Development of an LC-MS/MS Method for Simultaneous Determination of the Quaternary Ammonium Herbicides Paraquat, Diquat, Chlormequat, and Mepiquat in Plant-Derived Commodities

An optimized method for the determination of residues of the plant growth regulators chlormequat, mepiquat, paraquat, and diquat in plant-derived commodities using LC-MS/MS was developed. As an exemplary, but very challenging commodity, the focus was set on the oil seed chia and the coverage of incurred residues of these quaternary ammonium compounds. The study describes the evaluation of optimum extraction conditions, particularly of the extraction solvent and a potential thermal treatment. The final procedure was accomplished by the extraction solvent methanol/hydrochloric acid at room temperature without the application of an extra heating step. The LC-MS/MS measurement was performed on an Obelisc R column (SIELC) within 16 min of separation time. For compensating matrix effects and differences in recovery rates, the isotopically labeled internal standards (ILIS) diquat d(4), paraquat d(6), mepiquat d(3), and chlormequat d(4) were added to the samples prior to the extraction. The validation of the method for the matrices tomato, lemon, sultana, wheat, and chia seed was carried out according to the actual guidance document SANTE/11945/2015 by the European Commission. The precision and accuracy were determined in five replicates at residual levels of 0.010 and 0.100 mg/kg obtaining recovery rates between 77 and 120% and RSDs between 1 and 13%. By means of the analytical method developed, it is possible to meet the strict requirements for the analysis of organic and baby food.