Dechlorination of polychlorinated biphenyls in electron-capture negative-ion chemical ionization mass spectrometry

The electron‐capture negative‐ion chemical Ionization mass spectra of 36 35Cl mono‐ and di‐labelled penta‐ and hexachlorobiphenyls were studied. Under the conditions of the analyses, the dechlorination of these congeners probably proceeds through electrophilic hydrogen atom attachment followed by rapid loss of the chlorine atom at the same position. These dechlorination reactions occur regioselectively. When present, ortho‐chlorines are lost preferentially to meta‐ and para‐chlorines. The extent of the dechlorination reaction, as seen by the relative abundance of the [MH  Cl]− ions, decreases with increasing number of ortho‐chlorines.