Owing to its importance as a serum electrolyte, sodium is determined routinely in clinical laboratories. In the United States, the accuracy of these measurements is assessed through a number of proficiency testing programs. Clinical measurements are supported by the National Institute of Standards and Technology through the production of serum matrix Standard Reference Materials®
(909 and 956 series). For reference material certification, a gravimetric primary method has traditionally been used, but this method is time consuming. In this work, an alternative method has been developed in which sodium is determined by inductively coupled plasma mass spectrometry (ICP-MS) using a procedure, which is a cross between internal standardization and isotope dilution analysis. Diluted serum is spiked with 26Mg enriched isotope and the isotope ratio 23Na/26Mg measured in analog detection mode. The ratios are standardized by measuring a sodium primary standard (SRM 919a) spiked with 26Mg. As a relatively high concentration of sodium is measured, the sodium background from the ICP-MS instrument is comparatively small. The method has been successfully applied to the determination of sodium in SRM 909b (Human Serum), SRM 956a (Electrolytes in Frozen Human Serum) and serum pools from the College of American Pathologists.