Polycyclic Aromatic Hydrocarbons (PAHs)



Polycyclic aromatic hydrocarbons (aka polyaromatic hydrocarbons) and shortened to PAHs are ubiquitous contaminants (everywhere) in the environment and thats why you should know more about them. They are present in the air we breathe and the fish and food we eat. PAHs are derived in natural processes, diagenic processes (when sediment turned to rock) and most importantly from combustion processes. PAHs are present in low percentage amounts in crude oil, making them important for those working in the oil industry. Their prevalence in the environment makes their understanding important for all environmental practitioners so different sources of PAHs can be identified when conducting and reporting environmental measurements.

PAHs consist of two or more fused aromatic rings. They predominantly consist of carbon or hydrogen but may also contain oxygen, nitrogen and sulfur. When PAHs contain these other elements, they are called heterocyclic PAHs, but these a rarely measured or reported in most routine PAH studies. The standard suite of PAHs are the 16 priority PAHs (ones with regulatory requirements) which have been identified and measured in EPA analytical method which was first developed prior to 1977. These 16 PAHs have been the standard suite measured ever since. The 16 PAHs are some of the more abundant PAHs with ring structures consisting of 2 to 6 rings and no alkyl side chains. PAHs without alkyl side chains are referred to as parent PAHs. Analytical methods have improved in the last 40 years allowing separation and detection of these alkyl PAHs resulting in 100s of potential PAHs and substituted (alkyl) PAHs to be measured in samples.

“PAHs consist of two or more fused aromatic rings. They predominantly consist of carbon or hydrogen but may also contain oxygen, nitrogen and sulfur.”Court Sandau, PhD, PChem
One of the most studied PAHs is benzo(a)pyrene (BaP). The mechanism of toxicity of BaP has been well researched and begins with binding to the Ah-receptor, the same receptor as the PCDD/Fs due to their similar flat chemical structures. PAHs and PCDD/Fs also interact with the cytochrome P450 1A monoxygenases, a detoxifying enzyme system responsible for their biotransformation. This well studied Ah-receptor binding is the toxicity measure used during risk assessment to evaluate the toxic equivalents of PAHs and PCDD/Fs mixtures. The cytochrome P450 enzymes roll is to detoxify PAHs by making them more polar facilitating excretion. These hydroxylated PAHs can be measured in urine as part of human biomonitoring studies.

This plethora of PAHs in an environmental sample provide a chemical fingerprint that can be used to determine the source or sources of PAHs making PAHs common analytes for environmental forensics investigations. PAHs can be used to identify different crude oil spills in the environment and even different sources from different geological formations for geoforensics investigations. Examples are shown in the figures below where graphs of PAHs can be compared visually to examine sources or where data can be statistically analyzed using principal component analysis. For PCAs, all the data is used to assess similarities of samples where samples plotting closer together are related. Another common approach is to use diagnostic ratios and double ratio plots to group samples based on indicator PAHs. The figure below shows the double ratio plot commonly used for PAHs in the literature.

We have used extended PAH analysis (more than the 16 priority PAHs) to identify sources in air samples from oil recovery or from combustion sources, to identify mixed sources of oil from a release into the environment, and to assess types of combustion that occurred on historical sites. In order to identify sources of PAHs, these alkyl PAHs and heterocyclic PAHs can be invaluable. The science will be moving PAH assessment to new levels in the near future as researchers continue to identify more PAHs of environmental importance.

PAH Fingerprinting

PAH Fingerprinting

PAH double ratio plot

PAH Double Ratio Plot

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