Determination of Carbonyl Compounds Found in Electronic Cigarettes

Determination of Carbonyl Compounds Found in Electronic Cigarettes

By: Madison Parker

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Abstract:

Electronic Nicotine Delivery Systems (ENDS) and personal vaporizers are battery-

powered devices that aerosolizes nicotine so that it is readily available to the user. Food-grade

ingredients and traditional cigarette ingredients are used in these devices. There is very little

analytical data available that informs the public to the possible health effects of ENDS on the

user; however, it is known that these devices put out significant toxic carbonyl compounds. In

one experiment, electronic cigarettes were tested to determine their carbonyl compound output.

This was tested by testing 13 different brands of electronic cigarette solvent by capturing its

vapor using coupled silica cartridges impregnated with hydroquinone and 2, 4-

dinitrophenylhydrazine. They were then analyzed using high performance liquid

chromatography. Of the 13 brands tested, 4 brands did not generate any carbonyl compounds and

9 generated various carbonyl compounds. From this experiment, there were not specific carbonyl

compounds formed for every trial; however, it was determined that electronic cigarettes

incidentally produce high concentrations of carbonyl compounds11. In another study, the effect of

nicotine solvent and voltage output on carbonyl compound formation were tested. To determine

the effect of nicotine solvent on the carbonyl compound output, ten different electronic cigarette

liquids were used for the experiment with concentrations of nicotine varying from 18-24 mg/ml.

The ten different electronic cigarette liquids were placed in groupings based on the contents of

their humectants. One group was made up of purely propylene glycol, one group purely

vegetable glycerin, and another group a ratio of both propylene glycol and vegetable glycerin. In

order to see how the base humectant effects the carbonyl compounds, three controls were also

prepared for the experiment. In this experiment, it was observed that all electronic cigarette

liquids contained at least one carbonyl compound in the vapors produced by the electronic

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cigarette. In this experiment, formaldehyde, butanol, acetone, and acetaldehyde were the most

prevalent carbonyl compounds observed in the electronic cigarette vapors, while acrolein was

not detected at all12. In this same experiment, the effect of voltage on carbonyl compound

formation was tested by observing the carbonyl compound generation when increasing the

ENDS voltage to 3.2V, 4.0V, and 4.8V. From this experiment, it was observed that as voltage

increases, so does the amount of carbonyl compounds formed within the vapors. The most

significant increase in carbonyl compounds was observed in humectants that used propylene

glycol as a base in the e-liquid. In order to determine the harmful effects of electronic cigarettes

to its users, this paper evaluates the instrumentation of ENDS, analyzes the chemical action that

occurs during its use, and reviews available evidence that evaluates how carbonyl compounds are

generated during electronic cigarette usage.

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Introduction:

Electronic Cigarettes have been around since the 1960’s. Hubert A. Gilbert, in 1963, filed

a patent for the idea for the first electronic cigarette. At the time, smoking cigarettes in public

was normal social behavior and the toxic side effects of smoking tobacco was not as extensively

researched. At the time, there was not a need for “healthier” smoking options and smoking was

fairly accepted in society. In 2003, Han Lik, a Chinese pharmacist and a smoker, developed the

first usable electronic cigarette after his father passed away from lung cancer. Shortly after its

invention, the Chinese and European markets were the first to accept electronic nicotine delivery

systems. In 2007, the electronic cigarette was introduced into the American market3. Over the

years, the FDA and manufacturers have fought over the regulations of selling and producing

electronic cigarettes, due to their unknown health effects on users. Import bans have been placed

on the product and law suits have been filed to try and stop the spread of the popular product. To

this day, electronic cigarettes are still banned in certain states.

Originally, electronic cigarettes were created to help smokers quit their smoking

addiction. Now electronic cigarette-use has evolved into a large community that utilizes personal

vaporizers that can be modified to maximize the user’s smoking preferences. Electronic nicotine

delivery systems are designed to look like traditional tobacco cigarettes in order to simulate the

sensory, social, visual, and behavioral features of smoking4. The models can be filled with any of

the thousands of available “e-juice” flavors that range from traditional coffee, vanilla, cigar, or

more unique flavorings such as watermelon, mango, or cotton candy17. Some “e-juice” brands

aim to simulate traditional cigarette brands such as Camel or Marlboro1. Each “e-juice” contains

varying amounts of nicotine, propylene glycol, vegetable glycerin, and food-grade flavorings.

