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 character ised compounds each enriched in one of the isotopes. These mixtures we re used to calibrate isotope abundance measurements (using thermal ion ization mass spectrometry) of a sample of natural iron. These measurem ents 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 purifi ed using semi-preparative ion chromatography, achieving minimal (< 5%) loss. Three consecutive separation schemes were used, and the purifie d 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 i sotope carrier compound for the weighings. It was a free flowing powde r (particle size 1 mum) with an estimated level of impurities of 100 /- 100 mug g-1 (2s uncertainty) as obtained by spark source mass spect rometry. The stoichiometry and the uncertainty of the stoichiometry of the alpha-Fe2O3 were experimentally determined. The resulting five mi xtures had n(Fe-54)/n(Fe-56) isotope abundance ratios between the natu ral ratio (about 0.06370) and a ratio of 10. The relative uncertainty varied from two to 7 x 10(-4).