Space charge and mass discrimination effects on lead isotope ratio measurements by ICP-QMS in environmental samples with high uranium content

Abstract

The accuracy of Pb isotope ratio determinations by ICP-QMS in the presence of different U concentrations was investigated. The influence of U was very severe due to space charge and mass discrimination effects, which play an important role in the process of Pb signal acquisition. In this work, solutions of the isotope reference material NBS 981, spiked with different amounts of U, were analysed for Pb isotope ratios using an ICP-QMS and a MC-ICP-MS. The results obtained demonstrate the existence of a strong interference of U content in the analytical process, the same behaviour being exhibited by both pieces of equipment. In order to optimize an analytical procedure to perform Pb isotopic ratio determinations with an ICP-QMS, a method with external standardization and matrix match was developed. This method provided results in excellent agreement with those obtained by thermal ionization mass spectrometry (r² = 0.9999). The developed methodology was shown to have suitable analytical quality for Pb isotope measurements when they are used for identification of different Pb sources in environmental studies and applications.