criteria established by the Clean Water Act (Title 40 CFR part 125.58) to protect humans and wildlife are based on Aroclor values (EPA 1995). Unfortunately, the environmental weathering of Aroclors modulates mixture toxicity (Quensen et al. 1998). As such, carcinogenic risk-assessment guidelines recommend the calculation of congener-specific or total PCB data when available (EPA 1994c). Congener-specific analyses utilize the direct quantification of each unique PCB congener. The result is a precise description of PCB profiles, which can highlight physiological, spatial, and temporal changes that might not be apparent in Aroclor values. Both the Aroclor and the congenerspecific methods of PCB analysis rely on gas chromatography.
Gas-chromatography methods require the extraction of PCB from the environmental matrix and usually require the cleanup of the PCB extract by one or more methods before analysis. A gas chromatograph (GC) is used to separate individual PCB congeners or combinations of congeners based on physical properties, such as volatility and polarity. Over the past 20 years, open tubular capillary GC columns have replaced older packed GC columns for routine laboratory work. The capillary GC columns offer improved resolution, better selectivity, and increased sensitivity compared with packed GC columns. For that reason, many standard EPA methods have been rewritten to allow the use of capillary GC columns for the analysis of environmental samples. The use of open tubular capillary GC columns in a GC system is termed high-resolution gas chromatography (HRGC). All the analytical methods presented below for PCB analysis use HRGC.
Within a GC system, PCBs are then detected with electron capture detection (ECD), electrolytic conductivity detection (ELCD), or mass spectrometry (MS). For MS detection of PCBs, two systems are considered: low- or medium-resolution MS and high-resolution MS. The data output of a GC is called a chromatogram and individual PCBs are identified as peaks on the chromatogram. For GC-ECD and GC-ELCD, PCBs are identified by order of elution from the GC, also called retention time. For MS systems, PCBs are also identified by molecular mass. Certain coplanar PCBs, which are known or suspected carcinogens, are sometimes measured with high-resolution mass spectrometry (HRMS) techniques originally developed for the analysis of dioxins and furans in environmental samples. The HRMS technique provides lower limits of detection for PCB congeners but requires additional sample cleanup beyond that normally required and requires special instrumentation, the high-resolution mass spectrometer.
The utility of the two GC-based methods of PCB analysis (Aroclor and congener-specific), based on costs and benefits, is summarized in Table F-1. These two methods are discussed in detail in the following sections.