A novel magnetic multi-walled carbon nanotube-based magnetic solid-phase extraction combined with dispersive liquid-liquid microextraction method for the determination of four phenoxy carboxylic acid herbicides in food crops by using ultra-high performance liquid chromatography-tandem mass spectrometry

A simple, rapid and sensitive method for the determination of four phenoxy carboxylic acid herbicides (4-chlorophenoxy acetic acid, 2-methyl-4-chlorophenoxy acetic acid, 2,4-dichlorophenoxy acetic acid and 2-(2,4-dichlorophenoxy)propionic acid) in food crops was established by magnetic multi-walled carbon nanotube assisted magnetic solid-phase extraction combined with dispersive liquid-liquid microextraction followed by ultra-high performance liquid chromatography-tandem mass spectrometry. In our current study, magnetic multi-walled carbon nanotubes were successfully synthesized and characterized by using Fourier transform infrared spectrometry, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. The food crops were firstly pretreated with magnetic solid-phase extraction and the extracts were further purified by dispersive liquid-liquid microextraction. Various extraction factors were systematically optimized. Under the optimum conditions, satisfactory sensitivity levels were obtained with the method detection limits of 0.19-0.80 ng g(-1) and method quantitation limits of 0.61-2.66 ng g(-1). The recoveries of the four herbicides in different food crops ranged from 83.9 to 102.7% and the precision of the developed method was no more than 10.9%. Furthermore, the method was successfully applied to determine the four phenoxy carboxylic acid herbicides in real food crops (rice, millet, soy, oatmeal, and barley) with satisfactory results.