Validation of candidate reference methods based on ion chromatography for determination of total sodium, potassium, calcium and magnesium in serum through comparison with flame atomic emission and absorption spectrometry.

OBJECTIVES

We report on the validation of candidate reference methods based on ion chromatography for the determination of total sodium, potassium, calcium, and magnesium in serum. The current study is a continuation of previous investigations in which we were able to show the potential of ion chromatography to serve as a reference method principle. The validation consisted of comparison of the ion chromatographic methods with generally recognized reference methods based on flame atomic emission (for sodium/ potassium) and absorption (for calcium/magnesium) spectrometry.

METHODS

Sample preparation consisted of acidic dilution and pre-purification with reversed phase cartridges. Ion chromatography with suppressed conductivity was performed with polymeric stationary phases chemically modified with carboxylate and sulfonate functional groups for sodium/potassium and calcium/magnesium, respectively. Single-point calibration was used in combination with individual adaptation of serum sampling and diluting to give similar responses in samples and standards. In the study, more than 25 control sera were analyzed in parallel with the ion chromatographic methods and the traditional reference methods. The measurement protocol consisted of quadruplicate analysis of each sample per run, in 3 different runs. In each measurement series, internal quality control with certified reference materials from the National Institute of Standards and Technology and the Bureau Communautaire de Référence was applied. The performance criteria to fulfill were pre-defined (i.e., the maximum within-day and overall coefficient of variation, the confidence interval, the daily and overall deviation from the certified value and the maximum total analytical error).

RESULTS

The comparison is presented as difference plot (i.e., % deviation of the results obtained with ion chromatography from those obtained with the respective traditional reference methods). The mean deviations were +0.9% for sodium, +1.0% for potassium, +/- 0.0% for calcium, and +0.1% for magnesium. With the exception of 2 values each for sodium and potassium, all deviations were within the limit of 2 times the allowed total analytical error. From the data obtained for internal quality control during the different measurement campaigns, we further estimated the long-term precision and accuracy of our methods. The overall coefficient of variation (respectively the bias) amounted to 0.7% (+0.1%) for sodium, 0.9% (-0.3%) for potassium, 1.3% (-0.1%) for calcium, and 1.0% (+/- 0.0%) for magnesium.

CONCLUSION

From the precision and accuracy reached by our ion chromatographic methods, we conclude that they can be proposed as candidate reference methods for the determination of total sodium, potassium, calcium, and magnesium in serum.