Environmental pollution caused by the unreasonable use of pesticides has aroused wide concern among the public. To strengthen the research on pesticide residue detection technology is of great theoretical and practical significance for the rational use of pesticides and the protection of the environment and human health. As there are a wide range of pesticides whose physical and chemical properties are different and new pesticides are constantly emerging, higher standards have been put forward on the sample pretreatment. In recent years, due to its rapidness, easy to use, effectiveness, reliability and safety, the QuEChERS method, as a new extraction method, has become a new sample pretreatment technology that is extensively adopted at home and abroad and is also widely applied in the gas or liquid chromatographic analysis for various pesticides and veterinary drugs. In this paper, the QuEChERS-GC-MS method is used to quickly determine the pesticide residues in agricultural products.
Take edible parts of samples, mince and mix. Weigh 15 g (accurate to 0.01 g) of sample and place it into a 100 mL plastic centrifuge tube. Add 15 mL of 0.1% acetic acid / acetonitrile solution, 6.0 g of anhydrous magnesium sulfate, 1.5 g of sodium acetate and homogenize.
Centrifugate at 5000 r/min for 5 min. Take 10 mL of organic phase accurately into a 15 mL plastic centrifuge tube. Dry the solution under nitrogen stream. Vortex mix to dissolve the residue in 2.0 mL of 0.1% acetic acid / acetonitrile solution. According to the interference of sample matrix, select and weigh proper amount of absorbents like C18, PSA, graphite carbon black or NH2, and place into another 15 mL plastic centrifuge tube. Transfer 2 mL of the above dissolved solution to the centrifuge tube. Vortex mix for 2 min and centrifuge at 5000 r/min for 3 min. Take the supernatant with a disposable syringe and filter through 0.45 μm membrane for analysis.
Column: Bonna Agela DA-35MS capillary column, (30 m × 0.25 mm × 0.25 μm, P/N: 3525-3002);
Temperature programming in column box: 50 °C (for 2 min), 10 °C/min to 180 °C (hold on for 1 min), 3 °C/min to 250 °C (hold on for 1 min), 2 °C/min to 270 °C (hold on for 15 min);
Inlet temperature: 250 °C;
Carrier gas: He (>99.999%), constant flow, flow rate 1.0 mL/min;
Injection volume: 1 μL;
Injection type: Splitless injection. After 0.8 min switch the splitting valve on;
Electron impact ionization source: 70 eV;
Temperature of ionization source: 250 °C
Temperature of GC/MS interface: 250 °C
Selected ion monitoring: For each compound, select one ion for quantitative analysis and 2~3 ions for qualitative analysis.
All the ions to be determined in each group are monitored respectively at different time, according to their retention times.
Results and Discussion
Improvement of sample extraction
In a typical QuEChERS cleanup for analysis of multiple pesticide residues, 'shaking' the sample mixture is employed to extract target compounds, and 2 mL aliquot of the organic solution is taken for analysis. In this study, however, it is found that homogenization for sample extraction improves QuEChERS procedure with higher efficiency. This is useful for samples that contain high content of fibers. On the other hand, this study also shows that a concentration step is necessary to enhance the detectability of target compounds. 10 mL Aliquot was used in the procedure, which results in a 5 magnitude concentration comparing to 2 mL. The drying effect of anhydrous magnesium and sodium sulfates was compared. Before sample introduction to GC analysis, magnesium sulfate anhydrous is more effective to remove water content in the sample solution of reconstitution. It is helpful to reduce interfering background when using electron capture detector (ECD) in the GC analysis.
Improvement of sample cleanup
It is shown that Cleanert PSA is more effective to remove fatty acids in sample matrices while Cleanert C18 and graphite carbon black (PestiCarb) have good cleanup performance on pigments, sterols and vitamins. Cleanert NH2 is more adsorptive to interference than PSA. Therefore, besides the evaluation of PSA in this paper, Cleanert C18, NH2 and PestiCarb were also tested. Even though the amount of sorbents used cannot be specified explicitly for given sample matrices, it always falls into the range from 100 to 350 mg. The extract of spinach was clear and colorless, for example, with our improved QuEChERS procedure while the solution was greenish with the 'standard' QuEChERS. In the recovery experiment, we found that C18 has no impact upon recovery. But PSA, NH2 and PestiCarb could reduce recovery of some pesticides if the amount of sorbent used was too much since the analytes can be strong adsorbed. In practice, it is recommended that the type and the amount of sorbents should be screened and optimized based on the properties of target compounds and nature of matrices.
Recovery and precision experiments are carried out with spiked spinach, cabbage and yellow peach samples at three concentration levels. For each level, experiments are repeated six times. At the concentrations between 0.05 mg/kg~1.0 mg/kg, the recovery and RSD are 65%~120% and 1%~13.5%, respectively.