[Chemical sensors in medical use: ion-selective electrodes, limitations and potentials]

The electrolyte concentrations of potassium and sodium, for example, are routinely analyzed in diluted serum and plasma. In contrast, electrolyte activities are quantified by ISEs directly in undiluted whole blood and plasma and promise to be highly informative. Nevertheless, some drawbacks, such as the interaction between the biological matrix and the ISE surface, limit the accuracy of measurements. A symmetrical measuring arrangement allows one to localize and quantify the known sources of errors. These are minimized by adequate techniques. The membrane composition for sodium selective electrodes is optimized in particular. The interpretation of ISE values as compared to FAES (flame atomic emission spectroscopy) is closely related to the water concentration of the samples. Comparisons between ISE and FAES results obviously allow one to evaluate the water concentration of samples, as is shown by a linear regression plot.