Signal shapes and single‐boundary measurements in flow injection analysis with amperometric detection

Abstract

AbstractThe shapes of normal and reverse flow injection (nFI and rFI) signals have been studied using single‐channel manifolds, amperometric and visible spectrophotometric detection, and small and large (> 1.5 mL) injection volumes. The shapes of nFI and rFI signals are fundamentally different because in rFI the sample, being the carrier stream, is present in essentially infinite volume. For this reason, rFI signals are broader and are prone to double‐peak formation at lower injection volumes than are nFI signals. Contributions to the final signal from dispersion at the front and rear boundaries of the injected bolus frequently are, or appear to be, unequal. With loop injection, the rear boundary travels further than the front boundary and is dispersed more: injection by stream switching removes this factor. With spectrophotometric detection, correction for large and opposite refractive index effects at the two boundaries may be required. In amperometric detection, differences in acidity of carrier stream and injectate can cause significant changes in background current on switching and can distort the signal at low concentrations of monitorand. When these factors are eliminated, however, other factors causing peak height and peak shape differences remain: differences in dispersion rates of reactants, and of reaction rates, appear to be the major factors.