Physiochemical methods have long been employed as standard tests to determine the identity of essential oils. Genuinely produced oils typically exhibit characteristic values that are being used to measure the oils authenticity. After organoleptic evaluation, which involves examining the appearance, odour, and taste of an oil, physiochemical testing is next in line as it is economical and relatively easy to accomplish in comparison to some other testing methods.. Physiochemical specifications for essential oils encompass a range of parameters such as refractive index, specific gravity, congealing point, optical rotation and others.
These tests are usually conducted at ambient temperatures (20°C or 25°C) for oils that are naturally liquid at these temperatures. For some few oils which are solid at ambient temperatures like guaiacwood oil (Bulnesia sarmienti) the evaluation may be carried out at 40°C.
Specific Tests that are Commonly Used
Optical Rotation
is a phenomenon that occurs when polarised light rays pass through a transparent material, resulting in the rotation of the plane of polarisation. These rays interact with substances containing chiral molecules. Chiral molecules come in two versions that are similar in appearance but mirror images of each other. As light passes through a solution containing chiral compounds in varying proportions of each version these compounds will rotate the plane in which the polarised light oscillates.
It is quantified as degrees of angle of rotation and can be either leftward, known as leavo rotatory (minus), or rightward, called dextro rotatory (plus). When a sample contains both forms, they are referred to as "Optical Isomers."
The optical rotation of an essential oil is a result of the combined optical rotations of its individual constituents and is directly related to their proportions within the oil. In most cases, a range of values is quoted due to natural variations in composition.
For example, orange oil has dextro rotation due to d-limonene, while the other optical isomer of limonene, l-limonene, exhibits leavo rotation and can be found in mint oils as well. Both these limonene isomers, Dextro and Leavo, coexist in the dl-form, also known as dl-limonene or dipentene, which does not exhibit any optical rotation. A mixture of d- and l-forms is called "racemic".
It can be said that pure oil of a certain kind will typically move within a particular range and any deviation from this range is an indication that the oil may not be pure. Adulterated oils often contain compounds that have different optical activities compared to those found in natural oils. By assessing the optical rotation, discrepancies between the natural and adulterated components can be identified.
Here are some examples of typical ranges of natural oils (1):
Citrus bergamia (bergamot): +12 to +24 - Dextro Rotation
Lavandula angustifolia (Lavender): -5 to -12 - Leavo Rotation
Santalum album (Indian Sandalwood): -15 to -20 - Leavo Rotation
Citrus reticulata (Mandarin): +65 to +75 - Dextro Rotation
Rosmarinus officinalis (Rosemary): -5 to -20 - Leavo Rotation
To determine optical rotation, a polarimeter is used. While older models required about 25 ml of the oil for testing, newer instruments require only 3 to 5 ml of oil, and the measurement process is automatic and thus faster.
Specific Gravity
is also known as relative density and refers to the density of an oil in relation to the density of water. Density is determined by dividing the mass (weight) of a material by its volume at a known temperature. Water has a density of practically one, meaning 1 liter of water weighs 1 kilogram, or vice versa. When an oil is lighter than water and incapable of mixing with it (immiscible), it will form a separate upper layer that swims on the water. Since most essential oils are lighter than water and are immiscible with it, they will typically display this behaviour.
To ascertain the specific gravity of a sample under test, it is filled into a bottle with a known volume at a known temperature and accurately weighed. Notably, specific gravity is inversely proportional to the temperature, meaning the lower the temperature the more it increases.
To simplify the .process, specific gravity is usually measured these days by an electronic meter. The specific gravity measurement is typically conducted at 20°C. Most essential oils exhibit a lower density than water, resulting in a specific gravity value (called SG value) of less than 1. For example (1):
Lavandula angustifolia (lavender): 0.878–0.892
Citrus reticulata (mandarin) : 0.854–0.859
Some woody oils have a specific gravity above 1, causing them to collect beneath the distillation water during extraction, which means the water swims on top of the oil. For instance:
Syzygium aromaticum (clove bud): 1.041–1.054
Cinnamomum zeylanicum (cinnamon bark): 1.000–1.040
Here is an example of detecting possible adulteration through SG testing (1):
Rosewood essential oil typically possesses a specific gravity (SG) falling within the range of 0.872–0.887. It is mainly composed of 84–93% linalool, a compound with an SG of 0.87. If a sample of rosewood essential oil exhibits an SG significantly lower than 0.872, it might suggest the addition of extra linalool, which is a low-cost diluting agent.
