Separation and detection of OCPs were performed by an Agilent 6890 N gas chromatograph (Palo Alto, CA, USA) equipped with a split-splitless injector and an electron capture detector (ECD). A 30 m HP-5 Agilent fused-silica capillary column (0.32 mm i.d. and 0.25 μm film thickness) was employed. Nitrogen (purity 99.999%) was used as the carrier gas at the constant flow rate of 1.2 mL min−1. The temperatures of injector and detector were set at 260 and 320 °C, respectively. The injection port was operated at split mode with split ratio of (1:2). Oven temperature program was: initial temperature of 100 °C, increased to 220 °C at a rate of 20 °C min−1 (held 0.5 min), increased to 230 °C at a rate of 2 °C min−1 (held 3 min), increased to 260 °C at a rate of 5 °C min−1 and then held at 260 °C for 2 min.