PAH Analysis: Expanded Compounds of Concern and Advancements
By Lee Wolf, Regulatory Affairs Manager, ALS Environmental – USA
As a class of organic compounds, PAHs are characterized by bonded aromatic rings that do not typically carry other functional groups or branched groups substituted for hydrogen atoms. PAHs occur in fossil fuel materials such as oil, coal, tar and fuels. They are produced as a result of fuel burning. They are also found in products such as burned tobacco, incense, and some plant-based oils. To further understand the sources of PAHs, they may be classified as follows:
- Petrogenic – These are PAHs derived from petroleum inputs and generally associated with fossil fuels.
- Pyrogenic – These are PAHs which are derived from combustion sources.
- Biogenic – These are PAHs formed from natural biological processes.
The toxicity of PAHs is dependent upon the structure or arrangement of aromatic rings. For example, the toxicity of some PAH isomers (with the same formula and number of rings) can vary from being effectively nontoxic to being very toxic. The more toxic or carcinogenic PAHs may be small or large. The USEPA has identified seven PAH compounds as probable human carcinogens.
PAHs are lipophilic, meaning they will dissolve more easily in fats, oils, and lipids than in water, leading to a greater accumulation in a wide variety of organisms and animals, including humans. Due to the toxic nature of several of the more common PAHs, this accumulation can in turn lead to carcinogenic or mutagenic effects. Other negative health effects have been identified and studied, as well. Bioaccumulation of PAHs in marine organisms, such as edible shellfish, is commonly studied due to their pathways to humans. The effects of high prenatal and childhood exposure in humans are being studied as well.
Applications and Data Uses
In environmental testing, the analysis of samples for PAHs was included in some of the earliest analytical protocols and methods. Early EPA methods focused on 16 common PAHs, which were listed by the EPA as “priority pollutants” with analyses performed using GC/FID, GC/MS, and HPLC/UV for environmental monitoring and cleanup. The 16-compound list was often expanded to include 2-methylnaphthalene and dibenzofuran. As the need to assess a broader group of PAHs and related compounds increased, additional analytical alternatives were developed.
Prior to the last decade, analyses targeting a broader group of PAHs and related compounds were usually restricted to research efforts and major environmental disasters, in which new analytical alternatives were developed. Although PAH analysis is still used for environmental monitoring and cleanup, it is now commonly used for forensic/fingerprinting investigations, damage assessment from petroleum products and spills, studying pyrogenic mechanisms, the determination of biogenic vs. non-biogenic sources, and studying the environmental effects of alkylated PAHs.
Advances in instrumentation and the use of GC/MS–Selected Ion Monitoring (GC/MS-SIM) has resulted in much lower detection and quantitation levels, as well as the development of methods specifically targeting PAHs and alkylated PAHs for specific data evaluations. Many laboratories have established these specialized tests, which can provide detection and quantitation limits up to 100 times lower than common full-scan GC/MS analysis.
The study of environmental disasters related to petrogenic PAHs has led to the inclusion of additional PAHs and alkylated PAHs as routine target analytes in many environmental analyses. Alkylated PAHs are normal PAH structures, but have at least one hydrogen atom substituted with an alkyl group. These “extended” PAH and alkylated PAH analyses are commonly used to perform fingerprint analyses for forensic purposes. For example, the petroleum-related alkylated PAHs, such as alkylated naphthalenes, alkylated phenanthrenes, alkylated dibenzothiophenes, alkylated fluorenes, alkylated chrysenes, and certain unsubstituted PAHs, are much more significant components in crude oil compared to many of the priority pollutant PAHs.
Additionally, considering that oils and refined petroleum products from separate sources generally have differing relative amounts of PAHs, combined with alteration to normal distributions due to weathering, the ability to analyze a broad range of compounds becomes essential.
The analysis of pyrogenic PAHs, those generated in combustion of organic materials, is often used in the study of PAH formation mechanisms under various combustion conditions, including incomplete combustion. With pyrogenic materials the contribution of unsubstituted PAHs is much higher than the related alkylated homologue. Also, there is a more skewed distribution of alkylated PAHs in pyrogenic PAHs as compared to unweathered petrogenic alkylated PAHs. The data from extended PAH and alkylated PAH analyses can be evaluated to determine the sources of the present PAHs. In both petrogenic or pyrogenic studies, the ratio of PAHs is commonly used to identify sources of contaminant PAHs.
Expanding the PAH analytical techniques to include the determination of compounds indicative of biogenic PAHs, such as retene and perylene, can be used to determine the relative contribution or source of naturally occurring biogenic PAHs. These data can be used to distinguish biogenic sources of PAHs from petrogenic or pyrogenic sources. Examples of other analytes being studied, but not classically included in PAH analyses, are as follows:
- Biphenyl – An unsubstituted, 2-ring PAH that can be used for characterization and identification petrogenic sources.
- Carbazole – A nitrogen-containing PAH that is produced from coal tar and crude oil with several industrial uses. It is a suspected carcinogen with potential metabolic pathways identified.
- cis/trans-Decalin and alkylated decalins – Decalin is a bicyclic compound and is the saturated analog (C10H18) of naphthalene. Decalin is an industrial solvent.
- Dibenzofuran – An oxygen-containing PAH (two benzene rings fused to one oxygen-containing ring in the middle). It is present in low-% levels in creosote and commercial coal tars. It has been listed by the USEPA as a volatile hazardous air pollutant of potential concern and it a targeted analyte under EPA CERCLA and SARA (Superfund).
- Benzothiophene and alkylated benzothiophenes – Sulfur-containing PAHs that can be used for characterization and identification petrogenic sources. Benzothiophenes occur naturally in petroleum-related deposits.
- Dibenzothiophene and alkylated dibenzothiophenes – Similar to benzothiophene, dibenzothiophene is a sulfur-containing PAH, with two benzene rings fused to a thiophene ring. Alkylated dibenzothiophenes occur in heavier fractions of petroleum.
- Naphthobenzothiophene and alkylated naphthobenzothiophenes – Similar to dibenzothiophene but with a naphthyl group substituted for one of the benzene rings.
With the advancement in analysis technology and capability from a standard 16-compound analysis by GC-FID, GC/MS, or HPLC to today’s low-level GC/MS-SIM analyses that include greatly expanded compound lists, researchers, engineers, risk assessors, toxicologists, and other data users are using the analysis to study PAHs and sources extensively. This is evident in the examination of extended PAHs compounds list by the NOAA National Status and Trends program (NOAA NS&T), state programs (such as Washington Department of Ecology), and disaster assessment. The following table provides a comprehensive list of PAHs and related compounds discussed in this paper and of current interest.
PAHs, Alkylated PAHs, and Related Compounds of Environmental Interest
|Priority Pollutant PAHs|
|Other PAHs and Related Compounds|
|Discrete Alkylated PAHs|
|Alkylated PAHs Homolog Groups|