Application of the base-promoted esterification procedure to the chlorophenoxy acid herbicides of US EPA method 515.3

Determination of chlorinated phenoxyacid herbicides by gas chromatography is complicated by the fact that these compounds exhibit low volatilities due to the hydrogen bonding of their carboxylic acid and phenol functionalities. Furthermore, these functional groups can cause the parent molecule to adsorb on the stationary phase of the column, thus producing asymmetric peaks. Masking of these polar groups can significantly increase the volatility and reduce the adsorption effects of these compounds, yielding products that can undergo analysis by gas chromatography. A variety of reactions have been employed in the preparation of less polar derivatives. Two categories are examined in this work: silylation and alkylation. Within the category of alkylation, derivatization techniques examined include acid-catalyzed esterification, extractive alkylation, and base-promoted esterification. Of all the investigated techniques, base-promoted esterification is determined to be the most effective. This paper reports the application of this procedure in the development of a sensitive method for the determination of a variety of phenoxyacid herbicides at concentrations below the microgram-per-liter level in water.