Correction of Mass Spectrometric Interferences for Rapid and Precise Isotope Ratio Measurements of Calcium from Biological Samples Using ICP-Mass Spectrometry.

Stable isotope compositions of calcium (Ca) provide useful information concerning metabolic alterations of Ca in human and animal bodies. For the measurements of Ca isotope ratio, great care must be taken for the mass spectrometric interferences on Ca isotopes (42Ca+, 43Ca+, and 44Ca+) from doubly charged strontium (Sr) ions (84Sr2+, 86Sr2+, and 88Sr2+). To obtain reliable stable isotope data of Ca, we developed a new correction technique for the mass spectrometric interferences by mSr2+ ions based on standard addition method. Addition of a small fraction of Sr onto a Ca solution shifts the measured Ca isotope ratios on a three-isotope diagram (i.e., δ44Ca and δ43Ca) along a mixing line defined by both the true Ca isotope ratio and the Sr isotope ratio. Therefore, the true Ca isotope ratio of a sample can be obtained as the crossover point of mass dependent fractionation line and the mixing line. With the present correction technique, precise and accurate isotope ratio measurements can be made on analyte solutions having a CSr/CCa ratio (concentration ratio) of 0.03, which is 6 times higher than the CSr/CCa ratio applicable to the conventional correction technique.