The utility of rapid throughput single-collector sector-field ICP-MS for soil Pb isotope studies

Abstract

Lead (Pb) isotope tracing is widely used in environmental monitoring and public health studies, to determine the provenance of agricultural and archaeological materials, and in mineral exploration campaigns. Combined with elemental analyses, Pb isotope ratios are used to ‘fingerprint’ the sources and transport paths of Pb found in multiple targets including dust, metal filings, soil/regolith, plants, water and blood, to name a few. Low cost and high throughput are important factors in such studies - typically performed using solution-mode Inductively Coupled Plasma -Mass Spectrometers (ICP-MS) - but these undoubted advantages need to be balanced against data quality. Here we compare Pb isotopic compositions (including the low abundance 204Pb isotope) determined for unseparated soils measured by high-throughput single-collector sector-field inductively-coupled-plasma mass-spectrometry (SC-SF-ICP-MS) and quadrupole ICP-MS (Q-ICP-MS) with results from more traditional high-precision multi-collector-ICP-MS (MC-ICP-MS) analysis following Pb separation chemistry. The soils, from outcrops of young basalts near Melbourne (Australia), have Pb isotope compositions similar to those of the underlying lava flows, with minor offsets possibly related to dust import from Palaeozoic metasedimentary rocks which dominate the region. Except for one sample, contamination with industrial Pb appears to be absent to minor, a surprising finding given the proximity of the sample sites to a major metropolitan area. In these soils, comparison of data for fully digested and acid-leached soil, some from a range of sampling depths, allows small and variable contributions from anthropogenic Pb to be resolved. Despite their lower precision, isotopic compositions measured by SC-SF-ICP-MS and Q-ICP-MS largely reproduce the isotopic patterns established by MC-ICP-MS. However, analytical uncertainties are larger and performance is more variable for Q-ICP-MS. The results confirm SC-SF-ICP-MS as a viable high-throughput, intermediate precision tool for large-scale Pb isotope surveys.