GC-MS/MS Determination of Residual Amounts of 10 Acidic Herbicides in Tea

Tea sample (5.00 g) was soaked in 10.0 mL of water[which was adjusted to pH 2.0 with φ 50% HCl solution preliminarily] for 30 min, and extracted ultrasonically twice successively with acetonitrile (20.0 mL for each extraction) for 20 min in each extraction. After centrifugation, the supernatants were collected, combined and evaporated to near-dryness at 30℃ in a water-bath under reduced pressure and further to dryness by N₂-blowing at normal temperature. 5.0 mL of a mixture of acetonitrile, toluene and acetic acid (75+25+1, abbreviated as ATA mixture) were added to dissolved the residue, and the solution obtained was purified by passing through the SPE column of Cleanert Pestic Carb/NH₂ and the eluate was collected. The SPE column was then eluted with 15.0 mL of the ATA mixture, and the eluate was collected and combined with the first eluate, and the solution was evaporated to near-dryness by N₂-blowing under normal temperature. The residue was dissolved in 870 μL of acetone and the 10 acidic herbicides in the solution were derivatized by the PFBBr method in a water-bath at 60℃ for 1.0 h as described in an optimized procedure given in literature. The derivates formed were then extracted from the reaction mixture with 1.0 mL of n-hexane, and in which the derivates were determined by GC-MS/MS method representing residual amounts of the 10 acidic herbicides in the tea sample. As shown by the results, linear relationships between values of peak area and mass concentration of the 10 acidic herbicides were found in the definite ranges, with detection limits (3S/N) in the range of 0.022 3-1.54 μg·kg^-1. Test for recovery by standard addition method was made on the base of black tea, green tea and oolong tea, giving results in the range of 72.2%-109%, and RSDs (n=6) ranged from 0.10% to 5.2%.