Enantiomeric selectivity in the environmental degradation of dichlorprop as determined by high performance capillary electrophoresis

The chiral herbicide dichlorprop(2,4-dichlorophenoxy-2-propionic acid), which is sold and applied as the racemic mixture, was observed to degrade completely in soil within 31 days, with a half-life of 6.6 d. Degradation occurred with enantiomeric selectivity, indicating biologically mediated reactivity as opposed to strictly abiotic degradation. The S-(-)-isomer degraded significantly faster (t(1/2) = 4.4 d) than the R-(+)-isomer (t(1/2) = 8.7 d); this is contrary to other published results that show selective degradation of the R-(+)-enantiomer, although in other media. Soil samples taken from a field plot at increasing time intervals after application of Foxtril, a commercial herbicide formulation, were solvent-extracted and analyzed for total dichlorprop by capillary zone electrophoresis (CZE), using an acetate buffer at pH 4.7. Heptakis (2,3,6-tri-O-methyl)-beta-cyclodextrin, a chiral reagent, was then added to the buffer to effect separation of the (+)- and (-)-isomers of dichlorprop. Baseline resolution allowed calculation of relative concentrations (enantiomer ratios) of the two isomers. CZE is a fast and efficient technique for the analysis of ionic organic species (such as the anion of dichlorprop), including their enantiomers, in pesticide formulations as well as in environmental samples. It thus was possible to analyze Foxtril directly after dilution with water for ioxynil (2,6-diiodo-4-cyanophenol) as well as for dichlorprop. Ioxynil also was detected in the soil extract on the day of application. The hydrolysis product [methyl 2-nitro-5-(2,4-dichlorophenoxy) benzoic acid] of bifenox methyl ester, another herbicide component of Foxtril, was detected in the soil samples taken at 17 and 31 d.