High precision Rb isotope measurements by MC-ICP-MS

We present a high precision method to measure the Rb isotope composition of terrestrial rocks in this study. Rb was efficiently separated from matrix elements by a chromatographic technique using AG50W-X12 and Sr-spec resin columns. Rb isotope ratios were measured using a Thermo Scientific Neptune Plus MC-ICP-MS employing a sample-standard bracketing method with NIST SRM984 as the bracketing standard. Potential effects of acid molarities and concentration mismatch during instrumental analyses were rigorously evaluated. The delta Rb-87 values of the two in-house standards ICP-Rb and USTC-Rb relative to SRM984 measured in our laboratory were -0.31 +/- 0.05 parts per thousand (2SD, n = 276) and -0.15 +/- 0.03 parts per thousand (2SD, n = 78), respectively. Analyses of synthetic standard solutions obtained identical delta Rb-87 for the pure SRM984 solution (0 parts per thousand) with a precision better than 0.05 parts per thousand (2SD). We further measured the Rb isotope compositions of 16 reference materials, including 11 igneous rock standards, 2 soil standards, 1 loess standard, and 2 sediment standards using this method. The Rb isotope data of these geological standards are helpful for future interlaboratory calibration and data quality control. Based on replicated analyses of solution standards and rock standards, our long-term external precision for delta Rb-87 is better than 0.05 parts per thousand (2SD), which allows us to identify Rb isotope fractionation during various geological processes.