Fraction of Free-Base Nicotine in Simulated Vaping Aerosol Particles Determined by X-ray Spectroscopies

A new generation of electronic cigarettes is exacerbating the youth vaping epidemic by incorporating additives that increase the acidity of generated aerosols, which facilitate uptake of high nicotine levels. We need to better understand the chemical speciation of vaping aerosols to assess the impact of acidification. Here we used X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe the acid-base equilibria of nicotine in hydrated vaping aerosols. We show that, unlike the behavior observed in bulk water, nicotine in the core of aqueous particles was partially protonated when the pH of the nebulized solution was 10.4, with a fraction of free-base nicotine (alpha FB) of 0.34. Nicotine was further protonated by acidification with equimolar addition of benzoic acid (alpha FB = 0.17 at pH 6.2). By contrast, the degree of nicotine protonation at the particle surface was significantly lower, with 0.72 < alpha FB < 0.80 in the same pH range. The presence of propylene glycol and glycerol completely eliminated protonation of nicotine at the surface (alpha FB = 1) while not affecting significantly its acid-base equilibrium in the particle core. These results provide a better understanding of the role of acidifying additives in vaping aerosols, supporting public health policy interventions.

Automated summary

A new generation of electronic cigarettes with acidifying additives exacerbates the youth vaping epidemic. The behavior of nicotine in vaping aerosols was studied using X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. It was found that nicotine in the core of aqueous particles is partially protonated and further protonated by acidification. However, the degree of nicotine protonation at the particle surface is significantly lower. The presence of propylene glycol and glycerol completely eliminates nicotine protonation at the surface.