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Optimisation and validation of an analytical methodology for selected pesticides in waters by solid-phase extraction and liquid chromatography with ion-trap mass spectrometry detection

Multiresidue analytical methodologies are being developed for several classes of pesticides, using either GC or LC techniques coupled with MS detection. However, to reach ultra-low levels, as imposed by recent European Directives, specific extraction methods must be applied. This work intends to present the optimisation steps followed to implement a method to analyse pesticides of different families, as diuron, bentazone and cymoxanil. Among three types of SPE sorbents tested (EnviCarb, Easy and HR-P), the best results were achieved with HR-P. Concerning LC/MS with electrospray ionisation, the ionisation modes (negative and positive in a single run), the individual mass fragmentation, the parent ions for subsequent fragmentation, the needle voltage and the RF loading, the excitation amplitude, as well as the determination of optimal conditions for temperature of the drying gas and the pressure values for the drying and nebulisation gases, were optimised. As quantification method, attending the relatively low extracting recoveries, matrix-matched calibration was used, in order to achieve detection limits at ng L−1 levels. However, because high uncertainties are achieved, it is proposed that the optimised direct injection in LC/MS is the best approach. In these conditions, analysis could be performed in less than 10 minutes and the limits of detection were 0.48, 0.13 and 4.26 µg L−1 for diuron, bentazone and cymoxanil, respectively.