femtosecond laser ablation carries the potential to solve some of the difficulties encountered during the two prior decades since the introduction of laser ablation.     

A novel isotope analysis of oxygen in uranium oxides: comparison of secondary ion mass spectrometry,glow discharge mass spectrometry and thermal ionization mass spectrometry

The natural variation of the oxygen isotopic composition is used among geologists to determine paleotemperatures and the origin of minerals. In recent studies, oxygen isotopic composition has been recognized as a possible tool for identification of the origin of seized uranium oxides in nuclear forensic science. In the last 10 years, great effort has been made to develop new direct and accurate n(¹⁸O)/n(¹⁶O) measurements methods. Traditionally, n(¹⁸O)/n(¹⁶O) analyses are performed by gas mass spectrometry. In this work, a novel oxygen isotope analysis by thermal ionization mass spectrometry (TIMS), using metal oxide ion species (UO⁺), is compared to the direct methods: glow discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS). Because of the possible application of the n(¹⁸O)/n(¹⁶O) ratio in nuclear forensics science, the samples were solid, pure UO₂ or U₃O₈ particles. The precision achieved using TIMS analysis was 0.04%, which is similar or even better than the one obtained using the SIMS technique (0.05%), and clearly better if compared to that of GDMS (0.5%). The samples used by TIMS are micrograms in size. The suitability of TIMS as a n(¹⁸O)/n(¹⁶O) measurement method is verified by SIMS measurements. In addition, TIMS results have been confirmed by characterizing the n(¹⁸O)/n(¹⁶O) ratio of UO₂ sample also by the traditional method of static vacuum mass spectrometry at the University of Chicago.     

Determination of rubidium by isotope dilution—inductively-coupled plasma mass spectrometry as an alternative to thermal ionization mass spectrometry

The combined techniques of inductively coupled plasma mass spectrometry (ICP-MS) and isotope dilution yield as much as a three-fold improvement in precision for trace-level rubidium determinations in geological materials over conventional isotope dilution using thermal ionization mass spectrometry (TIMS). Rubidium determinations by TIMS, precise to 0.6% (1 s.d.), are hindered by uncorrectable fractionation effects, whereas fractionation can be monitored during ICP-MS determinations, providing results as precise as 0.17% (1 s.d.). Precise rubidium data are critical for high-precision RbSr geochronology.     

Measurement of isotope ratios in solids by glow discharge mass spectrometry

A commercial thermion mass spectrometer has been modified for glow discharge mass spectrometry. GDMS isotope ratio measurements on osmium, uranium, and boron containing samples are compared with TIMS measurements.     

Increasing the accuracy of mass isotopomer analysis through calibration curves constructed using biologically synthesized compounds

Mass isotopomer analysis is an important technique to measure the production and flow of metabolites in living cells, tissues, and organisms. This technique depends on accurate quantifications of different mass isotopomers using mass spectrometry. Constructing calibration curves using standard samples is the most universal approach to convert raw mass spectrometry measurements into quantitative distributions of mass isotopomers. Calibration curve approach has been, however, of very limited use in comprehensive analyses of biological systems, mainly suffering from the lack of extensive range of standard samples with accurately known isotopic enrichment. Here, we present a biological method capable of synthesizing specifically labeled amino acids. These amino acids have well‐determined and estimable mass isotopomer distributions and thus can serve as standard samples. In this method, the bacterium strain Methylobacterium salsuginis sp. nov. was cultivated with partially ¹³C‐labeled methanol as the only carbon source to produce ¹³C‐enriched compounds. We show that the mass isotopomer distributions of the various biosynthesized amino acids are well determined and can be reasonably estimated based on proposed binomial approximation if the labeling state of the biomass reached an isotopic steady state. The interference of intramolecular inhomogeneity of ¹³C isotope abundances caused by biological isotope fractionation was eliminated by estimating average ¹³C isotope abundance. Further, the predictions are tested experimentally by mass spectrometry (MS) spectra of the labeled glycine, alanine, and aspartic acid. Most of the error in mass spectrometry measurements was less than 0.74 mol% in the test case, significantly reduced as compared with uncalibrated results, and this error is expected to be less than 0.4 mol% in real experiment as revealed by theoretical analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass spectrometry of long-lived radionuclides

