Amperometric Detection of Nonelectroactive Cations in Electrolyte-Free Flow Systems at Zeolite Modified Electrodes

Abstract

An electrochemical detector was designed and constructed, based upon a thin-layer cell with a working electrode prepared from carbon paste modified with copper- (or silver-) exchanged zeolite particles. This electrode was then evaluated as a sensor for nonelectroactive cations in an electrolyte-free flow injection system and as an amperometric detector for suppressed ion chromatography. In the absence of supporting electrolyte, a higher signal-to-background ratio enhanced by about two order of magnitude the response obtained in electrolytic medium. Peak currents resulted from an important faradic component which was due to the electrochemical activity of the mediator-containing zeolites, while the capacitive counterpart was much lower. When applied to the analysis of alkali, alkaline earth metal and ammonium ions, in combination with a suppressed ion chromatography system, the zeolite-modified detector gave rise to reproducible amperometric responses. This resulted in chromatograms comparable to those obtained simultaneously by using a conductivity detector in series, and any extra-column effects were never observed. The most sensitive responses were achieved when using zeolites displaying the largest pore size, the highest ion exchange capacity, and the most mobile mediator, to ensure the highest exchange rates for the analytes within the zeolite framework.