The measurement of Pb isotope ratios in sub-ng quantities by fast scanning single collector sector field-ICP-MS

Abstract

This work describes an analytical protocol for the measurement of isotope ratios in samples containing small amounts of lead (i.e. ≤0.2ng total Pb) using the Nu Attom single collector-ICP-MS. The method was evaluated using the NIST SRM 981 isotopic standard and standard-sample bracketing for correction of the instrumental mass fractionation. The long-term precision (n=32, ±2SD) was 0.11% for the 208Pb/206Pb and 207Pb/206Pb ratios and 0.21–0.23% for the 20xPb/204Pb ratios (where x=208, 207, 206). The precision of the measured Pb isotope ratios reported here (for equivalent total Pb quantities) is comparable to multi-collector (MC) methodologies, using either an array of ion counters (Cocherie and Robert, 2007, Chem. Geol. 243, 90–104) or conventional Faraday collectors coupled with a high efficiency sample cone (Makishima and Nakamura, 2010, J. Anal. At. Spectrom. 25, 1712–1716). The accuracy of the measured Pb isotope ratios and the effect of the sample matrix were evaluated for a series of USGS rock standard digests and doped solutions of the NIST SRM 981 standard. For total cation concentrations <10μgmL−1 and concomitant low concentrations (<1.5μgmL−1) of easily ionisable elements (i.e. Na and K), matrix effects were not resolvable at the level of precision reported. However, significant matrix effects were associated with sample compositions typical of natural waters, and removal of (at least) the major cations is required for accurate isotope ratio measurements.