Deuterium/hydrogen ratio analysis of thymol, carvacrol, γ-terpinene and p-cymene in thyme, savory and oregano essential oils by gas chromatography–pyrolysis–isotope ratio mass spectrometry

Isotope ratio mass spectrometry online coupled with capillary gas chromatography (GC–Py–IRMS) on column INNOWAX is used in the origin specific analysis and the authenticity control of the phenolic essential oils (EOs). Isotopic data δ2HV-SMOW of thymol and carvacrol in natural essential oils were evidently more depleted than synthetic products (from −49 to 7‰ for thymol and −61‰ for carvacrol). δ2HV-SMOW values of p-cymene, γ-terpinene and thymol in authentic thyme oils (Thymus vulgaris L. and Thymus zygis L.) were found from −300 to −270‰, from −285 to −248‰ and from −259 to −234‰, respectively. δ2HV-SMOW values of carvacrol and p-cymene in authentic oregano oils (Origanum heracleoticum L., Coridothymus capitatus L. and Origanum compactum L.) varied from −223 to −193‰ and from −284 to −259‰, respectively. For authentic Satureja montana subsp. montana essential oils, the mean δ2HV-SMOW value for aromatic compounds were found to be the following: γ-terpinene −273‰ (SD = 4.6‰) and p-cymene −283‰ (SD = 3.0‰), thymol −245‰ (SD = 1.8‰) and carvacrol −226‰ (SD = 1.7‰). In addition, p-cymene was previously found as a precursor of the biosynthesis of thymol and carvacrol in thyme oil, thus, we considered p-cymene as an endogenous reference compound (ERC) for D/H ratio analysis. The isotopic fractionation factors αthymol/p-cymene = 1.05 and αcarvacrol/p-cymene = 1.08 were obtained and also used to control the authenticity of the phenolic EOs.