Refractive Index
The Refractive Index refers to when light transitions from air to a liquid (or vice versa). In this process it undergoes refraction, resulting in a change in its direction of travel and causing the light ray to appear 'bent'. When a light ray enters a denser medium, it alters its direction and enters at a smaller angle compared to the angle at which it approached the point of incidence. The relationship between the angle of incidence (i) and the angle of refraction (r) is termed the refractive index (RI).
To conduct the test, 2-3 drops of the oil under examination are placed between the two prisms of the instrument at a known temperature, and the degree of 'bending' is then measured. This testing method requires only a small amount of sample and is time-efficient.
To determine the Refractive Index, a refractometer is used. It is important to note that the refractive index provides only a qualitative assessment of the purity of essential oils and does not give information about the percentage purity.
The refractive index is always greater than 1 when light transitions from a less dense medium, such as air, to a more dense one, like oil.
Every essential oil exhibits a specific refractive index that falls within relatively narrow ranges. Falling out of these ranges can give a strong indication for possible adulteration. Some examples for the refractive index of natural essential oils are illustrated below (1):
Lavandula angustifolia (lavender): 1.457–1.464
Citrus reticulata (mandarin): 1.475–1.478
Syzygium aromaticum (clove bud): 1.528–1.537
Flash Point
Each essential oil possesses its own specific flash point. serves as an indicator of their flammability. This parameter is defined as the lowest temperature at which the vapor above a liquid can be ignited in the presence of air. Typical flash point values for essential oils fall within the range of 33°C to 77°C.
Some examples of specific flash point values for essential oils include:
Boswellia carteri (frankincense) with a flash point of 32°C
Citrus oils with flash points around 43°C
Santalum album (Indian sandalwood) with a flash point above 100°C
Cedrus atlantica (Atlas cedarwood) with a flash point of 110°C.
These variations in flash points are observed due to the differing chemical compositions and properties of each essential oil.
This information is essential to consider for various reasons:
When making soap. During the saponification process, soap can reach high temperatures exceeding 85°C. If all the essential oils used in soap-making have low flash points, there is a risk that they may evaporate completely during the process, resulting in minimal scent remaining in the soap. To address this issue, a practical solution is to blend the low flash point essential oils with others having higher flash points. This blending creates a more stable mixture that retains its aroma throughout the soap-making process.
Essential oils with low flash points tend to evaporate quickly when used in baths or vaporisers. To prolong the lasting effect of the scent in a room or during a bath, blending different essential oils can extend the longevity of the fragrance.
Safety during transportation. Essential oils with flash points below 60°C are considered flammable and require a "Flammable Liquid 3" warning sign when being shipped. As a result, couriers may charge higher shipping fees for these dangerous goods, and they cannot be transported by air. Oils like Grapefruit, Sweet Orange, Black Pepper, Peppermint and Chamomile, all have a flash point below 60°C.
Boiling Point
Boiling point refers to the temperature at which a liquid transforms into a gas at atmospheric pressure. It marks the specific temperature when the liquid undergoes a state change into its gaseous form.
Essential oils possess distinct physical characteristics, including their boiling points, which typically range between 160 and 240 °C.
Molecular weight plays a crucial role in essential oils, influencing various aspects such as the extraction method, physical properties like boiling point, and biological properties like absorption into the body.
A volatile substance is characterised by its ability to evaporate easily. Essential oils fall under this category, as they are volatile in nature. Among essential oils, the top notes have the lowest boiling points, causing them to evaporate most readily.
Steam distillation is an effective method when the boiling point of the substance to be extracted is higher than that of water, and heating the starting material to that temperature is not feasible due to decomposition or undesired reactions. Additionally, it proves useful when the desired substance is present in small quantities compared to non-volatile residues. This technique is commonly employed to separate volatile essential oils from plant material. For instance, it can be utilised to extract limonene (with a boiling point of 176 °C) from orange peels.
Different boiling points of individual oil components are utilised in gas chromatography. However, essential oil components tend to elute before reaching their actual boiling points (2).
Spathulenol: Boiling point of 297 °C - elutes between 150 and 200 °C
Limonene: Boiling point of 176 °C, - elutes between 105 and 115 °C.
Incensole acetate: Boiling point of 420 °C - elutes like other components at lower temperatures, but it requires an extended retention time due to its higher boiling point and size.
Sources:
(1) Sue Clarke - Essential Chemistry for Aromatherapy, Second Edition (2009)
(2) Fundamental Chemistry of Essential Oils and Volatile Organic Compounds, Methods of Analysis and Authentication by Nicholas J. Sadgrove, Guillermo F. Padilla-González, Methee Phumthum; DOI:10.1016/B978-0-12-811412-4.00013-8