Verifying the chiral switch of the pesticide metolachlor on the basis of the enantiomer composition of environmental residues

Abstract

Racemic metolachlor is currently being replaced by S-metolachlor, the herbicide enantiomerically enriched with the biologically active (S)-isomers. This 'chiral switch' is expected to reduce the amount of pesticide released into the environment as well as potential side-effects on non-target organisms. The switch is also expected to result in lower environmental concentrations and in a changed enantiomer/stereoisomer composition of the residues. To verify this switch, the enantiomer composition of metolachlor residues from a lake (Baldeggersee, Switzerland) receiving inputs from agricultural activities was measured from 1997 through 2001. Enantioselective GC-MS analyses indicated that pre-1998 inputs into the lake were from the racemic compound. In contrast, more recent inputs showed a clear excess of (S)-isomers, and eventually (2000/2001) consisted almost entirely of the new S-metolachlor. The data document the application of enantioselective analysis in environmental studies to apportion the relative contributions of old (racemic) and new (enantioenriched) herbicide on the basis of the enantiomer/stereoisomer composition. The data also show a rapid environmental response to the changed composition of a herbicide. In terms of absolute concentrations no direct evidence could be obtained for the replacement because of year-to-year variations in the inputs likely as a result of intensive rainfall during the application periods.