100 million unique chemical substances registered in CAS database

This year marks the 50th anniversary of CAS Registry and also the 100 Millionth CAS Registry Number. CAS Registry is world’s largest database of unique chemical substances. It identifies each chemical substances with a specific CAS registry number, index name, and graphic representation of its chemical structure. Information in CAS Registry about the many different types of substances includes: synonyms, molecular formulas, ring analysis data, structure diagrams, experimental and predicted property data, and literature references.

Substances included in CAS Registry are from journal articles, patents, chemical catalogs, and reputable web sources from the early 1800s to present. Organic chemicals with MW<1000 are dominant in CAS Registry. In addition to organic chemicals, CAS Registry also include alloys, coordination compounds, minerals, mixtures, polymers, salts, sequences, organometallics, proteins, and inorganic substances.

As chemical innovation continues at the current pace, it is expected chemical substances in CAS REGISTRY in 50 years would be more than 650 million. and the number would be significant higher with the past growth trajectory.

Such large number and fast increasing of chemical substances require high-throughput approaches be applied for hazard and risk assessment and chemical management.

Image credit: 2015 American Chemical Society

Image credit: 2015 American Chemical Society

References
CAS Content at a Glance (http://www.cas.org/content/at-a-glance)

Plastic microbeads in personal care products

Plastic (mainly polyethylene) microbeads are used in thousands of cosmetics and toothpaste around the world. Plastic microbeads used in personal care products are generally in particle sizes from 10 µm to 100 µm. Such particles serve the function of washing or rubbing a part of the body to remove dead cells from the surface of the skin. 

photo credit: http://marine-conservation.tumblr.com/post/86646501131/california-bill-would-ban-microplastics-in

Plastic microbeads are used in large quantities and are washed down the drain after use. However, they cannot be efficiently filtered or removed during sewage treatment. As a result, a large amount of plastic particles enter the environment, cause water pollution. Plastic beads eventually end up in the oceans, which are described as "plastic soup" due to a large amount there .   The microbeads are not ready to degrade and can pass through the marine food webs. High concentrations of micro plastic have been found in inland water such as the Great Lakes and in the oceans and also in the body of various marine creatures. 

It is estimated that over 90% of micro plastic in the environment are from the plastic microbeads used in personal care products (Eriksen, Marcus et al. (2013) Microplastic pollution in the surface waters of the Laurentian Great Lakes". Marine Pollution Bulletin 77: 177–182. doi:10.1016/j.marpolbul.2013.10.007). 

Due to the potential environmental problems such plastic microbeads are being removed from personal care products and replaced by naturally biodegradable alternatives by some manufactures.  However, there are still a far cry to say that all personal care products contain plastic microbeads. I recommend we check whether the personal care product we are using still contain environmental unfriendly plastic microbeads from this websites: 

http://beatthemicrobead.org/en/product-lists

Neonicotinoids pesticides

Recently, the word "neonicotinoids" as a group of insecticides appear frequently in various news reports because the concerns of their association with the increasing mortality rate of honeybees. Because of the potential impact on the pollinators including honeybees, neonicotinoids are at a stage of "near ban". High priority for conducting further risk assessment on neonicotinoids is given by chemical regulations agencies.

Let's have a closer look at neonicotinoids

As indicated by the name, neonicotinoids are related to nicotine.

nicotine

Neonicotinoids is a group of pesticides most widely used in the world. The group includes

acetamiprid

clothianidin

imidacloprid

nitenpyram

nithiazine

thiacloprid 

thiamethoxam. 

Neonicotinoids act on the central nervous system of insects. Neonicotinoids can bind to some receptors in the nervous system and prevent impulses transmitting between nerves. This biochemical action mechanism would make insecticides exposed to neonicotinoids unable to move and eventually die.

Neonicotinoids have high water solubility and break down slowly in soil. They are often applied to soil and be taken up by plants and provide protection from insects as the plant grows.

