Improved in situ isotope analysis of low-Pb materials using LA-MC-ICP-MS with parallel ion counter and Faraday detection

Abstract

A new analytical protocol is described allowing the measurement of Pb-isotope ratios, including the low abundance isotope ²⁰⁴Pb, in relatively small targets (∼100 micron) of low Pb content (1–10 ppm), such as might be encountered in the analysis of magmatic melt inclusions, for example. The analysis routine employed utilises multiple Faraday cups and two ion counters operated in static multi-collection mode. Concurrent analysis of the NIST 612 glass standard allows an external correction for mass bias, and simultaneous determination of ion counter gain, while Hg interference on ²⁰⁴Pb is corrected in real time by simultaneous acquisition of ²⁰⁰Hg in a second ion counter. Analytical precision approximates theoretical limits and it can be shown that analytical accuracy is limited primarily by ion counter gain for ratios involving ²⁰⁴Pb and the mass bias correction for all other ratios. We typically obtain precisions of <0.4% RSD (2 sigma) and accuracy of ≤0.2% on BCR2-g for ratios involving ²⁰⁴Pb. Two other glass reference materials (MPI-DING ATHO-G and KL2-G) were also analysed, and precision of ≤1.8% RSD (2 sigma) and accuracy of ≤0.9% were obtained for the glass with the lowest Pb abundance of approximately 2 μg g⁻¹. These results provide an improvement on the accuracy and precision of laser ablation ratio determinations, attributable to stable instrument conditions and high sensitivity.