Applications of thermospray liquid chromatography-mass spectrometry in photochemical studies of pesticides in water

Abstract

Thermospray liquid chromatography-mass spectrometry (TSP-LC-MS) in the positive- and negative-ion modes (PI and NI, respectively) was used for the characterization of the pesticides aldicarb, carbaryl, cyanazine, fenitrothion, linuron and parathion-methyl, including the corresponding photodegradation products. The LC analyses were performed on RP-18 columns using methanol-water (70:30 or 50:50) + 0.05 M ammonium acetate or 0.05 M ammonium formate. Photodegradation studies of pesticides in distilled water were performed using a suntest apparatus, except for fenitrothion, for which a high-pressure mercury lamp was used because of slow photodegradation. The main photodegradation pathways corresponded to dealkylation, deamination, dehalogenation and hydroxylation. Photodegradation experiments were carried out using solutions of the different pesticides at 10–200 ppm in distilled water. For carbaryl photolysis is very slow in distilled water, so 5% of acetone was needed as photosensitizer. When PI-mode TSP-LC-MS was employed, aldicarb sulphoxide, monolinuron, hydroxysimazine and fenitrooxon could be identified as breakdown products of aldicarb, linuron, cyanazine and fenitrothion, respectively. In NI-mode TSP-LC-MS aldicarb sulphoxide, 1-naphthol and p-nitrophenol could be identified as the main degradation products of aldicarb, carbaryl and parathion-methyl, respectively. As all the pesticides were dissolved in methanol for solubility reasons, methoxy analogues of the degradation products of cyanazine were also identified. In the PI mode of operation the base peak generally corresponded to [M + H] + for cyanazine and its photodegradation products and to [M + NH 4] + for linuron, aldicarb and fenitrothion and their corresponding degradation products. For carbaryl [M + NH 4] + was also the base peak, but its main photodegradation product, 1-naphthol, could only be identified under NI conditions. In the NI mode of operation different processes such as proton abstraction, (dissociative) electron capture and anion attachment, take place. Fragment ions such as [M] ·− for aldicarb sulphoxide, the main photodegradation product of aldicarb, [M - H] − for 1-naphthol and [M - CONHCH 3] − for carbaryl and anion attachment ions corresponding to [M + CH 3COO] − for aldicarb sulphoxide and 1-naphthol and to [M - CONHCH 3 + CH 3COOH] − for carbaryl were formed. A tentative photodegradation pathway for the different pesticides in water is postulated.