A modified multi-residue method for analysis of 150 pesticide residues in green beans using liquid chromatography-tandem mass spectrometry

Three methods of sample preparation and analysis of 150 pesticides in green beans were evaluated by LCMS/MS detection. The mass spectrometry parameters were optimized to give the best sensitivity, and two multiple reaction monitoring modes (MRMs) were chosen for quantification and conformation, based on the optimized declustering potential and collision energy. The first Quick, easy, cheap, effective, rugged, and safe (QuEChERS) method for pesticide residue analysis entailed extraction with acetonitrile whereas the second one used extraction with ethyl acetate, and the third one with acetone and partitioning by dichloromethane:petroleum ether (3:1). The extracts of the 3 methods were redissolved in methanol:buffer(1:1) solution (ammonium formate 10 mM), pH 4 as modification to increase injection volume to 25 μl without losing good peak shape separation). Stabilities of tested pesticides in 5 different calibration mixtures (pH 3, 4, 5, 6 and 7) stored for 2 weeks were studied. The total 150 pesticides showed 90-110 % recovery at pH 4 while the recovery rates of the different pesticides were pH-dependent proving that acidic pH was suitable for preparation and determination of multi-residue pesticides. The 150 pesticide signal intensities were studied at 3 different pH values and 4 concentrations to show the effect of mobile phase on pesticide intensity due to ionization power. The recovery rates at the 3 different concentration levels (0.01, 0.05 and 0.1 mg/kg) ranged from 70 to120% were carried out.