Photochemical Degradation of the Antimicrobial Sulfamethoxazole upon Solar Light Excitation: Kinetics and Elucidation of Byproducts Using LC/ ESI+/MS2 Technique

Abstract

The fate of the antimicrobial compounds in the environment is of great interest since their presence in the aquatic systems has raised environmental problems. More and more chemicals of this class are treated as emerging contaminants. The degradation of these compounds may result in the formation of a wide array of metabolites which can be more toxic than the parent substrate. Therefore, precise elucidation of all possible transformation products as well as a thorough study of their physico-chemical and biological properties is of great importance. The present work deals with the study of the photochemical behavior of sulfamethoxazole from the kinetic aspect as well as the elucidation of the products arising from the solar irradiation of the antimicrobial sulfamethoxazole in the aqueous solutions. HPLC/MS and HPLC/MS/MS with accurate mass measurements were used for this purpose. stainless steel equipped with two germicidal lamps (Mazda T815 15W) emitting selectively at 254 nm and symmetrically installed around the cylinder were used. The reactor, a quartz tube (d=2.5 cm) containing a maximum of 50 mL solution, was located in the centre of the container. The disappearance of the antimicrobial and the formation of the products were followed by high performance liquid chromatography using a Waters 2695 HPLC (Alliance) chromatograph system equipped with a Waters 2998 photodiode array detector. The experiments were performed by UV detection at either 250 nm or 280 nm and by using a reverse phase Nucleodur column (Macherey-Nagel, 100-5 C18 ec; 150-4.6 mm). The flow rate was 1.0 mL min-1 and the injected volume was 50 µL. The elution was accomplished with water, formic acid (0.1%) and acetonitrile (60/40 v/v). A Waters/Micromass LC/QTOF tandem mass spectrometer (Micromass, Manchester, UK), with an orthogonal geometry Z-spray ion source, was used for LC/ESI/MS and LC/ESI/MS/MS experiments. LC separation was performed using the gradient program reported in the literature [19]. Eluate was subjected to electrospray ionization (ESI) in the positive ion as well as negative mode and resulted in the formation of protonated molecules and deprotonation of the sample components. Scanning was performed in the range between m/z 60 and 600. The elemental composition of the recorded ions was further determined using MassLynx Elemental Composition software V4.1 (Micromass). C, H, N, O and S were selected as possible elements present. In LC/MS analyses, in order to assign the elemental formulas, the minimum and maximum atoms of each element were set as follows: C from 1 to 20; H from 1 to 20; N from 0 to 5; O from 0 to 10 and S from 0 to 2. No nitrogen rule, ring and double bond equivalents were applied. The maximum deviation was set to 10 ppm. Five scans were combined before the integration of the