2.4. Sample preparation
Approximately 2 g of tobacco sample was weighed into a 50 mL centrifuge tube. Next, 10 mL of pure water was added, and it was shaken for 30 s and then left to stand for 10 min. After that, acetonitrile (10 mL) and TPP standard solution (100 μL, 20 μg/mL) were added, and the tube was vortexed for 2 min. The tube was frozen at –20 °C for 10 min to avoid possible thermal decomposition of targeted pesticides in the salting-out procedure. Then a QuEChERS extract pouch was added and the mixtures immediately hand shaken for 30 s. Then 5 mL of toluene was added, and the tube was vortexed for 2 min. After the centrifugation step (4000 rpm, 5 min), the upper layer was collected. For the following clean-up step, 0.5 mL extract was added into an Eppendorf vial (1.5 mL) containing 80 mg magnetic graphene, and then the mixture was shaken vigorously for 1 min. The supernatant was collected with the aid of an external magnet and was supplied to on-line GPC–GC–MS2 for analysis. Blank tobacco (pesticide-free tobacco) samples were prepared in the same way but without the addition of the internal standard solution. For the matrix-matched working standard solution of this method, the blank tobacco supernatants (0.5 mL) were evaporated to dryness under a gentle stream of nitrogen at 35 °C, and reconstituted with 0.5 mL working standard solutions with different concentrations.
The extract was also treated by a traditional QuEChERS method (YQ/T 47.1-2014) for comparison [39]: 0.5 mL extract was added to an Eppendorf vial (1.5 mL) containing 150 mg MgSO4 and 50 mg PSA. The mixture was then shaken vigorously for 2 min and centrifuged for 3 min. The supernatant was collected and was supplied to on-line GPC–GC–MS2 for analysis.