The importance of the Avogadro constant for amount-of-substance measurements

One of the routes to re-determine the Avogadro constant requires ultra-high accuracy isotopic measurements of amount ratios of Si isotopes to 10 -5 reproducibilities on the ratio measurements and calibrating the results by synthetic isotope mixtures prepared to 2 10 -5 relative combined uncertainty. The route to these results is described. followed by a description of the improvement of out knowledge of the Avogadro constant N A through these measurements. The same route has recently been opened for other gases and has opened the way to the establishment of Primary Standards of Measurements' for gas isotopic measurements. It is described how these highly accurate measurements of ratios of amount of substance have considerably contributed to clearer thinking about concepts for measurements of amount of substance. The acquired expertise in measurement instrumentation and measurement procedures can now be extended to a more general use in measurements of isotope amount ratio (i.e. in other elements). It can also be combined with isotope dilution. This latter combination opens up a new route towards a realization of traceability of an amount-of-substance measurement to the SI and to the (measurement procedure and instrumentation leading to) Avogadro constant.