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 mat erials enriched in S-32, S-33 and S-34. The mixtures were made so as to clo sely resemble the (natural) isotopic composition of the materials to be "ca librated". This allowed a totally independent evaluation, on the same sampl es, of the relative combined uncertainty of: (a) the procedure to perform d irect measurements of the amount of substance ratios of gas isotopes in the redetermination of the Avogadro constant and (b) the gravimetric preparati on procedure. The result of both procedures, mass spectrometry and gravimet ry, agree to a relative uncertainty of 3 x 10(-4) for sulfur amount ratio m easurements of the major abundant isotopes. Thus it seems that a direct mea surement of isotopic gas mixtures (e.g. of natural isotopic composition) is now possible for sulfur-and probably also for other gaseous isotopes-witho ut necessarily having to rely on "calibration" by means of values provided by measurements of gravimetrically prepared isotope mixtures. However, synt hetic mixtures may be needed for validation and verification purposes, in p articular for quality assurance. (C) 2000 Elsevier Science B.V.