Triacetin Enhances Levels of Acrolein, Formaldehyde Hemiacetals, and Acetaldehyde in Electronic Cigarette Aerosols

Abstract

The health effects of inhaled electronic cigarette (e-cigarette) flavoring compounds are largely unknown. Earlier reports of their chemical reactivity have been conflicting, with some claiming, for example, that the degradation of flavoring chemicals in e-cigarettes to aldehydes is statistically insignificant. It is thus important to understand how these molecules react to afford enhanced aerosol products. The purpose of the current study was to investigate the origin of formaldehyde, acrolein, and acetaldehyde in e-cigarettes that contain the popular additive, triacetin (TA). By using ¹³C labeling and a combination of ¹H NMR and ¹³C NMR, we were able to identify that ester hydrolysis of TA occurs to form acetic acid (HOAc) during aerosolization. The released HOAc acts as a catalyst in the degradation of propylene glycol (PG) and glycerol (GLY), increasing the formation of formaldehyde hemiacetals, acrolein, and acetaldehyde. A solution of 10% TA in 1:1 PG/GLY e-liquid was aerosolized using two different e-cigarettes at two wattages. Each device exhibited a significant increase in aldehyde levels, of up to 185% compared to the aerosol from a 1:1 PG/GLY e-liquid. In addition, the GLY formaldehyde hemiacetal was more predominant within the presence of HOAc, indicating that GLY may be relatively more prone to degradation from protonation.

Figure 1

Isotopically labeled TA. TA derived from isotopically labeled GLY (4) and from isotopically labeled acetic anhydride (5).

Scheme 1. Two Pathways of TA-Proposed Thermal Degradation

TA forms acrolein, acetaldehyde, and formaldehyde. In e-cigarettes, formaldehyde further reacts with PG/GLY to form formaldehyde hemiacetals.^,

Figure 2

Overlay of the ¹H NMR spectra of aerosolized (red) PG/GLY e-liquid and (blue) 10% TA/PG/GLY e-liquid. The peaks of interest that increase in height are identified by the doublet at 9.55 ppm as the aldehyde resonance of acrolein, the multiplet at 6.35 ppm as the trans β hydrogen, doublet at 6.47 ppm as the cis β hydrogen, and doublet at 6.625 ppm as the α hydrogen resonance of acrolein; the quartet at 9.65 ppm as the aldehyde resonance of acetaldehyde; and last, the overlapping triplets at 6.20 and 6.17 ppm as the hydroxyl resonance of the primary formaldehyde hemiacetals corresponding to PG and GLY, respectively. Chemical peak identification by the addition of authentic standards was performed extensively and published in our previous work. These spectra were obtained using EC2 at 11 W.

Figure 3

Concentrations of compounds 1a–3 in the aerosolization of PG/GLY and increased levels with the addition of 10% TA. The blue bar represents the amount of product (mg solute/g solution consumed) formed from aerosolized PG/GLY e-liquid. The green bar represents the amount of products formed from aerosolized TA/PG/GLY e-liquid. The inset displays the results from EC2 at 9 W, expanded by 100 times. 1a, 1b, 2, and 3 represent PG formaldehyde hemiacetal, GLY formaldehyde hemiacetal, acrolein, and acetaldehyde, respectively. Errors bars were calculated by one standard deviation. The enhanced concentration of 1a–3 was significant under all conditions except in the case of 1a–b generated by EC1 at 55 W (see the Supporting Information).

Figure 4

Overlay of ¹H NMR spectra of aerosolized (red) PG/GLY e-liquid and (blue) 0.5% HOAc/PG/GLY e-liquid. The triplet at 6.20 ppm was identified as 1a. The triplet at 6.17 was identified as 1b. The doublet at 9.55 ppm, multiplet at 6.35 ppm, doublet at 6.47 ppm, and doublet at 6.625 ppm were identified as 2. The quartet at 9.65 ppm was identified as 3. In the presence of HOAc there is a visible increase in peaks corresponding to 1a–3. These spectra were obtained using EC2 at 11 W.

Scheme 2. TA in E-Cigarettes Leads to HOAc Formation and Subsequent Protonation of PG/GLY To Catalyze the Formation of Products Such as 1–3

This was confirmed via the use of ¹³C-labeled TA as the predominant pathway observed under the conditions used herein.