PCB calibration standard solutions, 13C-labeled surrogate standards (13C-PCB-14, 65 and 166), and injection standards(13C-PCB-9, 52, 101, 138 and 194) complying with US EPA method 1668A for PCBs analysis were purchased from Cambridge Isotope Laboratories in USA. Here, the mainly concerned PCBs were 20 individual PCB congeners and total PCBs (from Mono-CBs to Deca-CBs).