PCBs: Aroclors, Homologs and Congeners

With PCBs (polychlorinated biphenyls) being more in the news, you may hear the terms “Aroclors”, “homologs” and “congeners” used to describe the different ways that PCBs are measured.  Measuring the concentration of PCBs gets complicated because there are actually 209 different chemicals (referred to as congeners) included in the PCB chemical group.  Measuring all 209 congeners separately is research level analytical chemistry and is impractical for most purposes.  However, analytical chemists have developed a number of effective ways to measure PCBs that don’t require looking for all 209 different PCB congeners.

Measuring PCBs as Aroclors

The most common way to measure PCBs is as Aroclors.  Aroclor was the trade name of the commercial PCB mixtures manufactured by the Monsanto Chemical Company and sold in the United States.  An Aroclor PCB mixture might consist of over 100 different individual PCB congeners, although 10-20 might make up over 50% of the mixture.   When analytical chemists test a sample to see if it has an Aroclor PCB mixture in it, they look for a distinctive gas chromatographic pattern (sometimes called a chromatographic “fingerprint”) that is indicative of one of the Aroclors.  There were  nine common PCB Aroclor mixtures (1221, 1232, 1242, 1016, 1248, 1254, 1260, 1262, and 1268), and each of them has a distinctive gas chromatographic pattern.  Measuring PCBs as Aroclors relies on there being a relatively fixed composition of PCB congeners in the mixture.

When a chemist measures the amount of Aroclor in a sample, they will know the total amount of that Aroclor that is present, but will not know the identity or the concentration of the specific PCB congeners in the sample.  Provided the sample has not been subjected to conditions that might degrade or change the composition of the PCBs, knowing the type of Aroclor present and its concentration is usually sufficient for environmental assessment.

Homologs – For When Sample Weathering Has Occured

However, if an environmental sample has been subjected to conditions that might alter the congener composition of the sample, then it will be more accurate to test the sample by a different method.  Air samples, sediment samples, biota samples and water samples are the ones most likely to have had their congener composition changed by environmental conditions.  This can happen because the PCB congeners with fewer chlorine atoms tend to partition into air and water more readily than those with more chlorine atoms.  For this reason air and water samples are likely to be “enriched” with congeners with fewer chlorine atoms.  Biota samples can also be subject to bio-degradation with some congeners being selectively reduced and others remaining constant.

For samples whose congener makeup has been altered, testing for Aroclors will give erroneous results.  Testing for PCB homologs will give more reliable results for these samples.  Homologs are a way of grouping PCB congeners by the number of chlorine atoms they have; this can vary from one to ten.  All the PCB chemicals that have the same number of chlorine atoms are said to belong to the same homolog group.  There are 11 different di-chloro congeners in the 2-chlorine homolog group and there are 42 different tetra-chloro congeners  in the 4-chlorine homolog group, as examples.  Laboratory results for PCB homologs will list the the amount of PCB present in the sample by the number of chlorine atoms.

But, Sometimes Only Congener Analysis Will Do

In circumstances requiring more congener detail than can be provided by either Aroclor or homolog analyses, it is also possible to analyze samples for a subset of the full 209 congeners.  Analyzing samples for the full 209 congeners is, as I said before, still research level chemistry.  The NOAA PCB congener method cites 20 congeners to be reported, this is often used for sediment analysis. The USACE PCB congener method cites 22 congeners to be reported. The SW-846 8082 method cites 19 congeners to be reported. The WHO lists cites 12 congeners (those which the World Health Organization believes pose the greatest health concern – although this is disputed).  Congener data is particularly useful for forensic purposes, but the guidance available for interpreting the data is fairly limited.

Overall, in most instances measuring PCBs using the Aroclor method will be the best choice.  Where that method is inappropriate, looking at homologs is likely to be a good option, and where even more detailed results are needed, looking for PCB congeners will be necessary.  For homolog and congener testing make sure to select a laboratory with considerable experience with those analyses as they are challenging tests to perform.

For help selecting analytical methods or designing a PCB assessment program, please contact me at okun@oto-env.com.