High-precision molybdenum isotope analysis by negative thermal ionization mass spectrometry

Worsham E., Walker R., Bermingham K.

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 MoO3− species using a double filament assembly. The MoO3− ion beams are collected using Faraday cup detectors equipped with a mixed array of amplifiers utilizing 1011 and 1012 Ω resistors, which allows for in situ measurement and correction of oxygen isobars. The long-term external reproducibility of 97Mo/96Mo, 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 98Mo/96Mo for fractionation correction. The long-term external reproducibilities of 92Mo/96Mo, 94Mo/96Mo, 95Mo/96Mo, and 100Mo/96Mo 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.