Residue analysis of acephate and its metabolite methamidophos in open field and greenhouse pakchoi (Brassica campestris L.) by gas chromatography-tandem mass spectrometry

To analyze the dynamic degradation and final residues of acephate and its metabolite methamidophos, field-experiments with pakchoi (Brassica campestris L.) in open field and greenhouse were carried out in Beijing, China in 2004 and 2005. The degradation dynamics and final residues were determined by gas chromatography (GC) equipped with a pulsed flame photometric detector and GC coupled to mass spectrometry (MS)/MS after acephate was applied on open field and green house pakchoi (B. campestris L.). The dynamic degradation results showed that the half-lives of acephate and methamidophos in open field pakchoi were 1.36 days with dynamic degradation equation C( t ) = 133.01e( - 0.5107t ), and 2.86 days with C( t ) = 6.5753e( - 0.2422t ), respectively. While the half-lives of acephate and methamidophos in the greenhouse were 1.07 days with C( t ) = 59.134e( - 0.4353t ) and 0.79 days with C( t ) = 0.2703e( - 0.2595t ), respectively. The final residue analysis demonstrated that >50% of total methamidophos were resulted from the degradation of acephate 7 and 18 days after it was applied on the greenhouse pakchoi, respectively. While in the open-field pakchoi, >90% of total methamidophos was found to be the metabolite of acephate.