Lowering the Detection Limit of Solvent Polymeric Ion-Selective Membrane Electrodes. 2. Influence of Composition of Sample and Internal Electrolyte Solution

Abstract

The influence of the composition of the internal electrolyte solution on the response of Pb 2+ -and Ca 2+ -selective membrane electrodes is investigated. It is shown that the lower detection limit is improved by generating, in the 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 may cause analyte depletion at the membrane surface and, as a consequence, apparent super-Nernstian response. Such electrodes are not adequate to measure low analyte activities but can be used to determine unbiased selectivity factors. The results are interpreted in terms of a steady-state model, introduced in the companion paper, that describes the influence of concentration gradients generated by ion-exchange and coextraction processes on both sides of the membrane.

Theory and computer simulation of the time-dependent selectivity behavior of polymeric membrane ion-selective electrodes

“…The exact position of the super-nernstian response region mainly depends on the membrane parameters and can be drastically shifted for ISE configurations with built-in ion fluxes [26] (see also Appendix). Detailed explanations using schematics that illustrate the underlying ion-flux influences have been presented earlier [2][3][4]. Analogous phenomena are expected for the other two systems in Figure 2, but the effects are here hidden because the samples contain high levels of primary ions.…”

“…Transmembrane ion fluxes are, however, highly relevant at lower sample concentrations, especially in the presence of interfering ions [2][3][4][5]. The recent spectacular improvement of the lower detection limit of ISEs by orders of magnitude relies on the reduction of such fluxes [6,7].…”

“…It has been shown in a series of theoretical studies that the ion selectivity behavior of ISEs may change drastically under the influence of transmembrane ion fluxes in zero-current [3][4][5][21] or controlled-current experiments [19]. However, these treatments were usually restricted to steady-state conditions for the mass transport in the membrane phase.…”

Novel potentiometric and optical silver ion-selective sensors with subnanomolar detection limits

“…Unbiased selectivity coefficients can be obtained either by measuring the calibration curves for discriminated ions before the first contact of the membrane with the primary ion [27] or by using appropriate inner solutions [28]. The response slopes for the interfering ions must be recorded since close to theoretical values confirm that selectivity coefficients are not biased [29].…”

Multispectral imaging of ion transport in neutral carrier‐based cation‐selective membranes

“…In solutions of submicromolar concentrations, even infinitesimally small amounts of ions, leaching from the membrane, may dominate the measured membrane potential (27). Once this leaching was eliminated, the detection limits of ISEs could be extended from micromolar to nano-and picomolar concentrations and simultaneously the experimentally determined selectivity coefficients were also dramatically improved (3,26,(28)(29)(30)(31). Our group introduced methods to eliminate ion leaching and to control ion transport through ISMs by galvanostatically controlled current (32,33).…”