Measurement of Serum Sodium and Magnesium by Inductively Coupled Plasma Mass Spectrometry with Aluminum as Internal Standard

Background: Serum sodium and magnesium are important clinical laboratory tests and accurate testing results are needed for the correct interpretation of the tests. Standardization of serum sodium and magnesium measurements requires simple and reliable reference methods. Methods: Inductively coupled plasma mass spectrometry (ICP-MS) methods for the measurement of serum sodium and magnesium were developed. Aluminum was used as an internal standard. Serum samples were spiked with the internal standard, digested with 69% nitric acid at 150 degrees C and measured for Na-23/Al-27 and Mg-24/Al-27 ratios by ICP-MS. The measurements were calibrated by bracketing calibrators and serum sodium and magnesium concentrations were calculated by comparing the ion ratios of the serum samples with that of the calibrators. Results: The correlation coefficients between the measured ion ratios and the analyte concentrations were all > 0.9999 for both sodium and magnesium. The coefficients of variation of the measurements were 0.07%similar to 0.14% for sodium and 0.21%similar to 0.36% for magnesium. No significant interferences were detected and the analytical recoveries were 100.25% and 100.11% for sodium and magnesium, respectively. The accuracy of the measurements was also verified through measurement of certified reference materials and participation in international inter laboratory comparisons. Conclusions: New ICP-MS methods for serum sodium and magnesium have been developed. The methods are specific, precise, simple and low in cost and may be used as candidate reference methods in the standardization of serum sodium and magnesium measurements.