UC Davis

Electronic cigarettes (e-cigarettes) are battery-operated devices for nicotine delivery that operate by vaping or aerosolizing an “e-liquid” that contains propylene glycol (PG), vegetable glycerin (VG), nicotine, and different flavoring chemicals. E-cigarettes have been regarded as a “less-harm” alternative to combustible tobacco cigarettes, and their worldwide market share has been increasing exponentially in recent years. In the e-cigarette device vessel, the e-liquid is heated by an atomizer (metal coil) to create an e-cigarette aerosol mixture, which includes both gas and particle phases. Both nicotine-based and cannabinoid-based e-liquids are common in e-cigarette use. Due to their relative novelty, e-cigarettes have not been subject to significant regulatory action until the outbreak of e-cigarette or vaping use-associated lung injury (EVALI) starting from 2019 that killed more than 60 people. In the EVALI outbreak, cannabis vapes (mainly from extracted tetrahydrocannabinol oil) that were adulterated with vitamin E acetate (VEA) in the black market are thought, but not yet confirmed, to be causal agents. After the EVALI outbreak, e-cigarette flavors were banned in closed-tank systems.

During the heat-induced aerosolization process of e-liquid, many thermal degradation products have been identified and characterized (e.g., formaldehyde, acetaldehyde, acetone) that are produced from the thermal degradation of PG and VG, as well as flavorant mixtures. Previous research studies indicate that the production of thermal degradation products depends on puff regimen, coil temperature, e-liquid composition, and possibly other factors. However, knowledge gaps still exist regarding the large variety of thermal degradation products that remain unidentified or unquantified, and the intrinsic relationship between actual coil temperature and e-liquid composition to the thermal degradation of e-liquid. In addition, the thermal degradation mechanism of VEA and THC is still unknown.

In this work, high performance liquid chromatography (HPLC) coupled with electrospray ionization (ESI) high resolution mass spectrometry (HRMS) are used for the chemical analysis of thermal degradation carbonyl compounds and organic acids. Both carbonyls and acids are derivatized with 2,4-dinitrophenylhydrazine (2,4-DNPH) prior to mass spectrometry analysis. A novel theoretical chemistry model was developed to predict the analytical sensitivities of carbonyl(acid)-DNPH derivatives in ESI negative mode for the analyte compounds for which corresponding DNPH derivatives standards are unavailable. This characterization method enabled an untargeted analysis and the most comprehensive picture, to date, of the carbonyls and acids that are generated from both PG/VG and VEA/THC vaping systems. Over 40 thermal degradation carbonyls and acids were characterized from the thermal degradation of PG, VG, VEA and THC, while nearly 20 cannabinoids and derivatives were also identified by the same methods. PG, VG, and VEA were analyzed by gas chromatography to enable mass closure for the aerosolization process.

Moreover, this work systematically studies how changing the vaping parameters, including coil temperature, e-liquid composition and puff regimen, alters the production of aerosol mass and carbonyl degradation products. The thermal degradation mechanism of PG and VG is proposed from the results, including differences between heat-induced dehydration and oxidant-induced decomposition. The thermal degradation chemistry of THC and VEA is also studied, and the corresponding mechanisms proposed. In summary, this work provides important chemical-specific information that may be helpful for the fundamental understanding of chemistry in e-cigarettes and for guiding regulatory action. Corresponding toxicology and in vivo studies are needed to further evaluate the health risk of e-cigarette use.