Two spectrophotometric methods for the determination of cysteine in dietary supplements

Two rapid, sensitive, extraction-free spectrophotometric methods were developed for the determination of cysteine. The procedures were based on the addition reaction of cysteine with Aniline blue water soluble (Method A) or cysteine with Acid Fuchsin (Method B) in borax-sodium hydroxide medium, which formed a colorless thioether derivative, and resulting in a decrease in absorbance at wavelengths of 584 and 540 nm respectively. The cysteine complied with Beer's Law in the concentration range of 0.20~2.40 mg/L and 0.50~6.00 mg/L with good precision and accuracy, whose limits of detection were 0.122 mg/L at 584 nm for Method A and 0.113 mg/L at 540 nm for Method B, respectively. The proposed methods have been successfully applied to the determination of cysteine in dietary supplements. The analytical results of the actual samples were in accordance with those by the copper(II)-neocuproin reagent spectrophotometric method.