The capability of determining element concentrations at the trace and ultratrace level and isotope ratios is a main feature of inorganic mass spectrometry. The precise and accurate determination of isotope ratios of long-lived natural and artificial radionuclides is required, e.g. for their environmental monitoring and health control, for studying radionuclide migration, for age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, for quality assurance and determination of the burn-up of fuel material in a nuclear power plant, for reprocessing plants, nuclear material accounting and radioactive waste control. Inorganic mass spectrometry, especially inductively coupled plasma mass spectrometry (ICP-MS) as the most important inorganic mass spectrometric technique today, possesses excellent sensitivity, precision and good accuracy for isotope ratio measurements and practically no restriction with respect to the ionization potential of the element investigated—therefore, thermal ionization mass spectrometry (TIMS), which has been used as the dominant analytical technique for precise isotope ratio measurements of long-lived radionuclides for many decades, is being replaced increasingly by ICP-MS. In the last few years instrumental progress in improving figures of merit for the determination of isotope ratio measurements of long-lived radionuclides in ICP-MS has been achieved by the application of a multiple ion collector device (MC-ICP-MS) and the introduction of the collision cell interface in order to dissociate disturbing argon-based molecular ions, to reduce the kinetic energy of ions and neutralize the disturbing noble gas ions (e.g. of ¹²⁹Xe⁺ for the determination of ¹²⁹I). The review describes the state of the art and the progress of different inorganic mass spectrometric techniques such as ICP-MS, laser ablation ICP-MS vs. TIMS, glow discharge mass spectrometry, secondary ion mass spectrometry, resonance ionization mass spectrometry and accelerator mass spectrometry for the determination of long-lived radionuclides in quite different materials.     

Nuclear analytical application of thermionic mass spectrometry

Summary The relative uncertainty on the isotope abundance ratio measurements of uranium and plutonium samples by means of thermionic mass spectrometry at the Central Bureau for Nuclear Measurements (CBNM) in Geel, Belgium, has decreased to a level of about 2 · 10^–4.The improvement was mainly achieved through the preparation of synthetic isotope mixtures of uranium and plutonium, to a relative uncertainty of 0.01% (computed on a 2s basis) on the ratios of isotopes with major abundances. This allowed to determine some error sources more precisely, such as: — isotope fractionation, — non-linearity of the ion beam current measuring system.As a consequence CBNM is able to prepare certified uranium isotopic reference materials (U IRM's) for distribution, with a relative uncertainty of 0.07% (computed on a 2s basis) on the²³⁵U isotope abundance and to provide reference values on samples for the (European) Interlaboratory Measurement Evaluation Programmes (REIMEP).

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Nuklearanalytische Anwendung der Thermionen-Massenspektrometrie

     

Determination of <Superscript>238</Superscript>Pu in plutonium bearing fuels by thermal ionization mass spectrometry

Determination of ²³⁸Pu in plutonium bearing fuels is required as a part of the chemical quality assurance of nuclear fuels. In addition, the determination of ²³⁸Pu is required in nuclear technology for many other applications, e.g., for developing isotope correlations and while using ²³⁸Pu as a spike (tracer) in isotope dilution α-spectrometry (IDAS). This determination usually involves the use of α-spectrometry on purified Pu sample. In view of the random errors associated with the counting statistics and the systematic errors due to (1) in-growth of ²⁴¹Am in purified Pu sample and (2) tail contribution correction methodology in α-spectrometry, the precision and accuracy obtainable by α-spectrometry are limited. Thermal ionization mass spectrometry (TIMS) is generally used for the determination of different Pu isotopes other than ²³⁸Pu. This is due to the ubiquitous isobaric interference from ²³⁸U at ²³⁸Pu in TIMS. Recently, we have carried out studies on the formation of atomic and oxide ions of U and Pu by TIMS and developed a novel approach using interfering element correction methodology to account for the isobaric interference of ²³⁸U at ²³⁸Pu in TIMS. This methodology is based on the addition of ²³⁵U (enrichment >90 atom%) to Pu sample followed by the determination of ²³⁸U/²³⁵U atom ratio using UO⁺ ion and determination of Pu isotope ratios using Pu⁺ ion, from the same filament loading. The TIMS methodology was used for the determination of ²³⁸Pu in different Pu samples in U based nuclear fuels from PHWRs with ²³⁸Pu content about 0.2 atom%. The ²³⁸Pu determination was also carried out using α-spectrometry. This paper reports the results obtained by the two methods and presents the ments and shortcomings of the two approaches.     

High-precision measurement of mercury isotope ratios in sediments using cold-vapor generation multi-collector inductively coupled plasma mass spectrometry

