Simultaneous determination of herbicide mefenacet and its metabolites residues in river water by solid phase extraction and rapid resolution liquid chromatography-mass spectrometry with pre-column derivatization

A procedure based on solid phase extraction (SPE) has been developed for the simultaneous pre-concentration of herbicide mefenacet (MN) and its three photolysis degradation products. Three metabolites studied were hydroxylbenzothiazole (HBT), N-methylaniline (N-MA) and 2-benzothiazoloxyacetic acid (2-BAA). A trimethylsilylation derivatization method was applied for the analysis of HBT and 2-BAA which were derivatized to be corresponding derivatives D-1 and D-2, respectively, and a rapid resolution liquid chromatography-electrospray ionization mass spectrometry (RRLC-ESI-MS) system was used for the separation, identification and quantification of these four analytes. In the SPE pre-concentration step, three types of cartridges and four kinds of eluents were investigated. The mean recoveries of these four analytes were between 78.6% and 101.2% and relative standard deviations were between 3.2% and 9.2%. The limits of detection (LODs) obtained were 0.02 ng l−1 for MN and N-MA and 0.1 ng l−1 for HBT and 2-BAA which were less than the maximum residue limits (MRLs) in drinking water established by European legislation (0.1 μg l−1). The proposed method was applied to evaluate the presence and evolution with time of herbicide mefenacet and its degradation products in samples of Songhuajiang River of Heilongjiang province, China. The analyses, conducted from April to July of 2008, pointed to the presence of MN, 2-BAA, HBT and N-MA at maximum levels 1.0, 0.08, 0.1 and 0.3 μg l−1.