Flow injection chemiluminescent method for the successive determination of l-cysteine and l-cystine using photogenerated tris(2,2′-bipyridyl) ruthenium (III)

Flow injection configurations were developed for the individual determination of l-cysteine and l-cystine and for the mixture of both analytes. The method is based on the reaction of l-cysteine with tris(2,2′-bipyridyl) ruthenium (II) and peroxydisulphate under UV irradiation to produce chemiluminescence. Cystine is determined after reduction to cysteine in a copper-coated cadmium reductor mini-column in the flow system. The inclusion of a selection valve in the configuration allows the successive determination of the two analytes. Calibration was linear between 2×10−6 and 5×10−4 mol l−1 for cysteine and between 1×10−6 and 2×10−4 mol l−1 for cystine. When applied to pharmaceutical formulations, the procedure was free from interferences from common excipients. The results obtained for the assay of commercial formulations compared well with those obtained by an official method and demonstrated good accuracy and precision.