Proposed primary reference methods for the determination of some commercially important chiral aryloxypropionate herbicides in both free acid and ester forms

Abstract

The direct enantiomeric high-performance liquid chromatographic resolution of the free acid form of the herbicide 2-(4-chloro-2-methylphenoxy)propanoic acid (CMPP) was demonstrated using a second-generation α 1-acid glycoprotein (Chiral-AGP) chiral stationary phase (CSP). Analysis times were short, separations and retention times were reproducible during intermittent use and the baseline resolution of the enantiomers of the herbicide 2-(2,4-dichlorophenoxy)propanoic acid (2,4-DP) could also be achieved. This system was found to be far superior to the earlier separation using EnantioPac α 1-AGP CSP. Commercially important 2-butoxyethyl and 2-ethylhexyl esters and also model methyl and ethyl esters of racemic CMPP were resolved using an ionically bonded d-phenylglycine (Pirkle type 1-A) CSP. The same esters were chromatographed using a covalent d-phenylglycine CSP but the separations observed were inferior. These CSPs were capable of recognizing chirality at the phenoxypropionic moiety but no recognition of chirality associated with the 2-ethylhexyl ester was observed. The reproducibility and reliability of these methods of chiral analysis makes them suitable for both reference purposes and also for the routine determination of the newly introduced, optically active phenoxypropionate herbicides.