This Environmental Forensic Short Note is the first of a series that will provide our clients and friends with brief summaries of various tools used in the field of Environmental Forensics. This issue deals with chemical fingerprinting of dioxins and furans. Future issues will deal with other chemical fingerprinting topics as well as industrial historical reconstruction, aerial photogrammetry, geostatistical analyses, and other tools.

What are Dioxins and Furans?

The term “dioxins” commonly refers to chlorinated dibenzo-p-dioxins (CDDs) and “furans” to chlorinated dibenzofurans (CDFs). Sometimes “dioxin” also refers to the most studied and toxic of these compounds, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). CDD/CDFs are not created intentionally, but are produced inadvertently by a number of human activities including chemical manufacturing and incomplete combustion, as well as by natural processes such as forest fires and volcanoes. CDD/CDFs are among the most studied chemicals in terms of their formation processes, environmental occurrence, and toxicity.

The origin of dioxins and furans found in environmental media is often disputed in lawsuits and regulatory enforcement actions. Disputes arise in tort actions in communities downwind or downstream of an industrial source because there are many regional and local combustion sources that can result in soil or sediment concentrations exceeding regulatory screening levels. Moreover, the downwind/downstream concentrations and chemical “signature” from the industrial source (e.g., pentachlorophenol) can be similar to the concentrations and signatures resulting from these other common sources, particularly after environmental transport and chemical transformations.

Dioxins and Furan

CDDs and CDFs have a triple-ring structure that consists of two benzene rings connected by either one or two oxygens (CDFs and CDDs, respectively, Figure 1). These molecules have eight possible positions where substitution by a halogen such as chlorine can occur. Of environmental interest are the CDD/CDFs with four or more chlorines and specifically, those molecules with chlorine atoms on the 2,3,7, and 8 ring positions because they are believed to be particularly toxic. CDD/CDF homologues refer to compounds with the same number of chlorine atoms, regardless of position. For example, as shown in Table 1, there are 22 possible tetrachlorodibenzo-p-dioxin (TCDD) compounds in the TCDD homologue group. Only one of these, the 2,3,7,8-TCDD isomer, is considered toxic. The term “congener” refers to any individual CDD/CDF molecule, regardless of homologue class. There are 75 possible congeners of CDDs with 7 of those having chlorine atoms on the 2,3,7,8 positions and 135 possible CDF congeners with 10 congeners having the 2,3,7,8 substitution.

Homologues and Congeners

What is Dioxin and Furan Fingerprinting?

Chemical fingerprinting is a well-established technique for distinguishing different sources of contamination in the environment. It is particularly well suited for work with families of organic compounds that occur together, such as PCBs, polycyclic aromatic hydrocarbons, and CDD/CDFs. This is because (1) these classes contain many individual compounds, which together comprise a compositional pattern, also referred to as a “profile,” a “signature,” or a “fingerprint” and (2) the relative concentrations and/or ratios of an individual profile can be used as a marker of the original source material. However, because organic compounds can be transformed in the environment through chemical weathering and biological degradation, segments of the original patterns can be altered. Another complicating factor is that multiple sources often mix together and mask the individual signatures.

Environmental transformations must be considered when interpreting chemical source fingerprints. For example, the chemical fingerprint of a sample from an environmental medium such as soil, sediment, or indoor house dust typically represents many decades of input of CDD/CDFs that may have been chemically transformed during transport from their original source and after deposition and/or mixed with other sources.

What are some Common Fingerprinting Methods?

Chemical fingerprinting methods range from simple to complex. There are many chemical fingerprinting methods, which range from simple profile comparisons of individual samples to sophisticated multivariate analyses. Individual profile comparisons can be useful when the profiles are clearly different. However, the human eye has difficult detecting subtle patterns between histogram plots of 17 CDD/CDF congeners or, as an extreme example, the peaks of 80 semivolatile compounds on a gas chromatogram. Therefore, mathematical methods are often used to identify and distinguish fingerprints, as discussed below. These mathematical methods can also used to “unmix” the overlaid fingerprints from multiple sources.

The first fingerprinting step, and sometimes the only step, is the evaluation of individual samples by comparison of profiles of the relative concentrations of either the commonly reported 17 2,3,7,8-substituted congeners or the 10 homologue classes. Concentrations of CDD/CDFs found in environmental media can vary by orders of magnitude; therefore, normalization of the results is necessary so that the congener or homologue profiles from different locations and different media can be compared.

More detailed profile comparisons can be made using the selected ion current profiles (SICPs), also known as “mass fragmentograms” that are generated by the laboratory for each homologue class. These provide non-quantitative but useful relative concentrations of all detected isomers for a given homologue class (see Table 1), not just the 2,3,7,8-substituted  isomers. Samples that have been influenced by different sources of CDD/CDFs would show different dominant peaks and patterns of peaks on the SICPs for certain homologues.

To complement visual comparisons and/or ratio analyses described above, exploratory data analyses can be used to evaluate sources. Mathematical methods can be used to identify patterns (similarities and differences) in groups. Methods include, but are not limited to, double ratio plots, hierarchical cluster analysis, least squares analysis, discriminant analysis, principal component analysis (PCA), neural networks, and polytopic vector analysis (PVA).

Book coverA comprehensive review of chemical fingerprinting of CDD/Fs can be found in Shields, W.J., Y. Tondeur, L. Benton and M.R. Edwards. 2006. Dioxins and Furans. Chapter 14, In: R. Morrison and B. Murphy (eds.), Environmental Forensics: Contaminant Specific Guide. Academic Press, San Diego. The publisher has kindly allowed us to provide a copy of the chapter to readers of this newsletter. Click here for a download. The complete book can be purchased at http://books.elsevier.com.

Central to Exponent's environmental expertise is a deep capability in environmental forensics. We have applied our expertise and experience to a wide variety of situations - refineries, former manufactured gas plants, mines, smelters, foundries, pulp and paper mills, wood treatment facilities, oil spills, fuel terminals, and many manufacturing facilities with contaminants in air, groundwater, surface water, sediment, and soil. We have over 30 scientists and engineers with a variety of experience in environmental forensics. Points of contact for specific chemical classes are: Walt Shields (metals and dioxins); Brian Murphy (TCE and other chlorinated solvents, MGP wastes); Paul Boehm (Petroleum, PAHs, and PCBs); Gary Bigham (mercury) and Peter Mesard (perchlorate). Click here for more information on our Environmental Forensics services.

Please contact Walt Shields at Exponent if you would like additional information.