Determination of the absolute isotopic composition and Atomic Weight of a reference sample of natural iron

Absolute values have been obtained by means of thermal ionisation mass spectrometry for the iron isotope abundance ratios of a sample of metallic iron of natural isotopic composition. This was achieved by calibrating the mass spectrometric measurement procedure using five different synthetic isotope mixtures, prepared from carefully characterised enriched isotope carrier compounds, viz. 54Fe2O3 and 56Fe2O3. These mixtures were made up at three different n(54Fe)/n(56Fe) ratios, covering a ratio range of more than two orders of magnitude, in order to determine the extent of the isotope fractionation in the ion source. Two mixtures bracket the natural ratio, two mixtures have ratio values approximating to unity, and one mixture has a ratio of about 10. The total relative uncertainty on the ratio values of the mixtures varies between 2 and 7 × 10-4 (2s). Three different mass spectrometric measurement procedures were developed, all using a silica-gel/boric acid ionisation enhancer. Measurements were carried out on two different instruments. The n(54Fe)/n(56Fe) ratio of the synthetic mixtures and of the natural iron isotopic reference material (IRM) were measured using a mass spectrometer with Faraday detector. Both the n(57Fe)/n(56Fe) and n(58Fe)/n(56Fe) ratios of the natural iron IRM were determined using the same instrument but operated at a higher ionisation temperature (1430°C instead of 1350°C) and using an internal normalisation procedure. For the determination of the abundances of the minor isotopes in the enriched isotope carrier compounds, an instrument equipped with a calibrated ion counting device was used and the ionisation temperature was 1150°C. Using the latter instrument and method, the n(54Fe)/n(56Fe) ratio of the IRM was found to a gree within 4 × 10-5 with the calibrated value of the Faraday measurements, indicating that experimental conditions were well controlled in both cases. Compared to the current IUPAC data (Atomic Weights of the Elements, Pure Appl. Chem., 63 (1991) 975-990 and Isotopic Compositions of the Elements, Pure Appl. Chem., 63 (1991) 991-1002), which have not been changed since 1961 and which date back to uncalibrated measurements during the 1940s (G.E. Valley and H.H. Anderson, Phys. Rev., 59 (1941) 113; G.E. Valley and H.H. Anderson, J. Am. Chem. Soc., 69 (1947) 1871; J.R. White and A.E. Cameron, Phys. Rev., 74 (1948) 991), the uncertainties of the isotopic abundances obtained in this work have been improved by a factor of four to 15, and the Atomic Weight by a factor of six, even with a very conservative 2s uncertainty estimate. Samples of this new IRM (CBNM IRM-014) are now available from CBNM.