When developed, neonicotinoids were claimed to have low-toxicity to many beneficial insects, including bees. However,  toxic effects to bees and other beneficial insects have been realized recently. Bees and other pollinators expose to neonicotinoids through nectar and pollen where neonicotinoids are used in agriculture for pest control. Although the exposed levels are sub-lethal , they may impact some bees’ ability to navigate to flowers and nectar.

Despite some controlled studies, more information is still needed to link neonicotinoids to the bee colony collapse disorder and increased mortality rate of honeybees. However, to keep potential risk to bees and other beneficials low, Corn farmers in Ontario and Quebec are requested to use dust deflectors on their seed planters, and that the seed companies use a seed lubricant to minimize the dust that is emitted during planting.

References and more information:

http://www.theglobeandmail.com/report-on-business/new-usda-survey-reveals-alarming-increase-in-honeybee-mortality-rate/article24420306/
http://www.wsj.com/articles/over-40-of-honeybee-colonies-died-in-year-usda-says-1431531544
http://www.huffingtonpost.com/jon-entine/neonics-not-key-driver-of_b_6928578.html
http://www.bulletinofinsectology.org/pdfarticles/vol67-2014-125-130lu.pdf

Natural polychlorinated organic compounds produced by snow flee as predator repellent

A lot of organic pollutants posing negative impact on the environment and on human health are chlorinated. Well known chlorinated hazardous organic chemicals include the well known DDT, PCBs, HCB,  Mirex, Dioxin, etc. All of the 12 persistent organic pollutants (dirty dozens) initially listed in the Stockholm Convention for international regulations are chlorinated compounds. Many of the chlorinated compounds were used as pesticides.

Halogenated organic compounds are seldom found as natural products. Recently, researcher Schulz and colleagues in Germany found one natural chlorinated compounds from the snow flee, a winter-active species of springtail, produces unique polychlorinated octahydroisocoumarins to repel predators. Sigillin A, the active component with five chlorine atoms,  showed high repellent activity in a bioassay with predatory ants.

The snow flee is easily ignored as it is only a few millimeter long, just list some pallets of black pepper on the snow. But such tiny flees have their unique strategy to deal with their predators. They can synthesize their own pesticide Sigillin A to repel ants, spiders etc.

Snow flees are quite efficient at synthesizing Sigillin A. despite Sigillin A is such a complicated organic molecules. Sigillin A synthesized by a snow flee can be 0.2% of it tiny body weight.  It is still unclear whether Sigillin A is synthesized by snow flees themselves or by microorganisms in the body of a snow flee.

Effort has been make to synthesize this natural chlorinated in the lab. Chemists are looking forward to producing this natural chlorinated compound and use it for termite control.

However, natural generated does not mean it is environmental safe when synthesized and used in large quantity. It's environmental persistence, bioaccumulation potential, and toxicity needs to be carefully evaluated before large quantities is produced and used

Green Chemistry

Green chemistry , based on the definition by the USEPA, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances. 

There are 12 principles involved in Green chemistry

1. Prevent waste: Design chemical synthesis, purification, analysis processes to prevent waste. Leave no waste to treat or clean up.
2. Design safer chemicals and products: Design chemical products that are fully effective yet have little or no toxicity.
3. Design less hazardous chemical syntheses: Design chemical syntheses, purifications, analyses that generate substances with little or no toxicity to either humans or the environment.
4. Maximize atom economy: Design syntheses so that the final product contains the maximum proportion of the starting materials. Waste few or no atoms.
5. Use safer solvents and reaction conditions: Avoid using solvents, separation agents, or other auxiliary chemicals. If you must use these chemicals, use safer ones.
6. Increase energy efficiency: Run chemical reactions at room temperature and pressure whenever possible.
7. Use renewable feedstocks: Use feedstocks  (also known as starting materials) that are renewable rather than depletable. The source of renewable feedstocks is often agricultural products or the wastes of other processes; the source of depletable feedstocks is often fossil fuels (petroleum, natural gas, or coal) or mining operations.
8. Avoid chemical derivatives: Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.
9. Use catalysts, not stoichiometric reagents: Minimize waste by using catalytic reactions. Catalysts are effective in small amounts and can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and carry out a reaction only once.
10. Design chemicals and products to degrade after use: Design chemical products to break down to innocuous substances after use so that they do not accumulate in the environment.
11. Analyze in real time to prevent pollution: Include in-process, real-time monitoring and control during syntheses to minimize or eliminate the formation of byproducts.
12. Minimize the potential for accidents: Design chemicals and their physical forms (solid, liquid, or gas) to minimize the potential for chemical accidents including explosions, fires, and releases to the environment.

