Abstract

A new method for the measurement of calcium isotope ratios and total calcium, using a double focusing sector field inductively coupled plasma mass spectrometry (ICP-MS) instrument equipped with a shielded torch, is presented. The method was applied for the measurement of the ⁴⁴Ca/⁴³Ca, ⁴²Ca/⁴³Ca and ⁴⁴Ca/⁴²Ca ratios and total calcium by isotope dilution in urine samples from two different nutritional experiments set up to determine the calcium absorption from various foods. The isotope ratios were measured with a precision of 0.25%, 0.23% and 0.05% RSD for ⁴⁴Ca/⁴³Ca, ⁴²Ca/⁴³Ca and ⁴⁴Ca/⁴²Ca, respectively. The precision of the ⁴⁴Ca/⁴²Ca ratio is only limited by counting statistics, whereas the precision is limited by uncertainty from the needed background and interference corrections when the minor ⁴³Ca isotope is used for analysis. Uncertainty calculations on the determination of calcium absorption in the nutritional experiments are presented. These calculations show that, for a double stable isotope procedure (administration of enriched stable ⁴⁴Ca and ⁴²Ca), the overall uncertainty is controlled by the precision of the ICP-MS measurement of the isotope ratios (⁴⁴Ca/⁴³Ca, ⁴²Ca/⁴³Ca), whereas, for a mixed radioisotope–stable isotope procedure (administration of enriched stable ⁴⁴Ca and the ⁴⁷Ca radioisotope), the overall uncertainty is mainly controlled by the uncertainty in the measurement of total calcium by isotope dilution ICP-MS. From these results, it was concluded that, when optimizing the performance of nutritional experiments, the emphasis should be placed on different parameters depending on the exact analysis involved. The results also show that uncertainty calculations are very useful in pinpointing the critical parameters of an experiment.