Lowering the detection limit of solvent polymeric ion-selective membrane electrodes. 2. Influence of composition of sample and internal electrolyte solution

The influence of the composition of the internal electrolyte solution on th e response of Pb2+- and Ca2+-selective membrane electrodes is investigated. It is shown that the lower detection limit is improved by generating, in t he membrane, ionic gradients that lead to a flux of primary ions toward the inner reference electrolyte solution. If the ion flux is too strong, it ma y cause analyte depletion at the membrane surface and, as a consequence, ap parent super-Nernstian response. Such electrodes are not adequate to measur e low analyte activities but can be used to determine unbiased selectivity factors. The results are interpreted in terms of a steady-state model, intr oduced in the companion paper, that describes the influence of concentratio n gradients generated by ion-exchange and coextraction processes on both si des of the membrane.