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.
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