Enantiomeric distribution of key volatile components in Citrus essential oils

Citrus as many other plants present characteristic distribution of some enantiomers, thus it is often possible to use this parameter for identification, characterization, genuineness, and pharmacological activity assessment. In particular, it is possible to reveal adulteration of different nature, such as addition of synthetic compounds, or natural components of different botanical origin, with drastic changes in the biological and olfactory properties. This study is focused on the evaluation of the enantiomeric excesses of numerous samples of different Citrus species: C. deliciosa Ten., C. limon (L.) Burm., C. bergamia, C. aurantifolia (Christm.) Swing., C. latifolia Tan., C. sinensis (L.) Osbeck, and C. aurantium L. The enantiomeric distribution is determined by direct esGC and, depending on the complexity of the essential oil, by MDGC with a chiral column in the second dimension. The research is focused on the determination of fourteen chiral components which present specific distribution in the essential oils investigated. Particular attention is given to the trend of the enantiomeric distribution during the productive season, so to identify useful parameters for quality assessment also in consideration of the wide range of variability often reported in literature. The components investigated were the following: alpha-thujene, alpha-pinene, camphene, beta-pinene, sabinene, alpha-phellandrene, beta-phellandrene, limonene, linalool, camphor, citronellal, linalyl acetate, terpinen-4-ol, alpha-terpineol. The use of MDGC allowed the separation of the enantiomers of camphor and citronellal, otherwise not separated by conventional esGC; however for the separation of the enantiomers of alpha-pinene it was preferable to use conventional esGC. The MDGC system allowed to determine the enantiomeric distribution of camphene, alpha-and beta-phellandrene in lime essential oil for the first time. The results are discussed in function of seasonal variation and, when possible, in function of the extraction technology, with particular regards to lime oils.