Stereoselective separation of dimethenamid by cyclodextrin electrokinetic chromatography using deep eutectic solvents

Abstract

An Electrokinetic Chromatography (EKC) method was developed in this work enabling for the first time the separation of the four stereoisomers of the acetamide herbicide dimethenamid. A screening of different anionic cyclodextrins (CDs) revealed that the use of a single CD system did not allow the separation of the four dimethenamid stereoisomers while dual systems improved the chiral separation. The combination of 15 mM (2-carboxyethyl)-beta-CD (CE-beta-CD) with 10 mM methyl-gamma -CD (M-gamma -CD) originated the partial separation of dimethenamid stereoisomers. To obtain the baseline separation between all consecutive peaks, the effect of the addition of ionic liquids and deep eutectic solvents to the CDs dual system was investigated. While ionic liquids did not improve the chiral separation obtained with the CDs dual system, the addition of deep eutectic solvents showed generally beneficial effects on the separation in terms of resolution. The influence of the nature of the deep eutectic solvent was studied and the effects of the ready-made deep eutectic solvent and its components on the separation were compared. Choline chloride-D-fructose (ChCl-D-fructose) when added to the CDs dual system under optimized conditions (15 mM CE-beta-CD, 10 mM M-gamma -CD, 1.5 % ChCl-D-fructose (2:1) in a 100 mM borate buffer (pH 9.0), a separation voltage of 25 kV and a temperature of 20 ˚C) enabled separating the four stereoisomers of dimethenamid in 21 min with resolutions between consecutive peaks of 6.0, 2.1 and 1.5. The analytical characteristics of the developed method were evaluated and considered adequate to achieve the tereoselective analysis of dimethenamid-P in commercial agrochemical formulations. Results demonstrated the potential of the method to control the quality of these formulations and to determine the stereoisomeric purity of dimethenamid-P in these products.