Measuring the molar mass of silicon for a better Avogadro constant: reduced uncertainty

The current 3/spl times/10/sup -5/ relative combined uncertainty the isotope amount ratio measurements in natural Si must be improved in order to obtain a relative combined uncertainty on the molar mass smaller than 1/spl times/10/sup -7/. One of the problems is the isotopic effect, which accompanies adsorption/desorption processes of SiF/sub 4/ in the mass spectrometer inlet system. The value of M(Si) observed in 1994/95 was confirmed in this work to within 10/sup -7/M(Si), which means that the adsorption/desorption models developed are (quantitatively) correct to describe and predict isotopic effects due to memory at uncertainty levels of /spl les/1/spl times/10/sup -5/ R and /spl les/1/spl times/10/sup -7/ M(Si). It also shows that the previously measured isotope amount ratio values and derived molar masses have a lower uncertainty. The results obtained by using models compare well with those obtained after chemical cleaning of the mass spectrometer inlet system and reduce the relative combined uncertainty from 3/spl times/10/sup -7/ M(Si) to 1.3/spl times/10/sup -7/ M(Si). The new M(Si) for the "Avogadro Crystal" WASO 17.2 is 28.085 384 2 (35) g/spl middot/moi/sup -1/ as compared to 28.085 383 1 (89) g mol/sup -1/ previously obtained. The new value of the Avogadro constant is 6.022 136 7 (48)/spl times/10/sup 23/ mol/sup -1/ as compared to 6.022 136 5 (51)/spl times/10/sup 23/ mol/sup -1/ previously obtained.