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The “e-juice” is heated using a battery-powered device which aerosolizes the liquid mixture to

the user for inhalation1. There is little known about the long-term health risks associated with

electronic cigarette usage or the “e-juice” that it utilized. Due to a lack of combustion, these

products do not contain typical carcinogens that are known to be in tobacco products. In addition

to the nicotine used to help curb a smoker’s addiction, other compounds that are added to the

electronic cigarette liquid such as the humectant, flavoring, and other food-grade additives can

cause problems. As this fad continues to increase, so does the need for regulation and the

understanding of the long term effects.

Electronic Nicotine Delivery System

The electronic nicotine delivery system is a battery-powered alternative to cigarette

smoking. The device utilizes an atomizer to heat up the liquid mixture of nicotine dissolved in

propylene glycol. The propylene glycol acts as a humectant for the nicotine that users crave.

Characteristically, propylene glycol is a sweet, colorless, and odorless substance. When mixed

with the nicotine, it helps to preserve nicotine in the state needed for delivery. When it is inhaled

by the user, the resultant is a white cloudy smoke similar to cigarette smoke; however, it’s

odorless, which makes this form of smoking more attractive to its users.

First generation electronic cigarettes consist of a cartridge that holds the nicotine and

propylene glycol mixture and a battery that atomizes the liquid to be inhaled by the user. This

electronic nicotine delivery system is disposable and is powered when the user inhales. A model

of the first generation electronic cigarette can be seen in Figure I.

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Figure I. Model of a first generation electronic cigarette.

Figure II. shows the general set up of the second generation electronic cigarette. The

second generation electronic nicotine delivery system is powered by a lithium-ion battery. When

the user presses the control button, the device activates the atomizer. There are two different

types of atomizers: systems that are disposable and systems that can be rebuilt. Those that are

disposable are classified as clearomizers or cartomizers. Those that can be rebuilt are referred to

as rebuildable dripping atomizers (RDA) or rebuildable tank atomizers (RTA)2. Inside the

atomizer is a wick that soaks up the homogenous liquid. The wick is then wrapped around an

internal coil. The internal coil is nichrome wire made up of 80% nickel and 20% chromium that

is heated and incidentally heats the temperature of the electronic liquid to extremely high

temperatures. At the vaporization point, the aqueous solution of vegetable glycerin, propylene

glycol, flavoring, and/or nicotine within the tank is atomized to vapor. The vapor is then inhaled

through the mouthpiece by the user.

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Figure II. Model of the second generation electronic cigarette.

The newest generation of ENDS has progressed into box-mod devices that allow the user

to have absolute control over their smoking experience and is sometimes referred to as a personal

vaporizer. The personal vaporizer has LED displays and controls that allow the user to increase

or decrease the voltage of the device. The flexibility of the device allows the user to customize

their electronic liquid mixture to optimize their smoking capability. The box-mod/personal

vaporizer model can be seen in Figure III.

Figure III. Model of a box-mod personal vaporizer (3rd Generation).

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E-liquid

The two most widely used electronic cigarette bases are propylene glycol and vegetable

glycerin, usually referred to as glycerol. There are four main ingredients found in electronic

liquids: (1) a propylene glycol or vegetable glycerin base, (2) water, (3) flavoring, and (4)

nicotine. Propylene glycol and vegetable glycerin are commonly used as food additives and are

known to be safe for consumption. They are non-toxic organic compounds that hold the nicotine

and flavor in suspension. These particular bases are favored because they are characteristic for

the white clouds of vapor that are exhaled by the user. Every electronic liquid contains propylene

glycol, vegetable glycerin, or a customized ratio of both.

The organic molecule propylene glycol is generated from propylene oxide. It is odorless,

has low viscosity, and colorless. It is typically utilized to preserve foods, as solvents,

pharmaceutical products, and tobacco products. Vegetable glycerin comes from naturally

extracted plant oils such as coconut oil, palm oil, and soy. It is odorless, slightly tinted in color,

sweet, and typically more viscous than propylene glycol. It is found in food production,

cosmetics, and tobacco products.

