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It is possible to ‘read’ the formula of an essential oil using various techniques, the most common being the gas-liquid chromatograph (GLC). In this apparatus a minute amount of essential oil is injected into a temperature controlled, extremely fine, coiled, tubular column. The time taken (called the retention time) for each component to emerge from the other end of the column is different, depending on the molecule size. The quantity released is recorded, showing a peak on the trace (proportional to the quantity). This is a comparative test, not an absolute one, the retention time of known constituents having already been determined, to aid in the analysis.

As every batch of oil will vary in its percentages of components, one reading of each oil is kept as a ‘standard’. This standard can then be directly compared to that of another essential oil from the same plant.

This technique shows any added adulterant having a retention time not evident on the standard. However it is possible to adulterate an oil low in a certain constituent, simply by taking that constituent from another, usually cheaper, essential oil, to ‘correct’ its reading. Sometimes a synthetic replica of a component is used, and occasionally, where a high concentration of one ingredient is desired, the oil will have a percentage of its terpenes removed, as in peppermint oil (see chapter 4). Aromatherapists should be wary of such an oil as this ‘concentrates’ the active components. Alternatively an ingredient may be augmented, as in the case of eucalyptol added to eucalyptus oil.

FIGURE 2.3: Gas-liquid chromatographs

Of all tests carried out on essential oils, the most common, apart from the GLC, are infra red, optical rotation, specific gravity, mass spectrometry (very expensive, but excellent) solubility in alcohols and ester content.