Depending on e-liquid fill level, coil resistance and voltage settings, the coil temperatures of e-cigarettes ranges from 110 to 1008 °C^11,13. Typical wetted coil temperature is 200–400 °C, extremely high temperatures (around 1000 °C) have been measured for dry coils without any e-liquid¹⁴. In our experiments, we studied pyrolysis in the typical operating temperature range of e-cigarettes (250–400 °C) under both inert and oxidative conditions. Additional experiments were performed at 500 °C in both atmospheres to test thermal degradation pattern at elevated temperatures.

The composition of CBD pyrolysates obtained in inert and oxidative atmospheres are summarized in Tables 1 and 2, respectively. The obtained data clearly present the thermal instability of CBD. Depending on the temperature and atmosphere, 25–52% of CBD transformed into other chemical substances during the experiments. In the absence of oxygen, 23 pyrolysis products were observed, 15 of which were identified (see Table 1). These compounds represented 81–97% of the pyrolysis products (at lower temperatures, between 250–400 °C, 93–97%). In the presence of oxygen, 22 pyrolysis products were detected (see Table 2), the identified 15 components represent the 88–96% of the degradation products in the studied temperature range. Most of the degradation products (i.e. 18 compounds) appeared among both inert and oxidative conditions. Additional four products were observed exclusively in oxidative, while five degradation products exclusively in inert atmosphere.

The four most intense products, namely Δ⁹-THC, Δ⁸-THC, cannabichromene and cannabinol account for more than 95% of the decomposition products at 250 and 300 °C pyrolysis temperatures in inert atmosphere. Under oxidative conditions an additional product, cannabielsoin appeared. The ratio of the aforementioned five decomposition products is more than 80% under oxidative conditions up to 300 °C. All of these compounds formed by cyclization reaction (see Fig. 1). Cyclization of phenolic flavors to bicyclic compounds under simulated tobacco heating conditions at 300 °C was reported earlier¹⁵. The phenolic O of thymol or ethylvanillin link to a geometrically favorable position of a sterically adjacent side group of the molecule, forming a bicyclic compound. Analogously, in the present case one of the phenolic O of CBD linked to the tertiary carbon of the isopropenyl group, thus forming a sterically favored six-membered ring and resulting Δ⁹-THC molecule. However, Δ⁹-THC formed also under inert atmosphere. The related cyclization can be formally described as an intramolecular Markovnikov addition of the phenolic OH onto the double bond of the isopropenyl group. Thus, no oxidation step is needed by the formation of Δ⁹-THC. The psychoactive Δ⁹-THC was the main detected compound, accounting up to 42% and 70% of the decomposition products under oxidative and inert conditions, respectively, at all applied temperatures. One possible reason of the lower Δ⁹-THC amount in oxidative atmosphere measured in our study could be the higher decomposition rate of the formed Δ⁹-THC in the oxidative ambient. Increased degradation rate of Δ⁹-THC was published in cannabis resin sample exposed to air comparing to that of stored in a sealed plastic bag at ambient temperature¹⁶ indicating the role of oxygen in Δ⁹-THC decomposition.

Among THC isomers, Δ⁸-THC has also psychotropic effects according to the recent review of the Expert Committee on Drug Dependence of the World Health Organization (WHO)¹⁷. The Δ⁸-THC molecule formed by an additional isomerization to the cyclization during thermal treatment. Both Δ⁹-THC and Δ⁸-THC were formed at higher rate in inert atmosphere and at relative lower temperatures (250–300 °C). By increasing the temperature, the relative yield of THC decreased, while other decomposition reactions became more pronounced.

According to earlier studies, cannabinol derives by cyclization, dehydration, and aromatization of CBD, probably through a THC intermediate during long-term storage¹⁶. Notable amount of cannabinol was detected in the present study as a thermal degradation product of CBD. The relative amount of cannabinol was significantly higher in oxidative conditions at each studied temperature, indicating the effect of oxygen in the reaction mechanism. In addition to the driving force of the aromatization in both atmospheres, dehydrogenation by oxygen with water elimination may play additional role in oxidative atmosphere. The most intense cannabinol formation (10.4%) was observed at 400 °C in oxidative atmosphere.

Cannabichromene also formed through cyclization reaction. However, one of the phenolic O of the CBD molecule attacked the double bond of the cyclohexene ring at C substituted with methyl group in this case, while the cyclohexene ring opened up by C–C scission to form the chromene frame. Formation of cannabichromene was more pronounced in inert atmosphere compared to the oxidative condition at 250 °C. At higher temperatures there was no significant difference and the relative yield of cannabichromene was decreased.

Cannabielsoin was only detected under oxidative conditions. In this transformation, one of the phenolic O of CBD linked similarly to the double bond of its cyclohexene ring, but at the secondary carbon, while a hydroxyl group formed at the adjacent C substituted with methyl group via the oxidative medium. The relative intensity of cannabielsoin was around 3%, and its quantity was not much affected in the temperature range of 250–500 °C.

At higher temperatures, the share of the decomposition products formed through cyclization reaction was decreased, while the relative intensity of smaller molecules formed by C–C bond scission increased in the pyrolyzate. These identified products were formed by the scission of the bond connecting the pentylbenzenediol and the p-mentha-1,8-dienyl moieties of the CBD molecule. Menthatriene isomers appeared at 250 °C in oxidative atmosphere, while in inert atmosphere only above 400 °C, indicating that the presence of oxygen promoted the cleavage of the molecule. The relative amount of menthatrienes were the highest at 500 °C in both inert and oxidative atmospheres.