Indicator reaction for sensitive kinetic micro-determination of cysteine and cystine in a mixture without prior separation

A kinetic method is described for the micro-determination of cysteine and cystine in-mixtures, based on the inhibitory effect of cysteine (after reduction of the mixture) in the oxidation reaction of 4-methoxy-1,2-diamino benzene dihydrochloride by hydrogen peroxide in the presence of iron(III) as a catalyst, The reaction is followed spectrophotometrically by measuring the reaction rate, The method is sensitive, selective, simple and allows the determination of as little as 3 X 10(-7) mol l(-1) cysteine with very good accuracy and precision, This method was applied to the analysis of pharmaceutical products and biological samples. In the latter case we used the ion-exchange method with sodium hydroxide as an eluent to separate the amino acids. Sodium hydroxide as an eluent, eliminates many possible interferences caused by iron, chloride, oxalate, citrate and other ions.