Höfler Lajos, Bedlechowicz Iwona, Vigassy Tamás, Gyurcsányi Róbert E, Bakker Eric, Pretsch Ernö
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Hungary.
Anal Chem. 2009 May 1;81(9):3592-9. doi: 10.1021/ac802588j.
Ion fluxes across polymeric ion-selective membranes are a decisive parameter dictating the lower detection limit of potentiometric ion sensors. An applied current was earlier proposed to counteract such fluxes and reduce the detection limit to ultratrace levels. So far, however, the method has not been used in practical situations since the correct current amplitude requires prior knowledge of the sample composition. This paper explores the use of the stir effect to evaluate the optimal current by theory and experiments. It is shown that the traditionally used steady-state model assuming a uniform distribution of ion exchanger in the membrane, fixed with time, violates the electroneutrality condition. A modified steady-state model is introduced that allows for a concentration tilt of the ion exchanger and predicts that a stir effect can indeed be utilized to find the optimal current. Ideally, by choosing the optimal current and very long measurement times, the thermodynamic detection limit might be obtained. However, in practice the stir effect declines at low concentrations and the conditions are far from steady state. Therefore, the improvement of the lower detection limit achievable by galvanostatic control is only about 1 order of magnitude. A numerical finite-difference approximation is shown to trace the experimental potential responses of silver-selective electrodes well and to reproduce the stir effect adequately, even for different conditioning protocols. The stir effect is successfully used to improve the detection limit of electrodes with ill-optimized inner solutions; however, significant improvements beyond what is commonly feasible by chemical optimization does not seem to be easily achievable. The results indicate that with conventional membranes the possibility of improving the detection limit by current polarization is much more limited than assumed so far.
跨聚合物离子选择性膜的离子通量是决定电位离子传感器检测下限的关键参数。早前有人提出施加电流来抵消这种通量，并将检测限降低到超痕量水平。然而，到目前为止，该方法尚未在实际情况中使用，因为正确的电流幅度需要事先了解样品成分。本文通过理论和实验探索利用搅拌效应来评估最佳电流。结果表明，传统上使用的稳态模型假设膜中离子交换剂分布均匀且随时间固定，这违反了电中性条件。引入了一种改进的稳态模型，该模型考虑了离子交换剂的浓度倾斜，并预测搅拌效应确实可用于找到最佳电流。理想情况下，通过选择最佳电流和极长的测量时间，可能获得热力学检测限。然而，在实际中，搅拌效应在低浓度下会下降，且条件远非稳态。因此，通过恒电流控制可实现的检测下限的改善仅约为1个数量级。结果表明，数值有限差分近似能够很好地追踪银选择性电极的实验电位响应，并能充分再现搅拌效应，即使对于不同的预处理方案也是如此。搅拌效应成功用于提高内部溶液优化不佳的电极的检测限；然而，似乎不容易实现超出化学优化通常可行范围的显著改善。结果表明，对于传统膜，通过电流极化提高检测限的可能性比迄今所假设的要有限得多。
