Polychlorinated biphenyls (PCBs) which is a kind of chlorinated aromatic hydrocarbons formed by catalyzed and chlorinated under the high temperature conditon. Its raw material is biphenyls.

Because of its good flame resistance, low conductivity, anti-pyrolysis and chemical stability, it were widely used as impregnant in transformers and capacitors. According to statistics, only in China, during 1965 - 1974, nearly ten thousand tons of PCBs were produced, and 90% of them were used in transformers and capacitors. However, due to its enviromental toxicity and classification as a persistent organic pollutant, it was banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants in 2001. Further, according to the United State Environmental Protection Agency (EPA), PCBs have been shown to cause cancer in animals, and there is also evidence that they can cause cancer in humans. Therefore, it is urgent to measure the PCBs in transformer oil and subsequently find the way to dispose these PCBs.