Investigation of the dissociation pathways of metolachlor, acetochlor and alachlor under electron ionization – application to the identification of ozonation products

Abstract

With the future aim of using gas chromatography coupled with mass spectrometry to characterize the transformation products of ozonated herbicides: metolachlor, acetochlor and alachlor, an interpretation of their electron ionization mass spectra is presented. Fragmentation mechanisms are proposed on the basis of isotopic labelling and multiple-stage mass spectrometry experiments carried out on an ion trap mass spectrometer. We also give examples in order to demonstrate how the elucidation of such fragmentation mechanisms for herbicides may simplify the characterization of their ozonation products.

GC‐MSn and LC‐MS/MS couplings for the identification of degradation products resulting from the ozonation treatment of Acetochlor

“…Finally, ten degradation products were identified by GC-MS n and/or LC-MS/MS, and were labelled from A to J as reported in Tables 1 and 2. As stated in a previous study on chlorinated herbicides, CI using methanol as the CI reagent provided abundant MH + ions allowing the attribution of molecular weights for the degradation products with no doubt. [19] The presence of two isotopomer MH + ions in the ratio 75/25 in all CI mass spectra showed as an evidence that all degradation products issued from ozonation are monochlorinated, like the parent molecule. Comparison with the corresponding molecular weights of the degradation compounds issued from ozonation of acetochlor-d 11 permitted to establish the number of deuterium atoms that remained in the degradation compounds: this number of remaining deuterium atoms is reported in Table 1 for each structure proposed.…”

“…To be totally efficient, the use of these techniques implies to know as best as we can the ions that are formed when these transformation products are ionized. [19,20] A combined approach using both liquid chromatographytandem mass spectrometry (LC-MS/MS) and gas chromatographytandem mass spectrometry (GC-MS n ) has been chosen here to provide a complete analysis of acetochlor ozonation pathways, which covers the full range of expected polarities of degradation products under simulated water treatment conditions in industrial treatment plants. Most of the articles dedicated to herbicide degradation treatment efficiency report degradation kinetics.…”

Identification and ecotoxicity of degradation products of chloroacetamide herbicides from UV-treatment of water

“…The chromatographic separation was performed on a 60 m "Factor four VF-10-MS" (10% phenyl, 90% methylpolysiloxane) capillary column (internal diameter: 0.25 mm, film thickness: 0.25 µm) from Varian. The chromatographic separation and mass spectrometric analysis were performed as described by (Bouchonnet et al, 2011).…”

“…The relative abundance and suggested structures of the photo degradation products are presented in Table 2. The methodology for characterization of degradation products is fully described in previous published studies (Bourcier et al, 2010;Bouchonnet et al, 2011;Bouchonnet et al, 2012;Souissi et al, 2012;) in which we described how establishment of characteristic dissociation pathways of molecular ions permitted systematic structure elucidation of degradation products. The detailed structure elucidation of the photoproducts of metolachlor is described in and the similar identification of the same products of alachlor and acetochlor is supported in the the data and the mechanistic interpretations of the mass spectras are supported by the data shown in supporting information namely figures S6, S7 and S8 as well as tables S2 and S3.…”

“…This analytical strategy has been developed and applied to allow reliable identification of numerous degradation products. Our strategy combined three complementary major steps for generating data that could be used to identify the degradation products: collision induced dissociation for fragmentation mechanism establishment, high resolution measurements for elemental composition determination and use of isotope labelled molecules for structural confirmation (Bourcier et al, 2010;Bouchonnet et al, 2011;Bouchonnet et al, 2012;Souissi et al, 2012;. The labelled molecules allow the location of structural modifications induced by UV irradiation through the comparison of data with those of unlabelled molecules ).…”