Ruthenium stable isotope measurements by double spike MC-ICPMS

Abstract

We developed a new technique for precise measurements of ruthenium (Ru) stable isotope compositions by multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Instrumental mass bias and potential isotope fractionation induced during the chemical purification of Ru were corrected using a ⁹⁸Ru–¹⁰¹Ru double spike added prior to sample digestion. The separation and purification of Ru from natural samples is achieved by cation exchange chromatography followed by distillation of Ru as volatile oxides. A series of analytical tests demonstrates that this method results in very pure Ru cuts and accurate and reproducible Ru isotope measurements. The Ru isotope results are expressed relative to an Alfa Aesar™ Ru standard solution as the permil deviations of the ¹⁰²Ru/⁹⁹Ru ratio (δ^102/99Ru). The external reproducibility (2 s.d.) of the entire analytical procedure (including sample digestion, chemical purification and isotope measurement) that has been determined using several doped terrestrial rock standards is 0.05‰ for δ^102/99Ru. We obtained Ru stable isotope data for three different Ru standard solutions and five chromitite samples from two different localities in the Shetland Ophiolite Complex and from the Bushveld complex. All three standard solutions are isotopically distinct with δ^102/99Ru values up to ca. 0.9‰. The different chromitite samples also show different Ru stable isotope compositions with δ^102/99Ru values between ca. 0.2‰ and ca. 0.7‰. These natural Ru stable isotope variations can be readily resolved using the ⁹⁸Ru–¹⁰¹Ru double spike method presented here. Overall, these data show that Ru stable isotopes hold promise as a tracer of a wide range of geochemical and cosmochemical processes.