Chiral separation of metolachlor metabolites in a single, large volume injection to facilitate watershed tracer studies

Authors: BIANCA, MARLA - Oak Ridge Institute For Science And Education (ORISE); Owen, Dylan; Plummer, Rebecca; Rice, Clifford; McCarty, Gregory; Hapeman, Cathleen

Submitted to: ACS Agricultural Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2023
Publication Date: 2/15/2023
Citation: Bianca, M., Owen, D.C., Plummer, R.E., Rice, C., McCarty, G.W., Hapeman, C.J. 2023. Chiral separation of metolachlor metabolites in a single, large volume injection to facilitate watershed tracer studies. ACS Agricultural Science and Technology. 3(3):270–277. https://doi.org/10.1021/acsagscitech.2c00265.
DOI: https://doi.org/10.1021/acsagscitech.2c00265

Interpretive Summary: Pesticide degradates can be used as markers for environmental processes that link to agricultural practices. Especially when these degradates can be measured year round in groundwater fed streams and can be tied to nitrate loading. Two such compounds derived from the highly-used herbicide, metolachlor, have this potential; namely MESA and MOXA. Reported here for the first time is a sensitive method to distinguish each unique isomer of both compounds. The isomer-specific method for both compounds was validated for both concentration and isomer peak differences. A selected pair of these isomers was present in unbalanced amounts and they were found to be significantly related to drainage differences across the upper Choptank Watershed in the Delmarva region of Maryland. This information will be useful to scientists studying the fate of chemicals applied to agricultural fields.

Technical Abstract: 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.