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Optical rotation: application to oils analysis

by | Jan 20, 2022 | Popularization

Alexis St-Gelais, chimiste & Cindy Caron, chimiste – Popularization

Have you ever heard of the term optical rotation or seen it on a certificate of analysis, and wondered what was its purpose? If the answer is yes, you are in the right place to learn more about its use and meaning.

 In this post, we will detail what this parameter is by explaining what is optical rotation, how it is measured, and how it can be used in practice. We will conclude with an example by comparing two samples originally labelled as pure lavender essential oils.

Quick highlights

  • Optical rotation is a generic physicochemical property of organic liquids containing chiral molecules, such as essential oils and vegetable oils.

  • It can be monitored in routine to validate compliance with specifications and sort out odd samples.

  • It is a good addition to any standard certificate of analysis for essential oils, vegetable oils or related products.

What is optical rotation?

This value is an indication of the interaction of the substance or the mixture of substances and a particular kind of light called polarized light.

Polarized light and chiral molecules

In a nutshell, light consists of waves of energy travelling in a given direction. In non-polarized light, these waves oscillate randomly following a variety of planes. For instance, some waves can oscillate in parallel to the horizon, some other are perpendicular to it, and some oscillate in different diagonal directions.

In a polarized light, the waves have been filtered and only those oscillating in a given plane (for example, only those waves oscillating parallel to the horizon) are allowed through the filter (figure 1).

Figure 1. Schematic representation of polarized light. Arrows indicate the plate of oscillation of the energy wave
This polarized light can interact with substances that contain chiral molecules. Molecules featuring this property exist in two different versions that look-alike but that are mirror images of each other. The simplest example to understand chirality is that of your hands. They are identical, but mirror images of the other. If you keep both of your palms turned towards you, you will not be able to superimpose them perfectly. When polarized light moves through a solution that contains chiral compounds in different proportions of each version (in a way, there are more left than right hands or vice versa), those compounds will rotate the plane in which the polarized light oscillates (figure 2).
Figure 2. Effet of a chiral substance on polarized light, shifting the plane of energy waves (represented by arrows).

The magnitude of this effect is specific to each chiral molecule. If a liquid contains several different molecules, the overall effect is the result of the contribution of each of them. Therefore, in a complex organic liquid, as long as the general chemical breakdown remains constant, the resulting optical rotation should fall within a predictable range.

Measuring optical rotation

The polarimeter

The optical rotation is measured with an instrument called a polarimeter. In this apparatus, polarized light of known plane is emitted on one side of the instrument. It then moves through a glass cell of known length that contains the liquid under study (figure 3). The plane of polarization of the resulting light is then measured by a detector at the exit of the glass cell.

Figure 3. Glass cell of the polarimeter containing the substance under study and equipped with a thermometer to monitor temperature.

Unit of the result

Most of the time, the optical rotation is measured at a defined wavelength of yellow light (589 nm), emitted by a sodium light bulb. The instrument reading is normally presented as an angular value (expressed in degrees, as noted by the symbol °) that can either be positive or negative.

The + or − sign must be explicit for this measure.

For example, an essential oil could give a reading of +47.2°, or −3.0°. A plus sign denotes that the polarized light plane has been rotated clockwise (dextrorotary) and a minus sign denotes that the light plan has turned counterclockwise (levorotary).

Sample preparation for optical rotation

It can be necessary to dilute the substance to allow its examination. This is the case when:

  • The substance is opaque, such as for solids or for very coloured substances. Dilution in those cases allows the light to pass through.
  • The sample is costly or rare. Dilution can then reduce the amount of sample needed for the assay.
  • The substance has a very high optical rotation. The value can fall outside the working range of the apparatus. Therefore, dilution can reduce the magnitude of rotation of the raw reading.

When studying a dilute solution, it is commonplace to report the solvent used for dilution as well as the concentration, in g/100 mL (or %), at which the solution was tested. A calculation then allows to correct the raw reading and express the true optical rotation for the pure substance (figure 4).

Figure 4. Interface of the instrument to correct for the concentration of the substance in g / 100 mL.

In addition to the rotation reading, other information can be associated to this measure. It is typical to note the temperature of the substance when the measurement was done (e.g., 21 °C).

Therefore, you could see a result reported in the following way:

+47.2° (21 °C, methanol, c = 1.4)

This means that the substance induces a dextrorotary rotation of polarized light of 47.2 ° when measured at 21 °C, a reading that was obtained from a solution of the substance at a concentration of 1.4 g / 100 mL in methanol with proper correction to account for the dilution.

Optical rotation, essential oils and vegetable oils

The overall chemical composition of complex organic liquids such as essential oils or vegetable oils for a given species or variety is always similar. The resulting optical rotation should therefore always fall within a predictable range.

Existing standards

For many essential oils and some vegetable oils like castor oil, international or pharmacopoeia standards have been assembled over time. Many of them will include an expected optical rotation value interval, to which an individual result can then be compared for a routine compliance check.

How to proceed in the absence of a standard?

Distilleries and distributors can, of course, implement their own specification, especially for those species where the data is not covered by a standard. The range can be defined by monitoring over time the values obtained for various production batches and deducing a tolerance interval.

Therefore, if one knows what the typical reading should be for a normal substance, the measure of the optical rotation for a given batch can then be compared to the expected range for this substance. If the value is in accordance with the specification, this is one (of many possible) indication that the sample is also normal. On the contrary, an odd optical rotation value can indicate that there is a problem with the batch.

Example of application of optical rotation: lavender essential oils

We have examined two reportedly pure lavender essential oils, of undisclosed geographical origins.

The ISO 3515:2004 standard for Lavandula angustifolia essential oil of “other origins” states that, the optical rotation should be comprised between −12° and −6°.

Oil A gives a reading of −9.1° (figure 5), whereas oil B rather scores +1.1°.

Figure 5. Reading of the polarimeter for lavender essential oil A.

From these values, it is clear that sample B is irregular.

Further examination by GC reveals that sample B contains several synthetic ingredients and that this sample would better be described as a lavender artificial fragrance than as a pure essential oil.

In this example, optical rotation is therefore a parameter and a tool that allows to differentiate a fragrance from an actual essential oil.