Comparison of Pb Purification by Anion‐Exchange Resin Methods and Assessment of Long‐Term Reproducibility of Th/U/Pb Ratio Measurements by Quadrupole ICP‐MS

A comparison between HBr‐HCl and HBr‐HNO3 based anion chemistry is presented to test the efficiency of Pb purification in the preparation of samples for isotope ratio measurement by ICP‐MS. It was found that the small advantages in yield and blank offered by the HNO3‐based method were more than compensated by the more effective matrix removal of the HCl‐based method. Apart from very zinc rich matrices (e.g., sphalerite), a careful single pass purification using HBr and HCl removed more than 99.9% of the matrix. In preparation for the isotope ratio analysis, a small (2–5% m/v) liquid sample aliquot was analysed to determine U, Th and Pb concentrations by solution quadrupole ICP‐MS. This allowed accurate prediction of the expected ion signal and permitted optimal spiking with Tl, if desired, for mass bias correction. Long‐term results for international rock reference materials showed reproducibilities of better than 1% (Th/U) and 1.5% (U/Pb). For most geological applications, such analyses obviate the need for isotope dilution concentration measurements.