Modern analyses of ancient beer

A recent issue of the Journal of Agriculture and Food Chemistry published an interesting article on chemicals in beers from an 1840s’ shipwreck. 

(photo source: http://www.sci-news.com/, Image credit: John Londesborough et al.) 

In 2010, underwater archaeologists discovered an old schooner south of the Åland Islands at a depth of 50 m in the Baltic Sea. Archeological evidence suggests the shipwreck occurred during the 1840s, but the schooner’s name, its destination and its last port-of-call were not identified. Archeologist brought the cargo consisting of luxury items, including more than 150 bottles of champagne and five bottles that look like typical early 19th century beer bottles to the surface. One of these cracked in the divers’ boat. The liquid that foamed from the cracked bottle looked and, according to the divers, tasted like beer.

Researchers from Finland analyzed two bottles of the beer from the shipwreck  and six bottles of modern  beers (Leffe Brune, Koff Porter, Weihenstephan Hefe Weissbier, Paulaner Hefe Weissbier, Aldaris Porteris Alus, and Olvi Sandels) as reference. The founding provides clues for beer makers to resurrect the flavors of ages past.

The beer samples were degassed by ultrasonification and filtered (0.45 μm). An aliquot (5 μL) was loaded to an HPLC-MS/MS system (high performance liquid chromatography coupled with tandem mass spectrometry) and analyzed for hops related compounds (native or degradation products).

========================================================================
Sidenote (from Wikipedia): Hops are the female flowers of the hop plant, Humulus lupulus.

Hops are used primarily as a flavoring and stability agent in beer. Hops impart a bitter, tangy flavor to beer. Hops have antibacterial effect that favors the activity of brewer's yeast over less desirable microorganisms and for many purported benefits such as balancing the sweetness of the malt with bitterness and contributing a variety of desirable flavors
========================================================================

Above: Chemical structures of bitter chemicals identified in hops and freshly brewed beer; Below: Chemical structures of bitter chemicals identified in aged beer (Source: J. Agric. Food Chem. 2010, 58, 7930–7939)
Based on the chemical fingerprint in the beer, the two bottles of ancient beer were identified as two different kind of beer.  One was more strongly hopped than the other. 

High levels of organic acids, carbonyl compounds, and glucose indicated extensive bacterial and enzyme activity during aging. However, concentrations of yeast-derived flavor compounds were similar to those of modern beers, except that 3-methylbutyl acetate

was unusually low in both beers and 2-phenylethanol

and possibly 2-phenylethyl acetate

were unusually high in one beer. Concentrations of phenolic compounds were similar to those in modern lagers and ales

Besides hops related chemicals, the researchers also analyzed other chemical components in the ancient beer. Compared to typical modern lagers and ales, ethanol contents of the shipwreck beers were low (2.8-3.2%). Glycerol and ethanol had a ratio of 4.5% for both shipwreck beers, which is typical for a yeast fermentation product. Both beers were acidic, but their pH were ~1 below modern values. The color strengths were in the range of modern ales and lagers, and much lower than porters or stouts. Possibly been oxidized after over the 170 years, sulfur dioxide was not detected in shipwreck beers. Protein levels were very low in both beers.

