EFFECT OF TRANSMEMBRANE ELECTROLYTE DIFFUSION ON THE DETECTION LIMIT OF CARRIER-BASED POTENTIOMETRIC ION SENSORS

The detection limit of carrier-based ion-selective electrodes is expla ined by the presence of a locally elevated concentration of measuring ions at the sample-membrane phase boundary. Since ion-selective electr odes are responsive to phase boundary activities, such elevated concen trations render the potentiometric sensor insensitive to dilute bulk c oncentration changes. Different mechanisms for the continuous release of measuring ions from the membrane are conceivable. The extraction of inner electrolyte into the backside of the ion-selective membrane is predicted to lead to a concentration gradient of electrolyte across th e membrane and therefore to a net nux of measuring ions from the inner filling solution to the sample. This effect is described by an extend ed model that respects the relevant extraction and diffusion processes . The extent of coextraction at the backside is predicted on the basis of potentiometric measurements on the range of anion interference. Th ese predictions are found to relate well to experimental results with valino-mycin electrodes. The presence of lipophilic anions in the inne r electrolyte is found to increase detection limits owing to the incre ased extraction into the membrane. An upper limit apparently exists be yond which the detection limit is no longer increased upon increasing the inner filling solution concentration. Stirring the sample decrease s the detection limit owing to increased mass transport from the membr ane surface to the bulk sample.