Trans enantiomeric separation of MESA and MOXA, two environmentally important metabolites of the herbicide, metolachlor

Authors: BIANCA, MARLA - Oak Ridge Institute For Science And Education (ORISE); Rice, Clifford; LUPITSKYY, ROBERT - Tic Gums, Inc; Plummer, Rebecca; McCarty, Gregory; Hapeman, Cathleen

Submitted to: MethodsX
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2022
Publication Date: 10/19/2022
Citation: Bianca, M., Rice, C., Lupitskyy, R., Plummer, R.E., McCarty, G.W., Hapeman, C.J. 2022. Trans enantiomeric separation of MESA and MOXA, two environmentally important metabolites of the herbicide, metolachlor. MethodsX. 9. Article 10188. https://doi.org/10.1016/j.mex.2022.101884.
DOI: https://doi.org/10.1016/j.mex.2022.101884

Interpretive Summary: The collection and analysis of two abundant metolachlor metabolites, metolachlor ethanesulfonic acid (MESA) and metolachlor oxanilic acid (MOXA) can be used to estimate ground water transport of nitrate to surface waters. Chiral analysis of these compounds revealed a temporal stamp matching the switch in 1998 from racemic to S enriched metolachlor. Current 1-L water methods for these analytes are time, labor, and materially limiting. Reducing grab sample volumes to 10 mL was investigated. To compensate for less compound in the extracts, injection volumes of 250 uL were used. Six matched grab sample results comparing the 1 L and 10 mL procedures were less than 12% different. Average recoveries for the 10 mL method were 94.6% for the 4 isomer of MOXA and 98% for the 3 peaks of MESA. The 10 mL method increases the accessibility of this procedure for water age-dating on a larger scale. This information will be useful to scientists estimating nitrate run-off from agricultural fields.

Technical Abstract: Using grab sampling for collection of water for trace analysis of the herbicide metabolites, MESA and MOXA, typically require large, 1-5 L, volumes of water due to the low analyte concentrations. There are drawbacks to this method. Storage times are limited to two weeks since freezing is prohibited. Sample handling is difficult, due to weight of samples, lengthy extractions times, and transport issues. To overcome these drawbacks, a 10 mL sampling and extraction method was investigated by utilizing a new large volume injections method during analysis that overcame detection issues. Spike recoveries exceeded 98% and analyses of 6 field-collected samples using the 1-L and 10 mL methods averaged less than 12% of each other. This new method will allow more time-sensitive sampling to model flow dependent factors and lag time measurements for nitrate tracking.