Electrolytes in human serum. Two decades of SI traceability

The ions lithium, sodium, potassium, magnesium, calcium, and chloride are important constituents of the electrolytes in human serum. Since electrolytes play an important role in the human body, the concentrations of the above-mentioned ions in human serum are meaningful markers for several diseases. For this reason, PTB has developed primary methods of measurement for the SI-traceable determination of these ions in the past more than two decades, as well as a routine method based on ion chromatography. The equivalence of the results of the primary method on the one hand and the routine method on the other hand as well as the international comparability of the results was demonstrated through successful participation in CCQM key comparisons. As a consequence, PTB holds eight serum-related CMC claims in the according BIPM database. The routine method was continuously improved over time resulting in relative measurement uncertainties of less than 0.5 % nearly on par with the ones obtained with the primary methods. Important improvements discussed in this work are the optimization of the serum digestion, the gravimetric preparation of reference solutions starting from primary monoelemental solutions, and the development of the exact matching one-point calibration as well as the combination of chemical and CO${}_2$-suppression.