A study on matrix effect of QuEChERS-triple quadrupole tandem mass spectrometry in determination of pesticides in Citrus

Abstract: 【Objective】Currently, profenofos, chlorpyrifos, difenoconazole, imidacloprid, prochloraz, acetamiprid, carbendazim, enoylmorpholine, pyraclostrobin, malathion, etc. are commonly used in citrus cultivation. Pesticides with higher detection frequency, such as enoylmorpholine, have no limit re-quirements and relevant measurement method standards in China’s national standards. On the other hand, the determination methods of pesticides such as prochloraz and pyraclostrobin stipulated in the na-tional standards are gas chromatography and gas chromatography-mass spectrometry, respectively, and the detection sensitivity is not added, and the operation of these methods is complicated. The standard method for the determination of fresh amine requires the high-temperature hydrolysis of prochloraz and liquid-liquid extraction after GC-ECD determination, and pyraclostrobin needs to measure after multi-ple purifications using C18, Carb and NH2 solid phase extraction columns. The operation steps are com-plicated, time-consuming and laborious. In the operation process, it is necessary to use toluene with high toxicity, which is harmful to the environment and the operator. With the continuous development of QuEChERS technology combined with ultra-high performance liquid chromatography-triple quadru-pole mass spectrometer, it is currently widely used in the determination of a variety of pesticide resi-dues in fruits and vegetables due to its quick and easy operation and high sensitivity. However, for ultra-high performance liquid chromatography-triple quadrupole mass spectrometer, matrix effect is a com-mon phenomenon in mass spectrometry analysis, especially the complex matrix of citrus, due to the more pigment and essential oil content, its matrix effect is stronger, seriously affecting the accurate quantification of related pesticides in citrus. The matrix effects in the determination of commonly pesti-cides used in citrus were systematically studied by using the QuEChERS method and ultra-high perfor-mance liquid chromatography-triple quadrupole tandem mass spectrometry.【Methods】After the citrus sample was fully crushed and homogenized by an electric mixer, 5.00 g of sample was ccurately weighed in a 50 mL centrifuge tube, then 10 mL of acetic acid-acetonitrile solution (V∶V =1∶99) was added, and it was vortexed for 5 min. 3.0 g NaCl was added, and then it was vortexed for 1 min, centri-fuged at 5000 r·min^-1 for 3 min, 2 mL of supernatant was taken in a 10 mL centrifuge tube, add 200 mg of anhydrous magnesium sulfate and 80 mg of GCB filler were added, and the mix was vortexed to ex-tract 10 s, centrifuged at 5000 r·min^-1 for 3 min, and the supernatant was filtered by 0.22 μm micropo-rous membrane for analysis. Methanol-0.1% formic acid aqueous solution (containing 1 mmol·L^-1 am-monium acetate) was used as the mobile phase, and the positive ion multiple reaction monitoring  (MRM) mode was used , and the matrix was quantified by external standard method.【Results】In the extraction process, the appropriate amount of acetic acid could maintain the acidity of the extract envi-ronment and increase the extraction efficiency of related acidic pesticides. In positive ion mode, the ad-dition of formic acid and ammonium acetate could enhance the mass spectrometry response of pesti-cides to a certain extent. The 10 pesticides in citrus were mainly based on the matrix inhibitory effect. Compared with methanol-water solution as the mobile phase, the addition of formic acid and ammoni-um acetate could reduce the matrix effects of 10 pesticides to a certain extent. GCB and PSA had better effects on reducing the strong matrix effects of malathion, acetamiprid, enoylmorpholine and other pesticides, while GCB had a better effect on eliminating the matrix effects of malathion and enoylmorpho-line. The citrus blank matrix extract was used to prepare 10 pesticide standard solutions of different con-centrations, linear regression analysis was performed based on peak area and concentration; at the same time, 10 pesticides with appropriate concentration were added to the citrus blank matrix respectively for determination, through continuous decrease of the spiked concentration, and the spiked concentration corresponding to the signal and noise ratio (S/N) of 3 times and 10 times was used as the detection limit and quantitation limit. 10 pesticides were in the range of 0.1-100.0 ng. The correlation coefficient in the concentration range of mL was better, and the limit of quantification was better, which could meet the needs of determination of pesticide residues in citrus. According to the different quantitation limits of 10 pesticides, the quantitation limit, 5-fold and 10-fold quantitation limits were used as the three addi- tive concentration levels for the recovery test and precision determination. The citrus blank matrix ex-tract was used to prepare the matrix standard solution to minimize matrix effect. The six parallel experi-ments were performed for each spiked concentration level, with average recovery and relative standard deviation. The average recovery rate of 10 pesticides ranges from 70.5% to 102%, and the relative stan-dard deviation ranged from 3.32% to 5.76% .【Conclusion】The use of acidic acetonitrile extraction, GCB purification and methanol-0.1% formic acid aqueous solution (containing 1 mmol·L ^-1 ammonium acetate) as the mobile phase could effectively reduce the matrix effect. The method was quick and sim-ple, with good accuracy and precision, and was suitable for the rapid determination of pesticide residues commonly used in citrus samples.