A candidate reference method for serum calcium measurement by inductively coupled plasma mass spectrometry

Calcium is an important serum ion which is frequently assayed in clinical laboratories. Since quality assurance requires reference methods, the establishment of a candidate reference method for serum calcium measurement is important.An inductively coupled plasma mass spectrometry (ICP-MS) method was developed. Serum samples were spiked gravimetrically with aluminum internal standard, digested with 69% ultrapure nitric acid and diluted to test concentration. Then the (44)Ca/(27)Al ratios were measured by ICP-MS in hydrogen mode. The method was calibrated using 5% nitric acid matrix calibrators and the calibration function was established with bracketing method.The correlation coefficients between the measured (44)Ca/(27)Al ratios and the analyte concentrations ratios were all >0.9999. The coefficients of variation of the measurements were 0.27% and 0.16% for two spiked serum. The analytical recovery was 100.24%. The accuracy of the measurement was also verified through measurement of certified reference materials. Comparison with recognized reference method and international inter-laboratory comparisons gave satisfied results.New ICP-MS method is specific, precise, simple, and low in cost, and may be used as a candidate reference method in the standardization of serum calcium measurement.