Stable carbon isotope composition of monoterpanes in essential oils and crude oils

Twenty-five monoterpanes from six types of essential oils and hydrogenated turpentine oil have been identified and their stable carbon isotope composition determined. Monoterpanes in essential oils sourced from terrestrial higher plants display a δ13C value in the range of −34‰–26‰, and mostly between −29‰ and −27‰. The δ13C value of any single monoterpane is very consistent in different essential oils. Acyclic monoterpanes show closer isotope composition between −28.6‰ and −26.2‰, with an average value of −27.7‰. In contrast, the isotope composition of cyclic monoterpanes is more scattered with an average value of −28.6‰. Isotopic fractionation with 13C enrichment has been observed during both artificial and geological hydrogenation of monoterpenoids to monoterpanes, and this is more obvious for the acyclic monoterpenoids. In addition to higher plants, acyclic monoterpane 2,6-dimethylheptane in crude oil can also be originated from other organic inputs.