High-precision measurements of Mo isotopes by N-TIMS

Abstract

We have developed a new high-precision analytical method for measuring molybdenum (Mo) isotopes by negative thermal ionization mass spectrometry (N-TIMS). This method represents an improvement over existing methods, thanks to a new chemical separation method that yields a better purification of the Mo fraction. In addition, we have used a multi-dynamic method for data collection with three different magnet settings to avoid being sensitive to drifts in Faraday cup efficiencies. Our method resulted in a more reliable thermal emission of MoO3- for a given run and we have obtained Mo isotope ratios measured on a series of 26 standards with the following reproducibilities (2 SD): ±6.1, ±5.8, ±2.9, and ±9.6 ppm for 94Mo/96Mo, 95Mo/96Mo, 97Mo/96Mo, and 100Mo/96Mo, respectively. This represents an improvement by a factor of 2–3 compared with previously published methods, using N-TIMS or MC-ICPMS. This new analytical method will have applications in the field of cosmochemistry (nucleosynthetic anomalies), environmental geochemistry or nuclear forensics (mass-independent fractionation).