Solidification of a Switchable Solvent-Based QuEChERS Method for Detection of 16 Pesticides in Some Fruits and Vegetables

n-Octadecylamine was adopted as a cleanup agent to develop a novel solidification of a switchable solvent-based QuEChERS method. At higher temperatures (such as 55 °C), n-octadecylamine can melt into a liquid, allowing effective extraction of matrix interferences in acetonitrile solution (i.e., in dispersive liquid–liquid microextraction). At lower temperatures, n-octadecylamine carrying matrix interferences can rapidly solidify and easily separate from the acetonitrile solution. The results demonstrated that n-octadecylamine possessed a better ability to remove matrix interferences and reduce matrix effects than those of traditional solid-phase dispersive extraction cleanup agents of primary secondary amine and octadecyl bonded silica gel. By coupling it with gas chromatography–mass spectrometry, the proposed method was applied to the detection of 16 pesticides in cucumber. The recoveries were from 80.9 to 112.6% with relative standard deviations less than 12.9%. Satisfactory results were also obtained for the detection of 16 pesticides in pear, orange, apple, pepper, lettuce, and tomato.