Electrolytes in serum are important biomarkers for skeletal and cellular health. The levels of electrolytes are monitored by measuring the Ca, Mg, K, Na in blood serum. Many reference methods have been developed for the determination of Ca, Mg, and K in clinical measurements; however, isotope dilution thermal ionization mass spectrometry (ID-TIMS) has traditionally been the primary reference method serving as an anchor for traceability and accuracy to these secondary reference methods. The sample matrix must be separated before ID-TIMS measurements, which is a slow and tedious process that hindered the adoption of the technique in routine clinical measurements. We have developed a fast and accurate method for the determination of Ca, Mg, and K in serum by taking advantage of the higher mass resolution capability of the modern sector field inductively coupled plasma mass spectrometry (HR-ICP-MS). After a serum sample was diluted with an isotopic standard containing enriched 44Ca, 26Mg, and 41K, 42Ca+:44Ca+ and 24Mg+:26Mg+ ratios were measured in medium resolution mode (m/Δm≈4500), and the 39K+:41K+ ratio was measured in high resolution mode (m/Δm≈ 10000). Residual 40Ar1H+ interference was still observed but the deleterious effects of the interference were minimized by measuring the sample at K>100 ng g-1. The interferences of Sr++ at the two Ca isotopes were less than 0.25 % of the analyte signal, and they were corrected with the 88Sr+ intensity by using the Sr++:Sr+ ratio. The dilute-and-shoot (DNS) approach for ID-HR-ICP-MS determination of Ca, Mg, and K in serum was validated with the traditional sample digestion approach with ID-HR-ICP-MS measurement. DNS and digested samples of SRM 956c were measured with ID-HR-ICP-MS for quality assurance, and the results were in good agreement with the certified values.
Citation: Analytical and Bioanalytical Chemistry
Pub Type: Journals
Ca, Mg, K, electrolyte, serum, isotope dilution, high resolution, sector field, ICP-MS