These standards contains six key PCB isomers such as 2,4,4'-trichlorobiphenyl; 2,2',5,5'-tetrachlorobiphenyl; 2,2',4,5,5'-pentachlorobipheny; 2,2',3,4,4',5'-hexachlorobiphenyl; 2,2',4,4',5,5'-hexachlorobiphenyl and 2,2',3,4,4',5,5'-heptachlorobiphenyl dissolved in iso-octane.