Stepwise injection spectrophotometric determination of cysteine in biologically active supplements and fodders

A stepwise injection analysis system was developed to monitor the concentration of l-cysteine in biologically active supplements (BAS) and fodders. It is based on the rapid redox reaction of l-cysteine with 18-molybdo-2-phospahte heteropoly anion (18-MPA) and the detection of the formed heteropoly blue with a spectrophotometry. The method has relatively wide optimal ranges of pH and 18-MPA concentration − 5.0 to 7.3 and 0.4 to 2.0 mM, respectively. Under optimized operating conditions the performance of the SWIA system was linear up to a concentration of l-cysteine of 0.1 mM (R2 = 0.994) with a detection limit of 0.003 mM and a sample frequency of 20 h− 1. The SWIA system was employed to determine the concentration of l-cysteine in BAS and fodder. The obtained data were in good agreement with those measured by a HPLC method.