Iceberg lettuce is one of the most consumed leafy vegetables, which is often treated by different pesticides against pests and diseases. The aim of this study was to describe the fate of 25 pesticides (16 fungicides, 7 insecticides and 2 herbicides) based on quantitative analysis of the parent compounds and targeted screening of their (bio)transformation products. Mathematical models describing a decrease in pesticide residue levels were proposed for 24 pesticides using a first-order kinetic equation. These models provide the data needed to predict consumer exposure associated with the consumption of conventionally grown iceberg lettuce. At harvest, concentrations of most pesticides were dropped under the established EU maximum residue levels, except for flonicamid, fluazifop and pyriproxyfen. A total of 113 pesticide metabolites and degradation products were detected and tentatively identified in extracts prepared by an optimized extraction procedure, i.e., the acidified QuEChERS method. Several products of reactions such as hydrolysis, dealkylation, dehalogenation and/or oxidation-reduction, originated either from various physicochemical processes, or within Phase I pesticide metabolism were detected. Additionally, numerous conjugates with hexose, malonic acid or acetic acid formed during PhaseII of pesticide metabolism were found. In this way, a deeper understanding of specific pesticide degradation mechanisms is facilitated. In addition, it is easier to track the history of pesticide treatment.

Crop Research Institute Division of Crop Protection and Plant Health Drnovska 507 161 06 Prague Czech Republic

Demonstration and Experimental Center Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague Kamycka 129 165 00 Prague Suchdol Czech Republic

Department of Food Analysis and Nutrition University of Chemistry and Technology Technicka 3 166 28 Prague Czech Republic

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