ENVIRONMENTAL BEHAVIOR OF ACETAMIDE PESTICIDE STEREOISOMERS .1. STEREOSELECTIVE AND ENANTIOSELECTIVE DETERMINATION USING CHIRAL HIGH-RESOLUTION GAS-CHROMATOGRAPHY AND CHIRAL HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The chromatographic separation of enantiomers and diastereomers of som e acetamide pesticides was investigated using achiral and chiral high- resolution gas chromatography/mass spectrometry (HRGC/MS) and chiral h igh-performance liquid chromatography (HPLC). The compounds studied we re alachlor, acetochlor, metalaxyl, metolachlor, and dimethenamid and represent important herbicides or fungicides. Whereas alachlor is achi ral, all other compounds are axial- and/or C-chiral and consist of two or four stereoisomers (enantiomers and diastereomers). Since these st ereoisomers have different biological activities, stereo- and enantios elective determinations of these compounds in environmental studies se em important but have never been reported. In our study, chiral HRGC u sing a beta-cyclodextrin derivative showed varied resolution of diaste reomers and/or enantiomers; achiral HRGC showed no resolution of diast ereomers. Resolution of C-chiral enantiomers was easier than resolutio n of axial-chiral enantiomers (atropisomers). The analysis of differen t batches of metolachlor revealed a varied diastereomeric composition. Chiral HPLC using modified cellulose and phenylglycine columns also s howed some isomer resolution and was used for the isolation of fractio ns enriched with one or the other stereoisomer. The assignment of the (+)- and (-)-enantiomers was achieved for acetochlor and metalaxyl usi ng chiral HPLC in combination with chiroptical detection. A novel ther mal equilibration procedure allowed distinction among axial-chiral and C-chiral enantiomers.