THE USE OF A PH GRADIENT IN FLOW-INJECTION ANALYSIS IN DIFFERENTIATING ZINC AND CADMIUM IN A BINARY MIXTURE

A flow-injection system, designed to provide highly reproducible pH gr adients, was used to discriminate between fluorescent metal chelates o f zinc and cadmium. The chelating agent used, 8-hydroxyquinoline-5-sul phonic acid (HQS), has only minimal background fluorescence and its re action with metals is pH dependent. A pH gradient was generated within the flow-injection system and the fluorescence intensity of zinc, cad mium and mixtures of the two metal species in the presence of HQS, was measured as a function of time. With a pH gradient ranging from 4 to 12 and back to 4 over a period of 90 s, a doublet fluorescence respons e was produced for both cadmium and zinc. The distance between the two peaks of each doublet differed sufficiently to allow the resolution o f binary mixtures of the two metal species. Detection levels of 0.05 m uM for both zinc and cadmium were obtained. The method of steepest des cent was the mathematical procedure used to resolve the individual com ponents of the binary mixture.