A comparison between quadrupole, double focusing and multicollector ICP-MS instruments. Part I. Evaluation of total combined uncertainty for lead isotope ratio measurements

A quadrupole-based instrument (HP-4500), a double focusing single collector instrument (Element) and a single focusing multicollector (IsoProbe) were all evaluated for the precise and accurate measurement of lead isotope ratios using Tl as internal isotope ratio standard. The isotope ratio accuracy was evaluated using NIST 981 Common Lead Isotope standard reference material and an enriched Pb-204 spike. Mass bias correction was performed using an exponential model in all the three instruments and equations for the evaluation of the total combined uncertainty arising from the correction for the dead time and mass bias were developed. It was observed that, in the correction for mass bias, the uncertainty in the natural Tl ratio was the main source of uncertainty for the multicollector instrument while, for the quadrupole and double focusing single collector instruments, the measured isotope ratio was the main source of uncertainty.