**2.3. Analysis of the photolyzed solutions**

At specific time intervals, samples were removed from the photoreactor, and 5 mL aliquots of the photolyzed aqueous matrices were withdrawn for further analysis. In order to remove TiO<sup>2</sup> , samples were centrifuged at 8000 rpm for 15 min. OPP concentrations were determined after liquid-liquid extraction of the filtrated aqueous phases twice with 5 mL hexane or dichloromethane by vortex (1 min). Elimination of humidity in two combined extracts was accomplished by the addition of anhydrous Na<sup>2</sup> SO4 , whereas 1 μL of the organic extract was used for further analysis. Qualitative and quantitative determination of extracted pesticides' residues was performed on a Hewlett-Packard gas chromatographic (GC) system, model HP-5890, Series II, (Hewlett Packard, USA) equipped with a nitrogen-phosphorus detector (NPD). The chromatographic method and conditions applied are discussed in detail by Vagi et al. [14]. The validation of the method was carried out by the analysis of fortified water samples containing each target analyte at three different spiking levels in the range of 0.5–2 mg L−1 and prepared in triplicate individual solutions [15]. Percentage recoveries for the five selected insecticides were above 95.2 ± 2.4%, LOD values were 0.0050 μg L−1 for dimethoate and 0.0025 μg L−1 for the other four selected organophosphates (azinphos methyl, azinphos ethyl, disulfoton, and fenthion), and LOQ for dimethoate was 0.0165 μg L−1, while for the rest four OPPs 0.0083 μg L−1 [14].
