Determination of glyphosate and aminomethylphosphonic acid by sequential-injection reversed-phase chromatography: method improvements and application in adsorption studies

This paper describes a low-cost reversed-phase sequential injection chromatography method for the determination of glyphosate and aminomethylphosphonic acid in environmental samples. The method is based on the pre-column conversion of glyphosate to glycine by hypochlorite, followed by reaction with o-phthaldialdehyde in presence of 2-mercaptoethanol in borate buffer (pH 9.5) to produce the fluorescent 1-(2'-hydroxyethylthio)-2-N-alkylisoindole. In addition to producing detectable fluorescent indoles, the pre-column derivatization also decreases the polarity of the analytes, favoring their retention on a C-18 monolithic column. The isocratic reversed-phase chromatography enabled the separation of both glyphosate and aminomethylphosphonic acid derivatives from polar compounds such as organic acids, humic substances, and carbohydrates which are commonly found in waters and soil extracts. This separation minimizes the laborious sample preparation procedures prior to the analysis. The linear response was observed for concentrations between 0.10 and 12.8 mu M. The limits of detection and quantification were 0.03 and 0.10 mu M (glyphosate), and 0.015 and 0.050 mu M (aminomethylphosphonic acid). At the 0.10 mu M concentration level, the relative standard deviations were 21 and 25% for aminomethylphosphonic acid and glyphosate, respectively (n = 5). Recoveries between 80 and 120% were found in the determination of glyphosate and aminomethylphosphonic acid in spiked lake waters (0.80 to 6.4 mu M). The method was applied in the determination of kinetic and thermodynamic parameters related to the adsorption of glyphosate on two horizons of an Alfisol from the Parana State in South Brazil.