Analysis of orange juice for total cysteine and glutathione content by CZE with UV-absorption detection.

For total content of cysteine (Cys) and glutathione (GSH) a sample (500 microL) of neat orange juice or commercially available soft drinks is treated according to the procedure consisted of four essential steps: reduction of disulfide forms of the analytes to their thiol counterparts with Tris 2-carboxyethylphosphine, derivatization of thiols to their S-quinolinium derivatives with 2-chloro-1-methylquinolinium tetrafluroborate, separation of so-formed derivatives by capillary zone electrophoresis with ACN stacking, and detection and quantitation with the use of ultraviolet detector at 355 nm. Commercially available bare fused-silica capillary served as the CE column. The method is linear in a wide range of concentrations covering practical application. The coefficient of correlation between analyte concentrations within the range from 2.5 to 30.0 micromol/L and the detector response was for Cys and GSH 0.998 and 0.997, respectively. The imprecision for above-mentioned range was between 6.7 and 3.8, and 3.6 and 1.1% RSD for Cys and GSH, respectively. The method was applied to several samples of fresh juices and commercial orange beverages.