Comparison of a calixarene-based ion-selective electrode with two automated analyzers for the clinical determination of sodium in blood plasma

Neutral-carrier ion-selective electrodes based on methyl p-t-butylcalix[4]aryl acetate have been prepared that are responsive to sodium ions. The miniaturized catheter-type electrodes were obtained by dip-coating their porcelain tips in a PVC membrane cocktail. Examination of the general performance of the electrodes revealed excellent characteristics in terms of Nernstian response, selectivity, stability, reproducibility and response time. The results from the indirect potentiometric assessment of a large number of plasma samples with the electrodes showed a good correlation with the results from two automated analyzers (Technicon Smac 3, Hitachi 704) and with flame photometric data. Although inconsistencies were observed in the measurement of some plasma samples, the variance seemed to be method-dependent, and the overall performance of the electrodes showed promise as an alternative to the sodium glass electrode. Some factors influencing the standard potential of the measuring cell are discussed as a source of error.