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 pr imary ion in the inner reference solution of polymeric membrane ion-selecti ve electrodes (ISEs), the lower detection limit may be improved and unbiase d thermodynamic selectivity coefficients may be obtained. To this purpose, a cation-exchange resin is used here to keep the low concentration of the p rimary cation constant. Different compositions of the internal solution are required for obtaining optimal lower detection limits and unbiased selecti vity coefficients. All ISEs studied here, i.e., for K+, Ca2+, and NH4+, bas ed on valinomycin, ETH 5234, and nonactin/monactin, respectively, show impr oved lower detection limits in the range of 10(-7.6) (NH4+) to 10(-8.8) M ( Ca2+). Nernstian responses and, therefore, unbiased selectivity coefficient s ape obtained with the K+-ISE for the discriminated ions, Na+, Mg2+, and C a2+.