Electronic cigarettes are known for being customizable down to the flavor of their

electronic liquid; however, the chemicals used to flavor electronic cigarettes may not be as safe

as individuals’ believe5. Third generation personal vaporizers allow for the user to choose a

unique flavoring of electronic liquid to be vaporized. Flavorings can imitate common tobacco

products such as Camel and Marlboro, and some manufacturers have developed dessert-like

flavorings such as Pumpkin Spice, Watermelon, Swedish Fish, Marshmallow, or even Cotton

Candy to name a few. Most of these flavorings are food-grade ingredients that have been deemed

by the Federal Drug Administration as safe to consume; however, the FDA has not been able to

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state whether the food-grade ingredients are safe to inhale. These flavorings and additives make

the nicotine also found in the electronic liquid more addicting and appealing to its users.

Those who are regularly exposed to nicotine become dependent on the chemical16. If

exposure is discontinued, the user can experience withdrawal symptoms such as cravings,

depression, anxiety, the feeling of emptiness, and irritability6. In electronic cigarettes, nicotine is

present in the liquid form and held in suspension by a humectant, which is then heated and

aerosolized for the user to inhale. In its liquid form, nicotine is highly concentrated and

exceedingly toxic13. Users of personal vaporizers can also customize the concentrations of

nicotine utilized within the electronic nicotine delivery system. Liquid concentrations of nicotine

vary from 0 to 18 mg/ml and some were even found as high as 36-42 mg/ml. Dosing is

inconsistent and fluctuates by manufacturer. E-liquids containing “low doses” of nicotine

correspond to a concentration of 6-8 mg/mL, “Midrange” concentrations contain 10-14 mg/mL,

“High” concentrations correspond to 16-18 mg/mL, and “Extra-high” concentrations correspond

to 24-36 mg/mL of nicotine per mL of liquid1. All doses of liquid nicotine have the numerical

concentration printed on the container of the electronic liquid or on its original packaging;

however, some studies have determined that the actual concentration of nicotine within the

electronic liquid is hard to determine and often differs from what is stated on the packaging16.

Therefore, the user must be careful when loading their personal vaporizers due to the fact that

nicotine toxicity can occur when the liquid is consumed or applied to the skin13.

An ENDS user has the option to determine which base they would like to utilize as a

humectant in the third generation personal vaporizer. Users can use a pure propylene glycol base

or vegetable glycerin base. Often times, users create differing ratios of propylene glycol and

vegetable glycerin in order to maximize their smoking experience. Propylene glycol is utilized

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more often than vegetable glycerin as an e-liquid base for many reasons. Both structures can be

seen in Figure IVa. and Figure IVb.

Figure IVa. Structure of Propylene Glycol

Figure IVb. Structure of Vegetable Glycerin

Because propylene glycol is less viscous than vegetable glycerin it’s easier to load into the

reusable drip tank and there is less build-up deposited on the nichrome wire coil after the liquid

has been vaporized. Vegetable glycerin has a higher viscosity and density so it often creates

build up on the nichrome coil that heats up the electronic liquid over time. Due to vegetable

glycerin’s high viscosity, it takes more energy and a takes longer to reach the optimal

temperature needed to vaporize; however, the density of the vegetable glycerin allows the user to

create thicker vapor and tends to be a healthier option for the user.

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Chemical Reaction through which Propylene Glycol/Glycerol forms Carbonyl Compounds

In order for the vaporizer to work, the propylene glycol, flavoring molecules, and

nicotine molecules must be heated to their heat of vaporization without chemically degrading

them. It is estimated that the theoretical vaporization temperature of an electronic cigarette could

reach up to 350 ̊C. This temperature is high enough to cause physical alterations to the chemicals

within electronic liquids and cause chemical reactions to occur within the solvent. At such high

temperatures, the solution could undergo thermal decomposition which leads to the generation of

toxic aldehydes6. When glycerol (vegetable glycerin) is heated, it decomposes by a dehydration

mechanism to acrolein and water.

Eq. 1 C3H8O3

∆

→ C2H3CHO + 2H2O

Acrolein is typically found in the environment and in food products. It can be formed

from carbohydrates, animal fats, or by heating foods; however, when smoking tobacco products,

the produced acrolein exceeds or equals the total human exposure to acrolein from all other

sources. It is a colorless, poisonous, pungent, and the simplest unsaturated aldehyde. This

volatile organic compound can cause burning of the nose and throat and can cause damage to the

lungs. By a retro aldol condensation reaction, acrolein can further break down into acetaldehyde

and formaldehyde. This reaction only occurs in the presence of a catalyst, such as the hot metal

present in the e-liquid in the form of coils that heat the liquid. The nichrome wire present in the

atomizer of the electronic cigarette is known to have a low heat tolerance and give a metallic

taste to the user2. Acids and bases can also catalyze the reaction and are present in the electronic

liquid flavorings.

