Effective Correction of Mass Bias for Rhenium Measurements by MC-ICP-MS

The geochemistry of Re-Os and the recent use of Re as a non-traditional stable isotope both need accurate and precise quantification of ¹⁸⁷Re/¹⁸⁵Re ratios. This paper reports rhenium isotopic data obtained from the analysis of a standard solution and geological samples by MC-ICP-MS. We show that measured isotopic ratios are modified by matrix effects that cannot be accounted for by the standard solution bracketing technique. The bias resulting from measurements on a spiked (¹⁸⁵Re-enriched) sample is shown to alter the apparent Re concentration by several percent. When spiking samples and calibrators with tungsten, simultaneous measurement of tungsten and rhenium isotopes compensates for the matrix-induced modification of mass bias. Rhenium and tungsten are shown to have different fractionation factors. This may be due to the fact that the two elements fractionate in a different but systematic way, or that the reference isotopic ratios used for elemental Re and W are incoherent with one another. The consistency of fractionation through time can be used to obtain an empirical relationship between W and Re measured ratios from a standard solution to obtain a sample's fractionation-corrected ¹⁸⁷Re/¹⁸⁵Re spiked ratio on samples containing pg g⁻¹ levels of Re, even if some matrix capable of affecting mass bias remains in the final solution.