2.4. GC–MS analysis
The analysis of pesticides was performed by using Shimadzu GC-2010 system coupled with QP2010 mass spectrometer detector (Kyoto, Japan). The fused-silica capillary column (30 m × 0.32 mm) coated with 0.25-μm bonded film of DB-1 (J & W Scientific, Folsom, CA) was used. The GC column temperature program used was as follows: 60 °C for 1 min, then ramped to 150 °C at a rate of 20 °C min− 1 (held for 4 min) and then ramped to 290 °C at a rate of 15 °C min− 1 (held at this temperature for 5.17 min). The injector and detector temperatures were 250 °C, and all injections were made in the split/splitless mode (splitless time: 0.75 min). Helium was used as carrier gas at a flow rate of 1.2 mL min− 1. A 1 μL sample volume was injected at 280 °C. The mass spectrometer was operated in the electron impact mode with an ion source temperature of 250 °C and the electron impact energy was set at 70 eV. The MS scanned mass range m/z 40 and 300 was used for quantitative determinations of the studied pesticides. For quantitative determination using selective ion monitoring (SIM), pesticides were identified by ions with the following m/z values and quantified by the ions 87, 93 and 125 for dimethoate (retention time: 13.41 min); 109, 125 and 263 for methyl parathion (retention time: 14.95 min); 97, 153 and 231 for ethion (retention time: 17.63 min); 163 and 183 for permethrin (retention time: 20.15 min). Quantification was performed by calculating the absolute peak areas. Fig. 1 illustrates a standard chromatogram obtained under optimized conditions for all investigated pesticides.