Application of cyclodextrin-modified micellar electrokinetic chromatography to the separations of selected neutral pesticides and their enantiomers

The environmental chemistry of chiral pesticides is receiving increased attention - enantiomeric ratios are being measured and enantioselective degradation processes are being reported. The requisite analysis involves separation of the various enantiomers. Mixtures of three classes of chiral pesticides - organophosphorus, DDT congeners and methyl esters of phenoxy acids - were separately tested for separation, first by non-chiral micellar electrokinetic chromatography (MEKC). Generally, the components of each mixture were so strongly adsorbed by the micelles that they coeluted with the micelles. Then, different concentrations of an organic modifier, methanol or acetonitrile, were added. Only the five organophosphorus pesticides were separated efficiently; components of the other mixtures still migrated with the micelles. Each of six cyclodextrins (CD) - alpha-, beta-, gamma-, hydroxypropyl-beta-, dimethyl-beta- and trimethyl-beta-CD, were then added to the borate-SDS buffer, with and without the organic modifier, to test for separation of the non-chiral compounds and the enantiomers of the chiral racemates by CD-MEKC. The enantiomers of malathion, ruelene and dialifos were separated by hydroxypropyl-beta-CD, beta-CD and/or gamma-CD, while the enantiomers of isofenfos and fenamifos could not be separated. gamma-CD with methanol modifier allowed baseline separation of the three phenoxy acid methyl esters and of the enantiomers of fenoprop methyl ester, but none of the CDs separated the enantiomers of mecoprop and dichlorprop methyl esters. The use of gamma-CD with acetonitrile modifier resulted in excellent separation of six DDT congeners, o,p'- and p,p'-DDT, -DDD and DDE, as well as baseline separation of the enantiomers of the chiral members of this series, o,p'-DDT and -DDD. Finally, attempts were made to separate the four enantiomers of the herbicide metolachlor; three of the enantiomers were separated by gamma-CD with methanol. (C) 1997 Elsevier Science B.V.