What are Polycyclic Aromatic Hydrocarbons?
Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds consisting of two or more fused aromatic rings. Naphthalene (C10H8) forms the smallest parent PAH ring structure with two aromatic rings, while coronene (C24H12) forms the largest parent structure with six aromatic rings that can be detected by modern analytical techniques. PAHs are also known to contain atoms such as sulfur (ex. benzothiophenes), nitrogen (ex. acridine) and oxygen (dibenzofuran) within the structure of their aromatic rings (Figure 1). PAHs that contain atoms other than carbon and hydrogen in their ring structure are called heteroatom PAHs. PAHs are neutral, hydrophobic nonpolar molecules with physical and chemical characteristics, such as solubility in water, hydrophobicity and vapour pressure, varying with molecular weight. These characteristics cause differences in volatility, solubility and biogeochemical cycling. It is important to differentiate between their structural variations to respond to these chemical contaminants and understand their fate and transport in the environment. 2- and 3-ring PAHs are known as low molecular weight PAHs (LMW PAHs), and the 4-, 5-, and 6-ring PAHs are known as high molecular weight PAHs (HMW PAHs). This distinction relates to the physical characteristics of the PAHs and is used to define types of PAH sources. If the PAH contains only cyclic structures it is referred to as a ‘Parent PAH’, and 16 of these are often the target of US investigations based on US EPA guidance, while the Canadian Council of Ministers of the Environment (CCME) has a list of 13 (Figure 2). Other PAHs contain alkyl substituents, such as methyl and ethyl groups. These are referred to as the alkylated PAHs and can be a part extended PAH analysis.
How are they formed and where can they be found?
PAHs are ubiquitous contaminants derived from either diagenic processes such as the thermal maturation of sediment into rock or anthropogenic processes such as combustion of organic material. The diagenetic processes involve the formation of radical intermediates such as ethylene and acetylene that combine to form the parent PAH structures. Further maturation, aids in the production of alkylated PAHs through reactions with these radical intermediates. Anthropogenic processes include:
- Burning fuels such as coal, wood, petroleum, petroleum products, or oil.
- Burning refuse, used tires, polypropylene, or polystyrene.
- Coke production.
- Motor vehicle exhaust.
PAH sources produced from diagenic and anthropogenic processes have been found to contain hundreds of individual PAH compounds. Although diagenic and anthropogenic substances are comprised of many the same compounds, the distribution of these compounds and relative concentrations of individual compounds distinguish them. For anthropogenic sources, the high temperatures and low oxygen conditions promote the formation of parent PAHs over substituted PAHs. In contrast, alkylated PAHs are more abundant in diagenic sources over parent PAHs. This provides valuable information to environmental forensic chemistry since these sources have measurably different amounts of individual PAH compounds.
Importance of Monitoring PAHs
Due to these unique formation reactions, PAHs are used as forensic source identifiers for contamination that can occur via petroleum sources (diagenic formation) or anthropogenic sources (combustion).
One of the most studied PAHs is benzo(a)pyrene (BaP). The mechanism of toxicity of BaP has been well researched and cellular cascade of biochemical reactions begins with binding to the Ah-receptor. This is similar to the mechanisms associated with polychlorinated dibenzo-p-dioxin/polychlorinated dibenzofurans (PCDD/Fs) due to their flat chemical structure. 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. PAHs and PCDD/Fs also interact with the cytochrome P450 1A monoxygenases, a detoxifying enzyme system responsible for their biotransformation. The cytochrome P450 enzymes roll is to detoxify PAHs by making the chemicals more polar, therefore facilitating excretion. These more polar, hydroxylated PAHs can be measured in urine as part of human biomonitoring studies.
- Polychlorinated Biphenyls (PCBs)
- Incidental PCBs
- Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs, Dioxins)
- Ignitable Liquid Residue (ILR)
- Petroleum Hydrocarbons (TPH/PHC F1 to F4)
- Benzene, Toluene, Ethylbenzenes, Xylenes (BTEX)
- Volatile Organic Compounds (VOCs)
- Persistent Pesticides/ Persistent Organic Pollutants (POPs)
- Naphthenic Acids
- Legal Sampling
- Chain of Custody
- Study Design
- Data Analysis and Visualization
- Data Wrangling
- Multivariate Statistical Analysis
- Principal Component (PCA); Hierarchical Cluster (HCA)
- Science Communication
- Data Science/Big Data
- Multidimensional Gas Chromatography (GC×GC)
- Source Apportionment
- Chemical Fingerprinting
- Diagnostic Ratios
- Fate and Transport
- Arsonous Wildfires
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Chemistry Matters Consulting Services and Expertise
The Chemistry Matters team serves as subject matter experts for investigations with PAH data. This includes large complex sites (e.g., US Superfund cases), emergency spill response and environmental monitoring, air monitoring at oilfield operations and combustion facilities, and river floodplain site investigations. Each of these situations presents a different set of requirements and the nature of PAH data interpretation is tailored to suit the needs.
