The mass concentrations of different carbonyls/acids in air were calculated by the total mass concentration of the specific carbonyls/acids in the HPLC-HRMS analysis divided by the total volume of air that flowed through the DNPH cartridge during the vaping collection process.The method reported in this work offers unambiguous identification and a large quantification range for functionalized carbonyl compounds and organic acids. This is useful for studying e-cigarette thermal degradation chemistry, as well as other environmental chemistry topics . A total of nineteen DNPH hydrazones in the e-cigarette aerosol sample were observed : five simple carbonyls, six hydroxycarbonyls, four dicarbonyls, three acids, and one phenolic carbonyl. Hydroxycarbonyls comprised 3 of the top 6 most abundant compounds. Uchiyama et al., recently found that some compounds are emitted purely as gas-phase species , some as purely particulates , and some as both . Both the concentration and phase information is useful for estimation of exposure risk. Much of the chemical identification for DNPH hydrazones can be directly derived from the exact mass of the detected [M-H]- ions alone. As the formation of DNPH hydrazones replaces only one atom , it is straightforward to deduce the original molecular formula of the carbonyl or acid from the hydrazone formula. The chemical structures were confirmed as in 2.3.1. Figure 2.6a shows the total ion chromatography and SIC of select carbonyl-DNPH compounds, Figure 2.6b shows the corresponding integrated mass spectrum of TIC and each SIC. From the TIC, it is clear that ecigarette aerosol is a complex system which contains a large number of carbonyls/acids.

Co-elution is common in the TIC ; however, the SIC isolates the chromatographic peaks of the desired m/z, avoiding co-elution and misidentification. We also found that acetone-DNPH co-eluted with vanillin-DNPH in the chromatography. This will have led to an overestimation of the abundance of acetone using a chromatography method without HRMS, as vanillin-DNPH is not commercially available.Beyond molecular formulas, it is advantageous to confirm the exact bonding sites of carbonyls and other moieties to give insight to chemical mechanisms and aid in theoretical calculations of reaction energies, as these calculations are sensitive to structures. The chemical structure of DNPH adducts was identified by their neutral and radical losses in tandem multistage mass spectrometry using collision induced dissociation , 148,149 which often helps to elucidate the exact carbon location of the moiety-of-interest for small molecules. For example, alcohols adjacent to a beta carbon with an abstractable hydrogen can lose H2O by H-shift rearrangement, 150 while those bonded to aromatic or other non-abstractable sites do not show this loss in the negative ion mode. For nitroaromatics such as DNPH, the electron-withdrawing groups of NO2 exerts a strong stabilizing effect on anion radicals, and facilitates NO2-mediated rearrangements . For small ions like acetaldehyde-DNPH, there is no other reasonable carbonyl structure that exists for the molecular formula, and MSn confirms this structure with expected fragmentation of CH3NO and CH3CHO . However, cannabis grow equipment there are some ambiguous formulas such as C3H6O3, which may belong to structural isomers dihydroxyacetone and glyceraldehyde. Both of these hydroxycarbonyls are proposed to exist in e-cigarette aerosol after NMR analysis, but are impossible to distinguish with chromatography as they have the same UV-absorption and m/z.34 With MSn fragmentation, we found that dihydroxyacetone is the main product.

