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Measurement of Pb isotope ratios by continuous leach–inductively coupled plasma–mass spectrometry: quantification of precision and accuracy

Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, Ontario, Canada, K7L 3N6 (macfarlane{at}geol.queensu.ca)

Precision and accuracy have been quantified for the determination of lead isotope ratios using continuous leach–inductively coupled plasma–mass spectrometry (CL-ICP-MS) by optimizing acquisition parameters and minimizing the number of elements in the method. By analysing for only ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²⁰⁰Hg, and ²⁰²Hg, counting times can be increased allowing improvements in isotope ratio precision over normal CL-ICP-MS determinations, which measure these elements as part of a much larger suite. At total Pb concentrations of 1 ppb in solution, results show that precisions of 6.7% to 8.3% and 7.8% to 21% can be achieved for ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁴Pb, respectively, over the 0% to 30% HNO₃ concentrations used in CL-ICP-MS. Accuracy, as measured against the certified values for NIST 981, ranges from 0.5% to 1.4% and 1.4% to 21% for ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁴Pb, respectively. The improvement in precision and accuracy afforded by this technique allows greater differentiation of isotopically distinct reservoirs of Pb within natural samples collected for exploration and environmental geochemistry.

KEYWORDS: Pb isotope ratios, continuous leach, ICP-MS, sequential extraction, exploration geochemistry, environmental analyses