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

International audienceA straightforward method is described allowing to determine on a routine basis Rb and Sr concentrationsand 87Rb/86Sr isotope by using the isotope dilution method with isotopic tracers enriched in 87Rb andin 86Sr, respectively, and MC-ICP-MS for mass spectrometric measurements. The 87Sr/86Sr isotope ratiosare measured separately on unspiked sample aliquots, thereby alleviating the need for correcting thecontribution that would arise in case of total spiking with an 84Sr enriched tracer. The sample preparationinvolves sample dissolution with hydrofluoric acid, and the separation of Rb and Sr from matrixelements and from each other by combining extraction chromatography and cation exchange in nitricacid medium. The concentration and 87Rb/86Sr data measured on replicate dissolutions of several internationalgeostandards display the very good reproducibility typical of the isotope dilution method, evenat low analyte concentration. The within-run precisions of 87Sr/86Sr isotope ratios (2 S.E. from 1 10−5 to3 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, thesefigures are significantly worse at low Sr concentration, indicating that 87Sr/86Sr ratios should preferablybe measured by TIMS when Sr-poor samples are processed