Determination of herbicides, including thermally labile phenylureas, by solid-phase microextraction and gas chromatography–mass spectrometry

A method for the determination of 10 herbicides, including thermally unstable compounds, has been developed. The method uses solid-phase microextraction (SPME) with a polyacrylate fibre. Separation, identification and quantification were accomplished with gas chromatography–mass spectrometry. The herbicides chosen belong to different chemical groups and were alachlor, atrazine, chlorotoluron, diclofop, diflufenicam, ethofumesate, isoproturon, linuron, terbutryn and trifluralin. In the present work we studied the chromatographic behaviour of three phenylureas as a function of the medium and injection mode employed. The compounds generated as a function of the solvent used in direct injection of the phenylureas (ethyl acetate, methanol and methanol–water) and those obtained when injection was accomplished using the polyacrylate fibre were determined. The results allow us to propose a method for the determination of stable and thermally unstable herbicides as long as a preconcentration step involving SPME is carried out. In the proposed method, the limits of detection varied between 0.02 μg/l for ethofumesate and 0.11 μg/l for chlorotoluron. The method was applied to the determination of these herbicides in surface and ground water samples, performing quantification by standard addition calibration. The contents of chlorotoluron and atrazine found were significantly equal to those obtained using HPLC after a preconcentration step with styrene–divinylbenzene sorbents.