Preparation of two synthetic isotope mixtures for the calibration of isotope amount ratio measurements of sulfur

Two synthetic isotope mixtures for the calibration of sulfur isotope amount ratio measurements were gravimetrically prepared from high purity Ag2S materials enriched in S-32, S-33 and S-34. The mixtures were made so as to closely resemble the (natural) isotopic composition of the materials to be calibrated. This allowed a totally independent evaluation, on the same samples, of the relative combined uncertainty of: (a) the procedure to perform direct measurements of the amount of substance ratios of gas isotopes in the redetermination of the Avogadro constant and (b) the gravimetric preparation procedure. The result of both procedures, mass spectrometry and gravimetry, agree to a relative uncertainty of 3 x 10(-4) for sulfur amount ratio measurements of the major abundant isotopes. Thus it seems that a direct measurement of isotopic gas mixtures (e.g. of natural isotopic composition) is now possible for sulfur-and probably also for other gaseous isotopes-without necessarily having to rely on calibration by means of values provided by measurements of gravimetrically prepared isotope mixtures. However, synthetic mixtures may be needed for validation and verification purposes, in particular for quality assurance. (C) 2000 Elsevier Science B.V.