Simultaneous Enantiomeric Determination of Acidic Herbicides in Apple Juice Samples by Liquid Chromatography on a Teicoplanin Chiral Stationary Phase

Abstract

A novel and chiral liquid chromatography-ultraviolet method has been developed for simultaneous determination of mecoprop, fenoprop and diclofop acid herbicides. Optimization of mobile phase composition for enantiomeric separation on a teicoplanin stationary phase was done by experimental design and response surface analyses. When chiral separation of herbicide mixtures was optimized, multiple response analysis was necessary to determine combination of experimental factors, methanol percentage and triethylamine concentration in the aqueous buffer, which simultaneously optimize experimental responses. Under optimum conditions, acceptable resolution between the three enantiomeric pairs and between enantiomers of each studied herbicide was achieve in low analysis time. Applicability of the chromatographic method for simultaneous chiral herbicide determination was evaluated in commercial apple juice samples. Previously, a clean-up and preconcentration step on a polymeric solid phase extraction (SPE) cartridge (OASIS® MCX) was done. In order to get enantioresolution in the complex matrix, compatibility between the sample preparation procedure and the chiral liquid chromatography analysis was necessary. Thus, a focusing step was required after herbicide elution from the SPE sorbent to obtain an injection solution with low organic solvent composition. Detection limits were around maximum residue levels permitted by European Regulations in fresh fruit, allowing herbicide determination at enantiomer spiked levels from 0.075 to 0.750 μg g−1, with recoveries in the range 65–99 % and relative standard deviations of ≤4 %. Due to co-extracted endogenous matter, which partially overlapped with mecoprop enantiomers, mathematical deconvolution was the best choice as integration tool minimizing errors in peak area estimation.