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 (EI) 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 μg/kg, 20 μg/kg and
40 μg/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 EI 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
μg/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.
