Objective To establish and optimize a method for simultaneous determination for the contents of sodium,potassium,calci-um and magnesium ions in serum using microwave digestion combined with ion chromatography,and evaluate its performance.Methods Serum samples were pretreated by microwave digestion,and four cations were quantitatively determined using ion chromatography.The linearity,precision,recovery and accuracy of the established method were verified with reference to the EP6-A and EP15-A3 docu-ments issued by the Clinical and Laboratory Standards Institute(CLSI).Results The imprecisions of determinations for sodium,po-tassium,calcium and magnesium ions expressed as coefficient of variation(CV)were 0.10％to 0.36％,0.15％to 0.48％,0.22％to 0.87％and 0.21％to 0.73％,respectively.The recoveries with adding standard of the four cations were between 97.3％and 103.0％.Referred to RELA2020 and RELA2021,the deviations of the analysis results were-0.57％to-0.14％,-0.84％to-0.16％,-1.82％to-0.37％,and-0.60％to 0.34％for sodium,potassium,calcium,and magnesium,respectively,and all the comparisons were pas-sed.The limits of detection/quantification were 0.017 5/0.058 3 mmol/L,0.000 5/0.001 7 mmol/L,0.009 9/0.032 9 mmol/L and 0.001 1/0.003 5 mmol/L for the four cations,indicating that after pretreatment,no significantly interfere from other impurities for the results of determination was found.Conclusion A method for simultaneous determination of sodium,potassium,calcium and magnesi-um ions in serum was successfully established and optimized using microwave digestion combined with ion chromatography.The method showed good precision and accuracy,simple operation,short time consumption and low cost,thus it could be used for quality improve-ment of clinical electrolyte testing in serum.

Objective:To develop a candidate reference method for cortisol in human serum by isotope dilution liquid chromatography-tandem mass spectrometry (ID-LC/MS/MS).Methods:An isotope-labeled internal standard was added to samples, followed by alkalizing with sodium carbonate solution and liquid-liquid extraction with n-hexane/ethyl acetate. Isoelution with a methanol aqueous solution was employed for liquid chromatographic separation, while ESI in the positive ion and multiple reaction monitoring mode were used for mass spectrometry. The volume of sample, standard, and internal standard solutions were all controlled by weighing, and the results were calculated by bracketing method. The accuracy, precision, specificity, linearity, LOD and LOQ of this method were evaluated referring to the CLSI C62-A and C50-A guidelines and the domestic expert consensus documents.Results:The method demonstrated excellent accuracy and precision with the bias of ERM-DA192, ERM-DA193 and RELA 2021-2023 all being less than 2%. The recovery of added cortisol ranged from 98.2% to 101.1%. Both intra- and inter-assay imprecisions was <2%. The method was free from interference by structural analogue and showed a good linearity in the range of 25-1 600 nmol/L. The LOD and LOQ were 0.5 nmol/L and 1.0 nmol/L, respectively. A cortisol assay kit (chemiluminescence immunoassay) traced to this candidate reference method was used to determine 46 clinical serum samples concurrently, and the two methods exhibited good correlation.Conclusions:A candidate reference method for the determination of cortisol in serum was established, demonstrating high sensitivity, good repeatability and accuracy. This method can serve as a reference for the measurement traceability and accuracy evaluation of routine methods.