Improved detection limits and unbiased selectivity coefficients obtained by using ion-exchange resins in the inner reference solution of ion-selective polymeric membrane electrodes

By using a high concentration of an interfering ion and a low one of the primary ion in the inner reference solution of polymeric membrane ion-selective electrodes (ISEs), the lower detection limit may be improved and unbiased thermodynamic selectivity coefficients may be obtained. To this purpose, a cation-exchange resin is used here to keep the low concentration of the primary cation constant. Different compositions of the internal solution are required for obtaining optimal lower detection limits and unbiased selectivity coefficients. All ISEs studied here, i.e., for K+, Ca2+, and NH4+, based on valinomycin, ETH 5234, and nonactin/monactin, respectively, show improved lower detection limits in the range of 10-7.6 (NH4+) to 10-8.8 M (Ca2+). Nernstian responses and, therefore, unbiased selectivity coefficients are obtained with the K+-ISE for the discriminated ions, Na+, Mg2+, and Ca2+.