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

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Abstract
For total content of cysteine (Cys) and glutathione (GSH) a sample (500 μL) 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 μmol/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.

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