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

Complete separation of the trans-enantiomers of the two most abundant, persistent polar metabolites of metolachlor, metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA), was achieved using UPLC equipped with a reverse phase chiral column and trace detection with an electrospray triple quadrupole mass spectrometer. Various conditions that influenced the separation and instrumental signal were investigated to achieve the optimum separation and instrument response within an analysis time of less than 30 minutes. Different eluting solvent compositions for each metabolite were required for optimized separation of of the 4 enantiomers. Standard curves were responsive to less than 13 ng/mL and 8 ng/mL for the least plentiful MOXA and MESA enantiomers, respectively with a linear coefficient of determination greater than 0.998. Suitability of the method for quantification of the 4 mixed enantiomers of each was demonstrated using natural surface water samples collected from the Choptank River watershed in Eastern Maryland. • LC chiral separation parameters were varied to achieve optimal separation of the major enantiomers of the two metolachlor metabolites. • LC/MS-MS parameters were adjusted to maximize response and minimize analysis time. • Finished methods were used to quantitate enantiomers in archived stream water extracts from agricultural watersheds with corn/soybean production.