Regioselectivity of dechlorination of DDT and its metabolites in mass spectrometry

DDT and several of its metabolites were synthesized with their aromatic chlorines enriched with 37Cl. The mass spectra of these compounds were recorded under electron impact and under methane positive‐ion and electroncapture negative‐ion chemical ionization. The regioselectivity of the dechlorination reactions was determined by measurement of the relative proportions of the unlabeled aliphatic versus the labeled aromatic chlorines in the various fragment ions observed. For all the α,β‐unsaturated compounds studied under electron impact ionization, the regioslectivity is small, as both the aromatic and the aliphatic chlorines are lost. Under positive‐ion chemical ionization, the dechlorination is highly regioselective, the aliphatic chlorines being almost exclusively lost. Under electron‐capture negative‐ion chemical ionization, the regioselectivity is also very high and the reactivity of the various chlorines is often controlled by the position of the aromatic chlorines.