Refinement of high precision Ru isotope analysis using negative thermal ionization mass spectrometry.

A refined method for the isolation, purification, and high precision measurement of Ru isotope compositions in natural samples by negative thermal ionization mass spectrometry (N-TIMS) is reported. After chemical purification of Ru using ion exchange chromatography and microdistillation techniques, the Ru isotopic composition is measured as RuO- via N-TIMS. Data are corrected for oxide interferences using the simultaneously measured oxygen isotope composition, and subsequently for mass fractionation using an exponential law. Repeat analyses of an Ru standard solution demonstrate external reproducibility of Ru/Ru to ±6.4 ppm (2SD). This level of precision is more than two times better than prior techniques. Repeat analyses of gravimetrically prepared mixtures of a natural Ru standard and a Ru enriched spike show that isotopic differences of ≥13 ppm can be resolved by single measurements of a material using this method. Repeat analyses of diverse terrestrial materials (chromitites and Os-Ir-Ru alloys) are characterized by compositions that are identical to the standard, and the external reproducibility for these materials is also identical to that of the chemically pure standard, demonstrating that chemical separation/purification methods introduce no bias to the analysis. These materials likely define the Ru isotopic composition of the Earth's mantle.