Abstract
Procedures for the separation, purification, and high-precision analysis of mass-independent isotopic variations in molybdenum (Mo) using negative thermal ionization mass spectrometry are reported. Separation and purification of Mo from silicate and metal matrices are achieved using a two-stage anion exchange chromatographic procedure. Molybdenum is ionized as the MoO(3) (-) species using a double filament assembly. The MoO(3) (-) ion beams are collected using Faraday cup detectors equipped with a mixed array of amplifiers utilizing 10(11) and 10(12) Ω resistors, which allows for in situ measurement and correction of oxygen isobars. The long-term external reproducibility of (97)Mo/(96)Mo, the most precisely measured Mo isotope ratio, is ±5.4 ppm (2SD), based on the repeated analyses of the Alfa Aesar Specpure (®) Mo plasma standard and using (98)Mo/(96)Mo for fractionation correction. The long-term external reproducibilities of 92Mo/(96)Mo, (94)Mo/(96)Mo, (95)Mo/(96)Mo, and (100)Mo/(96)Mo are ±107, 37, 23, and 32 ppm (2SD), respectively. With this precision, smaller differences in Mo isotopic compositions can be resolved than have been previously possible.