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

A method for the measurement of calcium isotopes (Ca-42, Ca-43, and Ca-44) using quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is d escribed. Interferences from polyatomic ions such as (CO2+)-C-12-O-16 and ( ArH2+)-Ar-40 at the calcium masses are greatly minimized by operating the I CP-MS in the cool plasma mode. Relative standard deviations (RSD) for the C a-42:Ca-43 and Ca-44:Ca-43 ratios were found to be about 0.25%. Sample prep aration involved using ammonium oxalate at a pH of 8 to separate calcium fr om samples such as serum, urine, feces, and breast milk. The isotope ratio measurements were used to determine fractional absorption of calcium by a l actating woman after intravenous administration of Ca-42 and ingestion of C a-44.