Determination of the 87Sr/86Sr isotope ratio in USGS silicate reference materials by multi-collector ICP-mass spectrometry

Multi-collector ICP-mass spectrometry (MC-ICP-MS) was used for 87Sr/86Sr isotope ratio determination in newly introduced silicate reference materials from the US Geological Survey (USGS): granite G-3, andesite AGV-2, and basalt BCR-2. Next to the SrCO3 isotopic standard NIST SRM 987, also analogous USGS reference materials from the previous generation, and for which reference 87Sr/86Sr data obtained by TIMS are available, were analysed for validation purposes. Sample preparation consisted of acid digestion and subsequent isolation of Sr by means of a dedicated and commercially available crown ether-based resin. The Sr fractions thus obtained were analysed via MC-ICP-MS whereby mass discrimination was corrected for internally, while the isobaric interference at a mass-to-charge ratio of 86 caused by Kr impurities in the Ar gas was mathematically corrected for by using the signal for a Kr isotope free from spectral overlap. Finally, also the effect of the small amount of Rb that may still be present in the Sr fraction was corrected for mathematically on the basis of the signal intensity for 85Rb. The MC-ICP-MS results for G-2, AGV-1 and BCR-1 showed an excellent agreement with the corresponding TIMS values (<0.003% bias in all cases), such that it can be assumed that also the 87Sr/86Sr isotope ratio results obtained for the new reference materials are reliable.