Abstract

The performance of three different ICP-MS instruments for the determination of lead in biological reference materials by isotope dilution analysis has been evaluated. A quadrupole-based instrument (HP-4500), a double focusing single collector instrument (Element) and a single focusing multicollector instrument (IsoProbe) were all evaluated for precise and accurate measurement of lead isotope ratios and their application to lead isotope dilution analysis using enriched ²⁰⁴Pb. Detector dead time influenced the measured ratios only for the Element while mass bias was corrected using thallium added to each sample. Accurate lead concentration results were obtained for all three instruments. For the HP-4500 and the Element, the precision of the isotope ratios was observed to be independent of the ratio itself over two orders of magnitude, allowing the use of the random error propagation theory for uncertainty calculations. Finally, an uncertainty budget was also estimated for all measurements, showing that the measurement uncertainty and the concentration of the spike were the main sources of uncertainty. For the HP-4500, the main contribution to the total uncertainty came from the measurements, while for the IsoProbe the main contribution came from the uncertainty in the concentration of the spike. The Element provided results in between the other two instruments.