Determination of Acetanilide Herbicide Residues in Tea by Gas Chromatography-Mass Spectrometry with Two Different Ionization Techniques

An analytical method of solid phase extraction-gas chromatography-mass spectrometry with two different ionization techniques was established for simultaneous determination of 12 acetanilide herbicide residues in tea-leaves. Herbicides were extracted from tea-leaf samples with ethyl acetate. The extract was cleaned-up on an active carbon SPE column connected to a Florisil SPE column. Analytical screening was determined by the technique of gas chromatography GC -mass spectrometry MS in the selected ion monitoring SIM mode with either electron impact ionization El or negative chemical ionization NCI. It is reliable and stable that the recoveries of all herbicides were in the range from 50% to 110% at three spiked levels 10 mu g/kg 20 mu g/kg and 40 mu g/kg and the relative standard deviations RSDs were no more than 10. 9%. The two different ionization techniques are complementary as more ion fragmentation information can be obtained from the El mode while more molecular ion information from the NCI mode. By comparison of the two techniques the selectivity of NCI-SIM was much better than that of EI-SIM method. The sensitivities of the both techniques were high the limit of quantitative LOQ for each herbicide was no more than 2. 0 mu g/kg and the limit of detection LOD with NCI-SIM technique was much lower than that of EI-SIM when analyzing herbicides with several halogen atoms in the molecule.