Total evaporation technique for high-accuracy isotopic analysis of isotopically enriched molybdenum by negative thermal ionization mass spectrometry

摘要

The isotopic composition of isotopically enriched materials is essential for isotope dilution mass spectrometry and calibrated mass spectrometry, which are considered to be potential primary methods/authority methods of the highest metrological quality for the analysis of elemental content or isotopic compositions. In this study, a total evaporation (TE) technique was developed for accurate determination of isotopic compositions for seven enriched molybdenum materials with the utilization of negative ion-thermal ionization mass spectrometry (NTIMS). By using La(NO3)(3) as an activator, the sample was efficiently ionized into MoO3- for NTIMS measurement. To achieve highly precise results, the oxygen isotope interference in MoO3- on Mo isotopic analysis was corrected by using the IUPAC recommended isotopic compositions for atmospheric O-2. Various sources of uncertainty to TE-NTIMS, such as measurement repeatability, oxide interference correction, and the ion loss before and after measurement were discussed with respect to the determination of their uncertainty contribution and their influence on the results. As a result, the major isotope abundance for each enriched Mo material were determined as (k = 2): x(Mo-92) = 0.974 69 +/- 11 for enriched Mo-92, x(Mo-94) = 0.920 26 +/- 11 for enriched Mo-94, x(Mo-95) = 0.965 50 +/- 20 for enriched Mo-95, x(Mo-96) = 0.967 59 +/- 12 for enriched Mo-96, x(Mo-97) = 0.942 06 +/- 13 for enriched Mo-97, x(Mo-98) = 0.984 56 +/- 11 for enriched Mo-98, and x(Mo-100) = 0.993 655 +/- 12 for enriched Mo-100, respectively. These measurement results have been compared to those determined by a multi-collector inductively coupled plasma mass spectrometry via the mathematical iteration method in our previous work. Identical major isotope-abundance values were achieved within their combined standard uncertainties, indicating that the TE technique is a powerful and alternative tool for accurate determination of the isotopic composition for isotopically enriched materials. The potential influence owing to the variations of O isotopic compositions during NTIMS measurements were also investigated by applying the 'In-run O' isotope-abundance values reported in recent literatures for O isotope interference correction. The corrected Mo isotope abundances of either the major or the minor isotopes were in good agreement with that corrected by using the IUPAC atmospheric O-2 isotopic compositions.