In situ studies of pesticides photodegradation on soils using PD-TOFMS technique: Application to norflurazon and oxyfluorfen

As we have demonstrated that plasma desorption time-of-flight mass spectrometry (PD-TOFMS) is well adapted to the direct characterization of pesticides adsorbed on agricultural soils the technique has been applied for the first time to the study of their evolution under sunlight-like irradiation. Two pesticides have been selected: norflurazon which is the most documentated (both from the literature and from our previous experiments) and oxyfluorfen in order to assess the capability of the technique. The photodegradation process has been investigated both for a deposit onto a metallic substrate and for a soil impregnated with the product. For norflurazon degradation parameters have been extracted from the yield variation of ions representative of the molecule and breakdown products and particularly the time required for 50% dissipation of their initial concentration (DT50 values). The comparison between deposits and soils indicates clearly that the degradation is slower in the latter case with an increase of about 3.5 for the DT50 of the molecule, and about 2 for its breakdown products. These values are in agreement with the decays of other ions. As expected, the degradation is faster when the UV of the sunlight is unfiltered, more significantly for the breakdown products. This is also observed for the oxyfluorfen deposited onto aluminium although at a lower level (twice less). The trends are only qualitative for the impregnated soil but definitely there. A discussion is presented for the interpretation of the photodegradation process in both cases together with suggestions of improvement in the data acquisition. (C) 2008 Elsevier B.V. All rights reserved.