Published:Journal of Chromatographic Science,
ISSN 0021-9665 Volume
46, Number 3, March 2008, pp. 269-275
Monitoring Stereoselective Degradation of Metolachlor
in a Constructed Wetland: Use of Statistically Valid Enantiomeric
and Diastereomeric Fractions as Opposed to Ratios
Mohamed Y.Z. Aboul Eish[1,2] and Martha J.M.
Wells[1],
[1]Center for the Management, Utilization, and Protection of
Water Resources and Department of Chemistry, Tennessee Technological
University, Box 5033, Cookeville, TN 38505 and
[2]American
University of Sharjah, P.O. Box 26666, Sharjah, UAE
Environmentally contaminated aqueous samples are
examined for evidence of stereoselective degradation of metolachlor.
The unique chemical structure of metolachlor, a chloroacetamide
herbicide, consists of four stereoisomers due to axial and/or
C-chirality. The degradation of metolachlor is monitored over
time in agricultural runoff water that is applied to a subsurface
flow constructed wetland. Metolachlor stereoisomers are isolated
from aqueous samples by achiral reversed-phase solid-phase extraction
and analyzed by normal-phase high-performance liquid chromatography
using a chiral stationary phase. The analyses of 64 post-application
samples, collected over a period of four weeks, are reported.
The samples are filtered (0.45 µm) prior to analysis and
thereby represent metolachlor in solution and/or associated with
dissolved organic carbon. Sixteen samples demonstrate total racemic
metolachlor concentrations greater than 10 ppb. Of these 16 samples,
one sample is determined statistically to demonstrate enantioselective
degradation. Significant contributions made by this study include
the evaluation of stereoselectivity based on mathematically derived
fractions, rather than ratios, and statistical evaluation of
precision establishing the variability resulting from chromatographic
processes versus metabolic processes. The research demonstrates
that distribution of metolachlor between the solid phase composed
of chemical and/or biological particulates and the aqueous phase
is not primarily stereoselective, and that stereoselectively
enriched metolachlor does not dominate in the aqueous phase.
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