Separation of phenoxy acid herbicides and their enantiomers in the presence of selectively methylated cyclodextrin derivatives by capillary zone electrophoresis

The chiral and mutual separation of nine phenoxy acid herbicides including seven pairs of phenoxy acid enantiomers was examined by capillary zone electrophoresis using unmodified and selectively methylated cr-cyclodextrin (alpha-CD), beta-CD and gamma-CD derivatives as separation additives. The selective methylation of the secondary hydroxyl groups on the rim of the CD cavities produced remarkable selectivity changes for the phenoxy acid herbicides. For the chiral separation of seven racemic phenoxy acid herbicides, unmodified and methylated alpha-CDs exhibited higher enantioselectivities than the beta-CD additives and also much higher enantioselectivities than the gamma-CD additives. Among the alpha-CD chiral selectors, hexakis(2,3-di-O-methyl)-alpha-CD (2,3-DM-alpha-CD) especially exhibited quite high enantioselectivity at 10 mM for all seven phenoxy acid herbicides. For the nine phenoxy acid herbicides, unmodified alpha-CD at 5 mM produced their complete mutual separation within 14 min, but could not resolve all (seven) pairs of their enantiomers. The simultaneous (both chiral and mutual) separation of 2-(3-chlorophenoxy)propionic acid, 2-( 2,4,5-trichlorophenoxy)propionic acid, (2,4,5-trichlorophenoxy) acetic acid, 2-(2-chlorophenoxy)propionic acid and 2-phenoxypropionic acid could be readily attained, but 2-(2,4-dichlorophenoxy)propionic acid, 2-(4-chloro-2-methylphenoxy)propionic acid, (2,4-dichlorophenoxy)acetic acid (2,4-D) and 2-(4-chlorophenoxy)propionic acid (2,4-CPPA) could not be mutually separated by mixing 5 mM alpha-CD and 10 mM 2,3-DM-alpha-CD. However, 2,4-D and 2,4-CPPA could be completely separated with alpha-CD and 2,3-DM-alpha-CD at 2.5 mM each, though neither 2-(2,4-dichlorophenoxy)propionic acid nor 2-(4-chloro-2-methylphenoxy)propionic acid could be separated at all. (C) 1999 Elsevier Science B.V. All rights reserved.