No abstract

Aqueous solutions of metolachlor and metolachlor-d(6) were photolyzed with UV-visible radiations. The structures of 15 by-products of metolachlor were determined through gas chromatography-mass spectrometry analyses using electron and chemical ionization combined with multistage mass spectrometry. The photolysis by-products of metolachlor resulted mainly from dehalogenation and hydroxylation, in some cases accompanied by cyclization. In silico tests for toxicity prediction showed that the toxicity of some photolysis products is expected to be greater than that of metolachlor. Persistence studies showed that the by-product relative abundances vary in large amounts with the irradiation time. The post-photolysis evolution of the solution was also studied, in order to determine the persistence of the main by-products. It allowed to establish that most of the by-products can be found more than 12 h after the end of the photolysis, which is of a great concern as treated water is generally available for consumption only a few hours after treatment in most of industrial processes.

International audiencePyrimethanil is an anilino-pyrimidine fungicide particularly active against gray mold (Botrytis cinerea) and pear scab (Venturia inaequalis and Venturia pirina) on grapes, strawberries, tomatoes, fruits, vegetables and ornamental plants in greenhouses and open field situation.[1-5] According to the European Food Safety Authority, pyrimethanil has no evident mutagenic, genotoxic or carcinogenic potential, but a short-term toxicity study on rats and mice has shown an increase in liver weight accompanied by changes in the histopathology of the liver and thyroid.[6] To avoid these side effects, the concentration of pyrimethanil has been limited by legislation. The European Commission set the Maximum Residue Level of pyrimethanil at 10 mg/l in citrus fruits, 5 mg/l in pome fruits, strawberries, table and wine grapes and 0.05 mg/l in tree nuts (EC/600/2010).[7] It is thus important to increase our knowledge regarding the concentration of pyrimethanil in environmental matrices, its degradability and the factors affecting it. One of these factors is the photo alteration by sunlight, which is known to play a significant role in the degradation of this compound, due to the prolonged half-life (77 days approximately) of pyrimethanil in the environment.[8] Previous studies on the degradation of pyrimethanil were carried out mainly in a waste water treatment context and focused on the photo catalytic degradation using various salts as catalysts. Agüera et al.[9] used TiO2, Vanni et al.[10] and Anfossi et al.[11] added iron III, Navarro et al.[12] added ZnO and Gomis et al.[13] used thiopyrylium. Irradiation was carried out using a mercury or xenon lamp or direct sunlight. These studies have showed that a long irradiation time (between 150 and 1400 min, depending on the conditions and catalyst) is required for total disappearance of pyrimethanil. Degradation of pyrimethanil in water using the technique of photo-Fenton was investigated by Sirtori et al. They reported that the addition of sodium chloride accelerated the reaction and characterized four chlorinated photo-Fenton by-products.[14] The aim of the present study was the identification of UV-visible photo transformation products of pyrimethanil in water. Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and gas chromatography coupled with multi stage mass spectrometry (GC-MSn) were used for analysis, with the aim of covering a large range of polarities for the detection of the potential transformation products. Elucidation of the structures of photoproducts was carried out performing high-resolution measurements and collision-induced dissociation (CID) experiments. Photolysis mechanisms have also been proposed to explain the formation of photo products of pyrimethanil in water

With one exception, the structures proposed for the photoproducts on the basis of mass spectra interpretation have not been reported in previous studies. In silico toxicity predictions showed that two photoproducts are potentially more toxic than the parent compound considering oral rat LD50 while five photoproducts may induce mutagenic toxicity. With the exception of one compound, all photoproducts may potentially induce developmental toxicity.