Essential Oil Chemistry, Resources|

In recent years, the term “Chiral purity” is used increasingly in quality assessment of Essential oils. Assessment of Chiral purity has been formally required in the quality assessment methods defined in the British Pharmacopoeia monograph for Lavender Oil for nearly a decade and Chiral purity is set to be a recommended test in the latest draft of the ISO (International Standards Organisation) for Tea Tree Oil.

Chiral Purity refers to a comparative measurement of the relative proportions of naturally occurring stereoisomers of a chiral compound. Chiral compounds which are produced naturally will have a defined relative proportion of stereoisomers which can be used as a “signature” for the authenticity of the chiral chemical being analysed. The relative proportion of these allows detection of essential oils adulterated with synthetics analogues (exact copies) or with natural analogues derived from a different plant source. This protects genuine, naturally produced fragrance products from unfair competition as well as protecting consumers from fraudulent misrepresentation of the products they purchase.

The word chiral refers to anything which looks different to its mirror image. In short, asymmetrical objects are chiral while symmetrical objects are achiral. The human hand is a perfect example of a chiral object. In a mirror it’s looks the same but with reversed symmetry. In fact, the word chiral derives from the Greek word for hand (Kheir).

Molecules (chemicals) can also be chiral. Molecules are chains of atoms which can have branches and can twist and contort. As such, asymmetry is very common in molecules and is described as isomerism. Chiral isomers are called enantiomers. In chemical naming (nomenclature) enantiomers are described as either: + or – ; Dextro (Right), Laevo (Left); or R and S respectively. Even simple molecules like those founds in essential oils (e.g. terpenoids) can be chiral. Common chiral compounds are Terpinen-4-ol and a-Terpineol in Tea Tree Oil and Linalool and Linalyl acetate in Lavender.

Chirality in essential oils is analysed using Gas Chromatographic (GC) machines equipped with Chromatograpic columns that have internal coatings of specialised cyclodextrin and silica. These columns were not commercially available until the early 2000’s. Synthetically produced chiral chemicals are known to be racemic mixtures which are equal in relative abundance of enantiomers (i.e. 50:50 ratio). When chemicals are produced by plants the proportion the ratio of occurrence of the two enantiomers of a given molecule is governed by enzymatic processes defined by specific genetics. In this way, the enantiomeric ratio can be used an indicator of the genetic origin of a particular chemical.

Synthetic analogues of essential oil chemicals can often be produced at lower cost than their natural equivalents. Where the natural product attracts a high premium there is commercial incentive to adulterate the oil with synthetic analogues of the important quality indicating compounds to enhance their perceived quality by aroma but also by measurable standards using chemical analysis equipment such as GC. French High Alpine Lavender is an example of such an oil. Its quality in terms of antimicrobial efficacy and sweetness of aroma are determined by the overall abundance of Linalool and Linalyl acetate and their respective relative composition. Both Linalool and Linalyl acetate can be synthesised and have been commonly used to “standardise” oils derived from different species and cultivars of lavender which can be grown more costs effectively than high alpine lavender and are higher yielding. To thwart this process, internationally recognised pharmaceutical standards for Lavender oil in the EP and BP have been modified to include chiral purity as a defining requirement for quality assessment of Lavender oil for use in medicinal products.

The ratio of relative abundance for both Linalool and Linalyl acetate is defined over a ratio range. In different description of chirality in essential oils various terms are used to describe the same thing and this can be confusing! Chiral Purity, Enantiomeric distribution, Chiral ratio, and permutations thereof all mean the same thing.

Figure 1 and 2 below illustrate the chiral separation of stereoisomers of: Linalool and Linalyl acetate; and, Terpinen-4-ol respectively.

Chiral purity of Lavender is calculated in the BP/EP as the sum of % proportion of the enantiomer area in the sum total areas of the two enantiomers. So for example:

(-) Linalool Chiral purity in Figure 1 is 4.275/(111.95+4.275) x 100 = 3.69%
BP/EP specification = < 12.0

Chirality Figure 1
Figure 1.  Chiral GC profile of (+)Linalool, (–)Linalool and (+)Linalyl acetate, (–)Linalyl acetate
Terpinen-4-ol, Chiral purity is assessed differently by looking at the relative proportion of enantiomers directly or at a ratio of the two enantiomers.

Using values for Figure 2:
Where:
(+) Terpinen-4-ol = Peak Area 477.962
(-) Terpinen-4-ol = Peak Area 218.489
Summed peak area = 696.451
Relative proportion
(+) Terpinen-4-ol = (477.962/696.451)x100 = 68.63
(-) Terpinen-4-ol = (218.489/696.451)x100 = 31.37
Ratio  = 68.63/31.37 = 2.19

Chirality Figure 2
Figure 2. Chiral GC profile of (+)Terpinen-4-ol, and (–)Terpinen-4-ol

Chirality also affects Optical rotation. Oils heavily adulterated with synthetics will often be out of specification on Optical rotation. It is however also possible for products to be adulterated with selective synthetic enantiomers of common chemicals such as limonene which can artificially change the optical rotation back into specification. For this reason Chiral analysis by GC is necessary to unequivocally ascertain adulteration with synthetic molecules.

Ashley Dowell
Manager
Analytical Research Laboratory (ARL) Southern Cross Plant Science (SCPS)
Southern Cross University (SCU)
PO Box 157, LISMORE  NSW 2480 Australia

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