Aerosol droplet-size distribution and airborne nicotine portioning in particle and gas phases emitted by electronic cigarettes

Abstract

The reliable characterization of particle size distribution and nicotine delivery emitted by electronic cigarettes (ECs) is a critical issue in their design. Indeed, a better understanding of how nicotine is delivered as an aerosol with an appropriate aerodynamic size is a necessary step toward obtaining a well-designed nicotine transfer from the respiratory tract to the bloodstream to better satisfy craving and improve smoking cessation rates. To study these two factors, recent models of EC devices and a dedicated vaping machine were used to generate aerosols under various experimental conditions, including varying the EC power level using two different types of atomizers. The aerodynamic particle sizing of the resulting aerosol was performed using a cascade impactor. The nicotine concentration in the refill liquid and the aerosol droplet was quantified by liquid chromatography coupled with a photodiode array. The vaporization process and the physical and chemical properties of the EC aerosol were very similar at 15 watts (W) and 25 W using the low-power atomizer but quite distinct at 50 W using the high-power atomizer, as follows: (1) the mass median aerodynamic diameters ranged from 1.06 to 1.19 µm (µm) for low power and from 2.33 to 2.46 µm for high power; (2) the nicotine concentrations of aerosol droplets were approximately 11 mg per milliliter (mg/mL) for low power and 17 mg/mL for high power; and (3) the aerosol droplet particle phase of the total nicotine mass emitted by EC was 60% for low power and 95% for high power. The results indicate that varying the correlated factors (1) the power level and (2) the design of atomizer (including the type of coil and the value of resistance used) affects the particle-size distribution and the airborne nicotine portioning between the particle phase and the gas phase in equilibrium with the airborne droplets.

Figure 1

The universal system for analysis of vaping (U-SAV) machine. (a) The U-SAV machine allows the user to plug in a wide range of commercially available atomizers and batteries. (b) The atomizers can be tilted from 0° to 90°.

Figure 2

The experimental set-up for determining the particle-size distribution of aerosol generated by an electronic-cigarette. The emissions were generated by the U-SAV vaping machine and the particle-size distribution was measured by collecting and classifying the particles using a DLPI cascade impactor. The e-liquid collected on each stage of the cascade impactor was analyzed for nicotine concentration and density. Red arrow (1) indicates path of fresh air into the U-SAV machine. Green arrow (2) indicates path of aerosol particles through tubing from the U-SAV machine to the cascade impactor. cm centimeters, DLPI DEKATI low pressure impactor, LC–MS liquid chromatography–mass spectrometry, L/min liters per minute, MMAD mass median aerodynamic diameter, U-SAV universal system for analysis of vaping.

Figure 3

DLPI-collected data showing the effect of varying the power level on the frequency mass distribution of the e-liquid in an electronic cigarette. The results using the 15/25-W conditions (using low-power atomizer technology), and 50-W conditions (using high-power atomizer technology) are represented by a dot, triangle, and square, respectively. Experiments were carried out with an e-liquid composed of 50% propylene glycol and 50% vegetable glycerin with a nicotine concentration of 18 mg/mL (n = 5). DLPI DEKATI low pressure impactor, µm micrometers, mg/mL milligrams per milliliter, W watts.

Figure 4

DLPI-collected data showing the effect of power level on the nicotine mass distribution. The results from using the 15 W, 25 W, and 50 W power levels are represented by a dot, triangle, and square, respectively. Experiments were carried out with an e-liquid composed of 50% propylene glycol and 50% vegetable glycerin with a nicotine concentration of 18 mg/mL (n = 5). DLPI DEKATI low pressure impactor, µm micrometers, mg/mL milligrams per milliliter, W watts.

Figure 5

Portioning of nicotine in droplets of e-liquid during the vaporization process by power level (and, indirectly, by atomizer technology). The proportion of nicotine in the particle phase is obtained by a calculation that is based on the experimental data about e-liquid recovery and nicotine concentration in droplets. The result is an estimate by the research team and not a measurement. MMAD mass median aerodynamic diameter, µm micrometers, mg/mL milligrams per milliliter, W watts.