Adulteration of essential oils is very prevalent in the market. According to some estimates, more than 80% of the essential oils sold are adulterated in some ways. In the previous article, we discussed some of the common reasons and ways of adulterating essential oils. In this article, we will discuss some of the ways to check adulteration in essential oils.
In earlier days, essential oils were usually adulterated with addition of fatty oils. Such adulteration can easily be detected by placing a drop of essential oil on a filter paper.
Pure essential oils are comprised of volatile compounds and evaporate completely leaving either no traces or greasy patch. In case of presence of fatty oils, it will leave a greasy patch. In addition, standardisation of physical-chemical properties helped in detection of adulteration.
Appearance and colour of essential oils are one of the first checks to detect adulteration of essential oils. Usually, a particular essential oil will have certain colours and appearance in terms of viscosity. If the colour and appearance of an essential oils is very different from the usual colours and appearance, then probably it is adulterated.
For example, cardamom, rosewood, tarragon or turpentine oils are pale yellow in colour. Sandalwood is colourless to golden yellow and viscous. Vetiver and patchouli oils are yellow to reddish brown in colour and highly viscous. Any significant diversion in the appearance and colour of essential oils is a red flag and should be examined further.
However, it is not a very reliable method to detect adulteration since appearance and colour of oils may be affected by a lot of factors including biomass used for extraction of essential oil, storage methods, exposure to heat and air, etc.
In addition, it is a very subjective method and is not very reliable to detect advance levels of adulteration carried out these days. However, it is still a good first step based on elimination to detect adulteration.
Smelling an oil is an effective way to detect the adulteration; however, to do so one requires a trained nose. For such detection, it can be helpful to know how a particular high quality essential oil smells.
In terms of depth, a pure essential oil should have a depth of aroma. Any sample of oil that smells flat, stale or uninspiring either may be an adulterated oil or a pure oil gone bad.
Further quality essential oil should have smoothness in aroma. Any roughness or unevenness can indicate a low quality. Smoothness is non-fatiguing to the nose, whereas roughness will cause some degree of olfactory fatigue, especially with repeated inhaling.
Lastly, a high-quality oil should have many subtle, complex, minor fragrance notes, regardless of how prominent the dominant note may be. A good geranium oil, for instance, should not only present the typical rosy-sweet main note, but also faint yet distinct musky, slightly spicy-green side notes and mild citrusy-sweet top notes.
Before the development of the analytical methods, checking of physical-chemical properties of the oils was the best method to check adulteration of essential oils.
Some of the properties used to detect quality of oils are relative density, refractive index, optical rotation, freezing point, residue on evaporation, miscibility with ethanol, acid value, carbonyl value, peroxide value, water content, phenol content, etc.
Relative density, refractive index, optical rotation and flash points are physical properties and the values of a pure essential oil must be within the reference ranges.
Solubility tests indicates the presence of contaminants. Evaporation residue test detects the presence of nonvolatile or low volatile compounds. Similarly, chemical test helps to find out content of ethanol, carbonyl and phenol.
The main disadvantage of using physical-chemical properties to detect adulteration is that the values associated with the properties are too wide and can easily be manipulated nowadays. Further, parameters values from these tests tell us nothing about the composition of the tested oils. Hence, there has been a need for tests to find out the composition of the essential oils.
Most of the analytical methods used to find out the composition of essential oils rely on chromatographic procedures, which enable component separation and identification.
Thin layer chromatography is one of the oldest and easiest separation methods. In this method, a drop of the oil is placed at one end of a plate coated with a thin layer of an adsorbent as the stationary phase and placed in a glass chamber filled with a small amount of an appropriate solvent as a mobile phase.
The solvent is drawn up the plate through capillary forces and separates the oil into several fractions. These fractions or spots can be made visible by derivatization with a chromophoric reagent or by inspection under UV light. A reference oil of known purity and composition must be analysed simultaneously or under the same conditions for comparison. You can read further here here.
The main disadvantage of thin layer chromatography is low separation efficiency resulting in poor detection of constituents of essential oil. Essential oils are composed of hundreds of components with concentration ranging from over 90% to a few ppm. Hence, a high efficiency separation method is needed to find the purity of essential oils.
Gas liquid chromatography (GLC) or gas chromatography (GC) is most widely used analytical method for detection of constituents of essential oils.
The method involves vaporising the oil and injecting onto head of the chromatographic column. The oil is carried by inert gases – called mobile phase – over to the columns coated with a microscopic layer of liquid or polymer – called stationary phase.
Compounds present in the oil then interact with the stationary phase and get separated based on different strengths of interaction of the compounds with the stationary phase. The stronger the interaction, the longer the compound interacts with the stationary phase, and the more time it takes to migrate through the column, called retention time.
The comparison of retention times is what gives GC its analytical usefulness. The column is either held at constant temperature or more often programmed at a constant rising temperature rate.
The results obtained from the GC test are compared with the reference chromatographic profiles and percentage composition found in ISO standards and literatures. In case essential oil is adulterated, there will be additional or missing peaks or percentage composition will differs substantially.
Some of the main advantages of GC method is high separation efficiency, simple ease of use, ready availability of sophisticated inexpensive instrumentation and the large amount of qualitative and quantitative information.
However, GC method can not detect the chemical identity of detected peaks. In that case further information is needed besides the chromatographic data and mass spectrometry provides detailed information on the structure of the separated compounds.
Gas Chromatography Mass Spectrum or GC-MS separates and identifies most components of an essential oil. For further technical details of mass spectrometry, please read over here.
However, the reliability of GC-MS decreases in case of classes of compounds with similar mass spectra such as terpenes. Hence, in order to increase the reliability of the analytical results, retention indices must be taken into account as a second criterion for an unequivocal identification.
The use of retention indices in conjunction with the structural information provided by GC–MS is widely accepted and routinely used to confirm the identity of compounds. Usually a GC-MS run is performed for identifying the components and a second GC-FID is run for peak area, respectively, for percentage composition determination.
For long, organoleptic properties have been the main source of detection of adulteration of essential oils. However, later on with the advances in manufacturing capabilities to create synthetic components, it become more and more difficult to detect adulteration with the traditional organoleptic methods and using physical-chemical properties.
Development of GC and GC/MS analytical techniques helped to counter such adulteration to an extent. Many advances has been made further in the detection techniques like Chiral GC, 2D GC (GC/GC) and Nuclear Magnetic Resonance (NMR). NMR is one of the best and one of the most expensive methods available for the detection of authenticity of essential oils.
Kurt Schnaubelt. “The Healing Intelligence of Essential Oils: The Science of Advanced Aromatherapy.”
Kymberly Keniston-Pond. “Essential Oils 101.”