References: 

• John Londesborough, Michael Dresel, Brian Gibson, Riikka Juvonen, Ulla Holopainen, Atte Mikkelson, Tuulikki Seppänen-Laakso, Kaarina Viljanen, Hannele Virtanen, Arvi Wilpola, Thomas Hofmann, and Annika Wilhelmson J. Agric. Food Chem. 2015, 63, 2525−2536 DOI: 10.1021/jf5052943 Analysis of Beers from an 1840s’ Shipwreck 2015, 63 (9), pp 2525–2536
• Gesa Haseleu, Annika Lagemann, Andreas Stephan, Daniel Intelmann, Andreas Dunkel and Thomas Hofmann. Quantitative Sensomics Profiling of Hop-Derived Bitter Compounds Throughout a Full-Scale Beer Manufacturing Process. J. Agric. Food Chem., 2010, 58 (13), pp 7930–7939 DOI: 10.1021/jf101326v
• http://www.sci-news.com/othersciences/chemistry/science-beer-1840s-shipwreck-finland-02567.html

Purifying urban air with pavement containing titanium dioxide

A recent study published in the journal of Atmospheric Environment by Folli et al. demonstrated the effectiveness of cleaning NOx in urban air via photocatalytic oxidation using titanium dioxide as semiconductor photocatalyst added to pavement.

(source of image: www.mapfre.com)

The advantage of such technology is that photocatalystic oxidation only requires sunlight, existing oxygen and water in air. Therefore, remediation is continuous in day light.

In this study, researchers conducted a year-long field test in the city of Copenhagen. They employed two continuous air monitoring stations, one in the area with photocatalytic concrete pavers and the second one in the area without photocatalytic concrete and assessed the effectiveness of TiO2 containing pavement in removing NOx in the air.

The study indicates that a monthly abatement of NO was around 22% in the summer months;
NO noon abatement was >45% at the summer solstice, which corresponded to NOx noon abatement > 30%.

Personally, I think such technology is very promising although more research work needs to be done. Research questions worth to be addressed include but not limited to

• Does the technology also has the benefit of degrade other toxic chemicals in urban air such as polycyclic aromatic compounds?

• What's the over all benefit of such technology if widely used in urban pavement and building walls and how does the benefit of removal toxic chemicals compared to the cost. 

• What degradation products can be generated and how do they affect urban air and environmental quality? 

 In addition to the article by Folli et al. mentioned above http://dx.doi.org/10.1016/j.atmosenv.2015.02.025, the following articles on the removal of pollutants by adding TiO2 in cement are also informative on this topic.

• Folli, J.Z. Bloh, M. Strøm, T. Pilegaard Madsen, T. Henriksen, D.E. Macphee Efficiency of solar-light-driven TiO2 photocatalysis at different latitudes and seasons. Where and when does TiO2 really work? J. Phys. Chem. Lett., 5 (5) (2014), pp. 830–832 http://dx.doi.org/10.1021/jz402704n

• J. Ângelo, L. Andrade, L.M. Madeira, A. Mendes An overview of photocatalysis phenomena applied to NOx abatement J. Environ. Manag., 129 (2013), pp. 522–539 http://dx.doi.org/10.1016/j.jenvman.2013.08.006

• M.M. Ballari, H.J.H. Brouwers Full scale demonstration of air-purifying pavement J. Hazard. Mater., 254–255 (2) (2013), pp. 406–414 URL http://dx.doi.org/10.1016/j.jhazmat.2013.02.012

Chemistry of skin lipids

Human body is covered with skin surface lipids. The majority of skin lipids are sebaceous lipids which are secreted by sebaceous glands.  Epidermis also produce a tiny fraction of total extractable surface lipid. On skin areas rich in sebaceous glands such as forehead,  epidermal  lipids are 5-10 µg/cm3 compared with 150-300 µg of sebaceous lipids.

Chemicals that constitute skin surface lipids include non-polar lipids, mainly triglycerides, wax esters, squalene, fatty acids and smaller amounts of cholesterol, cholesterol esters and diglycerides

Carbon-carbon double bonds involve in many of these constituents.  Such chemicals are readily to react with ozone. On a molar basis, squalene is responsible for roughly 50% of the unsaturated carbon bonds in skin surface lipids. Hence, squalene is the most important individual constituent in terms of ozone consumption.