Glycerol Acrolein

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Eq. 2 C3H8O �

�

�� H3CCHO + HCHO

Eq. 3 C3H8O ��


Formaldehyde is a colorless, overpowering organic compound. The short term effect of

this compound on the body could be irritation of the eyes, throat, and nose. If exposed to this

toxic compound over a longer period of time, one could experience coughing, trouble breathing,

rawness of the throat and interior of the nose. The respiratory system could also be effected. It

has also been shown that with increased dosages of formaldehyde to the body, there is also an

increase in developing specific types of cancer8.

In an electronic cigarette that utilizes propylene glycol, the propylene glycol boils when

exposed to extremely high temperatures. With these specific conditions in the form of a catalyst,

the electronic liquid could dehydrate to form propionaldehyde.

Eq. 4 C3H8O2

��

�� C2H5CHO

Propionaldehyde is a colorless liquid that is accompanied by a fruity smell. When in contact with

the body it can irritate the skin, nose, throat, and lungs. When inhaled it could cause shortness of

breath, excessive coughing, and pulmonary edemas.

Glycerol

Glycerol

Acetaldehyde Formaldehyde


Propionaldehyde Propylene Glycol

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Effect of Aldehydes on the Body

An aldehyde is an organic compound that contains a –CHO group. It is a simple carbonyl

molecule that is formed by the oxidation of alcohol. The most common aldehydes are

formaldehyde, formed from methanol, and acetaldehyde, which is generated from ethanol.

Aldehydes such as acrolein, formaldehyde, acetaldehyde, and crotonaldehyde have been

documented to have acute effects on the human body8. Common aldehydes and their structures

can be seen below in Figure V.

Figure V. Common aldehydes and their chemical structures.

acrolein

Among these examples, acrolein was found to have the greatest impact7. Acrolein is found to be

2 to 3 times more toxic formaldehyde7. Occasional exposure to aldehydes may cause olfactory

and ocular irritation. Long-term contact may cause extreme irritation to the mucous membranes

and damage to respiration7. Chronic exposure can even cause irreversible damage to the

epithelial tissues lining the lungs and respiratory tract. A study was performed on rats to

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determine carcinogenicity of aldehydes. Rats were exposed to a concentration of formaldehyde

for a period of time. After that period of time, 103 rats were observed to have induced squamous

cell carcinoma. The same procedure was performed on mice. The mice were observed with nasal

tumors. These studies all gave evidence to reversible and irreversible damage to epithelium cells

lining the respiratory tract and the damage that can occur when exposed to aldehydes8.

Mechanism for Formation of Carbonyl Compounds by Glycerol and Propylene Glycol

The electronic liquids in the electronic cigarette tank are vaporized when they come into

contact with the nichrome wire and oxidized in the presence of oxygen from the surrounding air

to form formaldehyde, acrolein, glyoxal, methylglyoxal, and acetaldehyde9. The solid metal

oxide wire is used as a catalyst in this reaction. Because the vegetable glycerin has a high boiling

point, this is referred to as a heterogeneous catalyst9. Figure VI. shows the reaction that occurs

when the electronic liquid comes in contact with the heated nichrome wire.

Figure VI. Oxidation of vegetable glycerin and propylene glycol with the nichrome wire as a

catalyst

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The vegetable glycerin is oxidized to form acrolein. The propylene glycol is oxidized to form

methylglyoxal and then further oxidized to form formaldehyde and acetaldehyde whose toxicity

is well documented20.

Mechanism of Glycerin Dehydration Reaction to Carbonyl Compounds

Glycerin acts as a humectant for a homogenous mixture of flavoring, nicotine, and water.

