Optimising accuracy and precision of lead isotope measurement (206Pb, 207Pb, 208Pb) in acid digests of peat with ICP-SMS using individual mass discrimination correction

Abstract

Using ICP-SMS, a robust analytical protocol for accurate and precise determination of the isotopic Pb composition (²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) of acid digests of peat samples was developed. External precision better than 0.05% and 0.1% for Pb concentrations in the range of 1 µg l⁻¹ and 0.1 µg l⁻¹, respectively, were achieved for both ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb ratios. This precision has never before been achieved with ICP-SMS in a complex matrix containing such low concentrations of total Pb. Procedural Pb blank concentrations amounted to 0.003 µg l⁻¹ and had no influence on the accuracy of the Pb isotope ratios. Corrections for mass discrimination were accomplished using the certified isotopic reference material SRM 981. However, mass discrimination was found to be non-systematic, and varied among the masses both with respect to magnitude and direction. To accommodate this phenomenon, an individual mass discrimination correction was applied to each ratio resulting in improved accuracy. Using this approach, the mass discrimination for both Pb isotope ratios was <0.2%, compared to values on the order of ±0.35% if only a single Pb isotope ratio was used. The accuracy and precision of the ICP-SMS protocol was further evaluated using thermal ionisation mass spectrometry (TIMS) of selected samples and an in-house peat reference material. In general, the Pb isotope ratios determined using ICP-SMS deviated from the TIMS values by less than <0.1%. Given the throughput of the ICP-SMS compared to the TIMS (which requires chemical separation of Pb), the approach described offers great promise for environmental studies to fingerprint the predominant sources of anthropogenic Pb. Two applications are presented here: a bog profile from the Black Forest (SW Germany) consisting of eight thousand years of peat accumulation, and a depth profile through a snowpack collected from the same site during the winter of 2003.