Determination of L-cysteine in amino acid mixture and human urine by flow-injection analysis with a biamperometric detector

Based on the electrocatalytic oxidation of cysteine at pretreated platinum electrode and the flow-injection biamperometry for irreversible couple, a novel electrochemical detector is proposed for the selective determination of cysteine in amino acid mixtures and human urine samples. A thin-layer flowthrough cell was used to achieve large electrode surface area to volume ratio. Two identical pretreated platinum electrodes were mounted in the cell with an applied potential difference of 10 mV. By coupling two independent and irreversible electrode processes, namely, the oxidation of cysteine and the reduction of platinum oxide, the biamperometric detection scheme has been established. The resulting current is linear to cysteine over the range 4 x 10(-7)-4 x 10(-5) M with the detection limit 1 x 10(-7) M (15 pmol). The selectivity of the detector is tested by 55 foreign species including 26 ions, 11 amino acids, 6 vitamins, and 12 other compounds possibly found in urine. The detector performs well as a routine assay, showing high efficiency (180 samples/h) and good reproductivity shown by a RSD of 0.6% for eight repeated determinations of 2 x 10(-6) M cysteine. The urine samples are detected directly without the need of pretreatment or adding other reagents. (C) 2001 Academic Press.