No 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.

The processes determining the lower detection limit of carrier-based ion-selective electrodes (ISEs) are described by a steady-state ion flux model under zero-current conditions. Ion-exchange and coextraction equilibria on both sides of the membrane induce concentration gradients within the organic phase and, through the resulting ion fluxes, influence the lower detection limit. The latter is shown to improve considerably when very small gradients of decreasing primary ion concentration toward the inner electrolyte solution are created. By merely altering the concentration of the inner electrolyte, detection limits may vary by more than 5 orders of magnitude. Very large gradients, however, are predicted to lead to significant depletion of analyte ions in the outer membrane surface layer and thus to apparent super-Nernstian response. The currently recommended IUPAC definition of the lower detection limit leads to nonrealistic values in such cases. Small changes in the concentration profiles within the membrane may have large effects on the response of the ISE at submicromolar levels and enhance its sensitivity to interferences during trace determinations. The model studies presented here demonstrate that trace level measurements with ISEs are feasible but often require higher membrane selectivities than expected from the Nicolskii equation.In most cases described so far, the lower detection limit of solvent polymeric membrane-based ion-selective electrodes (ISEs) lies in the micromolar range. 1-3 Significantly lower values were found only when analyte ion concentrations were kept under control with the help of ion buffers, 4,5 whose effect is most likely due to their complexing the analyte ions that leach from the membrane. In their absence, the lower detection limit is governed by this leaching process, which may cause analyte ion concentrations at the phase boundary to be significantly higher than in the sample bulk. 6,7 One possible origin of this bias is the salt coextraction from the inner electrolyte solution into the membrane, which generates a primary ion flux toward the sample. 8 Based on experiments with various inner solutions of relatively high concentration and/or containing lipophilic salts, such coextraction processes were shown to be the reason the lower detection limit was shifted upward. 8 With other internal filling solutions, however, no correlation between calculated and observed detection limits was found, showing that additional effects must be taken into account. Indeed, there is experimental evidence for analyte ions being transported through the membrane owing to ion exchange and countertransport of interfering ions. 9,10 Very recently, detection limits in the picomolar range were obtained with inner solutions whose concentration of analyte ions was buffered to a low level while keeping that of the interfering ones high. 11 On the other hand, if the latter was kept low as well, no improvement was reached. 12 These observations were interpreted in terms of an ion-exchange process causing...