What do young people know about the nicotine in their e-cigarettes?

Introduction: In recent years, the nicotine in e-cigarettes has been available in either a 'free-base' (unprotonated) or 'nicotine salt' (protonated) form. Additionally, e-cigarette nicotine can be either 'synthetic' or 'tobacco-derived'. These dimensions of nicotine have implications for nicotine absorption, bioavailability and sensory experiences.