The purpose of this study is to examine the influence of nicotine flux, and nicotine form, on the rate and dose of nicotine delivery obtained from arterial blood measurements. This study involves a 4 x 2 crossover experimental design of four nicotine fluxes: 9, 18, 27, 35 μg/sec, and two nicotine forms (i.e., free-base and protonated). These nicotine fluxes are within the range reported for ENDS (3.1-111µg/sec). The investigators will isolate the effect of form on the pharmacokinetics of nicotine delivery by controlling for puffing behavior. The investigators will hold puff topography constant by controlling puff duration, inter-puff interval, and number of puffs using the LabVape PTL and confirm the data using eTop.

The flux/form conditions will be tested by participants in two lab visits separated by three weeks to minimize carryover effects. All sessions will be double-blinded. In the first visit, participants will use the ENDS device with one nicotine form and four fluxes in random order. Participants will be instructed to attend the lab for a second visit, to test the four fluxes but with the other nicotine form. The second visit will allow us to isolate the effect of nicotine form on nicotine delivery. The order of nicotine form in the two visits will be counter-balanced across participants. Outcome measures include arterial blood nicotine delivery, and puff topography.

Participants will be instructed to use the Subox mini C ENDS device connected to the LabVape PTL (which limits puff duration to 3 seconds) in four bouts separated by a 60min resting period. The e-liquids vaped are as follows: Propylene glycol(PG)/Glycerol (VG), 30/70 ratio by volume. Nicotine at different concentrations: (2, 4, 7 and 10mg/mL) Benzoic acid: approximately 1:1 molar ratio with nicotine. All these ingredients will be used to prepare the needed e-liquids to conduct the study.

All bouts will be directed; each bout will consist of 3 puffs in which puff duration is fixed to 3sec, as in 50, and inter-puff interval fixed to 30sec (CORESTA recommended method Nº 81).123 The puff duration and inter-puff interval will be fixed using the LabVape PTL. A puff topography device (eTop) will record the puffing topography to identify any deviation between directed and actual puffs drawn and to measure the puffing flow rate. Participants will be trained to follow the puffing cues prior to sampling using an unpowered ENDS device. For arterial blood sampling, a radial arterial line will be placed on the non-dominant side to provide access to blood samples during vaping sessions. Each blood sample (0.5cc) will be drawn manually by a trained nurse at every time point and stored in the freezer at -25°C. Blood nicotine samples will be assayed using LCMS/ MS with deuterated internal standards, as in 114. Blood will be sampled 30sec prior to the initial puff, 5, 15, and 25sec after each puff, and 60sec after the last puff of a bout. The obtained data will be used to calculate pharmacokinetic parameters of nicotine delivery under each condition (Cmax, Tmax, dCi/dt, and AUC). AUC from 0 to 160min for the four 3-puff directed bouts will be estimated using a non-compartmental model and trapezoidal rule. All measures will be corrected for baseline values by subtracting the blood nicotine concentrations by the initial value (at the start of each bout).