Essential Oils

Essential Oils Without the use of gas chromatography the analysis of essential oils would be extremely difficult. Prior to the technique being developed, only the major components of the oils could be separated, achieved by distillation with high efficiency columns. Even so, such columns rarely contained more than 100 theoretical plates (distillation plates), were very slow in operation, and took many days to complete an analysis. Due to the limited separation efficiency of the distillation column, even the major components were

Another typical example of the use of the GC/IR instrument is for the analysis of the essential oils of basil a chromatogram of which, is shown in figure 37. A sample of the oil was obtained by supercritical fluid extraction. Superfluid extraction is often used to obtaining samples of essential oils from botanical tissue. The technique is popular due to it being chemically ‘gentle’ and rarely causes thermal degradation of labile materials. The herb basil was extracted with liquid carbon dioxide at 60�C and at 250 atmospheres pressure. The extract (a solution of the essential oil

cm2. The method could separate all 6 enantiomers as their trifluoryl acetyl derivatives as shown in figure 47. The high efficiencies and the general versatility of this stationary phase, that provides strong dispersive and polar interactions, makes it especially useful for the separation of substances with multiple chiral centers and in the presence of metabolites. The use of a 5m retention gap method of injection (see page 19) allowed the direct injection of 7 ml of plasma. Essential oils (flavors and perfumes) also contain many chiral compounds and one enantiomer may be entirely responsible for a particular taste or odor whereas the complementary enantiomer has an entirely different olfactory effect. It is clear that the use of chiral chromatography can be one of the more useful techniques for the analysis of essential oils. A chromatogram of the essential oil vapor from White Pine leaves is shown in figure 48

isolating the major component of the mixture in pure form, the fractions containing the minor components still consisted of complex mixtures. As a consequence, this type of separation was virtually useless as it was often the minor component(s) that had physiological, toxic or organoleptic importance. The introduction of GC had a revolutionary effect on the separation and identification of the components of such complex mixtures and, for the first time, the real nature of many of the essential oils and industrial solvents that were in common use was disclosed. Even the early examples given by James and Martin (2) in one of their first public lectures on the subject, included dramatic separations of multi-component mixtures that fired the imagination of chemists world-wide. For the first time the complex nature of these important materials could be displayed both quantitatively and qualitatively. An example of the separation of an essential oil using modern GC equipment is

expected from open tubular columns and were used for the analysis of petroleum and fuel oils, etc. Metal columns, however, have some disadvantages as although easily coated with dispersive stationary phases (e.g., squalane, Apiezon grease etc.) they are not so easily coated with the more polar stationary phases such as CARBOWAX�. In addition, hot metal surfaces can cause decomposition or molecular rearrangement of many thermally labile materials such as the terpenes contained in essential oils. Metal can also react directly with some materials by chelation and adsorb polar material which results in asymmetric and tailing peaks. Nevertheless, metal columns are rugged, easy to handle and easy to remove and replace in the chromatograph consequently, their use has persisted in many application areas despite the introduction of fused silica columns

, well resolved into individual small peaks. It is also seen that the late small peak has retained its symmetry and is almost perfectly Gaussian in shape. Those familiar with cinnamon bark oil separated on GC capillary columns may wonder at the relatively few peaks that appear on the chromatogram. It should be pointed out that a UV detector was employed to monitor the separation and, thus, only, those substances that adsorb in the UV would be disclosed. As the majority of the substances in essential oils are UV transparent, only a limited number of the components will be detected. The example is given to illustrate the wide range of solutes that can be separated and that, providing adequate efficiency is available together with suitable apparatus, multi component mixtures can be separated by LC as well as GC. Separations on Small Bore Columns 2m Long Small bore column can have a fairly wide range of useful lengths and need not be 10 meters or more in length and have