Three different approaches for the separation of MCPA and 2,4-D by capillary electrophoresis

2,4-Dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA) are phenoxyalkyl acid herbicides widely used in agriculture. In many cases, commercial formulations contain a mixture of both compounds so, it is important to develop analytical methods for their simultaneous determination. Phenoxyalkyl acid herbicides can be analysed by capillary electrophoresis (CE) by means of capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC). Good results have been obtained for the separation of some phenoxyalkyl acid herbicides, but the CE separation of MCPA and 2,4-D present some difficulties and only few works have addressed this problem. In this work, the CE separation of 2,4-D and MCPA has been studied via the addition of an organic modifier (acetonitrile), the use of a micellar system (sodium cholate) and the addition of different cyclodextrins. The use of acetonitrile resulted in the total separation of both herbicides in 15 min but with low efficiency and reproducibility values. On the other hand, the use of a micellar system solved the reproducibility problem but resulted in poor efficiency. Finally, when using alpha-, beta- and gamma-cyclodextrins selectivity differences were observed and the best results were obtained with the use of alpha-cyclodextrin. In this case, 2,4-D and MCPA have been completely resolved with a high efficiency in less than 5 min. (C) 1999 Elsevier Science B.V. All rights reserved.