The large PCBs were cut into small pieces (2 cm x 2 cm) and 80 g of the PCB pieces were immersed in NaOH (10 M, 800 mL) for 24 h. The PCB pieces were subsequently washed thoroughly with distilled water until a neutral pH of the wash water was obtained . One randomly selected piece of the NaOH-treated PCBs was completely digested in aqua-regia (volume ratio of HCl to HNO3 = 3:1) for 24 h. This was carried out in duplicates. The aqua-regia solution was filtered and the filtrate was prepared for the determination of Cu concentration.