Abstract:

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.

Key words: acetanilide herbicides, electron impact ionization (EI), negative chemical ionization (NCI), tea-leaves , gas chromatography-mass spectrometry (GC-MS)