Molybdenum isotopic analysis by negative thermal ionization mass spectrometry (N-TIMS): effects on oxygen isotopic composition

Abstract

We developed a new, highly precise, and accurate Mo isotope analysis by thermal ionization mass spectrometry in negative ionization mode (N-TIMS). We discovered that the optimal condition to ionize Mo most efficiently into MoO₃⁻ was to load the Mo sample on a Re filament and cover the sample with La(NO₃)₃, thus yielding La/Mo = 5. To achieve highly precise Mo isotope analysis, determining the oxygen isotopic composition of MoO₃⁻ ions in each measurement by monitoring masses 149 (¹⁰⁰Mo¹⁶O₂¹⁷O⁻) and 150 (¹⁰⁰Mo¹⁶O₂¹⁸O⁻) ions and using the data for correcting for the O isotope interferences is important. After correcting the O isotopic interference and performing mass-dependent fractionation during the TIMS measurement, the acquired Mo isotopic ratios yielded the following reproducibilities (2SD): 47, 16, 10, 13, and 33 ppm for ⁹²Mo/⁹⁶Mo, ⁹⁴Mo/⁹⁶Mo, ⁹⁵Mo/⁹⁶Mo, ⁹⁷Mo/⁹⁶Mo, and ¹⁰⁰Mo/⁹⁶Mo, respectively. The reproducibilities have been improved by 1.3–2.7 times compared to those obtained in previous studies using multi-collector inductively coupled plasma mass spectrometry. The accuracy of our technique was confirmed by measuring two synthesized solutions with enriched ⁹²Mo, ⁹⁷Mo, and ¹⁰⁰Mo abundances and two iron meteorites, i.e., Henbury (IIIAB) and Albion (IVA). Moreover, we determined positive Mo isotope anomalies for a new iron meteorite, Tambo Quemado (IIIAB). Our N-TIMS technique can be applied to the studies of nucleosynthetic isotope anomalies in extraterrestrial materials as well as mass-dependent Mo isotopic shift in environmental samples.