Direct determination of lead isotope ratios inrainwater using inductively coupled plasma massspectrometry

Abstract

Inductively coupled plasma mass spectrometry (ICP-MS) is a suitable method for the direct determination of Pb isotope ratios in rainwater. The interaction of several instrumental bias factors are corrected by systematic measurement of an external reference standard solution (NIST SRM 981), and external precision is determined independently on the standard solution, a reference solution and five natural rainwater samples previously analysed for Pb isotope ratios by thermal ionization mass spectrometry (TIMS). External precisions of 0.50, 0.50, 0.70 and 0.30% (all given as 2σ) are achieved for²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁶Pb, respectively, within a wide concentration range (2–50 ppb). The bias between the measured and expected value is always below the external precision. The precision of the ICP-MS data is sufficient to solve environmental problems such as the origin of Pb contamination present in rainwater from the French Massif Central where at least four components are clearly involved in the rainwater Pb budget: gasoline, a natural and an industrial component, and an unknown Pb-enriched radiogenic component.