A method for the determination of trace amounts of acidic herbicides in water samples was developed. The analytical procedure involves in situ derivatization of analytes to their methyl esters with dimethyl sulfate, sampling using single drop microextraction (SDME) and gas chromatography-mass spectrometry (GC-MS). The effects of pH, ionic strength, extraction time, solvent of extraction as well as derivatization conditions were studied. Methyl esters were extracted using 2 µL of n-heptane. The response was linearly dependent on the concentration in the range 0.05-10.0 ng mL-1. Limits of detection were achieved at the level of 1.8-3.0 ng L-1. Derivatization-SDME/GC-MS analysis yielded good precision (RSD between 7.0 and 15.2%). The method was validated by analysis of spiked matrix samples.
acidic herbicides residues; direct derivatization-SDME; GC-MS
