Assessing the Breadth of Adduct Formation between Flavouring Molecules and Carrier Fluids within Electronic Cigarette Liquid Products

Introduction: A range of flavouring molecules are used in electronic cigarette liquids (e-liquids), some of which have been shown to form cyclic acetal adducts with e-liquid solvent components propylene glycol (PG) and vegetable glycerine (VG). The objective of this study was to identify the range of flavouring molecules which form adducts in e-liquid products. Methods: Common e-liquid flavouring molecules (N=36) from a range of chemical class groups were exposed to PG, VG or methanol and analysed by GC-MS over a timeframe of 4 weeks to identify possible reaction products. Results: Adduct formation was observed, with 14 of the flavouring molecules reacting with methanol, 10 reacting with PG and 9 reacting with VG. Where cyclic PG acetals formed, two diastereoisomeric peaks were observed in GC-MS chromatograms, compared to four peaks for VG acetals. All aldehyde-containing flavouring molecules analysed were reactive with PG, VG and methanol. Adducts formed by reaction with ketone-containing flavouring molecules were identified for the first time. Conclusions: A range of adduct products with unknown toxicological safety form in e-liquids through reaction of flavouring molecules and solvent components. PG and VG acetals are likely to form when any aldehyde flavourings are present in e-liquids. PG and VG solvents may also react with ketone-containing flavouring molecules. Implications: This study has established a range of flavouring molecules which are able to react with solvent components PG and VG in e-liquids under standard storage conditions. These newly identified adducts need to be further assessed to determine their toxicological safety.