Enantiomeric separation of chiral phenoxy acid herbicides by electrokinetic chromatography. Application to the determination of analyte-selector apparent binding constants for enantiomers

Abstract

The enantiomeric resolution of chiral phenoxy acid herbicides was performed by electrokinetic chromatography using a cyclodextrin as chiral pseudophase (CD-EKC). A systematic evaluation of several neutral and charged cyclodextrins was made. Among the cyclodextrins tested, (2-hydroxy)propyl β-cyclodextrin (HP-β-CD) was found to be the most appropriate for the enantioseparation of phenoxy acids. The influence of some experimental conditions, such as nature and pH of the background electrolyte, chiral selector concentration, and temperature, on the enantiomeric separation of phenoxy acids was also studied. The use of a 50 mM electrolyte solution in ammonium formate at pH 5 and a temperature of 40°C enabled the enantiomeric resolution of four of the six phenoxy acids investigated (2-phenoxypropionic acid, 2-(3-chlorophenoxy)propionic acid, 2-(4-chlorophenoxy)propionic acid, and 2-(2, 4-dichlorophenoxy)propionic acid) obtaining migration times ranging from 9 to 15 min. Mixtures of the two phenoxy acids not enantiomerically resolved (2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(2, 4, 5-trichlorophenoxy)propionic acid) and up to three of the phenoxy acids enantiomerically resolved were separated in about 15 min. Finally, the apparent binding constants for each enantiomer-HP-β-CD pair were calculated at two temperature values (20 and 40°C).