Monika Inkret,a* Staf Valkiers,a Michael Berglund,a Nineta Majcen,a Philip Taylora and Jure Zupanb
aEC-JRC-Institute for Reference Materials and Measurements, Retieseweg 111,
B-2440 Geel, Belgium
bNational Institute of Chemistry, Laboratory for Chemometrics, Hajdrihova 19,
SI-1001, Ljubljana, Slovenia
* Corresponding author: E-mail: monika.inkret@ec.europa.eu
Abstract
This work is a detailed study of the investigation of the optimization of the
procedure for the conversion of highly enriched 28Si single crystal into gaseous silicon tetrafluoride (28SiF4) with the highest
purity ever achieved. The gaseous 28Si-
F4 of highest possible purity is required for the determination of silicon molar
mass via performing silicon isotope ratio
measurements on highly enriched 28Si (> 99.99% of 28Si) using electron impact
gas mass spectrometry. The problem is
in avoiding the contamination of enriched 28Si samples with silicon of natural
isotopic composition. The maximal allowed
contamination is in order of 10–9 g of silicon of natural isotopic composition.
The sample preparation of highly
enriched 28Si samples is the largest contributor to the relative combined
uncertainty of silicon isotope ratio measurements,
which should not exceed 1 × 10–4. Accordingly this contribution affects the
relative combined uncertainty of silicon
molar mass. The experiments necessary to achieve this uncertainty in the
procedure were carefully designed to
avoid superfluous work and to reduce the costs which might be high due to the
required quantities of very expensive
enriched 28Si samples. The optimal sample preparation conditions (and execution
of these measurements) obtained within
this work are part of the large-scale experiments designed for the calibration
of Si isotope ratio measurements of
highly enriched 28Si, needed for the final determination of Si molar mass as one
of the input quantities needed for determination
of the Avogadro constant.
Keywords: sample preparation, optimization, enriched 28Si single crystal, isotope ratio measurements