Abstract:

Pyraquinil (Pyr), with a new skeleton of pyrazolo[1,5-a]quinazoline fused heterocycle, is a new chiral insecticide independently developed by South China Agricultural University in 2017. In previous studies, we found that pyraquinil can effectively control the lepidopteran pest population on cruciferous crops. Remarkably, the insecticidal activity of pyraquinil was 64-fold better than that of fipronil against the fipronil-resistant Plutella xylostella field population. Pesticides with new active mechanisms should be developed in the future to cope with the development of resistance to Plutella xylostella. Therefore, pyraquinil with new active sites has the potential to be the main rotation variety for the control of Plutella xylostella. Thus, pyraquinil has a broad prospect for application in the future. However, a chiral separation and analysis method for pyraquinil and oxidation products using high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) has not been established yet.
Based on the physical and chemical properties of the new chiral pesticide pyraquinil, we screened chiral chromatographic columns and optimized the mobile phase ratio in this study. The separation and analysis methods for pyraquinil were developed based on the QuEChERS (quick, easy, cheap, effective, rugged, safe) system. Simultaneously, we also established oxidation metabolites. These methods were used for the simultaneous determination of the chiral isomers of pyraquinil and oxidation products in pakchoi (Brassica rapa ssp. chinensis L.) and water spinach (Ipomoea aquatica Forsk) via HPLC-MS/MS. The Chiral INC column (250 mm×4.6 mm, 5 μm) was used for separation. Ammonium formate aqueous solution (2 mmol/L) and acetonitrile were used as the mobile phases. The column temperature was 28 ℃. The injection volume was 1 μL, and the flow rate was 0.5 mL/min. MS analysis was performed using an electrospray ionization source in the negative and multiple reaction monitoring modes. We found that under the optimized conditions, the resolution of the four isomers of pyraquinil were 1.63, 2.83, and 1.74, respectively, and the resolution of the isomers of the pyraquinil oxidation product was 5.82, which achieved baseline separation. Then, the absolute configuration and peak order of pyraquinil and oxidation product isomers were determined by derivatization. The order of the peaks was RS-Pyr, SS-Pyr, RR-Pyr, SR-Pyr, S-Pyr+O, and R-Pyr+O. The purification conditions of sample pretreatment were optimized; 1 g (0.835 g MgSO₄+0.150 g PSA+0.015 g GCB) was determined to be the optimal purification agent; and the average recoveries ranged from 80% to 110%. The chiral isomers of pyraquinil and oxidation products showed good linearity in the concentration range of 1.25 to 1250 μg/L and 2.5 to 2500 μg/L respectively. The square of the regression coefficient of the linear equation (R²) was greater than 0.99. The matrix effects of the pyraquinil and oxidation product isomers in pakchoi ranged from 6.1 to 30.6. In the water spinach, the matrix effect of the pyraquinil and oxidation product isomers were in the range of 0.7-26.8. The average recoveries of pyraquinil isomer at three spiked levels of 0.25, 5, 100 μg/kg in samples (pakchoi and water spinach) ranged from 90.2% to 110.6%. The oxidation product isomer average recoveries in samples (pakchoi and water spinach) spiked with 0.5, 10, 200 μg/kg ranged from 72.6% to 100.1%. Further, the relative standard deviations (RSDs) were 0.5%-9.4%. In water spinach, the intra-day and inter-day repeatability RSDs ranged from 0.5% to 8.7% and 1.0%to 8.6%, respectively. In pakchoi, the intra-day and inter-day repeatability RSDs ranged from 0.6% to 9.4% and 1.0% to 7.6%, respectively. These results indicate that the proposed method has satisfactory sensitivity, accuracy, and precision. This study can provide analytic technology for a novel chiral pesticide for environmental behavior studies, quality control, and pharmacodynamics evaluation, as well as significant technical support for the development and application of new pesticides.

Key words: high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), pyraquinil, Brassica rapa ssp. chinensis L., Ipomoea aquatica Forsk, chiral separation