Sample preparation procedures for accurate and precise isotope analysis of Pb in peat by multiple collector (MC)-ICP-MS

Abstract

A need exists in geochemistry to develop a peat sample preparation procedure for Pb isotope analysis by MC-ICP-MS that can isolate Pb within a wide range of concentrations (200 ng g⁻¹–250 µg g⁻¹) and digest both the large organic and the highly variable inorganic fractions. Dry ashing was followed by a tailored microwave acid digestion procedure, to avoid excessive HF addition, with 6 ml of HNO₃, 4 ml of H₂O and 0.16 ml HF found to be the optimal acid mixture for 50 mg of ash. The influences of dry and wet ashing sample preparation methods on isotope ratio measurements made by MC-ICP-MS were compared. No significant differences were found between the method results. Dry ashed samples, however, had slightly larger errors. We suggest that the main sources of imprecision in our procedure are sample preparation and sample heterogeneity. There was good agreement between samples prepared using our developed dry ashing/optimised microwave digestion procedure compared with TIMS, demonstrating its accuracy. The method reproducibility, based on multiple sample preparations, was generally <0.09%. The long-term reproducibility based on replicate NIST-SRM 981 measurements was a more conservative estimate of precision compared with short-term (or session) reproducibilities.