Alcohols can undergo a variety of changes, most of which are either oxidation or reduction

reactions. Primary alcohols can be oxidized to form an aldehyde structure. Oxidation is when

there is a loss of hydrogen and an addition of an oxygen or halogen. Primary and secondary

alcohols can be easily oxidized using catalysts such as acids and metals. The coil that is used to

vaporize the electronic liquid is made up of nichrome wire. The hot metal catalyzes the oxidation

reaction. The high temperatures that are reached within the electronic cigarette cause thermal

degradation to occur, which is the probable catalyst for this oxidation reaction. The use of a

heterogeneous catalyst significantly reduces the activation energy of the transition states and

increases the rate of the reaction. Glycerin has been found to dehydrate to acrolein; however, the

mechanism does not just produce acrolein but other carbonyl compounds such as acetaldehyde,

propanal, and acetone. From the reaction, carbon dioxide and carbon monoxide were identified

in small quantities10. Glycerin readily forms a homogenous mixture with water due to its three

hydroxyl groups that readily form a hydrogen bond with water molecules. When glycerin is in its

purest form, its boiling point is 290 ̊ C. When water is mixed with glycerin to form a

homogenous solvent, the boiling point decreases. Figure VII. shows the reaction mechanisms

possible for the dehydration of glycerin.

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Figure VII. Pathways of dehydration of glycerol and its proposed products.

Figure VII. shows that there are two specific pathways of dehydration that glycerin can

undergo-a 1-2 dehydration and a 1-3 dehydration. The 1-2 dehydration occurs when the

secondary or primary hydroxyl group is protonated. If the secondary hydroxyl group is

protonated, acrolein will be formed, if the terminal hydroxyl group is protonated, acetol will be

formed. When the terminal hydroxyl group is protonated, has an unstable transition state is

formed; however, this state is stabilized due to the conjugation of the weak basic sites4. From this

pathway, acetol is formed. If this product was dehydrated again, the product that would form

would be thermodynamically unstable. Because of its unstability, acetol is the major product of

this dehydration pathway. This unstable transition state is the reason that the dehydration

pathway yields a large acrolein output. Acrolein is formed when the secondary hydroxyl group is

protonated. The hydroxy propanal that is formed undergoes a second dehydration to form

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acrolein. If an aldol or retro aldol condensation reaction occurs, acetaldehyde, formaldehyde, and

acrolein are favorable products.

In a 1-3 dehydration of glycerin, the carbon backbone is deconstructed and the products

formed are formaldehyde and vinyl alcohol. The mechanisms for the carbon backbone

deconstruction and decomposition to formaldehyde and acetaldehyde can be seen in Figure

VIII. The vinyl alcohol goes through keto-enol tautomerization to acetaldehyde, this aldehyde

can further oxidized to form acetic acid. In the experiments performed, both acetaldehyde,

formaldehyde, and acetic acid were present in the vapors produced by electronic cigarettes.

Figure VIII. Mechanism for the deconstruction of the carbon backbone that occurs due to high temperatures

Electronic cigarettes are heated to high temperatures in order to reach the vaporization

temperature of the solvent so that it can be aerosolized to the user for inhalation. Formaldehyde

is known to be unstable at such increased temperatures. When this occurs, formaldehyde

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thermally decomposes to carbon monoxide and hydrogen. The hydrogen that is formed at these

high temperatures are responsible for reducing products formed in the reaction pathway.

Mechanism of Propylene Glycol Dehydration to Carbonyl Compounds

Propylene glycol decomposes at high temperatures via three different reaction

pathways15. These pathways can be seen below in Figure IX.

Figure IX. Scheme of the three reaction pathways of propylene glycol

In the first pathway, propylene glycol (1) dehydrates to an allyl alcohol (5). The reaction

barrier for this pathway is fairly high compared to the other pathways15. TDue to the higher

reaction barrier, this pathway is not as favored as the other two. The allyl alcohol is further split

into formaldehyde and acetaldehyde by bond scission.

In the second pathway, Propylene glycol is dehydrated to form propylene oxide (2) as an

intermediate; however, if a hydrogen shift occurs, propylene glycol will further decompose to

acetone (3). The mechanism for this decomposition can be seen in Figure IX. in the first

mechanism. In this mechanism, a hydrogen ion comes out and the propylene oxide structure

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rearranges it’s double dond to form acetone. Acetone was found in electronic cigarette vapors in

multiple studies. This shows that this pathway can be favored at high temperatures. The propylene

glycol can also decompose to propanal, or propionaldehyde (4). This can be seen in Figure X.

below the first mechanism. In this mechanism, a hydride shift occurs and the propylene oxide

rearranges it’s structure to form propionaldehyde.. The propylene glycol is in equilibrium with the

protonated form; however, at high temperatures, entropy favors dehydration which will be

stabilized by the formation of the enol15. The reaction barrier to form propionaldehyde is the lowest

among the pathways, therefore, this pathway is the most favorable and the main product formed

in the thermal degradation of propylene glycol.

