Analysis of PCB Congeners vs. Arcolors in Ecological Risk Assessment

Teresa Bernhard, NAVFAC

Steve Petron, CH2M Hill

# Abstract

An issue of growing interest in the field of ecological risk assessment
is whether analyses of polychlorinated biphenyl (PCB) contamination in
environmental media should focus on Aroclors, which are commercial
mixtures of PCB compounds, or more specifically on congeners, which are
individual PCB compounds. As is often the case in such debates, the
answer seems to boil down to "It depends." This issue paper presents a
general description of PCBs and then discusses the differences in costs,
data quality, and significance of results for the two analytical
methods, as well as their pros and concs as tools in ecological risk
assessment.

# What are PCBs and why are they important in assessing risk?

Polychlorinated biphenyls (PCBs), all of which are man-made, are among
the most stable organic compounds known. Each PCB consists of a biphenyl
molecule with a specific number of attached chlorine atoms (see diagram
on the next page). There are 209 distinct PCB compounds (known as
congeners) with from 1 to 10 chlorine atoms on a biphenyl molecule. The
number and placement of the chlorine atoms on the biphenyl molecule
determines how the congener is named and dictates its environmental fate
and toxicity. PCBs generally occur as mixtures of congeners; the most
common commercial mixtures are called Aroclors. Aroclor names reflect
the percent chlorine (by weight) of the mixture (e.g., Aroclor 1242 is
42% chlorine by weight), with the more chlorinated mixtures generally
being the most persistent and toxic.

Two mechanisms allow PCB concentrations to change in the environment:
degradation and weathering. Under normal environmental conditions, PCBs
are slow to degrade. Microbial degradation depends on the position of
the chlorine atom on the biphenyl molecule and the degree of
chlorination. Higher chlorinated compounds (those with five or more
chlorine atoms) are more persistent in the environment and are not
readily transformed by bacteria. The number and position of the chlorine
atoms on the biphenyl rings also influence how biological organisms
incorporate and are affected by exposure to PCBs. PCBs with hydrogen
atoms on two adjacent carbon atoms are more readily metabolized than
those with hydrogen atoms adjacent to the chlorine atoms. PCBs are
highly soluble in lipids and are known to biomagnify in upper trophic
levels. Congeners with higher chlorine contents (and higher log K~ow~
values) tend to bioaccumulate the most and, depending on structure,
metabolize the least. The toxicity is influenced by the presence or
absence of chlorines ortho to the phenyl ring. Since congeners tend to
bioaccumulate and biomagnify, evaluations of potential adverse effects
to ecological receptors are generally focused on upper trophic level
organisms. Because of the presistence of PCBs in environmental media,
analyzing the presence and concentration of PCBs is important in
conducting ecological risk assessments. The growing issue, however, is
whether such analyses should focus on mixture of PCBs (Aroclors) or on
individual PCB congeners. As discussed here, a number of factors need to
be considered in making that decision.

![](media/image1.png){width="2.451388888888889in"
height="1.2347222222222223in"}

# What are the Differences between Aroclor and Congener Analyses? 

Analytical methods for Aroclors include the EPA approved Method 8082,
using capillary column with GC/ECD. This method typically has
quantitation limits in the range of 36 to 540 μg/kg wet weight in
sediment. Lower reporting limits can be achieved through improved sample
cleanup, and concentration steps. On the other hand, homologue-based[^1]
quantification methods using GC/MS and congener-specific analyses can
achieve lower quantitation limits (0.02 to 0.6 μg/kg) but at generally
higher costs. Table 1 compares the various methods, quantitation limits,
and approximate costs associated with these various PCB analyses.

#### Table 1. Methods, Quantitation Limits, and Costs Associated with PCB Analysis

  -------------- ----------------- --------------- ------------------------
  **Method**     **Quantitation    **Approximate   **Comments**
                 Limits in         Cost            
                 Sediments (μg/kg) (\$/sample)**   
                 \*\***                            

  Aroclor using  36 - 540          75 - 300        May not meet DQOs.
  GC/ECD                                           Aroclor analysis may
                                                   over- or under- estimate
                                                   PCB concentrations
                                                   because it is not a
                                                   measurement of
                                                   individual congeners but
                                                   is instead is a pattern
                                                   recognition estimate.
                                                   Individual congeners
                                                   will weather, degrade,
                                                   and bioaccumulate at
                                                   different rates. Aroclor
                                                   analysis may severely
                                                   underestimate toxicity.

