Strontium Spike Calibration in the Isotope Dilution Method: New Approaches and Solutions

The paper discusses methods for determining the parameters of a spike solution (concentration of the element and its isotope composition) and provides theoretical verification of these methods with reference to strontium as an example. The Sr isotope composition of rocks and minerals is widely used in geochemical and isotope–geochronological research. Earlier theoretically justified techniques for the calibration of a spike of a given chemical element relied on the known isotope composition of the element in the reference solution and the concentrations of the element in both the reference solution and the spike solution. In other words, it was only the isotope composition of the spike that should have been certified. The authors propose a new method for certifying a strontium spike: determining its parameters from results of the isotopic analysis of two mixtures of the spike with a reference solution. In this case, the ratios of the three stable isotopes (84Sr, 86Sr, and 88Sr) are used, and the Sr concentration in the spike solution should not necessarily be precisely known. The isotope composition of one of the mixtures is specified by the geometric mean of the isotope compositions of the reference solution and the spike, and the isotope composition of the other is the arithmetic mean of these indicators. The former is aimed at determining the concentration of the element in the spike, and the latter is used in evaluating the isotope composition of the spike. The combined use of these mixtures makes it possible to obtain all parameters of the spike: its Sr concentration and Sr isotope composition. The authors believe that the most accurate way to determine these parameters is to use all four stable isotopes of Sr. One of the promising ways to determine the parameters of the spike solution is to use a reference solution enriched in the 87Sr isotope and with preserved natural ratios of other isotopes. The application of reference solution enriched in the 87Sr isotope and a spike enriched in the 84Sr isotope enables the parameters to be evaluated.