Terpene composition of essential oils

A review is given of techniques for sampling, extraction, separation and analysis of essential oils, and the effects of each upon the finally determined essential oil composition. It is concluded \that presently~available techniques such as steam-distillation solvent extraction vapour trapping and even solid-sampling gas chromatographic injection procedures impose so many characteristic effects upon the volatile terpenoids in a plant sample that it is not practicable to obtain instantaneous measurement of volatiles as they are released to the atmosphere. In addition many of the well-known techniques and approaches to essential oil analysis lead to many losses and artifacts and are so protracted as to mitigate against the feasibility of a routine analytical procedure for use in a survey of essential oils. A recommended routine analytical procedure was developed for use in surveys of essential oils and its effectiveness is illustrated by the analysis of essential oils from 7 endemic Tasmanian plants. Details are given of the development of syringe-headspace gas chromatographic analysis which is a novel method of identifying and monitoring components in vapours from comminuted plant tissue as they are released to the atmosphere at room temperature. The syringe-headspace technique is recommended as a means of directly using verified plant species from a botanical gardens as a source of reference terpenes which otherwise may be too unstable to be stored as pure compounds. This technique allows the relative retention time of an authentic terpenoid to be measured in plant material for direct comparison with that of an unidentified compound. It thereby enables the worker to justify spending considerable effort in synthesizing isolating or purchasing the suspected terpenoid. Successive injections of vapour from a single sample of comminuted foliage by the syringe-headspace technique often exhibit changes in the proportions of some components with respect to one another. Such changes were observed between terpenoids having a common hypothetical precursor according to the biosynthetic scheme by Ruzicka. This technique is a novel means of directly measuring biosynthetic changes. It considerably extends the usefulness of Zavarin's earlier advocated procedure for utilizing 'quantitative co-occurrence' which is a basically different procedure to supplement the results of tracer studies. Both the routine analytical procedure for analysis of essential oils and the syringe-headspace technique were used to compare the compositions of terpenoids in 19 species of conifer susceptible to attack by the Woodwasp Sirex noctilio. An investigation was also made of earlier inferred changes in the compositions of essential oils following the wounding of trees of Pinus radiata. Widely varying compositions are reported for the first time for oils distilled from bark of felled.trees. Variations in oil composition from a single injured tree trunk ranged from 15.7 to 20.3 percent a-pinene 54.8 to 68.2 percent 8-pinene 9.0 to 18.3 percent limonene and 2.5 to 6.7 percent myrcene. The range of compositions of oils from within and between trees of a species was very wide so that in a comparison of each species no single insect-attractive optimum composition could be envisaged. It appeared more likely that S. noctilio could be attracted to one of the 'temporarily-released' components which appeared to cause qualitative changes in many oils."