Aerosols from e-cigarettes were generated using a smoking/vaping device specifically conceived and designed for the study (Institute for Work and Health, Lausanne, Switzerland) (Fig. 4). It is a three-channel linear piston-like smoking machine with adjustable puffing frequencies and volumes, controlled by a computer interface. Cartridges and special traps were placed between the clearomizer and the suction syringe-pump to trap volatile organic compounds (VOCs), carbonyls and cannabinoids in the generated aerosol. In a first step, the syringe pump draws aerosol through the clearomizer and then through the sampling cartridge. In a second step, the aerosol moves toward an outlet pipe. The sampling cartridge is located close to the mouthpiece (<5 cm) to limit the loss of aerosols by impaction or condensation inside the tubing. In this study, the smoking machine was set to generate 70 mL puffs with a frequency of 3 puffs per minute19. To prevent overheating of the clearomizer caused by too frequent ignition, three e-cigarettes were used alternately in one minute. The puff duration, defined as the time during which the e-cigarette push button is pressed, was 3 seconds. Given that the puff duration is an essential parameter that determines the aerosol temperature, the e-cigarette button was pressed by an electric actuator to ensure precise timing control. The temperature of the e-liquid near the coil was monitored at a sampling rate of 3 Hz (type k thermocouple and MAX31855 MAXIM data logger). The temperature sensor was located inside the wick fibers, near the center of the coil without direct contact with the coil. The electric current through the coil was monitored by a current sensor (INA169 Analog DC Current Sensor) connected between the battery and the coil, at a sampling rate of 3 Hz.
Figure 4: Operating diagram and picture of the vaping device used to sample the gases and aerosols generated by the e-cigarettes.
((A) Diagram of the main parts of the vaping device with a focus on the temperature sensor location, (B) Photograph of the linear actuator, the glass syringe, and the first e-cigarette channel).
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Volatile organic compounds (VOCs) analysis
To sample VOCs, charcoal sorbent tubes (SKC Anasorb CSC 400/200) were inserted between the e-cigarette mouthpiece and the corresponding electric valve (see Fig. 4). The tubes were removed after 20 cycles of 70 mL puffs at 3 seconds per puff, with a frequency of 2 puffs per minute. After collection, the front and the back of each tube were desorbed in 5 ml of CS2 (Sigma Aldrich, Buchs, Switzerland).
All of the calibration points were prepared by spiking the front of the charcoal sorbent tube. In this study, only the front was analyzed, assuming that none of the tube was saturated because all of the sample contents were always below the highest calibration point of the measured compound.
A first screening of VOCs was performed by gas chromatography (HP Agilent 6890) coupled to a mass spectrometer (HP Agilent 5973). Compounds identified with mass spectra libraries were quantified by gas chromatography coupled to a flame ionization detector (GC-FID) (Varian 3800, column CP-SIL 8CB 60 m, 0.25 mm i.d., 0.25 μm film thickness), split set to 20%, an injection temperature of 200 °C and an oven temperature gradient of 50 °C to 165 °C. Five standards were prepared from 200 to 900 μg/tube for each compound. Due to the high expected amount of VOCs and to prevent saturation of the solvent, tobacco and cannabis samples were desorbed in 10 ml of CS2.
Phenols and propylene glycols were analyzed by GC-FID (Agilent 6890, column Supelcowax 30 m, 0.25 mm i.d., 0.25 μm film thickness), split set to 20%, with an injection temperature of 250 °C and an oven temperature gradient of 160 °C to 200 °C. Five standards were prepared from 0.2 to 1.1 mg/tube for phenols and 0.2 to 25 mg/tube for propylene glycol.
Regarding VOCs sampling, carbonyls were trapped using LpDNPH S10 cartridges (Supelco, 350 mg, 3 mL) coated with 2,4-dinitrophenylhydrazine (2,4-DNPH). For tobacco and cannabis cigarettes, the LpDNPH H30 cartridges (Supelco, 1 g, 6 mL) were used to avoid overloading the cartridge carbonyl capacity. To sample, cartridges were also inserted between the e-cigarette mouthpiece and the corresponding electric valve (Fig. 4). To quantify carbonyls, the method was based on the reaction of carbonyls with 2,4-DNPH to form the corresponding hydrazone derivative. After sampling, the cartridges were desorbed using 3 ml of acetonitrile. Eluate was filtered using syringe filters with PTFE membrane (Acrodisc CR 13 mm PN 4422T HPLC certified), and the volume was adjusted to 3 ml with acetonitrile in a 4 ml glass vial. Five calibration points were prepared in acetonitrile at concentrations ranging from 1.6 to 53 ng/μl. Samples were analyzed by HPLC (Varian ProStar Model 410) connected to a UV detector (Varian ProStar Model 335). Carbonyls were separating using a Spherisorb ODS 1 column (150 × 4.6 mm, 3 μm film thickness) at a rate of 0.8 ml/min, and kept at 30 °C. The gradient elution was obtained by mixing two eluents: A) methanol/water (5:5, v/v); and B) methanol. The initial proportion of solvent A was 100% for 3 minutes; then, the proportion of solvent B linearly increased up to 100% in 27 min and was maintained for 5 min. The injected volume was 10 μl, and the UV spectra were set at 365 nm.