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

Abstract

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 (α FB ) of 0.34. Nicotine was further protonated by acidification with equimolar addition of benzoic acid (α FB = 0.17 at pH 6.2). By contrast, the degree of nicotine protonation at the particle surface was significantly lower, with 0.72 < α FB < 0.80 in the same pH range. The presence of propylene glycol and glycerol completely eliminated protonation of nicotine at the surface (α 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.

Quantification of Free Radicals from Vaping Electronic Cigarettes Containing Nicotine Salt Solutions with Different Organic Acid Types and Concentrations

“…pH measurements of e-liquids followed the protocols reported in previous studies. ,,, The acidity and basicity of e-cigarette solutions have also been measured with other techniques such as proton NMR and X-ray spectroscopy . The pH values discussed here do not necessarily represent absolute hydronium activity in solution .…”

“… 11 , 32 , 34 , 35 The acidity and basicity of e-cigarette solutions have also been measured with other techniques such as proton NMR 36 and X-ray spectroscopy. 37 The pH values discussed here do not necessarily represent absolute hydronium activity in solution. 13 Instead, they are discussed within a relative context in this work and are provided to extrapolate our results to commercial e-liquids with proprietary formulations for which pH values have been measured using similar methods.…”

Probing Complex Chemical Processes at the Molecular Level with Vibrational Spectroscopy and X-ray Tools

“…(B) N K-edge NEXAFS spectra of aerosols generated by nebulizing aqueous solutions containing 0.5 M nicotine at pH 2, 6, and 10.4. Adapted from ref . Copyright 2023 American Chemical Society.…”