This work was supported by the US National Institutes of Health (grant R01ES025257). Research reported was supported by the National Institute of Environmental Health Sciences and the Food and Drug Administration Center for Tobacco Products.
Introduction: The distribution of nicotine among its free-base (fb) and protonated forms in aerosolised nicotine affects inhalability. It has been manipulated in tobacco smoke and now in electronic cigarettes by the use of acids to de-freebase nicotine and form ‘nicotine salts’.
Methods: Measurements on electronic cigarette fluids (e-liquids) were carried out to determine (1) the fraction of nicotine in the free-base form (α fb) and (2) the levels of organic acid(s) and nicotine. Samples included JUUL ‘pods’, ‘look-a-like/knock-off’ pods and some bottled ‘nicotine salt’ and ‘non-salt’ e-liquids.
Results: α fb= 0.12 ±0.01 at 40°C (≈ 37°C) for 10 JUUL products, which contain benzoic acid; nicotine protonation is extensive but incomplete. Discussion: First-generation e-liquids have α fb ≈ 1. At cigarette-like total nicotine concentration (Nictot) values of ~60 mg/mL, e-liquid aerosol droplets with α fb≈ 1 are harsh upon inhalation. The design evolution for e-liquids has paralleled that for smoked tobacco, giving a ‘déjà vu’ trajectory for α fb. For 17th-century ‘air-cured’ tobacco, α fb in the smoke particles was likely ≥ 0.5. The product α fbNictot in the smoke particles was high. ‘Flue-curing’ retains higher levels of leaf sugars, which are precursors for organic acids in tobacco smoke, resulting in α fb ≈ 0.02 and lowered harshness. Some tobacco cigarette formulations/designs have been adjusted to restore some nicotine sensory ‘kick/impact’ with α fb≈ 0.1, as for Marlboro. Overall, for tobacco smoke, the de-freebasing trajectory was α fb ≥ 0.5 → ~0 →~0.1, as compared with α fb= ~1 →~0.1 for e-cigarettes. For JUUL, the result has been, perhaps, an optimised, flavoured nicotine delivery system. The design evolution for e-cigarettes has made them more effective as substitutes to get smokers off combustibles. However, this evolution has likely made e-cigarette products vastly more addictive for never-smokers.
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Duell, A. K., Pankow, J. F., & Peyton, D. H. (2019). Nicotine in tobacco product aerosols: ‘It's déjà vu all over again.’ Tobacco Control, tobaccocontrol-2019–055275.