TG analysis of the cannabis material
For a more thorough identifying of the decarboxylation reaction, it is important to understand the weight loss. In the first experiment, Type 1 sample was used to monitor THCA decarboxylation. The derivative TG (DTG) curve depicted the weight loss of sample as a function of temperature. The experimental results of the decarboxylation process for THCA to THC using different heating rates are shown in Fig.
2. In this case, Fig.
2 (a) suggests that the maximum mass change is not as clear as the temperature increase of the experiment. The entire weight loss was described as a continuous process with a temperature range of 30 to 180°C. The beginning of the starting mass loss of the decarboxylation reaction was observed at around 100°C, which is higher than reported in the literature [
8]. The observed mass loss was integrated at temperatures as high as 180°C, resulting in a 13.17% weight change. At 2°C/min heating rates, DTG curve indicates a more complex chemistry reaction of the THCA to THC interconversion. This could be attributed to slower heat transfer rates under lower heating rates, and led to an accelerated occurring of undetermined side reactions.
The maximum mass change is clearly shown in Fig. 2 (b, c, d, and e) as the heating rates and temperature of the experiments increase. The results show that heating rates are directly related to the maximum rate temperature of THCA to THC conversion. The temperature of the maximum reaction rate ranged from 142.03 to 156.44°C as the heating rates increased, and it was discovered that the residual mass began to decrease. The results show that the higher energy flux introduced at higher heating rates significantly increased the conversion temperature of the cannabis samples. It should be noted, however, that the heating rates have an effect on the onset of the initial mass loss of the THCA to THC. The decarboxylation onset temperature ranges from 105 to 125°C. Furthermore, the side reactions were significantly decreased.
To compare the temperature of the maximum reaction rate in the cannabinoid composition of cannabis plant. The results of the second experiment for the CBDA to CBD weight loss process are shown in Fig. 3. Despite the fact that both samples were tested under identical condition, it was discovered that the temperature of the maximum reaction of the type 2 sample was significantly lower than that of the type 1 sample. However, DTG curve observed a similar changes of weight loss process at the same heating rate. The process conditions (temperature, cannabinoid composition, heating rate) that affect the decarboxylation process vary significantly. These results suggest the complex matrix of the cannabis plant (e. g. cannabinoids, non-cannabinoids, cellulase, xylanase) that could significantly alter the decarboxylation reaction.
HPLC determination of cannabinoids
The HPLC analysis on cannabis sample collected after TG analysis at 2–10°C/min heating rates range, confirmed decarboxylation conversion of THCA to THC and CBDA to CBD (Fig. 4). The cannabinoids content in the non-thermally treated cannabis sample was also obtained by HPLC. The results confirmed the THC dominant type or CBD dominant type of the strain.
The cannabis sample subjected to TG measurement at different heating rates in the range of 30–180°C showed different decarboxylation process according to the HPLC analysis. The type 1 sample after performed TG run showed THCA/THC ratio ≈ 1, evidencing incomplete decarboxylation reaction at these heating rates range (Fig. 5a). A significant loss of THCA and THC concentration was observed at elevated heating rates. Nevertheless, no any degradation products were detected by HPLC. On the contrary, the type 2 sample showed complete decarboxylation proven by the absence of CBDA according to the HPLC analysis (Fig. 5b). The decarboxylation conversion efficiency of CBDA was the highest at 6°C/min. The analytical method for determining the conversion of decarboxylation see the ESI. Furthermore, during this process, there is almost no decarboxylation reaction of THCA. The results showed that the composition variability of the cannabis plant is another variable to be considered, since it will influence the heat transfer mechanism and decarboxylation reaction kinetic. Cannabis plant with lower THCA content show higher activation energy (Ea), when compared with high THCA content cannabis plant. The results indicate that more THCA dissolves in the cannabis material matrix and that the thermodynamics of the reaction can be significantly altered by the presence of various chemicals and enzymes.