Stable isotope fractionation associated with the synthesis of hexachlorocyclohexane isomers for characterizing sources.

The stable isotope fingerprints of hexachlorocyclohexane (HCH) isomers have potential for identifying sources as they are related to the synthesis processes and isotopic compositions of raw materials. However, the isotopic fractionation associated with the synthesis processes has not been investigated. Therefore, photochemical synthesis experiments using benzene and chlorine gas were conducted to characterize the associated isotopic fractionation under different conditions. Different patterns of isotopic fractionation factors (αC, αCl, and αH) were observed in each experiment. The large variability of αH is related to the accumulating secondary hydrogen isotope effects or the rearrangement of C-H bonds at the cyclohexane ring. An increase of δ13C and δ37Cl values of HCH isomers was observed during synthesis, which is related to the C-Cl bond formation in the radical dichlorination forming HCH and the subsequent chlorine substitution forming heptachlorocyclohexanes. The large variability of δ2H values is related to the secondary and primary hydrogen isotope effects. Different δ13C, δ37Cl and δ2H values among HCH isomers were observed, indicating that conformational complexity of HCH caused by arrangement of C-Cl bonds in planar and axial positions also influence the isotope values. The understanding of isotopic fractionation during HCH synthesis can be indicative for source identification in the field.