Measuring 0.01 Per Mille to 0.1 Per Mille Isotopic Variations by MC-ICPMS - Testing Limits for the First Time with Pb Delta-iCRMs

A blind exercise involving Pb-isotope measurements by MC-ICPMS of 0.01 per mille to 0.1 per mille level isotopic variations 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 ~natural Pb-isotopic composition ("delta zero" or "delta0") and the same natural Pb progressively enriched in 207Pb (with d207Pb values certified 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 delta7/6 per mille and delta 7/8 per mille 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 per mille levels (two participants delivered accurate results above this threshold systematically for delta7/6 per mille, delta7/8 per mille and delta7/4 per mille). Below this limit, at ~0.01 per mille and ~ 0.03 per mille 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 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 per mille (k=2).

QUETEL Christophe;  PONZEVERA Emmanuel;  RODUSHKIN Ilia;  GERDES Axel;  WILLIAMS Ross;  WOODHEAD Jon; 

2009-05-05

ROYAL SOC CHEMISTRY

JRC49066

0267-9477,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC49066,   

10.1039/b821403b,