An evaluation of Rb-Sr isotope dilution analyses with a 86Sr-enriched tracer and Multiple Collection-ICP-MS

A straightforward method is described allowing to determine on a routine basis Rb and Sr concentrations and Rb-87/Sr-86 isotope by using the isotope dilution method with isotopic tracers enriched in Rb-87 and in( 86)Sr, respectively, and MC-ICP-MS for mass spectrometric measurements. The Sr-87/Sr-86 isotope ratios are measured separately on unspiked sample aliquots, thereby alleviating the need for correcting the contribution that would arise in case of total spiking with an Sr-84 enriched tracer. The sample preparation involves sample dissolution with hydrofluoric acid, and the separation of Rb and Sr from matrix elements and from each other by combining extraction chromatography and cation exchange in nitric acid medium. The concentration and Rb-87/Sr-86 data measured on replicate dissolutions of several international geostandards display the very good reproducibility typical of the isotope dilution method, even at low analyte concentration. The within-run precisions of Sr-87/Sr-86 isotope ratios (2 S.E. from 1 10(-5) to 3 10(-5)) are not as good as those typically achieved by TIMS, but the data show a good reproducibility (0.002% RSD) and are accurate, based on their fair agreement with recommended values. However, these figures are significantly worse at low Sr concentration, indicating that Sr-87/Sr-86 ratios should preferably be measured by TIMS when Sr-poor samples are processed. (C) 2018 Elsevier B.V. All rights reserved.