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 (aC, aCl, and aH) were observed in each experiment. The large variability of aH is related to the accumulating secondary hydrogen isotope effects or the rearrangement of C-H bonds at the cyclohexane ring. An increase of 613C and 637Cl 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 62H values is related to the secondary and primary hydrogen isotope effects. Different 613C, 637Cl and 62H 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.

Automated summary

The isotopic fingerprints of HCH isomers have potential for identifying sources. This study conducted photochemical synthesis experiments to investigate isotopic fractionation associated with the synthesis processes. Different patterns of isotopic fractionation factors were observed, and the variability of isotopic values was related to secondary hydrogen isotope effects and the conformational complexity of HCH caused by the arrangement of C-Cl bonds. The understanding of isotopic fractionation during HCH synthesis can be indicative for source identification in the field.