THE GRAVIMETRIC PREPARATION OF SYNTHETIC MIXTURES OF IRON ISOTOPES

Five synthetic isotope mixtures were prepared with accurately known n(Fe-54)/n(Fe-56) abundance ratios, by weighing and mixing two characterised compounds each enriched in one of the isotopes. These mixtures were used to calibrate isotope abundance measurements (using thermal ionization mass spectrometry) of a sample of natural iron. These measurements have led to a new calibrated value for the iron atomic weight [R. Maeck, Ph. D. Thesis, University Antwerpen (1992); P.D.P. Taylor, R. Maeck and P. De Bievre, Int. J. Mass Spectrom. and Ion Processes, 121 (1992) 111]. The starting materials (Fe in the form of metal and 99.8% enriched, and Fe in the form of Fe2O3 and 99.9% enriched) were purified using semi-preparative ion chromatography, achieving minimal (< 5%) loss. Three consecutive separation schemes were used, and the purified materials were converted to Fe(NO3)3 and then transformed into alpha-Fe2O3 by heating to 950-degrees-C. This alpha-Fe2O3 was used as the isotope carrier compound for the weighings. It was a free flowing powder (particle size 1 mum) with an estimated level of impurities of 100 +/- 100 mug g-1 (2s uncertainty) as obtained by spark source mass spectrometry. The stoichiometry and the uncertainty of the stoichiometry of the alpha-Fe2O3 were experimentally determined. The resulting five mixtures had n(Fe-54)/n(Fe-56) isotope abundance ratios between the natural ratio (about 0.06370) and a ratio of 10. The relative uncertainty varied from two to 7 x 10(-4).