Key parameters for carbamate stability in dilute aqueous-organic solution

Within the framework of the determination of pesticides of the carbamate family, the isocyanates prove to be excellent derivatizing agents of the hydrolysis products of pesticide residues. The optimum conditions for derivatization require the use of an aqueous organic solution (water-acetonitrile) with the presence of triethylamine. Under these conditions, the derivatives formed (ureas or carbamates) must be stable to enable a reliable and representative determination. Our objective, therefore, was to study the parameters, such as solvent, structure of the derivatizing agent, pH, nature of the catalyst, and the optimum conditions needed for stability. The kinetic and statistical analysis of 24 compounds (an analysis of variance based on 142 measurements) demonstrate that neither the effects of pH, nor of the group introduced by the derivatizing agent make it possible to simultaneously stabilize all of the formed derivatives. To ensure reliable and repeatable determination of the carbamate (obtained after derivatization), it is necessary to expulse the carbamate, in the organic phase, by adding NaCl to the aqueous-organic medium. (C) 2001 Elsevier Science Ltd. All rights reserved.