Measuring the molar mass of silicon for a better Avogadro constant: increased certainty

Summary form only given, as follows.Summary form only given. At 3/spl middot/10/sup -5/ combined uncertainty the abundance ratio measurements of Si isotopes in natural Si must still be improved in order for the uncertainty on the molar mass to become smaller than 1/spl middot/10/sup -7/. One of the problems are the isotopic effects which result from adsorption/desorption processes of SiF/sub 4/ in the ion source of the mass spectrometer. In earlier work, some adsorption of SiF/sub 4/ gas was detected when measuring samples of widely varying isotopic composition (in our case enriched /sup 28/Si, /sup 29/Si and /sup 30/Si isotopes). These were removed from the internal surface of the spectrometer by means of a chemical process and prior to each measurement. In the work presented now, abundance ratios of the Si isotopes were measured by applying measurement procedures based on the knowledge gained from newly developed adsorption/desorption models. The latter enable to perform measurements under proven conditions of absence of memory. The values as observed in 1994/95 were confirmed, which means that: 1. adsorption/desorption models developed are (quantitatively) correct to describe and predict isotopic effects at the 3/spl middot/10/sup -5/ uncertainty level. 2. The measured abundance ratio values and derived molar masses have acquired an increased certainty, now equal to a combined uncertainty of 2/spl middot/10/sup -7/ M(Si).