Analytical chemistry challenges in electronic cigarettes studies

Abstract

Electronic cigarettes (ECIGs) are battery-powered devices that generate inhalable aerosol by vaporizing liquids consisting of propylene glycol (PG), vegetable glycerin (VG), nicotine, additives, and flavoring compounds. Since their introduction, ECIGs have gained wide popularity that continues to rise, reaching an epidemic level among youth. In line with this popular surge, research has intensified to address the knowledge gaps in ECIG toxicity by determining toxins in the main liquid constituents and their pyrolysis products in the remaining liquids, aerosols, and gases. This work came with many challenges that are mainly due to the presence of non-aqueous viscous carriers consisting of PG and VG mixtures at different ratios. In this thesis, after giving a small introduction about the work in the first chapter, a systematic review was presented in the second chapter. This review highlights the challenges faced by researchers when detecting the main constituents of the e-liquids (nicotine and flavors), additives (caffeine, synthetic cannabinoids, and pyrazine), contaminants (tobacco-specific nitrosamines and emerging chemicals of concerns), and aerosols emissions (carbonyls, ³benzene, toluene, ethylbenzene, and xylene known as BTEX, polyaromatic hydrocarbons, and phenols). In the third chapter, we present a novel method based on using headspace gas chromatography to determine the volatile flavorants in the e-liquid while minimizing the interferences that are usually generated from the associated matrix. In particular, the presented method is used to quantify menthol and other menthol-derived flavoring compounds in mentholated JUUL pods samples (e-liquids). Three different brands: Menthol, Classic Menthol, and Cool Mint that were purchased in different years 2017, 2018, and 2020 were tested. Results showed different concentrations of flavoring compounds in the various tested pods indicating a change in the chemical composition of the JUUL mentholated pods after the flavor enforcement policy imposed by the US Food and Drug Administration (FDA) in 2020. The following chapter included both a review of the different methods that were used to calculate the ratio of the free base nicotine (fb) of nicotine in the different e-liquid mixtures and the development of a new method for the same purpose based on potentiometric titrations and the calculation of physical and chemical parameters of the different solutions. In fact, the determination of the fraction of fb was found important to policy makers as the prevalence of the fb form in ECIGs has been associated with the harshness sensory of inhalable aerosols. This work culminated in developing a chemical-based mathematical model that can predict the fraction fb after imputing a few parameters like the dielectric constant of the e-liquid and the concentration of nicotine on the label. The outputs of the model were compared to the fraction fb that has been reported in the 3 literature based on liquid-liquid extraction (LLE) and 1H NMR methods. The reported and computed values were highly correlated with a percentage difference ranging between 1.41-2.57% for 1H NMR and 6.64-9.13% for LLE. The conclusion presented in the last chapter, included a summary of the challenges associated with working with non-aqueous media, the description and results of the menthol containing products, the list of the new physical and chemical parameters that were determined for the first time and were used in the predictive model in addition to proposing a future work plan.