Chiral Separation of Metolachlor Metabolites in a Single, Large Volume Injection to Facilitate Watershed Tracer Studies

Single injection, chiral analysis of two abundant metolachlor metabolites, metolachlor ethanesulfonic acid (MESA) and metolachlor oxanilic acid (MOXA), was accomplished using 10 mL of water samples, modified from the original 1 L method for MESA analysis alone. These compounds revealed a temporal stamp corresponding to a conversion from racemic to S-enriched metolachlor that can be used to reduce uncertainty and equifinality in modeling watershed cropland response. They offer a dual tracer for watershed processing of nitrate that is derived from a common source and provides utility for sampling and consistency in tracing both time-weighted and long-term processes related to hydrology. Six matched grab sample results compared the 1 L and 10 mL procedures with less than a 12% difference. Average recoveries for the 10 mL method were 94% for the four isomers of MOXA and 96% for the three peaks of MESA. The 10 mL method increases the accessibility of this procedure for water age dating on a larger scale.