MASS-SPECTROMETRIC METHODS FOR DETERMINING ISOTOPIC COMPOSITION AND MOLAR-MASS TRACEABLE TO THE SI, EXEMPLIFIED BY IMPROVED VALUES FOR NITROGEN

The isotopic composition of atomic nitrogen and its molar mass (numeri cally equal to its atomic weight) M(N) were re-measured using recently improved accurate gas isotope-ratio mass spectrometry. We found in on e sample of pure nitrogen, M(N) = 14,0067214g . mol(-1) with a combine d uncertainty u(c) = 0,0000018g . mol(-1) (an average of four partiall y independent values). Another sample, atmospheric nitrogen, similarly from four values, yielded M(N) = 14,0067278g . mol(-1) with u(c) = 0, 0000015g . mol(-1) corresponding to an isotope substance ratio n(N-14) /n(N-15) = f(N-14)/f(N-15) = 271,87 with u(c) = 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 isot opic measurements for ongoing redeterminations of the Avogadro constan t, Although the measurements were not calibrated against synthetic N-i sotope mixtures, the results compare favorably, within quoted uncertai nties, with those selected by the International Union of Pure and Appl ied Chemistry (IUPAC) as the hitherto ''best measurements from a speci fic source'' and those representative of terrestrial compositions. The y suggest a new standard of reliability for molar-mass determinations. They also establish the best realization so far of traceability of ni trogen isotopic measurements to the mole in the Systeme International d'Unites (SI), as well as to the Avogadro constant.