Chemical Analysis of Exhaled Vape Emissions: Unraveling the Complexities of Humectant Fragmentation in a Human Trial Study

Abstract

Electronic cigarette smoking (or vaping) is on the rise, presenting questions about the effects of secondhand exposure. The chemical composition of vape emissions was examined in the exhaled breath of eight human volunteers with the high chemical specificity of complementary online and offline techniques. Our study is the first to take multiple exhaled puff measurements from human participants and compare volatile organic compound (VOC) concentrations between two commonly used methods, proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) and gas chromatography (GC). Five flavor profile groups were selected for this study, but flavor compounds were not observed as the main contributors to the PTR-ToF-MS signal. Instead, the PTR-ToF-MS mass spectra were overwhelmed by e-liquid thermal decomposition and fragmentation products, which masked other observations regarding flavorings and other potentially toxic species associated with secondhand vape exposure. Compared to the PTR-ToF-MS, GC measurements reported significantly different VOC concentrations, usually below those from PTR-ToF-MS. Consequently, PTR-ToF-MS mass spectra should be interpreted with caution when reporting quantitative results in vaping studies, such as doses of inhaled VOCs. Nevertheless, the online PTR-ToF-MS analysis can provide valuable qualitative information by comparing relative VOCs in back-to-back trials. For example, by comparing the mass spectra of exhaled air with those of direct puffs, we can conclude that harmful VOCs present in the vape emissions are largely absorbed by the participants, including large fractions of nicotine.

Figure 1

Example of sampling progression using the PTR-ToF-MS for participant 4, collection 3. The acetone/propanal (m/z 59.0491, C₃H₇O⁺) trace was selected (black trace), and the intensity is shown as a function of the PTR-ToF-MS cycle number (each cycle is 1 s). After sampling room air and clean air, a participant exhaled a baseline puff (segment 1) into the bag and the ion trace increased. Once the trace plateaued, the participant inhaled from the vape device and exhaled into the bag (segment 2). Finally, a syringe was directly connected to the vape device and 20 mL of vape aerosol was collected then subsequently pushed into the bag using a syringe pump (segment 3).

Figure 2

Combined mass concentrations of the species detectable by PTR-ToF-MS for all participant visits. The average mass per puff (μg VOC puff^–1) is shown for baseline breath (no vape, blue), exhaled puff (after inhaling from the vape device, red), and direct injection of vape aerosol (gray). The variability in the emissions is represented by error bars (one standard deviation) calculated using each participant’s puff topography data (Table S1). Note that the vertical scale is logarithmic. Participants 1, 4, 7, and 8 used closed vapes (circled numbers, striped bars), whereas participants 2, 3, 5, and 6 used open vapes (solid bars). A linear version of this figure is presented in the SI (Figure S3).

Figure 3

PTR-ToF-MS results of selected VOCs from participant exhaled puffs as a function of e-liquid flavor class. These results were averaged among all 40 participant trials. Error was calculated using each participant’s puff topography data which varied by visit (Table S1).

Figure 4

Typical unit mass resolution PTR-ToF-MS mass spectra from (a) exhaled vape puff of participant 3 in trial 3, (b) direct vape injection from vape smoked by participant 3 in trial 3, (c) heated propylene glycol (PG) (Fisher, > 99%, CAS 57-55-6), and (d) heated glycerol (GLY) (Fisher, > 99%, CAS 56-81-5). Both humectants were aerosolized separately using participant 5′s open vape (SMOK Alike). The peak indicated with (†) corresponds to DMAC at m/z 88, a known impurity of Tedlar bags. The identity of labeled ions can be found in Table 1.

Figure 5

Reported mixing ratios for methanol, 1,2-propadiene, propene, acetaldehyde, acetone, and toluene by PTR-ToF-MS (a, b) and GC (c, d) from the control experiments with heated pure solvents. In panels (a) and (b) “acetone” refers to m/z 59, which is the sum of acetone and propanal; “toluene” refers to m/z 93, which is the sum of glycerol and toluene; and “1,2-propadiene” refers to the sum of 1,2-propylene and the C₃H₅⁺ fragment ion from PG ([M+H-2H₂O]⁺).