Final report of the key comparison CCQM-K98: Pb isotope amount ratios in bronze

Isotope amount 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 results. The importance of traceability and comparability in isotope ratio analysis has already been recognized by the Inorganic Analysis Working Group (IAWG) within the CCQM. While the requirements for isotope ratio accuracy and precision in the case of IDMS are generally quite modest, 'absolute' Pb isotope ratio measurements for geochemical applications as well as forensic provenance 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 and therefore initiated at the IAWG meeting in Paris in April 2011. The analytical task within such a comparison was decided to be the measurement of Pb isotope amount ratios in water and bronze. Measuring Pb isotope amount ratios in an aqueous Pb solution tested the ability of analysts to correct for any instrumental effects on the measured ratios, while the measurement of Pb isotope amount ratios in a metal matrix sample provided a real world test of the whole chemical and instrumental procedure. A suitable bronze material with a Pb mass fraction between 10 and 100 mg•kg-1 and a high purity solution of Pb with a mass fraction of approximately 100 mg•kg-1 was available at the pilot laboratory (BAM), both offering a natural-like Pb isotopic composition. The mandatory measurands, the isotope amount ratios n(206Pb)/n(204Pb), n(207Pb)/n(204Pb) and n(208Pb)/n(204Pb) were selected such that they correspond with those commonly reported in Pb isotopic studies and fully describe the isotopic composition of Pb in the sample. Additionally, the isotope amount ratio n(208Pb)/n(206Pb) was added, as this isotope ratio is typically measured when performing Pb quantitation by IDMS involving a 206Pb spike. Each participant was free to use any method they deemed suitable for measuring the individual isotope ratios. However, the majority of the results were obtained by using muIti-collector ICPMS or TIMS. The key requirements for all analytical procedures were a traceability statement for all results and the establishment of an uncertainty budget meeting a target uncertainty for all ratios of 0.2 %, relative (k=1). Additionally, the use of a Pb-matrix separation procedure was encouraged. The obtained overall result was excellent, demonstrating that the individual results reported by the NMIs/DIs were comparable and compatible for the determination of Pb isotope ratios. MC-ICPMS and MC-TIMS data were consistent with each other and agree to within 0.05 %. The corresponding uncertainties can be considered as realistic uncertainties and mainly range from 0.02 % to 0.08 % (k=1). As stated above isotope ratios are being increasingly used in different fields. Despite the availability and ease of use of new mass spectrometers, the metrology of unbiased isotope ratio measurements remains very challenging. Therefore, further comparisons are urgently needed, and should be designed to also engage scientists outside the NMI/DI community. Possible follow-up studies should focus on isotope ratio and delta measurements important for environmental and technical applications (e.g. B), food traceability and forensics (e.g. H, C, N, O, S and 87Sr/86Sr) or climate change issues (e.g. Li, B, Mg, Ca, Si). Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.