Development and validation of a modified QuEChERS ‐ LC – MS / MS method for seven herbicides in cereals

A modified QuEChERS method coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) was developed for the simultaneous determination of seven herbicide residues (pinoxaden, monosulfuron-ester, pyribambenz-propyl, quintrione, pyraclonil, tripyrasulfone, and tefuryltrione) in cereals (wheat, brown rice, and corn). A major challenge encountered during method development was the unexpectedly low recoveries(52.1-62.3%) of quintrione and tefuryltrione when using acetonitrile as the extraction solvent. Mechanistic investigation suggested that these two compounds tend to form coordination complexes with endogenous Mg²⁺/Ca²⁺ present in cereal matrices, leading to their poor extractability. To address this issue, 1% formic acid in acetonitrile was employed as the extraction solvent. The added H⁺ competitively protonates the basic sites(e.g., nitrogen-containing heterocycles) of the target molecules, effectively disrupting the metal complexes and releasing the analytes into the extract, thereby restoring recoveries to 88.6-91.5%. Following systematic optimization of QuEChERS parameters(water addition, extraction time) and purification sorbents (PSA+C18), matrix effects were significantly reduced to acceptable levels(80.8-118.9%). Under optimal conditions, the seven herbicides exhibited good linear relationships(r²>0.9945) within 5-80 μg/L. The limits of detection (LODs, S/N = 3) ranged from 0.01 to 1.08 μg/kg, and limits of quantification(LOQs, S/N=10) from 0.02 to 3.23 μg/kg. At three spiked levels(5, 10, 50 μg/kg), average recoveries ranged from 78.5% to 103.5% with relative standard deviations(RSDs, n=6) of 1.9-8.8%. The method was applied to 20 cereal samples from Fujian province; pinoxaden was detected in one wheat flour sample(0.23 μg/kg), and monosulfuron-ester in two brown rice samples(0.18 and 0.31 μg/kg). The developed method is simple, sensitive, and accurate, meeting the detection requirements of the corresponding temporary maximum residue limits in GB 2763-2026.