The fractional contribution and  levels of skin lipids change with human ages. For example, the amounts of omega-7 unsaturated fatty acids

 have been found to increase with increasing age

Reactions between ozone and skin lipids generally follow the Criegee mechanism

(Youtube education material on Criegee mechanism of ozonolysis)

The ozone-skin lipids reaction products cover a range of volatility. These more volatile products are found primarily in the gas-phase while the less volatile primary and secondary products are found primarily in the condensed phase (e.g., skin, hair, clothing surfaces,  airborne particles,surface film).

Based on the Criegee mechanism, the ozone/squalene reaction is anticipated to produce aldehydes and organic acids with 27, 22, or 17 carbon atoms and five, four, or three double bonds, respectively and hydrogen peroxide (H2O2), organic peroxides (ROOR) , and short-lived highly reactive products including hydroxyl (OH.), hydroperoxyl (HOO.), and alkyl peroxyl radicals (ROO.). These radicals will react with squalene and primary and secondary products producing additional carbonyls, dicarbonyls, and hydroxycarbonyls.  Given the number of double bonds in these less volatile products, they themselves readily react with ozone to generate still more products.

The reaction could generate products with high hydrophilicity, which increases their transdermal penetration and redox activity. Such products from ozonolysis could pose potential risk  to human health.

image source: http://www.cdc.gov/niosh/topics/skin/

Some of the primary and secondary ozone/lipids reaction products can condense on existing particles or nucleate to form new particles or secondary organic aerosols (SOA) with size ~10-1000 nm. Based on chamber experiments, the size of SOA formed depends on ozone concentrations with larger size at higher O3 concentrations. 

The reaction of O3 with lipids at skin surface involves the deposition of ozone to skin surface and the chemical reaction.  Various studies suggest the actual reaction is very fast and the overall rate limit step is the deposition of ozone to the surface. 


References and further information: 

• Weschler CJ, Roles of the human occupant in indoor chemistry. Indoor Air. 2015 http://dx.doi.org/10.1111/ina.12185
• Apostolos Pappas. Epidermal surface lipids.2009 1(2): 72–76
  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2835894/pdf/de0102_0072.pdf

Firework as a source of perchlorate-a contaminant of emerging concern

Potassium Perchlorate (KClO4) as an oxidizer is commonly used in fireworks to produce temperatures of 1700 to 2000°C. Such high temperature make it possible to excite the electrons surrounding the nuclei of metal atoms and when the absorbed energy is emitted, different atoms give out specific colors. 

Firework is an important source of perchlorate to aquatic environment. Monitoring data show over four percent of public water systems have detected perchlorate.  In a human biomonitoring campaign, perchlorate was found in all human urine samples collected from the U.S. The chemical was also found in cow's milk in California with an average level of 1.3 µg/L, which may have entered the cows through feeding on crops that had exposure to water containing perchlorates. Studies have indicated that perchlorate can disrupt the thyroid’s ability to produce hormones needed for normal growth and development.

Because of the adverse effect on the health and frequent detection in public water systems at levels causing a public health concern, the U.S. Environmental Protection Agency issued a "regulatory determination" in 2011 that perchlorate meets the Safe Drinking Water Act criteria for regulation as a contaminant. Prior to issuance of its regulatory determination, the U.S. EPA issued a recommended Drinking Water Equivalent Level (DWEL) for perchlorate of 24.5 µg/L. Such regulation provides
opportunity for health risk reduction for the between 5.2 and 16.6 million people who may be
served drinking water containing perchlorate.

References and more to read:

http://water.epa.gov/drink/contaminants/unregulated/perchlorate.cfm
http://water.epa.gov/drink/contaminants/unregulated/upload/FactSheet_PerchlorateDetermination.pdf
http://pubs.acs.org/doi/abs/10.1021/es504444e
Blount, B. C.; Valentin-Blasini, L.; Osterloh, J. D.; Mauldin, J. P.; Pirkle, J. L. Perchlorate exposure of the US Population, 2001−2002. J. Exposure Sci. Environ. Epidemiol. 2007, 17 (4), 400−407.

