The determination of the isotopic composition of strontium

If the process of instrumental isotopic fractionation in thermal-ionization mass spectrometry is modeledin terms of Rayleigh's distillation, the values of the isotopic ratios of strontium in a sample material canbe calculated by distributions of measured ratios obtained by instrumental runs in which the process ofisotopic fractionation in the sample reservoir more closely fitted the modeled behavior. Such runs can berecognized because are known, in the modeled conditions, the values of the slopes of distributions ofmeasured values of isotopic ratios which are characterized by an equivalent difference in nominal massbetween the isotope at the numerator and the isotope at the denominator (distributions 88Sr/87Sr vs.87Sr/86Sr, and 88Sr/86Sr vs. 86Sr/84Sr). The instrumental runs that more closely fitted the theoreticalconditions are characterized by the lowest deviations between the observed value of the slope of thedistribution and the respective theoretical value. This model of calculation was used to determine thetrue/initial values of the isotopic ratios of strontium in the NIST SRM 987 by two independent sets ofinstrumental runs. In the first set the 86Sr, 87Sr and 88Sr peaks were sampled, and the 87Sr/86Sr ratio andthe 88Sr/87Sr ratio were the ratios characterized by an equivalent difference in nominal mass between theisotope at the numerator and the isotope at the denominator. In the second set, the 84Sr, 86Sr and 88Srpeaks were sampled, and the 86Sr/84Sr ratio and the 88Sr/86Sr ratio were the ratios characterized by anequivalent difference in nominal mass. The first set of instrumental runs gives calculated true/initial88Sr/86Sr values in the range 8.275 ± 0.019, and 87Sr/86Sr values in the range 0.7060 ± 0.0008 (errors are1s). The second set gives 88Sr/86Sr values in the range 8.249 ± 0.019 and 84Sr/86Sr in the range0.05734 ± 0.00017 (1s).