Direct measurement of lead isotope ratios in low concentration environmental samples by MC-ICP-MS and multi-ion counting
Résumé
The Pb isotope composition of environmental samples containing very low Pb concentrations (100pg ml− 1: dissolved atmospheric dust, groundwater, river water, rainwater, etc.) can be determined by MC-ICP-MS without the need for chemical purification and without the use of a desolvating nebulizer. Using a Faraday cup configuration and Tl-based mass bias correction, reproducibilities of 0.10% (± 2σ) were determined for 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios and 0.01% (± 2σ) for 207Pb/206Pb and 208Pb/206Pb ratios, at Pb concentrations as low as 10ng ml− 1. Sample matrix does not have a significant effect on the analysis if the total ion charge is less than 400mg l− 1. Apart from geothermal water and seawater, a wide range of environmental samples can be measured in this way. In addition, for studies that do not require the use of 204Pb, Pb concentrations as low as 1ng ml− 1, can be measured at reproducibilities of 0.10% (± 2σ) for 207Pb/206Pb and 208Pb/206Pb ratios.
For very low Pb samples (e.g. 100pg ml− 1), a new sample-bracketing method employing a multi-ion counting (MIC) system allows measurement all four Pb masses and 202Hg simultaneously. Reproducibility of this method is ± 0.25% (± 2σ) for 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios and ± 0.15% (± 2σ) for 207Pb/206Pb and 208Pb/206Pb ratios using unprocessed samples, thus saving time and avoiding sample contamination. This level of precision is adequate for environmental samples in which the Pb isotope composition varies widely. The method is demonstrated by direct analyses of reference rainwater samples for which conventionally analysed Pb measurements were made by TIMS after Pb purification.