Optical Rotation Test as Quality Indicator for Essential Oil
Have you ever seen Optical Rotation (OR) listed on the COA (Certificate of Analysis) and wonder what it means? This article will explain the importance of Optical Rotation in determining the quality of Essential Oil.
Light consists of waves of energy that travels in certain direction. Light can oscillate randomly following various planes which is referred to unpolarized light. For instance, some waves can oscillate in parallel or perpendicular to the horizon and in different diagonal directions. Meanwhile, polarized light is a state where the waves have been filtered and oscillates in a certain plane that are allowed through the filter .
Light that can interact with substances containing chiral molecules is polarized light. Chiral molecules display properties in two different versions that are similar but mirror image of each other. A simple example is our hands, they are identical (shape, size, color), but mirror images of the other. In other words, when both of your palms are directed at the same thing and superimposed, it is impossible to be perfectly superimposed .
Optical Rotation and Polarimeter
Optical rotation is the value that indicates the interaction between the sample and the polarized light . We can check the Optical Rotation of the sample, including essential oil, using Polarimeter.
Essential oil is a complex mixture of many chemical compositions which are optically active which make this Optical Rotation measurement possible. The overall chemical composition of a given type of essential oil is always similar and the Optical Rotation reading should fall within a predictable range. Any changes in the overall chemical composition will affect the Optical Rotation .
The light source emits light through a polarizer where the angle of the light is restricted. Then, the angle where light rotates as it passes through the measured sample is calculated. When the plane facing the light source appears to rotate clockwise, it is called dextrorotatory (from Latin = “dexter” which means right). On the other hand, if the plane rotates counter-clockwise during measurement, it is called levorotatory (from Latin = “laevus”, means left). The term for representing optical isomers (enantiomers) d- or l-, is derived from these words .
The result of the Polarimeter is usually presented as an angle value (expressed in degrees, symbol o) which can be positive or negative. The positive sign (+) indicates that the plane of polarized light has been rotated clockwise (dextrorotatory). While the minus sign (-) indicates that the the plane has rotated counter-clockwise (levorotatory).
The importance of Optical Rotation in Determining Quality of Essential Oil
The word “essential” in essential oil refers to the “essence of” the aroma of the original plant, for example Patchouli oil from Patchouli leaves which is widely use in the perfumery and cosmetic industries. Essential oil is a complex mixture whose composition can be affected by every step of production, from growing and harvesting conditions of the plants to extraction methods and storage condition. This creates a challenge in grading the quality of essential oil. In addition, approximately 80% of the essential oils on the market that claim to be pure and natural are somehow adulterated or diluted or mixed.
So, how can one be sure that the particular batch of essential oil is pure? The good news is that plants of any given species and origin produces the same enantiomeric ratio, which can be measured. The Optical Rotation reading can be used as one indicator in the authentication in essential oils. For example, citrus has d-limonene while pine has l-limonene. They have the same physicochemical properties, but very different biological and organoleptic properties. The chiral analysis can detect if mixing of other species or the addition of nature-identical synthetic compounds in essential oil is done .
At Mitra Ayu, our internal Quality Control Lab runs the Optical Rotation analysis on all of our Essential Oil products. Below are some examples of the Optical Rotation readings for Nutmeg oil, Patchouli oil and Massoia Bark oil.
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