Quantitation of chlorophenoxy acid herbicides by high-performance liquid chromatography with coulometric detection

Abstract

Chlorophenoxy acids are toxic compounds used as selective herbicides in agriculture. To meet the need to determine trace levels of such pesticides in ground and drinking water, a highly selective and sensitive method using HPLC in combination with electrochemical detection has been developed. The four phenoxy acids 4-chloro-2-methylphenoxyacetic acid (MCPA), 4-(4-chloro-2-methylophenoxy)butyric acid (MCPB), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were investigated coulometrically in the oxidative mode. To determine the optimum working potential, hydrodynamic voltammograms were recorded at potentials between +0.1 and +1.0 V. As electrochemical activity is present only at higher potentials, the influence of photolysis was investigated with a view toward improving selectivity and sensitivity. After photoconversion by UV irradiation, detection ensued at potentials 0.3 V lower than without photolysis. For efficient preconcentration prior to HPLC-electrochemical detection, a solid-phase extraction system with C 18 cartridges was employed, resulting in high recovery and good reproducibility. The method developed meets the requirements with regard to the limits for pesticides in ground and drinking water and may be useful for the analysis of environmental herbicide residues.