A rapid and sensitive method for simultaneous determination of hexanoic acid 2-(diethylamino)ethyl ester (DA-6) and mepiquat chloride (MQ) residues in soil and cotton samples was developed using ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). DA-6 and MQ were extracted with water–0.6% formic acid containing 5.0 mL 0.2% NH4Cl (cotton seeds 2% NH4Cl). Then, extracts containing the target compounds were analysed using UPLC-MS/MS with an electrospray ionization source in positive mode. The determination of the target compounds was achieved in less than 2.0 min. The limits of detection (LOD) were below 0.005 mg kg−1, while the limits of quantification (LOQ) did not exceed 0.015 mg kg−1 in all three matrices. Quantification was performed by calibration curves of standards from 0.005–1.0 mg kg−1 with r2 > 0.997. Recovery studies were evaluated at three spiked levels 0.5, 1.0 and 2.0 mg kg−1 for soil and cotton samples, and the overall average recoveries ranged from 76.3% to 100.0% with relative standard deviations 4.7–11.8% (n = 5).
