Comparison of Current Chemical and Stereochemical Tests for the Identification and Differentiation of Pelargonium graveolens L'Hér. (Geraniaceae) Essential Oils: Analytical Data for (-)-(1S, 4R, 5S)-Guaia-6,9-diene and (-)-(7R,10S)-10-epi-γ-Eudesmol

 Commercial geranium oil samples, steam-distilled oils of authenticated plant samples, and a reference sample were investigated by GC/MS to determine the validity and applicability of a series of chemical and stereochemical tests that have been proposed in the literature to identify the country of origin, phytochemical identity or authenticity of geranium oils. The chemical tests evaluated include the ratio of the concentrations of geraniol to citronellol and the presence or absence of certain sesquiterpenes, viz., (-)- guaia-6,9-diene and (-)-10-epi-γ-eudesmol. The stereochemical tests include the stereochemical distribution of i) citronellol, ii) menthone and isomenthone, and iii) rose oxides. The most reliable chemical test was the presence or absence of the sesquiterpene probes. The stereochemical tests proved to be less reliable. Most of the tests could be used to classify geranium oils into general types; however, none of the tests provided a foolproof method to distinguish cultivars or country of origin. During this study, the ambiguity in the absolute stereochemistry of (-)-10-epi-γ-eudesmol and (-)-guaia-6,9-diene was addressed, and these two sesquiterpenes could serve as effective markers for the authentication of P. graveolens essential oils.