A QuEChERS-LC-HRMS-based method for the quantification of six sulfonamides and trimethoprim residues in milk

The present study describes the development and validation of an analytical method based on QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) extraction and purification with dispersive solid-phase extraction (dSPE) before analysis, followed by liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) for the determination of six (6) sulfonamides and trimethoprim in milk. The validated analytical method demonstrated good validation characteristics, including trueness (recoveries ranging from 73.0% to 114.3% at four fortification levels), linearity (coefficients of determination, R², above 0.9965 in all cases), and precision (within-laboratory reproducibility below 15.3%). The method exhibited high sensitivity, with limits of quantification (LOQs) ranging from 0.7 to 2.4 µg kg^-1, enabling reliable quantification at concentrations below 3% of the maximum residue limits (MRLs) established by European legislation. Matrix effects were generally limited for most analytes, while stability studies confirmed the robustness of the analytical procedure during storage. The validated method was successfully applied to 16 milk samples of different origins and processing types, revealing no quantifiable residues of the target sulfonamides and trimethoprim. The results demonstrate that the proposed QuEChERS/LC-HRMS method provides a reliable, sensitive, and robust analytical tool for the routine monitoring of sulfonamide residues in milk.