Large US Superfund sites have numerous historic PAH impacts and form a complex scenario, often along with other chemicals of concern. Our work in these cases involves providing evidence for the source of PAHs using source identification techniques described above and presented using data visualization, as well as the review and extraction of information relating to historic industrial processes and site investigations.
Our involvement in oil spills ranges from emergency response using PAH results to help direct clean-up activities and fast response to regulators. Distinguishing PAHs from the release oil from other sources is important in these cases to ensure that environmental results are correctly interpreted. Over longer periods of time, we have provided the interpretation of PAH data to describe how weathering mechanisms are providing natural attenuation of oil residuals, which is important for determining closure endpoints.
The Chemistry Matters team has over 20 years of experience providing consultation services for complex PAH monitoring studies. Through strategic partnerships, we provide our clients with cutting-edge technology to solve complex issues in environmental forensics, chemometrics, and petroleum fingerprinting. Chemistry Matters can provide full-service subject matter expertise from analysis to data interpretation to communication of results.
PAH Source Identification and Fingerprinting
As these chemical contaminants are ubiquitous in the environment, it is important from an environmental consulting perspective to understand where they are derived from. For instance, regions such as the Oil Sands may exhibit increased levels of PAHs not due to industrial contamination, but due to diagenic processes that have occurred naturally. The plethora and varying distribution of PAHs present in an environmental sample provide unique chemical fingerprints for environmental forensics investigations. Source identification for PAHs can be especially difficult when there are multiple source inputs and when these inputs are of similar origin. To breakthrough these challenges, the consultants at Chemistry Matters use customized statistical analysis and receptor modeling tools to differentiate between sources. A common approach is to use diagnostic ratios and double ratio plots to group samples based on indicator PAHs (Figure 3). Quantification of source contributions to a set of samples can be conducted using Receptor modeling, which we have used for describing PAH sources in samples ranging from oil spills to air monitoring.
Fate and Transport
The fate and transport of PAHs is directly influenced by their physical-chemical properties. PAHs exhibit a large degree of hydrophobicity, which increases with increasing molecular weight. The volatility of these compounds is also a function of molecular weight, where volatility decreases with increasing molecular weight. Due to their insolubility in water, their mobility is limited in the aquatic environment. Although their mobility is limited in water, their prevalence in the aquatic environment stems from their high affinity to sorb onto suspended organic solids and organic-rich sediments. The degree at which these compounds will be transported is a function of their molecular weight. Low-molecular weight PAHs (2-3 ring structure) exhibit a reduced amount of hydrophobicity with increasing volatility resulting in them being present in both the aquatic environment and the atmosphere. Due to these intrinsic properties, lower molecular weight PAH compounds exhibit more biological uptake and biodegradation. Alternatively, higher molecular weight PAHs are limited to the suspended organic solids or sediments. PAH transport to the atmosphere is commonly associated anthropogenic sources, such as emissions generated from combustion of organic matter. The extent of atmospheric transport is dictated by the volatility of the PAH, with lower molecular weight PAHs being more readily volatilized in the atmosphere.
At Chemistry Matters, we have extended our expertise in PAH analysis by analyzing data sets beyond the routinely measured 16 US EPA and 13 CCME PAHs to identify sources in our forensic investigations. In order to identify sources of PAHs, alkylated PAHs and heterocyclic PAHs are incorporated into our investigations. These compounds pose a greater environmental threat as they exhibit increased hydrophobicity, decreased solubility in water and increased resistance to in-situ degradation. There is also a lack of historical evidence regarding these compounds as conventional analytical methods (16 US EPA Parent PAHs) could only resolve the parent compounds in environmental samples. These outdated analytical methods no longer reflect the current technology and are still heavily relied on in litigious cases. At Chemistry Matters, we employ novel analytical methods that provide additional context towards the complexity associated with the PAH distribution in samples that cannot be achieved with conventional methods. We utilize the extremely high peak capacity of Comprehensive Two-dimensional Gas Chromatography (GC×GC) as an analytical method for the analysis of not only parent PAH, but their alkylated isomers (Figure 4). Chemistry Matters is the only consulting company worldwide that can provide expertise in alkylated isomer identification and quantification utilizing these novel GC×GC approaches.
The Chemistry Matters team is actively involved in publishing research in peer-reviewed journals that highlight the advantages of using GC×GC compared to 1D GC in litigious investigations involving PAHs. Partnership with Chemistry Matters, allows the use of GC×GC plots and statistical analysis to generate compelling visuals that can be understood by attorneys, judges, regulators, and the general public.
Sandau, C., Idowu, I., Richards, P., Stetefeld, J., Tomy, G. “Chemical fingerprinting of Polycyclic Aromatic Compound (PACs) sources in sedisments using gas chromatography mass spectrometry”. 26th International Symposium on Polycyclic Aromatic Compounds (ISPAC), 9 September 2019, Örebro, Sweden.