Estimation of mass discrimination factor for a wide range of m/z by argon artificial isotope mixtures and NF₃ Gas

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 f_MD values in extensive m/z region from 28 to 71. Our result shows that f_MD 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 f_MD 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 %.