Mass-spectrometric methods for determining isotopic composition and molar mass traceable to the SI, exemplified by improved values for nitrogen

Abstract

The isotopic composition of atomic nitrogen and its molar mass (numerically equal to its atomic weight) M(N) were re-measured using recently improved accurate gas isotope-ratio mass spectrometry. We found in one sample of pure nitrogen, M(N) = 14,006 721 4 g mol-1 with a combined uncertainty uc = 0,000 001 8 g mol-1 (an average of four partially independent values). Another sample, atmospheric nitrogen, similarly from four values, yielded M(N) = 14,006 727 8 g mol-1 with uc = 0,000 001 5 g mol-1 corresponding to an isotope substance ratio n(14N) /n(15N) = f(14N)/f(15N) = 271,87 with uc = 0,11. The latter is in good agreement with the currently accepted IUPAC value of 272,0 with uc = 0,3. The measurements were carried out by means of the same rigid measurement procedures and instrumentation which are used in the isotopic measurements for ongoing redeterminations of the Avogadro constant. Although the measurements were not calibrated against synthetic N-isotope mixtures, the results compare favorably, within quoted uncertainties, with those selected by the International Union of Pure and Applied Chemistry (IUPAC) as the hitherto "best measurements from a specific source" and those representative of terrestrial compositions. They suggest a new standard of reliability for molar-mass determinations. They also establish the best realization so far of traceability of nitrogen isotopic measurements to the mole in the Système International d'Unités (SI), as well as to the Avogadro constant.