Dechlorination of polychlorinated biphenyls in electron impact mass spectrometry: Regioselective analysis using 35Ci‐labelled compounds

Four tetrachlorinated bipbenyls were chlorinated with 35Cl2 to yield seventeen polychlorinated biphenyls (PCBs) substituted once or twice with chlorine enriched in 35Cl. Gas chromatographic analysis of the mixtures allowed the identification of the labelled PCBs produced by comparison of their relative retention times with published values. This also permitted the assignment of the position of the labels in these molecules. Mass spectrometric determination of the 35Cl/35Cl ratio of the ions corresponding to successive dechlorination of these PCBs in positive‐ion electron impact (EI) allowed quantification of the residual 35Cl label in these ions. Chlorines in the ortho position are lost preferentially to those in meta or para positions. The first dechlorination reaction was found to be totally regioselective for at least two PCB congeners.