An on-line Hg reduction technique using stannous chloride as the reductant was applied for accurate and precise mercury isotope ratio determinations by multi-collector (MC)-ICP/MS. Special attention has been paid to ensure optimal conditions (such as acquisition time and mercury concentration) allowing precision measurements good enough to be able to significantly detect the anticipated small differences in Hg isotope ratios in nature. Typically, internal precision was better than 0.002% (1 RSE) on all Hg ratios investigated as long as approximately 20 ng of Hg was measured with a 10-min acquisition time. Introducing higher amounts of mercury (50 ng Hg) improved the internal precision to <0.001%. Instrumental mass bias was corrected using ²⁰⁵Tl/²⁰³Tl correction coupled to a standard-sample bracketing approach. The large number of data acquired allowed us to validate the consistency of our measurements over a one-year period. On average, the short-term uncertainty determined by repeated runs of NIST SRM 1641d Hg standard during a single day was <0.006% (1 RSD) for all isotope pairs investigated (²⁰²Hg/¹⁹⁸Hg, ²⁰²Hg/¹⁹⁹Hg, ²⁰²Hg/²⁰⁰Hg, and ²⁰²Hg/²⁰¹Hg). The precision fell to <0.01% if the long-term reproducibility, taken over 11 months (over 100 measurements), was considered. The extent of fractionation has been investigated in a series of sediments subject to various Hg sources from different locations worldwide. The ratio ²⁰²Hg/¹⁹⁸Hg expressed as δ values (per mil deviations relative to NIST SRM 1641d Hg standard solution) displayed differences from +0.74 to −4.00‰. The magnitude of the Hg fractionation per amu was constant within one type of sample and did not exceed 1.00‰. Considering all results (the reproducibility of Hg standard solutions, reference sediment samples, and the examination of natural samples), the analytical error of our δ values for the overall method was within ±0.28‰ (1 SD), which was an order of magnitude lower than the extent of fractionation (4.74‰) observed in sediments. This study confirmed that analytical techniques have reached a level of long-term precision and accuracy that is sufficiently sensitive to detect even small differences in Hg isotope ratios that occur within one type of samples (e.g., between different sediments) and so far have unequivocally shown that Hg isotope ratios in sediments vary within approximately 5‰.     

Iron isotope fractionation between liquid and vapor phases of iron pentacarbonyl

Iron isotope fractionation between liquid and vapor iron pentacarbonyl was measured in a closed system at ∼0 and ∼21 °C to determine if Fe isotope analysis of iron pentacarbonyl vapor is viable using electron-impact, gas-source mass spectrometry. At the 2σ level, there is no significant Fe isotope fractionation between vapor and liquid under conditions thought to reflect equilibrium. Experiments at ∼0 °C indicate iron pentacarbonyl vapor is ∼0.05 per mil (‰) greater in ⁵⁶Fe/⁵⁴Fe than liquid iron pentacarbonyl, which is just resolvable at the 1σ level. Partial decomposition of iron pentacarbonyl vapor or liquid to an iron oxide or iron metal shows that significant isotopic fractionation occurs, where the decomposed product has a lower ⁵⁶Fe/⁵⁴Fe ratio as compared to the starting iron pentacarbonyl. It follows that methods to decompose iron pentacarbonyl must be quantitative to obtain accurate isotope values.     

Revised δ34S reference values for IAEA sulfur isotope reference materials S‐2 and S‐3

Revised δ³⁴S reference values with associated expanded uncertainties (95% confidence interval (C.I.)) are presented for the sulfur isotope reference materials IAEA‐S‐2 (22.62 ± 0.16‰) and IAEA‐S‐3 (−32.49 ± 0.16‰). These revised values are determined using two relative‐difference measurement techniques, gas source isotope ratio mass spectrometry (GIRMS) and double‐spike multi‐collector thermal ionization mass spectrometry (MC‐TIMS). Gas analyses have traditionally been considered the most robust for relative isotopic difference measurements of sulfur. The double‐spike MC‐TIMS technique provides an independent method for value‐assignment validation and produces revised values that are both unbiased and more precise than previous value assignments. Unbiased δ³⁴S values are required to anchor the positive and negative end members of the sulfur delta (δ) scale because they are the basis for reporting both δ³⁴S values and the derived mass‐independent Δ³³S and Δ³⁶S values. Published in 2009 by John Wiley & Sons, Ltd.     

Characterization of neutron transmuted zinc traces in pure copper materials by isotope dilution mass spectrometry

The neutron transmutation doping (NTD) of highly pure copper with zinc was investigated as a promising means of achieving controlled gradation of the zinc content in the range 1–20 μg g^–1. The doping process leads to the enrichment of two stable isotopes ⁶⁴Zn and ⁶⁶Zn in a ratio which differs from that of natural isotopic distribution. Mass spectrometric investigations by thermal ionization mass spectrometry (TIMS) were performed to validate the results obtained by gamma spectrometry. The investigations included both determination of the isotopic ratios of the doped zinc isotopes and the analysis of the accumulated zinc contents by isotope dilution (ID) analysis. Thereby a sample-specific correction of the blank could be performed because the isotope ⁶⁸Zn was not influenced, because of the transmutation process. The results obtained by TIMS prove the strict proportionality of the doped zinc content, in the range 5 to 20 μg g^–1, to the neutron fluence. Comparison with gamma spectrometric results showed a very good agreement within the uncertainties.     

