Measuring 0.01 parts per thousand to 0.1 parts per thousand isotopic variations by MC-ICPMS-testing limits for the first time with Pb delta-iCRMs

A blind comparison on Pb-isotope delta-scale measurements by MC-ICPMS of 0.01 parts per thousand to 0.1 parts per thousand level was organised, involving five laboratories. Test samples were obtained from the series of candidate ERM-3810 delta-isotopic Certified Reference Materials (delta-iCRMs), and comprise four pairs of a material with similar to natural Pb-isotopic composition ('delta zero' or 'delta-0') and the same natural Pb progressively enriched in (207)Pb (with delta(207)Pb values certified to similar to 0.1% relative uncertainty, k = 2). Participants were free to apply the measurement strategy of their choice. A result was considered 'acceptable' only when, simultaneously, there was agreement within stated uncertainties with the corresponding reference value and the relative uncertainty stated by the participant was < 100%. This study illustrates the high degree of difficulty inherent to these delta-scale measurements by 'routine' MC-ICPMS methodologies (in this case, three participants reported 55% of their results which were deemed accurate, and the other two reported none). The closer to unity the isotope ratio value the better the results became ('acceptable' results mostly for delta 7/6 parts per thousand and delta 7/8 parts per thousand measurements). This first experiment of its kind demonstrates that Pb delta-scale isotopic measurements by MC-ICPMS can be reliably carried out down to 0.05 parts per thousand levels (two participants delivered accurate results above this threshold systematically for delta/6 parts per thousand, delta 7/8 parts per thousand and delta 7/4 parts per thousand). Below this limit, at similar to 0.01 parts per thousand and similar to 0.03 parts per thousand levels, results are no longer consistent or reproducible and appear to be susceptible to a number of effects introducing error (such as short term changes in mass discrimination) which are either not well understood, or not controlled and/or not corrected for at a sufficiently low level of uncertainty. These results also suggest that 'routine' methods for absolute (calibrated) Pb-isotope ratio determination by MC-ICPMS produce relative combined uncertainties on results which are unlikely to be better than 0.05 parts per thousand (k = 2).