HIGH-ACCURACY ISOTOPE ABUNDANCE MEASUREMENTS FOR METROLOGY

The uncertainty achievable in the direct determination of a number of fundamental constants, for example the Faraday's constant F, the unive rsal gas constant R, and Avogadro's constant N(A), depends directly on the uncertainty with which the molar masses of the substances used in the experiments (e.g. silver, argon and silicon) can be determined. P rerequisite for an improvement of the relative uncertainties from appr oximately 1 . 10(-6) at present to < 3 . 10(-7), as is aimed at in the Avogadro project carried out jointly by the Physikalisch-Technische B undesanstalt and the Institute for Reference Materials and Measurement s (IRMM), is an improved determination of the isotope abundances. For this purpose, in close cooperation between IRMM and the company Finnig an-MAT, Bremen, a conventional mass spectrometer has been considerably improved in particular for the measurement of isotope abundance ratio s. A detailed description of this instrument is given, and its charact eristics are compared with those of the conventional mass spectrometer . It turns out that most of the systematic effects previously observed (such as mass fractionation effects, tail correction or memory effect ) can be reduced to values, which are so small that individual calibra tion with synthetic mixtures can be dispensed with for quite a number of applications. This is demonstrated by several examples. Previous me asurements of synthetic mixtures of enriched Si isotopes and on Si sin gle crystals were repeated and relative uncertainties of < 3 . 10(-7) attained for the molar masses. The new values are compared with those found in previous measurements. The consequences for the determination of ''amount of substance'' in practice are also discussed.