Stable isotopic analysis of pyrogenic organic matter in soils by liquid chromatography–isotope‐ratio mass spectrometry of benzene polycarboxylic acids

Pyrogenic organic matter (PyOM), the incomplete combustion product of organic materials, is considered stable in soils and represents a potentially important terrestrial sink for atmospheric carbon dioxide. One well‐established method of measuring PyOM in the environment is as benzene polycarboxylic acids (BPCAs), a compound‐specific method, which allows both qualitative and quantitative estimation of PyOM. Until now, stable isotope measurement of PyOM carbon involved measurement of the trimethylsilyl (TMS) or methyl (Me) polycarboxylic acid derivatives by gas chromatography–combustion–isotope ratio mass spectrometry (GC‐C‐IRMS). However, BPCA derivatives can contain as much as 150% derivative carbon, necessitating post‐analysis correction for the accurate measurement of δ¹³ C values, leading to increased measurement error. Here, we describe a method for δ¹³ C isotope ratio measurement and quantification of BPCAs from soil‐derived PyOM, based on ion‐exchange chromatography (IEC‐IRMS). The reproducibility of the δ¹³ C measurement of individual BPCAs by IEC‐IRMS was better than 0.35‰ (1σ). The δ¹³ C‐BPCA analysis of PyOM in soils, including at natural and artificially enriched ¹³ C‐abundance, produced accurate and precise δ¹³ C measurements. Analysis of samples that differed in δ¹³ C by as much as 900‰ revealed carryover of <1‰ between samples. The weighted sum of individual δ¹³ C‐BPCA measurements was correlated with previous isotopic measurements of whole PyOM, providing complementary information for bulk isotopic measurements. We discuss potential applications of δ¹³ C‐BPCA measurements, including the study of turnover rates of PyOM in soils and the partitioning of PyOM sources based on photosynthetic pathways. Copyright © 2011 John Wiley & Sons, Ltd.     

An evaluation of a single-step extraction chromatography separation method for Sm-Nd isotope analysis of micro-samples of silicate rocks by high-sensitivity thermal ionization mass spectrometry

A single-step separation scheme is presented for Sm–Nd radiogenic isotope system on very small samples (1–3 mg) of silicate rock. This method is based on Eichrom^® LN Spec chromatographic material and affords a straightforward separation of Sm–Nd from complex matrix with good purity and satisfactory blank levels, suitable for thermal ionization mass spectrometry (TIMS).This technique, characterized by high efficiency (single-step Sm–Nd separation) and high sensitivity (TIMS on NdO⁺ ion beam), is able to process rapidly (3–4 h), with low procedure blanks (<10 pg) and very small sample (1–3 mg). Replicate measurements by TIMS on ¹⁴³Nd/¹⁴⁴Nd ratios and Sm–Nd concentrations are presented for eleven international silicate rock reference materials, spanning a wide range of Sm–Nd contents and bulk compositions. The analytical results show a good agreement with recommended values within ±0.004% for the ¹⁴³Nd/¹⁴⁴Nd isotopic ratio and ±2% for Sm–Nd quantification at the 95% confidence level. It is noted that the uncertainty of this method is about 3 times larger than typical precision achievable with two-stage full separation followed by state-of-the-art conventional TIMS using Nd⁺ ion beams which require much larger amounts of Nd. Hence, our single-step separation followed by NdO⁺ ion beam technique is preferred to the analysis for microsamples.     

Precise determination of cadmium and lead isotopic compositions in river sediments

A method for the accurate determination of Cd and Pb isotope compositions in sediment samples is presented. Separation of Cd and Pb was designed by using an anionic exchange chromatographic procedure. Measurements of Cd isotopic compositions were carried out by multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), by using standard-sample bracketing technology for mass bias correction and Pb isotopic ratios were determined by thermal ionization mass spectrometry (TIMS). The factors that affect the accurate and precise Cd isotope compositions analysis, such as instrumental mass fractionation and isobaric interferences, were carefully evaluated and corrected. The Cd isotopic results were reported relative to an internal Cd solution and expressed as the δ^114/110Cd. Five Cd reference solutions and one Pb standard were repeatedly measured in order to assess the accuracy of the measurements. Uncertainties obtained were estimated to be lesser than 0.11‰ (2s) for the δ^114/110Cd value. Analytical uncertainties in 2s for Pb isotopic ratios were better than 0.5‰. The method has been successfully applied to the investigation of Cd and Pb isotope compositions in sediment samples collected from North River in south China.     

Determination of lithium isotopic composition by thermal ionization mass spectrometry in seawater

The isotopic composition of lithium in seawater has been determined by thermal ionization mass spectrometry (TIMS) based on the use of lithium hydroxide as the ion source. Isotopic measurements in a reference material supplied by IAEA (L-SVEC Li₂CO₃) were made to check the reproducibility of the method and ⁶Li indicates mobilization of light isotope of lithium form the sediment.