Experimental investigation of glycerol byproducts in electronic cigarettes: A molar conversion rate approach

This study quantifies the conversion behaviour of hazardous aldehydes formed by the thermal dehydration of glycerol in electronic cigarette devices. The analysis focuses on acrolein, acetaldehyde, and formaldehyde, generated during controlled vaporisation experiments conducted with both pure glycerol and commercial e-liquids. Aerosols were produced using a customised vaping machine operated at various power outputs (10–80 W), airflow rates (1.1–13 L/min), and coil configurations, including cotton and ceramic materials. The molar conversion rate, defined as the number of moles of aldehyde produced per mole of glycerol vaporised, was introduced as a characteristic process variable linking operating conditions to chemical degradation. Under optimal conditions corresponding to nucleate boiling, conversion rates remained low and stable — approximately 15 µmol/mol for acrolein, 14 µmol/mol for acetaldehyde, and 76 µmol/mol for formaldehyde. In contrast, operation in the film boiling regime led to exponential increases, with conversion rates up to 10 to 100 times higher. Devices equipped with ceramic coils did not exhibit this increase, indicating improved thermal regulation. This molar conversion rate framework advances current emission assessment methods by normalising chemical yields to vaporised mass, thereby allowing meaningful comparison between devices, operating powers, and e-liquid compositions. It also provides a process-based basis for interpreting emissions in relation to precursor vaporisation and device operation. Finally, distinguishing boiling regimes is shown to be critical for device reliability and safety, as the transition from nucleate to film boiling marks the onset of overheating and sharp increases in toxicant formation.