Determination of Uranium to Calcium and Strontium to Calcium Ratios in Corals by Inductively Coupled Plasma Mass Spectrometry

Abstract

A method has been developed to determine Ca (about 40% m/m in corals), Sr (about 1–2%) and U (3–5 µg g ^-1 ) in the same aliquots. All the samples were spiked with ⁴⁵ Sc, ⁸⁹ Y and ²³⁶ U. None of these isotopes are present in significant amounts in corals. The Ca and Sr were determined using two different approaches. Firstly, ⁴⁵ Sc and ⁸⁹ Y were used as spikes to measure ⁴³ Ca and ⁸⁶ Sr, respectively, by the isotope dilution technique (external isotope dilution). It was established that the behaviour of the various elements in the plasma varied from day to day and cannot be modelled with good accuracy ( i.e ., better than 1%). Therefore, a second approach was developed using ⁴⁵ Sc and ⁸⁹ Y as internal standards. This method yielded extremely good reproducibility within a run and also on a day to day basis, providing the ICP-MS instrument is properly tuned. Values for %RSD of 0.2–0.3% for Ca and Sr measurements were routinely obtained, as were calibration curves with correlation coefficients better than 0.9998. The U was determined by classic isotope dilution. Analyses on a standard solution of coral powder over 5 d yielded %RSDs of 0.25% for Sr:Ca and 0.5% for U:Ca. Accuracy was assessed using the certified reference material CCH-1 and a coral standard (NC20), which were both analysed by ICP-MS and ICP-AES.