25 Drug Discovery, Pharmaceuticals & Cannabis Testing
Figure 3: Determination of the oxidation stability order of different cannabinoids.
Arrhenius dependency and calculation of oxygen consumption
Due to measuring the oxygen uptake of a sample via a corresponding pressure drop in a closed test chamber, a variety of investigation options with the RSSOT are possible. For simple comparison and automatic evaluations of measurement curves, the OxyLogger 100 desktop software is available.
Arrhenius dependency and calculation of activation energy
Typically, the induction period of a sample shows an Arrhenius dependency on the applied test temperature. In our example of the MCT oil you can see the pure comparison of the measurement curves from OxyLogger 100 at different temperatures (120°C, 140°C, 160°C) in Figure 4.
Where
∆[O2 ]t Oxygen consumption pmax Maximum pressure pt
R T Pressure after a specifi ed time
Real gas constant R=8.314 kg • m2 • s(-2) • mol(-1) • K(-1) Measurement temperature
Vtot Volume of test chamber, 28.5 mL Vliq Sample volume
With OxyLogger 100, the result is available automatically. When choosing the calculation with a right click, the calculation is automatically applied at the end point of the measurement. However, every point on the pressure curve that is of interest can be added when chosen. For MCT oil, you can see the OxyLogger 100 calculation, including one additional pressure/time point, in Figure 6.
Figure 5b: Arrhenius graph and calculation of activation energy from measurement at different temperatures of MCT oil containing CBD
Oxygen uptake calculation
Thanks to the closed test chamber and the general measuring principle of the Rapid Small-Scale Oxidation Test, all values for calculating the oxygen consumption according to the following equation are readily available if the volume of the sample (or weight and density in case of solid food products) is known:2
Figure 4: Measurement curves of pure MCT oil at different temperatures.
We carried out the same study with MCT oil containing CBD. For both – the pure oil and the oil with CBD – we plotted the derived IPs in a corresponding Arrhenius graph (see Figure 5) with OxyLogger 100. The software automatically calculated the activation energy for the oxidation reaction from the slope of the respective graph. As you can see below, the activation energy that is necessary is signifi cantly higher for the pure oil without contained cannabinoid (see Figure 5).
Figure 6: OxyLogger 100 calculation of oxygen consumption of MCT oil at 120°C.
Further investigation possibilities: Oxidation Induction Time and shelf-life estimation
The Oxidation Induction Time (OIT) is another way to evaluate the pressure time curve derived from a measurement with RapidOxy 100. It is the point of time where the pressure drop shifts signifi cantly (e.g., when all antioxidants are consumed and the raw product starts to oxidise, or initial peroxides are formed and the oxidation reaction of a sample becomes very rapid). To determine the OIT, you need to typically conduct the measurement until a pressure drop higher then pmax-10% (e.g., to 40%).
Figure 5a: Arrhenius graph and calculation of activation energy from measurement at different temperatures of pure MCT oil
You can estimate shelf life with OxyLogger 100 by extrapolating the Arrhenius graph of the results (IP or OIT can be selected) to room temperature. By default, the software provides the estimate for 40°C, 25°C, and 20°C, but it can be customised to specifi c temperatures used during storage, transport, etc. by simply adding the respective temperature value.
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