MEASURING THE MOLAR-MASS OF SILICON FOR A BETTER AVOGADRO CONSTANT - REDUCED UNCERTAINTY

The current 3 x 10(-5) relative combined uncertainty the isotope amoun t ratio measurements in natural Si still must be improved in order to obtain a relative combined uncertainty on the molar mass smaller than 1 x 10(-7). One of the problems is the isotopic effect, which accompan ies adsorption/desorption processes of SiF4 in the mass spectrometer i nlet system, The value of 1(Si) observed in 1994/95 was confirmed in t his work to within 10(-7) M(Si), which means that the adsorption/desor ption models developed are (quantitatively) correct to describe and pr edict isotopic effects due to memory at uncertainty levels of less tha n or equal to 1 x 10(-7) R and less than or equal to 1 x 10(-7) M(Si). It also shows that the previously measured isotope amount ratio value s and derived molar masses have a lower uncertainty, The results obtai ned by using models compare well with those obtained after chemical cl eaning of the mass spectrometer inlet system and reduce the relative c ombined uncertainty from 3 x 10(-7) M(Si) to 1.3 x 10(-7) M(Si). The n ew M(Si) for the ''Avogadro Crystal'' WASO 17.2 is 28.085 384 2 (35) g . mol(-1) as compared to 28.085 383 1 (89) g . mol(-1) previously obt ained, The new value of the Avogadro constant is 6.022 136 7 (48) x 10 (23) mol(-1) as compared to 6.022 136 5 (51) x 10(23) mol(-1) previous ly obtained.