Measurement of calcium stable isotope tracers using cool plasma ICP-MS

A method for the measurement of calcium isotopes (42Ca, 43Ca, and 44Ca) using quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is described. Interferences from polyatomic ions such as 12C16O2+ and 40ArH2+ at the calcium masses are greatly minimized by operating the ICP-MS in the cool plasma mode. Relative standard deviations (RSD) for the 42Ca∶43Ca and 44Ca∶43Ca ratios were found to be about 0.25%. Sample preparation involved using ammonium oxalate at a pH of 8 to separate calcium from samples such as serum, urine, feces, and breast milk. The isotope ratio measurements were used to determine fractional absorption of calcium by a lactating woman after intravenous administration of 42Ca and ingestion of 44Ca.