Figure X. Mechanism of the rearrangement of propylene oxide in the event of a hydride shift

Propylene glycol has been known to produce more carbonyl compounds than glycerol

when vaporized. After reviewing both mechanisms, it can be assumed that this occurs due to the

amount of carbonyl compounds produced for each molecule of humectant. The dehydration of

propylene glycol has the possibility to yield formaldehyde and propionaldehyde. The

propionaldehyde can further decompose to acetone. Therefore, this reaction mechanism presents

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the formation of two carbonyl species for every one molecule of propylene glycol. The glycerin

only forms one carbonyl molecule when dehydrated.

Determination of Carbonyl Compounds Generated from E-Cigarettes by HPLC

In this experiment, carbonyl compounds from electronic cigarette vapor were captured

using coupled silica cartridges impregnated with hydroquinone and 2, 4-dinitrophenylhydrazine

and were analyzed using high performance liquid chromatography. A test group of 13 electronic

cigarette brands were analyzed in this way. Of the 13 brands tested, 4 brands did not generate

any carbonyl compounds and 9 generated various carbonyl compounds. From this experiment,

there was not a prominent carbonyl compound that was always formed; however, it was

determined that electronic cigarettes incidentally produce high concentrations of carbonyl

compounds11.

An HPLC instrument was set up with two LC20AD pumps, photodiode array detector,

and an auto-sampler. The column used allowed for a 2.7μm particle size and was 150mm x

4.6mm. The column temperature was set for 40 ̊C and the injection size was 10μL. The flow rate

of the mobile phase was 0.7 mL/min. In order to generate vapor, a smoking machine was

employed. Before the collection of the vapors from the electronic cigarette machine, a

hydroquinone cartridge (HQ-cartridge) and a 2, 4-dinitrophenylhydrazine cartridge (DNPH-

cartridge) were connected to the machine to capture the vapors in solid form. The cartridges were

placed between the mouthpiece of the electronic cigarette and the smoking machine in order to

collect the carbonyl compounds from the vapors. The smoking machine was set to 55mL puff

volume, 2-s puff duration, 30-s puff interval, and 10 puffs. The cartridges were removed after

each run and were rinsed with acetonitrile containing 1% phosphoric acid in the opposite

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direction the smoking machine was used until the total volume reached 4.5 mL. After 10

minutes, ethanol was added to the solution and it was then analyzed by HPLC11.

From this experiment, multiple simple carbonyl compounds were detected in the vapors

of electronic cigarettes. Major carbonyl compounds found in electronic cigarette vapors were

formaldehyde, acetone, propanol, glyoxal, acetaldehyde, and methylglyoxal11. Figure XI. shows

a sample chromatograph from one of the trials.

Figure XI. Chromatogram of carbonyl compounds found in e-cigarette vapors. (Where

FA=formaldehyde, AA=acetaldehyde, ACR=acrolein, GA=glyoxal, AC=acetone,

MGA=methylglyoxal, and PA=propanol)11

The concentrations of each carbonyl compound that was found in the electronic cigarettes were

compared against each other for each electronic cigarette brand. These comparisons can be seen

in Figure XII.

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Figure XII. Graphs of the concentrations of carbonyl compounds found in 10 e-cigarettes using

the same brand of e-liquid11.

The concentrations of all the major carbonyl compounds that were produced during the

experiment from all 13 brands of e-liquid tested can be seen in Table I.

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Table I. The concentrations of key carbonyl compounds that were produced from the 13 e-

cigarette brands tested11

From Figure XII. and Table I. the statistical analysis shows that there were large statistical

differences in the carbonyl compounds produced among the different products and the carbonyl

concentrations. Of the 13 e-cigarettes tested, nine produced carbonyl compound groups and the

other four (J, K, L, M) did not. This evidence highly suggests that not one specific carbonyl

group is produced; however, from the results it was noted that formaldehyde was measured at

high concentrations in the electronic cigarette vapor. Two new carbonyl groups that were

observed that are not prevalent in traditional cigarette smoke were glyoxal and methylglyoxal.

Both are known to be mutagenic aldehydes. Methylglyoxal, also known as pyruvaldehyde,

inhibits the metabolism of formaldehyde and increases the chance of formaldehyde-induced

cytotoxicity11.

