MEASURING AMOUNT RATIOS OF GAS ISOTOPES BY 2 PRIMARY METHODS

Procedures developed for accurate measurement of amount of substance r atios for gas isotopes, as used to improve our knowledge of the Avogad ro constant and checked for quantitative conformity against kinetic ga s theory, were used to measure synthetic mixtures of enriched xenon ga s isotopes, prepared by (micro)-gravimetry as is usual in the preparat ion of synthetic gas compound mixtures. This allowed a totally indepen dent evaluation, on the same samples, of the relative combined standar d uncertainty of (a) the procedure to perform direct measurements of a mount of substance ratios of gas isotopes in the redetermination of th e Avogadro constant and (b) the gravimetric preparation procedure. The results of both procedures, mass spectrometry and gravimetry, agree t o a relative uncertainty of 2x10(-3) for amount measurements of the ma jor abundant isotopes. Thus it seems that a direct measurement of isot opic gas mixtures (e.g. of natural isotopic composition) is now possib le for xenon isotopes-and probably also for other gaseous isotopes - w ithout necessarily having to rely on ''calibration'' by means of value s provided by measurements of gravimetrically prepared isotope mixture s. However, synthetic mixtures may be needed for validation and verifi cation purposes, in particular for quality assurance (QA).