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- Isotope ratio (2)
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Organisationseinheit der BAM
Isotope amount ratios (hereafter referred to as simply isotope ratios) are proving useful in an ever increasing array of applications that range from studies unravelling transport processes, to pinpointing the provenance of specific samples as well as trace element quantification by using isotope dilution mass spectrometry (IDMS). These expanding applications encompass fields as diverse as archaeology, food chemistry, forensic science, geochemistry, medicine and metrology. However, to be effective tools, the isotope ratio data must be reliable and traceable to enable the comparability of measurement.
The importance of traceability and comparability in isotope ratio analysis has already been recognized by the Inorganic Analysis Working Group (IAWG) within the CCQM. Three pilot studies have focused on the quality of isotope ratio determinations (P48 “U isotope ratios in urine”, P75 “stable isotopes in Methionine”, P105 “87Sr/86Sr in wine”). Moreover, isotope ratio measurements are fundamental to IDMS amount of substance determinations. For example, when Pb quantification using IDMS is undertaken, this requires the measurements of Pb isotope ratios. While the requirements for isotope ratio accuracy and precision in the case of of IDMS are generally quite modest, “absolute” Pb isotope ratio measurements for geochemical age dating and source rock characterization as well as forensic provenance and fingerprinting studies require Pb isotope ratio measurements of the highest quality. To support present and future CMCs on isotope ratio determinations, a Key Comparison was urgently needed. Therefore, it was decided at the IAWG meeting in Paris in April 2011 that a Key Comparison on the determination of Pb isotope ratios in a pure Pb solution and in a bronze sample should be organized and accompanied by a pilot study.
Measuring Pb isotope amount ratios in a pure Pb solution, while seemingly straight forward, rigorously tests the ability of analyst to correct for any instrumental effects (such as mass discrimination and blank correction) on the measured ratios. Pb, present in trace amounts in a metal matrix sample (e.g. Pb in bronze), provides a real world test of the whole chemical and instrumental procedure, from chemical separation and sample purification to analysis and subsequent correction of appropriate instrumental effects on the separated samples.
A suitable bronze material with a Pb mass fraction between 10 and 100 mg·kg-1 was available at BAM. A high purity solution of Pb with a mass fraction of approximately 100 mg·kg-1 was also available. By comparing the Pb isotope ratio results obtained for the bronze sample with the Pb isotope ratio results from the Pb solution, potential biases arising from the processing of the bronze sample could be effectively identified and separated from the instrumental effects arising from the measurement and data processing protocol.
For the first time, an international comparison was conducted on the determination of the purity of a high purity element. Participants were free to choose any analytical approach appropriate for their institute’s applications and services. The material tested was a high purity zinc, which had earlier been assessed for homogeneity and previously used in CCQM-K72 for the determination of six defined metallic impurities. Either a direct metal assay of the Zn mass fraction was undertaken by EDTA titrimetry, or an indirect approach was used wherein all impurities, or at least the major ones, were determined and their sum subtracted from ideal purity of 100 %, or 1 kg/kg. Impurity assessment techniques included glow discharge mass spectrometry, inductively coupled plasma mass spectrometry and carrier gas hot extraction/combustion analysis. Up to 91 elemental impurities covering metals, non-metals and semi-metals/metalloids were quantified. Due to the lack of internal experience or experimental capabilities, some participants contracted external laboratories for specific analytical tasks, mainly for the analysis of non-metals. The reported purity, expressed as zinc mass fraction in the high purity zinc material, showed excellent agreement for all participants, with a relative standard deviation of 0.011 %. The calculated reference value, w(Zn) = 0.999 873 kg/kg, was assigned an asymmetric combined uncertainty of + 0.000025 kg/kg and – 0.000028 kg/kg. Comparability amongst participating metrology institutes is thus demonstrated for the purity determination of high purity metals which have no particular difficulties with their decomposition / dissolution process when solution-based analytical methods are used, or which do not have specific difficulties when direct analysis approaches are used. Nevertheless, further development is required in terms of uncertainty assessment, quantification of non-metals and the determination of purity of less pure elements and/or for those elements suffering difficulties with the decomposition process.
An international comparison study on the accurate determination of the molar mass M(Si) of silicon artificially enriched in 28Si (x(28Si) > 0.9999 mol mol−1) has been completed. The measurements were part of the high level CCQM-P160 pilot study assessing the ability of National Metrology Institutes (NMIs) and Designated Institutes (DIs) to make such measurements at the lowest possible levels of measurement uncertainty and to identify possible difficulties when measuring this kind of sample. This study supports the molar mass measurements critical to disseminating the silicon route to realizing the new definitions for the kilogram and the mole. Measurements were also made by one external research institute and an external company. The different institutes were free to choose their experimental (mass spectrometric) set-ups and equipment, thereby enabling also the comparison of different techniques. The investigated material was a chemically pure, polycrystalline silicon material. The subsequent modified single crystalline secondary product of this material was intended for the production of silicon which was used for two additional spheres in the context of the redetermination of the Avogadro constant NA, required for the revision of the International System of Units (SI) via fundamental constants which came into force from May 2019. The CCQM pilot study was organized by Physikalisch-Technische Bundesanstalt (PTB). Aqueous silicon solutions were shipped to all participating institutions. The data analysis as well as the uncertainty modelling and calculation of the results was predefined. The participants were provided with an uncertainty budget as a GUM Workbench® file as well as a free software license for the duration of the comparison. The agreement of the values of the molar mass (M(Si) = 27.976 942 577 g mol−1) was excellent with ten out of 11 results reported within the range of relative uncertainty of 1 × 10−8 required
for the revision of the SI.
Lead (Pb) isotope amount ratios are commonly used in applications ranging from archaeology and forensic sciences to terrestrial and extra-terrestrial geochemistry. Despite their utility and frequency of use, only three certified isotope amount ratio reference materials are currently available for Pb: NIST SRMs 981, 982 and 983. Because SRM 981 has a natural Pb isotopic composition, it is mainly used for correcting instrumental mass discrimination or fractionation. This means that, at present, there are no other certified isotope reference materials with natural Pb isotopic composition that could be used for validating or verifying an analytical procedure involving the measurement of Pb isotope amount ratios.
To fill this gap, two new reference materials, both certified for their Pb isotopic composition, have been produced together with a complete uncertainty assessment. These new reference materials offer SI traceability and an independent means of validating or verifying analytical procedures used to produce Pb isotope amount ratio measurements.
ERM-EB400 is a bronze material containing a nominal Pb mass fraction of 45 mg/kg. ERM-AE142 is a high purity solution of Pb with a nominal mass fraction of 100 mg/kg. Both materials have been specifically produced to assist analysts in verifying or validating their analytical procedures. Note that while one of these reference materials requires the chemical separation of Pb from its matrix (ERM-EB400), the other does not (ERM-AE142). Details on the certification of these isotope reference materials are provided in this report.