Even though several fragmentation pathways for these isomers are similar and 269.05→ 239.04 , the H2O loss and C2H4O2 loss that is expected for glyceraldehyde-DNPH were observed to be negligible in the mass spectrum . The preferred formation of dihydroxyacetone over glyceraldehyde supports the radical-mediated oxidation pathways suggested by Diaz et al., as radical abstraction of the H in VG should lead preferentially to a secondary alkyl radical compared to the primary radical . The initiating radicals are suggested to be reactive oxygen species such as hydroxyl radical, and as such, the degradation products can be described by processes that occur in atmospheric chemistry. Some of the products identified here can be expected from the thermal degradation of PG and VG , which is in agreement with the proposed mechanism, while others are likely to be flavoring additives . A shared product ion after fragmentation of the DNPH hydrazones is C6H3N4O3 – , which is the modified DNPH after the O-rearrangement loss of the original carbonyl/acid. Other similar loss pathways are those of the DNPH itself, including loss of HONO, NO2, and NO . There are also distinctive fragmentation pathways for each ion, which are summarized in Table 2.2.While the process of ionization in ESI is complex, it has been demonstrated that there are key factors influencing the ionization efficiency of different compounds. For example, for the same family of compounds, there is a relationship between negative ion electrosprayionization response and pKa of the dissociation equilibrium HA ⇆ A – + H+ , which is directly related to basicity. We calculate the basicity in terms of ΔGdeprotonation , because the deprotonated [M-H]- ion is usually detected in the ESI negative mode. Our calculations of the electrostatic potential maps of carbonyl-DNPH hydrazones show that they have a primary acidic proton ; thus, they are excellent candidates for which gas phase basicity can be used to parameterize ionization efficiency in the ESI negative mode.

We emphasize that the theoretical chemistry results in this work only provide a relative indication of sensitivity, not absolute calibration factors, and only for the same family of compounds that are protonated or deprotonated. The relative theoretical sensitivities are then anchored by absolute ESI calibrations for the carbonyl-DNPH compounds where standards are commercially available.The trend of ΔGd and ESI sensitivity arises from the intrinsic relationship between deprotonation efficiency and the ability of the aromatic product ion to stabilize the negative charge initially formed on the N atom . Acrolein is the most sensitive compound in ESI negative mode because it has conjugated double bonds, i.e., additional pi orbitals for the negative charge to be delocalized. Also, ketones have lower sensitivities than aldehydes because the electron donating group on both sides of the C=N bond slightly destabilizes the negative ions. A limitation of this model occurs for compounds that have similar ΔGd. In this situation, other factors like molecular volume and polarity may also play an important role for these compounds. Despite the limitations, this method is applicable to the compounds found in e-cigarette aerosol and enables the first estimation of concentrations for complex carbonyls that have not yet been quantified with acceptable uncertainty. Furthermore, this computational technique offers an advantage compared to the time expenditure, costs, and chemical usage of synthesizing standards.The calculated concentrations of e-cigarette constituents characterized in this work are shown in Table 2.2 as mass per volume or mass per ten puffs analyzed. The most abundant compounds in the blu e-cigarette aerosol for our study conditions are hydroxyacetone, formaldehyde, acetaldehyde, lactaldehyde, acrolein, and dihydroxyacetone. While, within uncertainty, the exact order of abundance is not definitive, it is clear that hydroxycarbonyls are just as important assimple carbonyls to the composition of the e-cigarette aerosol. Hydroxyacetone has been found to be a major, sometimes dominant, emission in other e-cigarette brands and e-liquids, as quantified by gas chromatography. The agreement of the high abundance of hydroxyacetone lends support to the theoretical approach in this work, which enables all carbonyls and acids to be quantified by the same method. The high abundance of hydroxyacetone may be due to its multiple formation pathways in Scheme 2.2 and its possible role as an impurity in e-liquid, e.g., Sleiman et al., found hydroxyacetone in concentrations of < 1% of the sum of PG and VG in the e-liquids they used. We were not able to test the e-liquid in this work due to cartridge design; thus, are unable to comment on the extent of hydroxyacetone impurity in the e-liquid, if present. Dihydroxyacetone and lactaldehyde, in contrast, have not been regarded as major e-cigarette emissions until their unambiguous identification in this work. Their formation pathways from PG and VG are highly feasible, so their higher abundance is not unexpected. It’s not clear why these compounds have not been reported earlier; we suspect analytical challenges may be a reason. As we discussed previously, lactaldehyde-DNPH co-eluted with formaldehyde-DNPH in the TIC . Thus, HPLC-UV, one of most frequently used instrument for studying carbonyl compounds in e-cigarette aerosol, indoor grow cannabis will not be able to identify and quantify lactaldehyde. However, the HPLC-HRMS method overcomes co-elution challenges by distinguishing compounds based on their exact mass from the SIC and mass fragmentation patterns. Dihydroxyacetone-DNPH appeared to be baseline-separated in HPLC-UV, with a retention time slightly shorter than DNPH itself; however, its unambiguous identification is not possible without HRMS and/or authentic standards. Furthermore, both of these compounds are quite polar, and thus, not conventionally compatible with gas-chromatography.

