Selective flow-injection biamperometric determination of sulfur-containing amino acids and structurally related compounds

A simple and selective flow-injection method for the determination of cysteine and methionine is proposed. The method is based on the reaction of the amino acid with chloramine-T and the subsequent reaction of the excess of chloramine-T with an acidic solution of iodide producing a stoichiometric amount of tri-iodide (diminution of the flow-injection analysis (FIA) peak) which is determined biamperometrically using two platinum electrodes polarized at 100mV. The calibration graph is linear up to 10?gml-1 of cysteine and 30?gml-1 of methionine; the limit of detection is 0.06 and 0.1?gml-1; the repeatability (also as percent of a series of 70 continuous FIA peaks of 5?gml-1 of cysteine or 15?gml-1 of methionine) is 1.1 and 0.7 for cysteine and methionine, respectively. The sample throughput is 220h-1 in both cases. A large study on the influence of foreign substances is performed especially focused on the rest of L-amino acids. The method is applied to the determination of cysteine, methionine and N-acetyl-cysteine, penicillamine and cystine as structurally related compounds in pharmaceutical and veterinary samples. Finally, the proteins methionine enkephalin and oxytocin are determined on the basis of their contents in both amino acids.