Analytical Sciences

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

Yu-ki TANAKA,*,** Yuko MIKUNI-TAKAGAKI,*** Kouki HIDAKA,*** Satoko WADA-TAKAHASHI,*** Ryota KAWAMATA,*** and Takafumi HIRATA**

*Graduate School and Faculty of Pharmaceutical Sciences, Chiba University, 1-8-1 Inohana, Chuo, Chiba 260-8675, Japan
**Geochemical Research Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
***Graduate School of Dentistry, Kanagawa Dental University, 82 Inaoka-cho, Yokosuka, Kanagawa 238-8580, Japan

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 C_Sr/C_Ca ratio (concentration ratio) of 0.03, which is 6 times higher than the C_Sr/C_Ca ratio applicable to the conventional correction technique.

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