Functional characterization of flavorings in electronic cigarette refill liquids by nuclear magnetic resonance spectroscopy

Abstract

The chemical content of electronic cigarette liquids (e-liquids) was determined by NMR spectroscopy. The study aimed to quantify the abundance of propylene glycol, glycerol and nicotine and reconcile the functional types of non-exchangeable hydrogen of flavorings in e-liquids. Propylene glycol, glycerol, nicotine, menthol, vanillin and benzaldehyde were detected in e-liquids by 1D and 2D-NMR spectroscopy. Short- and long-range coupling of carbonyl, aryl and alkyl hydrogen was also observed by 2D ¹H–¹H and ¹H–¹³C NMR. Propylene glycol and glycerol (from 30 : 70 to 65 : 35) accounted for more than 97% of e-liquids by mass with nicotine levels comparable to those reported by the manufacturer. E-liquids were classified into five clusters based on the relative distribution of non-exchangeable hydrogen types in flavorings. The mean relative abundance of flavorings ranged from 2.4 to 4.7% for the five clusters. Alkyl hydrogen was dominant in clusters II–V, representing 35–55% of non-exchangeable hydrogen, followed by saturated oxygenated hydrogen (32–33%). In cluster I, saturated oxygenated hydrogen was predominant. The highest abundance of aryl hydrogen was estimated for clusters I and IV, while carbonyls accounted for up to 1.5% of non-exchangeable hydrogen. Tobacco-flavored e-liquids had an average flavoring molar ratio of 1.8% with saturated oxygenated hydrogen being the predominant group with considerable quantities of aryl (6.8%) and carbonyl (1.5%) hydrogen atoms. Overall, this analysis allowed for the qualitative and quantitative determination of flavorings in e-liquids. This is needed to improve protocols aiming to determine the chemical composition of e-liquids and the feasibility of toxicological investigations.