Lowering Detection Limits Toward Target Ions Using Quasi-Symmetric Polymeric Ion-Selective Membranes Combined with Amperometric Measurements

Nagy, Xénia and Höfler, Lajos (2014) Lowering Detection Limits Toward Target Ions Using Quasi-Symmetric Polymeric Ion-Selective Membranes Combined with Amperometric Measurements. ANALYTICAL CHEMISTRY, 88 (19). pp. 9850-9855. ISSN 0003-2700

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Abstract

An amperometric method is reported that compensates for the interference from marginally discriminated interfering ions when using traditional polymeric ionselective membrane (ISM) electrodes. The concept involves utilizing two ISMs in a three-compartment electrochemical cell configuration. The two ISMs are identical in composition except for the addition of an ionophore to one of the membranes. Initially, all three compartments contain the same concentration of interfering ion and the membrane does not contain primary ions. Reference electrodes are placed into each of the two outer compartments. Theoretically, at this point, there is no potential difference between the two reference electrodes. When the concentration of an interfering species is increased in the sample compartment, the phase-boundary potentials of both sample solution|ISMs change similarly. However, when the primary ion is added to the sample, an asymmetry will emerge, and the membrane with the ionophore will exhibit a larger phase-boundary potential change. At low concentrations the difference in membrane potentials can be too small for reliable potentiometric detection. Current, which can be routinely measured on pA levels, can be used instead to detect the small primary ion concentration changes with a significant lowering of detection limits compared to classical potentiometry. The theory of this method is described by Nernst-Planck-Poisson finite element simulations, and experimental verification is demonstrated using ammonium ISM. It is shown that amperometric measurements enable 200 nM ammonium to be detected in the presence of 0.1 mM of potassium, detection capability that is not possible via conventional potentiometry.

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