Dispersive Liquid-Liquid Microextraction Combined with Gas Chromatography-Mass Spectrometry for the Determination of Multiple Pesticides in Celery

A rapid and sensitive method was established to simultaneously determine multiple pesticide residues in celery through gas chromatography-mass spectrometry (GC-MS). Samples were extracted through a modified quick, easy, cheap, effective, robust, and safe method (modified QuEChERS) and then refined and preconcentrated through dispersive liquid-liquid microextraction (DLLME) by using CHCl₃ as extractive solvent and acetonitrile (ACN) as dispersive solvent. The main factors, including type of extraction solvent, volume of extraction solvent, volume of dispersive solvent, extraction time, salt concentration, vortex velocity, and pH of aqueous solution, influencing DLLME were initially evaluated by performing single-factor variable experiments; three significant factors, particularly volume of extraction solvent, volume of dispersive solvent, and extraction time, were thoroughly analyzed through response surface methodology. The following optimized extraction conditions were obtained: 100 μL of CHCl₃, 900 μL of ACN, and 1.62-min extraction time. The optimized method was validated with average recoveries ranging from 70.8 to 93.2 % (with relative standard deviations of <15 %) at three spiked levels for all of the pesticides. Good linearity with determination coefficients of >0.9974 was obtained on the basis of the matrix-matched calibration curve of each pesticide; limits of detection ranging from 2.4 to 14.2 μg/kg indicated high sensitivity. Malathion with concentrations varying from 0.009 to 0.012 mg/kg was detected in all of the samples; other pesticides were not detected.