The determination of the isotopic composition of strontium

If the process of instrumental isotopic fractionation in thermal-ionization mass spectrometry is modeled in terms of Rayleigh's distillation, the values of the isotopic ratios of strontium in a sample material can be calculated by distributions of measured ratios obtained by instrumental runs in which the process of isotopic fractionation in the sample reservoir more closely fitted the modeled behavior. Such runs can be recognized because are known, in the modeled conditions, the values of the slopes of distributions of measured values of isotopic ratios which are characterized by an equivalent difference in nominal mass between 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 theoretical conditions are characterized by the lowest deviations between the observed value of the slope of the distribution and the respective theoretical value. This model of calculation was used to determine the true/initial values of the isotopic ratios of strontium in the NIST SRM 987 by two independent sets of instrumental runs. In the first set the 86Sr, 87Sr and 88Sr peaks were sampled, and the 87Sr/86Sr ratio and the 88Sr/87Sr ratio were the ratios characterized by an equivalent difference in nominal mass between the isotope at the numerator and the isotope at the denominator. In the second set, the 84Sr, 86Sr and 88Sr peaks were sampled, and the 86Sr/84Sr ratio and the 88Sr/86Sr ratio were the ratios characterized by an equivalent difference in nominal mass. The first set of instrumental runs gives calculated true/initial 88Sr/86Sr values in the range 8.275 ± 0.019, and 87Sr/86Sr values in the range 0.7060 ± 0.0008 (errors are 1s). The second set gives 88Sr/86Sr values in the range 8.249 ± 0.019 and 84Sr/86Sr in the range 0.05734 ± 0.00017 (1s).