A comparison of the absolute emission concentrations of thermal degradation products between studies is not straightforward, even for the same brand of e-cigarettes, as the puffing regimens and apparatus of reported works are all different and individual puffing parameters have non-linear effects on the thermal degradation chemistry. Klager et al., also reported high variability of carbonyl concentrations for the same brand, puffing-regimen, and flavor, suggesting that the factors driving the thermal degradation chemistry are not yet fully understood. Our work should be primarily viewed as a demonstration of a new method to the chemical characterization of our specific e-cigarette model at the stated puffing conditions, with noted insights into the thermal degradation mechanism. Formaldehyde, acetaldehyde, and acrolein are known to produce pathological and physiological effects on the respiratory tract. They are known to cause sensory irritation, inflammation, and changes in pulmonary function; formaldehyde is also carcinogenic. The average daily dose of aldehydes can be calculated by the amount of aldehydes per puff multiplied by the average number of puffs a user inhales per day. For example, the median puffs per day for e-cigarette users can be assumed to be 250171, so the average daily exposure dose of formaldehyde is 37.5 µg/day for this e-cigarette device, e-liquid, and operating conditions. The California Office of Health Hazard Assessment Chronic Reference Exposure Levels for formaldehyde is 9 µg/m3 , which could be translated to an acceptable daily dose of 180 µg/day and is higher than the e-cigarette aerosol exposure for formaldehyde in this work. In addition, OEHHA has a No Significant Risk Level recommendation of 40 µg/day which is intended to protect against cancer; this NSRL level is close to the exposure dose of formaldehyde in this work. The average exposure dose of acrolein for blue-cigarettes is 15.2 µg/day according to Table 2.2, which is higher than the OEHHA chREL value . Logue et al. used a similar approach to estimate health impacts and found that both formaldehyde and acrolein can exceed maximum daily doses derived from occupational health guidelines. Differences in results are likely due to the different devices, e-liquids, and puffing regimens used.While the reported emissions in this work may not be generalized to all e-cigarettes and use scenarios, it is informative to compare the aldehyde emissions normalized by nicotine, since ecigarette users transitioning from traditional tobacco products will self-titrate nicotine intake when using e-cigarette products. In this work, the nicotine yield is 10.4 ± 1.9 μg/10 puffs. We did not observe evidence of nicotine oxidation174 under the puffing conditions of this work, which will impact the ratio. The formaldehyde/nicotine ratio is 144 ±32 μg/mg nicotine, which is 4 times higher than the formaldehyde/nicotine ratio in combustible cigarettes . The acrolein/nicotine ratio measured in this work in close to that of tobacco products , while the acetaldehyde/nicotine ratio and propionaldehyde/nicotine ratio are lower than that in combustible cigarettes. Logue et al. observed similar trends using different e-cigarette products; however, the results were not normalized for nicotine so a direct comparison is not possible. Thus, we find e-cigarettes do not necessarily emit lower carbonyl compounds than tobacco products, but the comparisons may change depending on the specific e-cigarettes or tobacco products, or different puffing/smoking regimens. Although hydroxycarbonyls are abundant in e-cigarette aerosol, a general lack of toxicological data precludes health risk assessment. Smith et al. found that exogenous exposure to dihydroxyacetone is cytotoxic and will cause cell death by apoptosis. Glycolaldehyde is also suspected to have biological toxicity. For hydroxyacetone and lactaldehyde, toxicology data are currently unavailable on many toxicology databases like Hazardous Substances Data Bank , European Chemicals Agency and Research Institute of Fragrance Materials .