Chemicals in fireworks (2)

In addition to the chemicals mentioned the previous post,  there are other chemicals in fireworks and support their functions

Combustion agents, which generate high temperature after the fireworks are ignited. The major chemical component of combustion agents include fuel and oxidants that support combustion. Besides black powder, some metals (Al, Mg, Fe, Zn), petroleum components, phosphor, sulfur organometal also serves as fuels of fireworks. The combustion of these fuels requires the support of oxygen. However, at the fast rate of combustion with fireworks, the oxygen in air is consumed faster than can be refilled by surrounding air. Therefore, different oxidants are also added to fireworks to support the abrupt chemical reactions.

The most common oxidizer is potassium nitrate, which decomposes to potassium oxide, nitrogen gas, and oxygen gas.

Sometimes more explosive oxidizers, which produced temperatures of 1700 to 2000°C and made possible the creation of much more intense colors. These oxidizers are the chlorates and perchlorate

decomposition of potassium nitrateThe chemicals as oxidants generally involve nitrate, chlorate, bromorate, and perchlorate.

Fireworks use chlorate and perchlorate as the oxidizer often include catalyzers . The function of catalyzers is to decrease the required temperature for certain chemical reactions. Catalyzers often include some transition elements, for example, CoO2, Cr2O5, CuO, TiO2, PbO2, Pb3O4.

Besides the chemical components that take part in chemical reactions during firework combustion, there are also other inner component such as sulfate, phosphate, carbonate and natural resin that make the different chemicals stay together.

Sometimes fireworks are used during the day time. In such case, smoke from fireworks plays a more important role than the light from fireworks. Such fireworks used during the day time also includes smoke generating agents which forms many aerosol particles with different sorption and refraction on the light to give different colors. Such chemicals that form aerosol particles include yellow phosphor, white phosphor, hexachloroethane, alumimum powder, zinic oxide, and dyes

With so many chemicals in fireworks, what's the environmental impact caused by these chemicals in fireworks? Let's continue next time.

Chemistry of fireworks

This week unfolds the Lunar year of Sheep.  Lunar new year has been celebrated with fireworks for almost a thousand years. What's the science in fireworks. Let's explore by starting with black powder. 

Black powder was one of the greatest inventions of ancient China. It was invented  by alchemists experimenting with a naturally-occurring salt, potassium nitrate while looking for an elixir of immortality. But in handling and heating the sensitive substance they inevitably discovered its explosive properties. 

ancient book documenting how to make black powder

Although the first known account of the use of gunpowder as a weapon dates to 1046 in China, the use of black powder to make fireworks was documented during the Song Dynasty around the 1200s. 

There are various chemical components in fireworks. The major chemical component is the lighting agent. The ideal lighting agent requires a long time of lighting from chemical reactions. Flare of firework needs to be supported with heated solid and liquid microparticles that release energy via chemical reaction. The temperature of flare from lighting agents can be over 2000 Celsius. The effieciency of lighting agent depends on the content of magnesium. The most commonly used lighting agent in fireworks contains 55% magnesium, 40% sodium nitrate and ~5% synthetic resin. The time of lighting can last from 30 seconds to a few minutes.

Besides lighting agent, there is also a component called flashing agent in fireworks. Flashing agent generate bright light in a short time (0.1 s).  One type of flashing agent include aluminum-magnesium alloy, which is relatively safe. Another type of flashing agent additionally include potassium perchlorate (KClO4).

The other component of firework is called coloring agent. Coloring agents are comprised of different metal salts. Chemical reaction by the lighting and flashing agent generate large amount of heat. Such energy will excited the electrons of metal elements.  This high-energy excited state does not last for long, and the excited electrons of the elements quickly release their energy. The amount of energy released can be characterized by a particular wavelength of light and varies from element to element.  Higher energies correspond to shorter wavelength light, whose characteristic colors are located in the violet/blue region of the visible spectrum. Lower energies correspond to longer wavelength light, at the orange/red end of the spectrum.

