Experimental and theoretical insights in the improvement of the detection limit of ion-selective electrodes.

摘要

Earlier works have shown that elimination of zero-current ion fluxes in the ionselective membrane leads to improvement of the low detection limit of ion-selective electrodes (ISEs) for several orders of magnitude. Two experimentally most attractive methods are described here. Immobilization of ionophore to the polymer matrix diminishes ion fluxes, while at the same time reduces number of cocktail components, and prevents membrane components from leaching out. Newly synthesized plasticizer-free methyl-methacrylate decyl-methacrylate polymer matrix was tested, and slower diffusion through the polymer was observed that lead to reducing ion fluxes. Successful immobilization of modified calcium selective ionophore onto the same polymer matrix has lead to diminishing ion fluxes and improvement of the low detection limit. On the other hand, optimization of the inner solution by buffering of the primary ion to a very low concentration level was examined on the [9]-mercuracarborand-3 based iodide selective electrode. An electrode with nanomolar detection limit was manufactured. This electrode is the first anion-selective electrode reported with such detection limit.; Since it is established that the composition of the inner solution is crucial for obtaining optimal low detection limit, an experimental method for testing the level of optimization of the inner solution is developed. Simple two point calibration on two different rotational speeds while electrode is immersed in the sample that does not contain primary ions provides very accurate information of the level of inner solution optimization. Theory that explains so-called stirring effect was also presented.; Theoretical understanding of mechanisms that lead to low detection limit is very important for successful advance. We are presenting the dynamic diffusion model based on the solution of the one-dimensional transmission problem with a non-linear jump condition on an interface. It gives an approximate solution of the diffusion equation in the aqueous and organic diffusion layers, and treats the sample-membrane interface condition on the basis of current ion-exchange and ISE selectivity theory. This method is able to trace real-time potential response, as well as to treat so-called memory effects, and potential drifts.; In addition to studying the low detection limit, possibility of improvement of the upper detection limit was also examined. It is known that too strong complex formation between primary ion and ionophore can induce stronger coextraction that results with decreasing of the upper detection limit. An attempt to reduce complex formation constant of chloride-selective electrode by adding 1-decanethiol in the membrane cocktail containing [9]-mercuracarborand-3 as ionophore was made. It was shown that the complex formation constant was lowered while achieving improvement of the upper detection limit.