Separation and determination of di- and tricarboxylic acids in fruits by capillary zone electrophoresis with amperometric detection

The method for separation and determination of several di- and tricarboxylic acids, including oxalic, malic, tartaric and citric acids (OX, MA, TA and CI), found in fruit juices by capillary zone electrophoresis with amperometric detection at a copper electrode was developed. Based on cyclic voltammetry (CV) experiments, the detection mechanism for the analytes at a copper electrode was proved to be the formation of complexes between di- and tricarboxylic acids and Cu(II) ion on the electrode surface. Separation was carried out in a pH 6.2 phosphate buffer solution (PBS) containing 0.4mmol/l cetylpridinium bromide (CPB) and 20mmol/l β-cyclodextrin (β-CD) as an electro-osmotic flow (EOF) modifier to reverse the direction of the EOF and a complexing reagent for CPB to improve the separation, respectively. At the working potential of 0.12V (versus SCE), the calibration curves for the analytes were linear with about three orders of magnitude and the detection limits (S/N=3) for them were between 4 and 8fmol. By the method developed here, di- and tricarboxylic acids in four fruit juices, apple, grape, pear and orange juices, were successfully separated and determined