High-precision potassium isotopic analysis by MC-ICP-MS: an inter-laboratory comparison and refined K atomic weight†
In the last a few years, several groups proposed new methods of high-precision K isotope analysis using Multiple Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) either through “collision gas” or “cold plasma” methods. Here we report a detailed method analyzing K isotopes in high-precision using Neptune Plus MC-ICP-MS in cold plasma and conduct an inter-laboratory comparison. The precision of measurements of 41K/39K ratios of a single run in this study can routinely reach ∼0.05 per mil (95% confidence interval; n = 10). The long-term (20 months) robustness and reproducibility of this method have also been evaluated (0.11 per mil; 2 standard deviation; n = 890). We also report K isotopic compositions of 20 geological reference materials and compare this method with recent methods from other laboratories. These independent measurements of the same reference materials agree well with each other within reported analytical uncertainties. Because of the newly improved methods and observed K isotopic fractionation among reference materials, the International Union of Pure and Applied Chemistry (IUPAC) recommended values for the weight of potassium and the atomic fractions of K isotopes need to be revised.