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 n ovel electrochemical detector is proposed for the selective determination o f cysteine in amino acid mixtures and human urine samples. A thin-layer flo wthrough cell was used to achieve large electrode surface area to volume ra tio. 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 b een 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). T he 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 efficie ncy (180 samples/h) and good reproductivity shown by a RSD of 0.6% for eigh t repeated determinations of 2 x 10(-6) M cysteine. The urine samples are d etected directly without the need of pretreatment or adding other reagents. (C) 2001 Academic Press.