Abstract: In modern agricultural production, pesticides play a crucial role in preventing and controlling agricultural insect attacks, regulating crop growth, and ensuring the yield and quality of agricultural products. However, the unscientific use of pesticides can lead to excessive pesticide residues, causing serious impacts on the human nervous, immune, cardiovascular, and reproductive systems. Therefore, conducting pesticide residue monitoring and risk assessment on food is of great significance. Most existing analysis methods for food matrices cannot balance simplicity with the number of target substances, especially for scenarios involving a large scale of samples. As an essential agricultural product in people's lives, the quality and safety of vegetables have a significant impact on the health of the general public. Therefore, a simple, sensitive, and high-throughput method for the simultaneous detection of 191 pesticides in bok choy was established by combining magnetic solid-phase extraction with ultra-high performance liquid chromatography-electrostatic field orbital trap high-resolution mass spectrometry (UHPLC-Orbitrap-HRMS). In this study, the extraction solvent, salt composition, and the dosage of the two types of magnetic adsorbents were optimized. The sample was extracted using 1% formic acid in acetonitrile, and 2 g of magnesium sulfate, 0.5 g of sodium chloride, 0.5 g of sodium citrate, and 0.25 g of disodium hydrogen citrate were used for stratification, followed by centrifugation. After purification with Fe₃O₄ nanoparticles modified with N-propylethylenediamine (Fe₃O₄-PSA NPs) and magnetic multi-walled carbon nanotubes (MG-MWCNTs), the target substances were separated on a Waters Acquity BEH C₁₈ colum (100 mm×2.1 mm, 1.7 μm) using the mobile phase consisting of 0.1% formic acid aqueous solution (containing 2.5 mmol/L ammonium formate) and methanol, with a flow rate of 0.3 mL/min and a column temperature of 40 ℃. Heated electrospray ionization (HESI) in positive and negative ion modes was used for detection with primary mass spectrometry full scan/secondary mass spectrometry data dependent scan. Quantitation was achieved using a matrix-matched external calibration method. The results demonstrated that all pesticides in the optimized method exhibited good linear relationships, with correlation coefficients (r²)≥0.992 2. The limits of detection (LODs) of the 191 pesticides were ranging from 0.1 to 6.7 μg/kg, and the limits of quantification (LOQs) ranged from 0.5 to 20.0 μg/kg. The LOQs were all below their respective maximum residue limits (MRLs), indicating that this method was reliable and sensitive. The average recoveries of the 191 pesticides were between 72.7% and 120.5%, with the relative standard deviations (RSDs) between 0.1% and 14.9%, demonstrating the accuracy and precision of this method. Finally, the detection of real bok choy samples was carried out, and six target substances were detected in ten samples, namely chlorantraniliprole, dimethomorph, indoxacarb, metalaxyl, pyraclostrobin and thiophanate-methyl. Among them, the detection rates of dimethomorph and thiophanate-methyl were relatively high, at 60% and 40%, respectively. In addition, pesticide residues were detected in all samples but did not exceed their respective MRLs. The results of this method are consistent with those of the national standard method. Compared with other methods, this method has comparable or better LOQs, recoveries and RSDs, as well as a large number of target substances, making it suitable for high-throughput detection of multiple pesticide residues. The research results indicate that this method has the advantages of sensitivity, accuracy, simplicity, and a large number of target substances. It can be used for rapid detection of the 191 pesticides in bok choy, providing a new method for rapid monitoring of multi-pesticide residues and risk assessment of a large scale of bok choy samples.

Key words: bok choy, pesticide residues, magnetic solid phase extraction (MSPE), magnetic nanomaterials, QuEChERS method, ultra-high performance liquid chromatography- electrostatic field orbitrap high-resolution mass spectrometry (UHPLC-Orbitrap-HRMS)