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

The enantiomeric resolution of chiral phenoxy acid herbicides was performed by electrokinetic chromatography using a cyclodextrin as chiral pseudophas e (CD-EKC). A systematic evaluation of several neutral and charged cyclodex trins was made. Among the cyclodextrins tested, (2-hydroxy)propyl beta -cyc lodextrin (HP-beta -CD) was found to be the most appropriate for the enanti oseparation of phenoxy acids. The influence of some experimental conditions , such as nature and pH of the background electrolyte, chiral selector conc entration, and temperature, on the enantiomeric separation of phenoxy acids was also studied. The use of a 50 mm electrolyte solution in ammonium form ate at pH 5 and a temperature of 40 degreesC enabled the enantiomeric resol ution of four of the six phenoxy acids investigated (2-phenoxypropionic aci d, 2-(3-chlorophenoxy)propionic acid, 2-(4-chlorophenoxy)propionic acid, an d 2-(2,4-dichlorophenoxy)propionic acid) obtaining migration times ranging from 9 to 15 min. Mixtures of the two phenoxy acids not enantiomerically re solved (2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(2,4,5-trichloroph enoxy)propionic acid) and up to three of the phenoxy acids enantiomerically resolved were separated in about 15 min. Finally, the apparent binding con stants for each enantiomer-HP-beta -CD pair were calculated at two temperat ure values (20 and 40 degreesC).