Estimation of Mass Discrimination Factor for a Wide Range of m/z by Argon Artificial Isotope Mixtures and NF₃ Gas

摘要

Absolute isotope ratio is a critical constituent in determination of atomic weight. To measure the absolute isotope ratio using a mass spectrometer, mass discrimination factor, fMD, is needed to convert measured isotope ratio to real isotope ratio of gas molecules. If the fMD could be predicted, absolute isotope ratio of a chemical species would be measureable in absence of its enriched isotope pure materials or isotope references. This work employed gravimetrically prepared isotope mixtures of argon (Ar) to obtain fMD at m/z of 40 in the magnetic sector type gas mass spectrometer (gas/MS). Besides, we compare the nitrogen isotope ratio of nitrogen trifluoride (NF3) with that of nitrogen molecule (N2) decomposed from the same NF3 thermally in order to identify the difference of fMD values in extensive m/z region from 28 to 71. Our result shows that fMD at m/z 40 was −0.044 % ± 0.017 % (k = 1) from measurement of Ar artificial isotope mixtures. The fMD difference in the range of m/z from 28 to 71 is observed −0.12 % ± 0.14 % from NF3 and N2. From combination of this work and reported fMD values by another team, IRMM, if fMD of −0.16 % ± 0.14 % is applied to isotope ratio measurement from N2 to SF6, we can determine absolute isotope ratio within relative uncertainty of 0.2 %.