The colors you see exploding in the sky are produced by the elements with the characteristic emissions listed in the table (source: http://scifun.chem.wisc.edu/chemweek/fireworks/fireworks.htm)

	Color	Compound	Wavelength (nm)
	red	strontium salts, lithium salts lithium carbonate, Li2CO3 = red strontium carbonate, SrCO3 = bright red	652
	orange	calcium salts calcium chloride, CaCl2	628
	yellow	sodium salts sodium chloride, NaCl	610-621
	green	barium compounds + chlorine producer barium chloride, BaCl2	589
	blue	copper compounds + chlorine producer copper(I) chloride, CuCl	505-535
	purple	mixture of strontium (red) and copper (blue) compounds	420-460
	silver	burning aluminum, titanium, or magnesium

More coming in the next post. ..

Chemicals associated with E-Cigarettes

Electronic cigarette is also referred as e-cig or e-cigarette, which is a battery-powered vaporizer which has a similar feel to tobacco smoking.

Third generation of e-cigarette that have organic light-emitting diode displays and buttons to adjust wattage or voltage.

Credit: Shutterstock/C&EN

Electronic cigarettes do not contain tobacco, although they do use nicotine from tobacco plants. They do not produce cigarette smoke but rather an aerosol. In general, they have a heating element that atomizes a liquid solution known as e-liquid.  E-liquid, also referred as e-juice or simply "juice", is a liquid solution that when heated by an atomizer produces vapor. The main ingredients of e-liquids are usually a mix of
propylene glycol (PG),

glycerin (G)

and/or polyethylene glycol 400 (PEG400),

sometimes with differing levels of alcohol mixed with concentrated or extracted flavourings;

E-cigarette fluid or “e-juice” comes in thousands of flavors, including pineapple custard and Scooby snack.

Credit: Associated Press

and optionally, a variable concentration of tobacco-derived nicotine.ingredients but without nicotine.

The solution is often sold in bottles or pre-filled disposable cartridges, or as a kit for consumers to make their own eJuices. Components are also available to modify or boost their flavour, nicotine strength, or concentration of e-liquid. Pre-made e-liquids are manufactured with various tobacco, fruit, and other flavors, as well as variable nicotine concentrations (including nicotine-free versions). Surveys suggested that the most liked e-liquids had a nicotine content of 18 mg/ml, and largely the favorite flavors were tobacco, mint and fruit. The flavorings may be natural or artificial.

Flavoring substances not identified in a natural product intended for human consumption, whether or not the product is processed. These are typically produced by fractional distillation and additional chemical manipulation of naturally sourced chemicals, crude oil or coal tar.

Most artificial flavors are specific and often complex mixtures of singular naturally occurring flavor compounds combined together to either imitate or enhance a natural flavor. These mixtures are formulated by flavorists to give a food product a unique flavor and to maintain flavor consistency between different product batches or after recipe changes. The list of known flavoring agents includes thousands of molecular compounds, and the flavor chemist (flavorist) can often mix these together to produce many of the common flavors.

Chemical	Odor
Diacetyl	Buttery
Isoamyl acetate	Banana
Benzaldehyde	Bitter almond
Cinnamaldehyde	Cinnamon
Ethyl propionate	Fruity
Methyl anthranilate	Grape
Limonene	Orange
Ethyl decadienoate	Pear
Allyl hexanoate	Pineapple
Ethyl maltol	Sugar, Cotton candy
Ethylvanillin	Vanilla
Methyl salicylate	Wintergreen

References and more to read:
http://cen.acs.org/articles/93/i7/Boom-E-Cigarettes-Sparks-Calls.html
http://en.wikipedia.org/wiki/Electronic_cigarette#Atomizer
http://health.howstuffworks.com/wellness/smoking-cessation/10-facts-about-e-cigarettes.htm
http://en.wikipedia.org/wiki/Flavor