Determination of glyphosate residues in plants by precolumn derivatization and coupled-column liquid chromatography with fluorescence detection

A rapid method was developed for the trace-level determination of glyphosate in olives. After extraction of the glyphosate with water-dichloromethane and simultaneous removal of the olive oil, an aliquot of the aqueous extract is derivatized with 9-fluoroenylmethyl chloroformate (9-fluorenylmethoxycarbonyl chloride; FMOC-CI) to produce a highly fluorescent derivative. A 2 mL aliquot of this extract is injected directly into a coupled-column liquid chromatography system with fluorimetric detection (LC/LC-FD). The procedure was validated by recovery experiments at 3 spiking levels; recoveries ranged from 80 to 97% with relative standard deviations of 3-6%. The limits of detection and quantitation were estimated to be 0.01 and 0.05 mg/kg, respectively. The method was also applied to other plant materials, i.e., tomato plants, strawberry plants, and pear trees (branches, leaves, and fruits) suspected to be contaminated by glyphosate. In all these cases, the extraction was performed in aqueous media. The derivatization reaction was modified by increasing the FMOC-CI concentration, to ensure a quantitative reaction between analyte and reagent in the presence of high levels of coextractives, which also react with FMOC-CI. The final determination was by LC/LC-FD, yielding a rapid, selective, and sensitive method for the determination of glyphosate residues in these samples. The method was tested with real-world samples after application of glyphosate to the surrounding area of crops.