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From this experiment, the cartomizer that was utilized was examined after the conclusion

of the experiment. The cartomizers used in this experiment operated with a nichrome wire to heat

the electronic liquid mixture to vaporization temperature so that it could be delivered in aerosol

form. After the experiment, the nichrome wire was observed to have changed color from white to

black. The cartomizer used in this experiment can be seen in Figure XII.

Figure XII. The cartomizer used from the experiment with blackened deposits from thermal degradation of e-liquids used. The left shows a cartomizer that produced low concentrations of carbonyl compounds while the right shows a cartomizer that produced high concentrations of carbonyl compounds11.

From what is known about the contents of the electronic liquid used in electronic cigarettes, it

can be assumed that the propylene glycol and glycerin came in contact with the metal, which

catalyzed an oxidation reaction to form the carbonyl compounds acetone, acetaldehyde,

formaldehyde, acrolein, glyoxal, and methylglyoxal.

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The Effect of Nicotine Solvent and Battery Output Voltage on Carbonyl Compounds

Present in Electronic Cigarettes

Previous experiments that determined the levels of carbonyl compounds found in e-

cigarettes were performed on first generation electronic cigarettes. Since those experiments were

performed, the electronic cigarette market continued to enhance the product and rapidly

introduce the “second generation” electronic cigarette and “third generation” electronic cigarette

which is also referred to as a personal vaporizer. This newest instrumentation allows the user to

fully customize their smoking experience. The user can determine what ratio of propylene glycol

to glycerin they would like to use in the tank, along with the concentration of nicotine. The

individual can also increase the vaporization temperature by changing the battery output voltage.

In this experiment, ten nicotine solvents and three control solutions made up of pure propylene

glycol, pure glycerin, or a mixture of both solutions, were analyzed for twelve particular

carbonyl compounds. The electronic cigarette voltage was slowly increased during the

experiment from 3.2V to 4.8V. The carbonyl compounds were measured using HPLC method.

The purpose of the experiment was to determine how battery output voltage and the nicotine

solvent effect the concentration of carbonyl compounds produced in the vapors of the newest

electronic cigarette model.

Ten different electronic liquids were used for the experiment with concentrations of

nicotine varying from 18-24 mg/ml. The ten different e-liquids were placed in groupings based

on the contents of their humectants. Products A1-A3 were glycerin based, products A4-A6 were

a mixture of glycerin and propylene glycol, and products A7-A10 were purely proplene glycol

based. In order to see the how the base humectant effects the carbonyl compounds, three controls

were also prepared for the experiment. The controls were made by dissolving liquid nicotine in

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analytical-grade solvents. Control 1 (C1) was a ratio of 88.2% glycerin, 10% redistilled water,

and 1.8% nicotine. Control 2 (C2) was made up of 44.1% glycerin, 44.1% propylene glycol, 10%

redistilled water, and 1.8% nicotine. Control 3 (C3) was composed of 88.2% propylene glycol,

10% redistilled water, and 1.8% nicotine. Each test was performed with a 70mL puff volume,

1.8s puff duration, and puff intervals of 17s. Each test consisted of 30 puffs from each electronic

cigarette. The trial was ran in two series of 15 puffs with a 5 minute break in between series. For

the experiment testing battery output voltage effect on carbonyl compounds found in electronic

cigarettes, the electronic cigarette generated vapor at the battery voltages 3.2V, 4.0V, and 4.8V12.

The controls were utilized for this trial and each voltage was performed three times for each

control for a total of nine runs. Table II. shows the electronic liquid brands, the label

information, and nicotine content for each brand that was utilized for the experiment.

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Table II. Ingredient list with nicotine concentrations for each e-liquid product used12.

Silica gels were impregnated with 2, 4-dinitrophenylhydrazine in order to extract the carbonyl

compounds from the aerosol phase to the solid phase to be examined. These gels were placed in

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between the mouthpiece of the electronic cigarette and the smoking machine in order to trap the

carbonyl compounds that are present in the electronic cigarette vapors. The gels were rinsed with

1mL of acetonitrile. The solvent was then analyzed using HPLC. The elution gradient was made

up of acetonitrile and water and the separation was carried out at 40 ̊ C. Table III. Shows the

carbonyl compounds that were present in the vapors generated by the electronic cigarettes in the

experiment12.

