Inductively coupled plasma mass spectrometric analysis of calcium isotopes in human serum: A low-sample-volume acid-equilibration method

Background: Analytical methods for measuring the calcium isotope distribution in enriched human serum samples that use low blood volumes, simple preparation methods, and rapid analysis are important in clinical studies of calcium kinetics. Previously, sample preparation by oxalate precipitation typically required 500 muL of serum. This method was time-consuming, and the blood volume required was limiting in circumstances when only a small amount of serum could be obtained. Methods: Serum was collected from humans who were administered Ca-42, and 20 muL of serum was mixed with 2 mL of 0.22-0.67 mol/L HNO3 at room temperature for between 1 min and 16 h. The Ca-42/Ca-43 ratio in the supernatant was measured by a magnetic sector inductively coupled plasma mass spectrometer (ICP-MS). Calcium isotope ratios from these equilibration solutions were compared with data from oxalate-precipitated serum samples to determine the optimum equilibrium time and the. effect of acid concentration on equilibrium. Results: Various amounts of aggregated particles developed in different acid-serum mixtures. These affected the time required for isotope equilibration in the mixture. The shortest equilibrium time needed for the calcium isotopes varied from 1 to 6 h for samples acidified with 0.22-0.45 mol/L HNO3. Data obtained from these solutions were consistent with data from oxalate-precipitated calcium. The precision of Ca-42/Ca-43 ratio measurements was better than 0.5%. Conclusions: We have developed a simple; rapid sample preparation technique for ICP-MS analysis in which 20 muL of serum can be used for accurate measurement of the calcium isotope distribution in a sample with good precision and a rapid analysis time. (C) 2003 American Association for Clinical Chemistry.