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An analytical method for the determination of diflufenican, mepanipyrim, pretilachlor, and fipronil in water samples was developed using single drop microextraction in the direct immersion mode and gas chromatography-mass spectrometry. A factorial fractionated design of type 2(6-1) at two levels was performed, to study the influence of experimental variables such as ionic strength, pH, agitation speed, extraction time, drop volume, and sample volume. To establish the optimal conditions for the variables that were significant, a Doehlert design was performed. The optimum conditions of extraction were 1 μL of heptane immersed in 4.0 mL of sample with continuous agitation at 500 rpm for 30 min at room temperature. The developed method proved to have good linearity for the range studied. The detection limits were 0.07 μg L(-1) for diflufenican, 0.03 μg L(-1) for mepanipyrim, 0.08 μg L(-1) for pretilachlor, and 1.39 μg L(-1) for fipronil. The method was validated on river water samples, showing the absence of matrix effect and recoveries ranged from 90.1 to 107.8 %. The results show that the method developed is accurate, sensitive, rapid, simple, and low cost, so it is recommended for application in the analysis of these different classes of pesticides in water samples.

Um método para a determinação de quantidades traços de herbicidas ácidos em amostras de água foi desenvolvido. O procedimento analítico envolve derivatização in situ dos analitos para seus ésteres metílicos com sulfato de dimetilo, amostragem usando microextração com única gota (SDME) e cromatografia gasosa-espectrometria de massa (GC-MS). Os efeitos do pH, força iônica, tempo de extração, solvente de extração assim como condições de derivatização foram estudados. Ésteres metílicos foram extraídos com 2 µL de n-heptano. A resposta foi linearmente dependente da concentração na faixa de 0,05-10,0 ng mL . A análise por derivatização-SDME/GC-MS rendeu boa precisão (RSD entre 7,0 e 15,2%). O método foi validado pela análise de amostras enriquecidas.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.

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