Table III. Carbonyl compounds present in the ten e-liquid solutions12

Table III. shows that all electronic liquids contained at least one carbonyl compound in the

vapors generated by the electronic cigarette. This phenomena could have occurred due to the

high temperatures needed to vaporize the electronic liquid. At these high temperatures, the

solvents could have been catalyzed by the metal coil used to heat the liquid and the solvents

could have undergone thermal decomposition. The humectants present in the bases, propylene

glycol and glycerin, could have been oxidized to form the toxic carbonyl compounds. In this

experiment, formaldehyde, butanol, acetone, and acetaldehyde were the most prevalent carbonyl

compounds observed in the electronic cigarette vapors, while acrolein was not detected at all12.

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The effect of battery output voltage on the carbonyls released in the electronic cigarette

vapors were measured by increasing the battery voltage for each control and measuring the

carbonyl groups using the silica gels saturated in DNPH. Each control was ran three times at

each voltage. The amounts of acetone, acetaldehyde, and formaldehyde that were measure for

each run and each control at each battery voltage output can be seen in Figure XIII.

Figure XIII. The effect of the battery output voltage on carbonyl compound yields from e-cigarettes12

Figure XIII. shows that when the voltage was increased from 4.0V to 4.8V, the amount of

formaldehyde in electronic cigarettes that used a propylene glycol and glycerin mixture base or

purely propylene glycol increased significantly. The acetaldehyde was also significantly

increased in those mixtures when the voltage was increased. Similarly, the amount of acetone

produced experienced a statistically significant increase from 3.2V to 4.8V in the control that

used the base mixture of glycerin and propylene glycol. Glycerin was not as affected by battery

output as the base mixture propylene glycol; however, in this experiment, an increase in voltage

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showed an increase in carbonyl compound yield. Propylene glycol is known to be less viscous

than glycerin. This means that it has a lower optimum temperature that it can be aerosolized.

When voltage is increased, and temperature is increased faster, the reaction rate of the oxidation

of propylene glycol will be increased, which produces more toxic carbonyl compounds. These

results also propose that propylene glycol is more vulnerable to the thermal degradation than

glycerin.

Conclusion:

The vaping community is quickly emerging. Between 2012-2013, the sale of electronic

products increased 320% for disposable electronic cigarettes, 72% for starter kits, and 82% for

cartridges18.Within the next year, revenue from electronic cigarettes are expected to double to

over $1.7 billion and projected to pass traditional cigarette sales by 204719. With its increasing

popularity, the electronic cigarette has rapidly evolving technology that gives the user more

freedom with their personal vaporizing experience. There is still a lot to learn about the chemical

reactions that are taking place within the electronic nicotine devices and how the by-products of

these reactions could affect the user’s body short-term and long term. The refill solutions for

these ever-evolving systems contain aldehydes, heavy metals, volatile organic compounds, food-

grade flavoring, and humectants. Research has only scratched the surface of the chemical

reactions that take place among all these additives. At the high temperatures that are required to

vaporize these solutions, unpredictable behaviors among the compounds take place and

carcinogenic carbonyl compounds are being formed and inhaled17. The inconsistency of the

carbonyl compounds that are formed from the electronic cigarette vapors suggests that at high

temperatures there is a lot more interaction among the compounds within the solvents. From the

studies performed it has been observed that at these high temperatures, the electronic liquid is

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catalyzed by the nichrome wire that incidentally touches the electronic liquid as it is heated to its

vaporization temperature. By the metal coil, the solvent is oxidized to form formaldehyde,

acetaldehyde, acrolein, and acetone. Increase in battery output voltage also proved that these

toxic compounds can be produced in extremely high concentrations. The mechanism reaction for

the oxidation of the solvent to form aldehydes has been determined; however, when food

additives and flavorings are added to the solvent, there is a possibility of more interaction within

the solvent and more toxic by-products being produced due to an acid catalyst being present.

While it is known how the body is affected when these additives are consumed, it is not known

how the body is affected when these additives are inhaled.

Aldehydes have been identified as cytotoxic and carcinogenic and highly toxic to the

body when exposed over a long period of time. In order to further the research on electronic

cigarette reactions and obtain precise results, more research should be performed to determine

the behaviors of electronic cigarette users. With this information, experiments can be ran

similarly to the electronic cigarette user’s behavior so that results are more comparable. Also by

standardizing the analysis of aerosol generation and collection of carbonyl compounds, this

would allow for better comparisons of electronic cigarette vapor and cigarette smoke.

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