Enantiomeric separation of acidic herbicides by capillary electrophoresis using vancomycin as chiral selector

In this study vancomycin has been used as chiral selector for the enantiomeric separation of several free acid herbicides, namely aryloxypropionic (mecoprop, fenoprop and dichlorprop), N-benzoyl-N-(3-chloro-4-fluorophenyl)-2-aminopropionic acid (flamprop) and aryloxyphenoxypropionic (haloxyfop, fluazifop, diclofop and fenoxaprop). The partial filling method was used in order to increase the sensitivity of the electrophoretic method; vancomycin was not present at the detector path during the detection due to its positive charge and to the absence/reduction of the electroosmotic flow at the operating pH. The pH of the BGE, the capillary temperature and the vancomycin concentration influenced both effective mobility and resolution of the studied compounds. The increase of vancomycin concentration caused a general increase of migration time, resolution and selectivity, the best results have been achieved when a 6 mM concentration of chiral selector was used. Using such a concentration of vancomycin, baseline resolution was obtained for all the studied enantiomers in 4.5-8.4 min. The optimized CE method has been tested for the analysis of haloxyfop free acid metabolite in a soil sample spiked with racemic haloxyfop ethoxyethyl ester solution. The metabolite resulted to be a mixture of R and S enantiomers where the R form was about 72% (peak areas ratio). The method showed good precision for both migration time and corrected peak areas with a detection limit of 5 x 10-7 M.