Simultaneous determination of phenyl- and sulfonyl-urea herbicides in river water at sub-parts-per-billion level by on-line preconcentration and liquid chromatography–tandem mass spectrometry

A method based on on-line preconcentration followed by liquid chromatography-electrospray ionization–tandem mass spectrometry (LCESI– MS/MS) was developed for the determination of three sulfonyl-urea (thifensulfuron, metsulfuron, chlorsulfuron) and two phenyl-urea (isoproturon and chlortoluron) herbicides in water at sub-ppb concentration ranges. Preconcentration was accomplished using on-line enrichment on a C18 cartridge; the procedure was optimized by an evaluation of the breakthrough volumes for the target analytes. Subsequently, LC-ESI–MS/MS was adopted for analytes separation and detection. In particular, a selective reaction monitoring (SRM) approach, based on the detection of a peculiar fragment for each analyte, was chosen for MS/MS analysis, in order to enhance selectivity. Normalization to the response of a phenyl-urea herbicide (chloroxuron), used as an internal standard, was also adopted to achieve a reproducibility enhancement. The described method was applied to the analysis of the target analytes in river water samples and LOD values ranging between 8 and 30 ppt were obtained.