TOF-MS and Quadrupole Ion-Trap MS/MS for the Discovery of Herbicide Degradates in Groundwater

摘要

Time-of-flight mass spectrometry (TOF/MS) and quadrupole ion-trap mass spectrometry/mass spectrometry (QIT/MS/MS) were combined for the identification and discovery of two new 2nd amide degradates of acetochlor, alachlor, and metolachlor in groundwater. These degradates were hypothesized to be environmentally important intermediates in the degradation of these acetanilide herbicides with over 100 million pounds applied annually to soil. The strategy involved four steps. First was the theoretical hypothesis that the secondary-amide ESA of the acetanilide herbicides were present in groundwater because of similarities of structure and likelihood of degradation and persistence (called the discovery process). Second was the QIT/MS/MS analysis of several samples for the molecular ion and the characteristic fragmentation and diagnostic ions. Third was the synthesis of a standard and verification of standard with retention time and MS/MS spectra followed by accurate mass analysis and molecular formula with TOF/MS. Finally, the last step involved the discovery of these 2nd amide ESA degradates in groundwater with HPLC and QIT/MS/MS. The analytical procedure described herein has the potential for widespread use in the discovery of degradates for many herbicides used on crops in the United States. The QIT/MS/MS is powerful for structural elucidation because of the capability of MS~n. For example, two degradates with the same exact mass could be differentiated because of differences in MS/MS fragmentation. The TOF/MS analysis was useful for the assurance of synthesis of pure standards of the secondary-amide degradates. Analysis of 82 shallow groundwater samples from the Midwestern United States showed that the secondary amide of ESA degradates of acetochlor, alachlor, and metolachlor occur at detection frequencies of 21-26%.