Monoterpene hydrocarbons from citrus essential oils

Isotope Ratio Analysis by HRGC-MS of Monoterpene Hydrocarbons from Citrus Essential Oils... 

In principle, it is possible to obtain the isotope ratio from MS data. The authors have previously shown, in fact, a practical use for the isotope ratio from mass spectrometry. (10) However, more detailed analytical conditions need to be found to achieve more precise measurement. The intensity of each molecular ion peak is not very strong. In addition, the isotope peak is approximately 10% of that of the molecular ion peak in the case of monoterpene hydrocarbons in citrus essential oils. 

The modified Ir data for monoterpene hydrocarbons from the three species of citrus oils were subjected to a multivariate analysis. According to principal component (PC) analysis, the citrus essential oils examined were clearly discriminated, with 93.3% of the accumulation contribution ratio of both PCI and PC2 (Fig. 4A). As shown in Fig. 4B, p-pinene (PCI, 0.530 PC2, -0.80), a-terpinene (0.685 0.062) and P-phellandrene (0.324 0.810) were large for the absolute value of the eigenvector of monoterpene system hydrocarbons, followed by terpinolene (0.367 -0.010). These compounds greatly contributed to the modified Ir discrimination of the citrus essential oils. 

Most essential oils contain a significant proportion of terpenes (monoterpenic and sesquiterpenic hydrocarbons). For example, their level in some citrus essential oils is 95% or more. These substances are not usually essential to the smell and aroma character of essential oils, as the most important odoriferous compounds are alcohols, aldehydes, ketones, esters and other compounds. Furthermore, terpenic hydrocarbons are a reason for the limited solubility of essential oils in diluted ethanol and are often the cause of deterioration of essential oils that easily oxidise or polymerise. By removing hydrocarbons from essential oils, concentrates are obtained. Monoterpene-free or sesquiterpene-free essential... 

Citrus oil dominates this class of essential oil. It is obtained by the cold press method with the exception of lime oil, which is also prepared by steam distillation of essential oil separated during the production of juice.106,107 Aside from bergamot, these oils are primarily monoterpene hydrocarbon mixtures of which (if)-limonene (3) is usually the dominant compound. Since odor contribution of this monoterpene compound is low, it is often removed by distillation or repeated solvent extraction. The resulting oil rich in odor-active compounds is called terpeneless oil and is used extensively. In the case of bergamot and lemon oils, psoralen derivates like bergaptene (64) causing photosensitivity are problematic, and those for fragrance use are rectified to remove it (Table 8). 

In comparison with essential oils from conifers the enantiomeric ratio of monoterpenic hydrocarbons is more stable in citrus oils and thus it can give information about their origin, extraction techniques and genuineness of the oils. The main component of monoterpenes in those types of oils is (+)-limonene of a high enantiomeric purity for most of the citrus oils. 

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