Environmental behavior of acetamide pesticide stereoisomers. 1. Stereo- and enantioselective determination using chiral high-resolution gas chromatography and chiral HPLC.

The chromatographic separation of enantiomers and diastereomers of some acetamide pesticides was investigated using achiral and chiral high-resolution gas chromatography/mass spectrometry (HRGC/MS) and chiral high-performance liquid chromatography (HPLC). The compounds studied were alachlor, acetochlor, metalaxyl, metolachlor, and dimethenamid and represent important herbicides or fungicides. Whereas alachlor is achiral, all other compounds are axial- and/or C-chiral and consist of two or four stereoisomers (enantiomers and diastereomers). Since these stereoisomers have different biological activities, stereo- and enantioselective determinations of these compounds in environmental studies seem important but have never been reported. In our study, chiral HRGC using a β-cyclodextrin derivative showed varied resolution of diastereomers and/or enantiomers ; achiral HRGC showed no resolution of diastereomers. Resolution of C-chiral enantiomers was easier than resolution of axial-chiral enantiomers (atropisomers). The analysis of different batches of metolachlor revealed a varied diastereomeric composition. Chiral HPLC using modified cellulose and phenylglycine columns also showed some isomer resolution and was used for the isolation of fractions enriched with one or the other stereoisomer. The assignment of the (+)- and (-)-enantiomers was achieved for acetochlor and metalaxyl using chiral HPLC in combination with chiroptical detection. A novel thermal equilibration procedure allowed distinction among axial-chiral and C-chiral enantiomers.