Effect of Transmembrane Electrolyte Diffusion on the Detection Limit of Carrier. Based Potentiometric Ion Sensor

摘要

The detection limit of carrier-based ion-selective electrodes is explained by the presence of a locally elevated concentration of measuring ions at the sample-membrane phase boundary. Since ion-selective electrodes are responsive to phase boundary activities, such elevated concentrations render the potentiometric sensor insensitive to dilute bulk concentration 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 the membrane and therefore to a net flux of measuring ions from the inner filling solution to the sample. This effect is described by an extended 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. These predictions are found to relate well to experimental results with valinomycin electrodes. The presence of lipophilic anions in the inner electrolyte is found to increase detection limits owing to the increased extraction into the membrane. An upper limit apparently exists beyond which the detection limit is no longer increased upon increasing the inner filling solution concentration. Stirring the sample de-creases the detection limit owing to increased mass transport from the membrane surface to the bulk sample.