Polychlorinated Biphenyls (PCBs)
Polychlorinated biphenyls (PCBs) are a group of chlorinated aromatic hydrocarbons which were manufactured in industrially since 1927. PCB mixtures were produced by chlorinated the biphenyl carbon structure resulting in mixtures of PCBs depending on the degree of chlorination. There is a potential for up to 209 different structural congeners, of which, about 120 are present in the varying commercial mixtures. The number of PCBs present in commercial mixtures and the environment make PCBs good candidates for environmental forensics in order to determine potential sources. Common commercial mixtures include Aroclor formulations which were produced by Monsanto. The formulations are named according to the degree of chlorination based on weight. The most common PCB formulation encountered in the literature is Aroclor 1254 which would be the formulation with 54% chlorine by weight and the 12 represents the number of carbons in the biphenyl backbone.
Unfortunately, PCBs are commonly measured using Aroclor methods (EPA Method 8082) whereby a reduced set of PCBs are identified and quantified and patterns assessed to determine the Aroclor pattern concentrations in the sample. These relative amounts of Aroclor concentrations are not fingerprints, merely reporting the amount of Aroclor that would be present in the sample based on the reduced set of PCBs in the sample. For environmental forensics studies, it is important to have as much detail about the individual PCBs present as possible. EPA method 1668 provides much lower detection limits and is able to quantify all PCBs in the samples. There is an increasing trend to measure more PCB congeners to allow more detailed PCB fingerprinting and provides more compositional information to conduct source apportionment analysis. This is particularly necessary for investigations involving sediments where PCBs are being altered by microbes or where alternative PCBs sources, such as incidental PCBs, may be present.
Some of the main desirable properties of PCBs included their chemical stability, insulating properties and high thermal resistance. These same properties are what make PCBs so persistent in the environment. PCBs are commonly measured as part of biomonitoring studies as their lipophilic properties allow them to bioaccumulate and biomagnify in the food chain.
PCB congener data is important for environmental forensic investigations. The congener data can be plotted out visually in order of PCB number and compared to production formulations or used in statistical analysis to identify association to potential sources. We have used PCB data from sediment samples to determine likelihood of sources. In the figure above, we were able to use cosine theta analysis and compare PCBs in sediment to individual Aroclor formulations. Using this approach we were able to identity a previously unknown party that was responsible for the release into the waterbody.