  Homologues     0.02 - 0.2        1,000           
  using GC/MS                                      

  Homologues     0.5 - 5           500             Interferences may
  using GC/ECD                                     overestimate total PCB
                                                   concentration
  -------------- ----------------- --------------- ------------------------

**Table 1. Methods, Quantitation Limits, and Costs Associated with PCB
Analysis**

(Continued)

  -------------- ----------------- --------------- ------------------------
  **Method**     **Quantitation    **Approximate   **Comments**
                 Limits in         Cost            
                 Sediments (μg/kg) (\$/sample)**   
                 \*\***                            

  NIST 18        0.5 - 5           250             Interferences may
  congeners                                        overestimate total PCB
  using GC/ECD                                     concentration

  Congener       0.02 - 0.2        1,000           
  NIST/WHO[^2]                                     
  list using                                       
  GC/MS                                            

  Congener       0.5 - 5           250             May not be able to
  NIST/WHO list                                    detect non-ortho
  using GC/ECD                                     (indicates position of
                                                   chlorine molecule on the
                                                   ring-clockwise ortho,
                                                   meta, para) congeners.
  -------------- ----------------- --------------- ------------------------

\*\* Reporting limits can often be altered by cleanup and concentration,
often with increased cost.

## 

## What are the pros and cons of the different methods?

PCB congener data offer many advantages over Aroclor data for
environmental risk analysis. Congener-specific analyses generally offer
lower detection limits and a higher information content from the
analytical technique than do Aroclor analyses. The specific advantages
of congener analysis are:

-   It is easier to detect and discard results biased by interference
    caused by chemicals that coelute (come out of the column at the same
    time and therefore make it hard to determine type of compound) with
    PCBs;

-   Quantitation of individual congeners is more accurate than
    estimating Aroclors;

-   Composition of weathered, degraded, and metabolized PCB mixtures can
    be measured and interpreted easier using congener versus Aroclor
    analysis;

-   Aroclor concentrations can be estimated using congener
    concentrations (dependent on the list of congeners);

-   The toxicity of PCBs is congener-specific, and, therefore,
    measurement on an Aroclor basis may not accurately measure toxicity.
    Estimated risk based on congeners versus Aroclors may indicate the
    areal extent of contamination is substantially reduced. This finding
    may lead to significantly reduce cleanup costs.

The cons of using congener-specific analyses include cost and the
general lack of toxicity data for most congeners. Most of the currently
available ecotoxicity data are for Aroclor mixtures. However, toxicity
equivalency factors (TEFs) for fish, birds, and mammals have been
developed for one dozen congeners (i.e., PCB-77, 81, 105, 114, 118, 123,
126, 156, 157, 167, 169, 189). Congener-specific analyses demand greater
effort in terms of data reduction, quality assurance, and processing. A
problematic area in congener analyses is comparability between
laboratories. Sources of this variability include differences in
coelution patterns, different lists of congeners are analyzed at
different laboratories (labs are not yet synchronized with similar lists
of analytes), and normal interlaboratory variation. Coelution of
analytes is a major impediment in PCB congener-specific analyses. The
use of dual column chromatography ECD or single column GC/MS-SIM
(selective ion monitoring) screens out interferences, minimizes
coelution of PCB congeners, and increases reliability of the data.
Congener data generated by a single column GC-ECD should be interpreted
cautiously. A strong quality assurance program, including reference
materials (matrix specific when possible), is essential in conducting
reliable congener-specific analyses.

### When should Aroclor and/or Congener Analyses be performed?

The history, potential remedies, and risk management issues for each
site being evaluated should be considered before selection of either
Aroclor or congener-specific methods. The data quality objectives should
be defined, including analytical reporting limits (not detection limits)
and methods needed to achieve these limits. In addition, the intended
end use of the data should be defined. Actual toxicity data exist for
Aroclors, but congener-specific methods generally require the use of
TEFs to determine potential adverse effects to ecological receptors
(using total PCB equivalents).

Aroclor analyses may be appropriate when:

-   The area will have a presumptive remedy to prevent exposure to
    ecological receptors (precluding use of any PCB analyses);

-   The historical information suggests there are no sources of PCBs
    (precluding use of any PCB analyses);

-   The Aroclors are detected at non-site-related sources;

-   The use of TEFs is not defensible when used to predict adverse
    effects (e.g. using TEFs based on ingestion to predict dermal
    toxicity);

-   The project is in the initial stages of investigation to determine
    presence or absence of PCBs or a preliminary estimation of risk.

Congener analyses may be appropriate when:

-   PCB hot spots have been identified;

-   Weathering, biotransformation (metabolism and variable
    bioaccumulation) have occurred;

-   Lower reporting limits are required;

-   Fingerprinting to determine source is necessary;

-   Adverse effects (toxicity) have been observed and retrospective
    analysis using TEFs will help determine causation;

-   Cleanup will be based on congener-specific TEFs.

# Point of Contact

Teresa Bernhard

NAVFAC

Washington Navy Yard

1322 Patterson Ave.

Washington, DC 20374-5065

<bernhardt@navfac.navy.mil>

# Acronyms

**DQO** data quality objective

**ECD** electron capture detector

**GC/ECD** gas chromatography / electron capture detector

**GC/MS** gas chromatography / mass spectrometer

**GC/MS-SIM** gas chromatography / mass spectrometer - selective ion
monitoring mode

**NIST** National Institute of Standards and Technology

**PCB** polychlorinated biphenyl

**TEF** toxicity equivalency factor

**WHO** World Health Organization

# Glossary

**Aroclor:** A common trade name for mixtures of PCBs. The mixtures have
been widely used as coolants and lubricants in transformers, insulators,
and other electrical equipment because of their highly stable
properties. Because of their stability in the environment together with
the toxicity and their propensity to biomagnify up the food chain,
Aroclor mixtures can cause severe impacts to human and ecological
systems.

**Biomagnification**: A measure of cumulative bioaccumulation from the
media source through two or more steps in a food chain.

**Coelute**: More than one compound comes out of the column at the same
time, making it difficult to determine the type of compound analyzed
Coeluting compounds can make it difficult to accurately measure specific
compounds. Often coelution can be eliminated as a problem if different
ions for each coeluting peak can be monitored.

**Congener**: Any single, unique, well-defined chemical compound in the
PCB category is called a \"congener.\" The name of a congener specifies
the total number of chlorine substitutes and the position of each
chlorine. There are a total of 209 congeners.

**GC/ECD (Gas Chromatography / Electron Capture Detector)**: Analyzes
Aroclor compounds but not individual congeners. One of the many
instruments used in analytical measurement. It is very powerful in
analyzing chlorinated compounds, but weak in quantifying specific
congeners. The electrons emitted from ECD ionize the carrier gas eluded
from GC, and the ionized gas subsequently reduces the current and is
expressed as a response curve.

**GC/MS (Gas Chromatograph / Mass Spectrometer)**: An instrumental
analysis especially useful in analyzing for specific PCB congeners. The
instrument consists of is a gas chromatograph coupled with a mass
spectrometer to produce a 3-D dataset that is not available with
traditional GC detectors (i.e., GC-ECD). The gas chromatography
separates samples into fractions, and the mass spectrometer produces
characteristic spectra. GC/MS operates under scan mode or selective ion
monitoring mode (SIM). The scan mode produces the maximum qualitative
information of the mass data, while SIM samples at a predetermined mass
value to give maximum quantitative information.

**GC/MS-SIM (Gas chromatography / Mass Spectrometry - Selective Ion
Monitoring mode):** A mode to run a GC/MS that increases sensitivity of
the instrument by looking for only specific ions as opposed to scanning
through a broad range. Spending more time looking for a few ions as
opposed to hundreds increases the instrument sensitivity.

**Homolog**: Subcategories of PCB congeners having equal numbers of
chlorine substituents. For example, the \"Tetrachlorobiphenyls\" (or
\"Tetra-PCBs\" or \"Tetra-CBs\" or just \"Tetras\") are all PCB
congeners with exactly 4 chlorine substituents that may be in any
arrangement.

**Log K~ow~**: Quantification of the degree of partitioning of a
contaminant between a mixture of octanol and water (i.e., immiscible
liquids). A chemical's propensity to biomagnify up the food chain is
proportion to the log K~ow.~

**Meta**: Indicates the position of a chlorine molecule on the ring.
Meta indicates the second carbon (clockwise direction) and para
indicates the third carbon location.

**Ortho**: Indicates the position of chlorine molecule on the ring. The
ortho position indicates the chlorine is located on the first available
carbon on the ring.

**Para**: Indicates the position of chlorine molecule on the ring. Para
indicates the third carbon location.

**Quantitation Limit**: The lowest concentration of an analyte that can
be reliably quantified within specified limits of precision and
accuracy.

**Toxicity Equivalency Factors (TEF)**: Scaling factors that estimates
the toxicity of dioxin-like PCBs to that of dioxin
(2,3,7,8-tetrachlorodibenzodioxin).

**Trophic level**: A functional classification of taxa within a
community food web that is based on feeding relationships.

# Informative Web Page Links:

For more information on PCBs and congener-specific analyses, the
following links are provided:

<http://www.epa.gov/pcb>

<http://www.epa.gov/toxteam/pcbid/index.html>

# Laboratories

It is imperative when selecting a lab for PCB analyses to use one that
not only performs congener-specific analyses, but that also has
experience with delivering quality products. The cleanup procedures and
concentration steps make congener-specific analysis very lab dependent.
Find a lab that has a good reputation performing these analyses. Two
examples of quality labs include Battelle Marine Sciences Lab in
Washington State and the CH2M HILL Applied Sciences Lab in Oregon.
Identification of these two examples does not constitute official Navy
endorsement of these laboratories.

[^1]: Homologue-based methods are defined by the chlorination of the
    molecule (e.g., monolog).

[^2]: NIST list refers to National Institute of Standards and
    Technology; WHO refers to World Health Organization.