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A rapid, simple method for screening polychlorinated dibenzo-p-dioxin and dibenzofuran classes in shellfish tissue at pg g-1 wet mass concentrations is described. The method does not require a clean room facility and is based on saponification followed by extraction into hexane, clean-up using gel-permeation chromatography and sulfuric acid treatment, and measurement using capillary gas chromatography-low resolution mass spectrometry with selected ion monitoring. Selected ion monitoring using multiple ions eliminates interferences not removed by clean-up or chromatography. The detector response factor is constant for isomers within a class, e.g., tetrachlorodibenzo-p-dioxin isomers gave a mean response of 0.977 +/- 0.075 area counts fg-1, but varied significantly between classes. Thus one isomer serves as a 'standard' for all members of its class. Recoveries of added polychlorinated dibenzo-p-dioxins and dibenzofurans (20-500 pg g-1 wet mass) averaged 95.6 +/- 6.9 and 99.0 +/- 5.7%, respectively. The limits of detection (five times the noise level) are 20 pg g-1 tetrachlorodibenzo-p-dioxins and dibenzofurans, 20 pg g-1 pentachlorodibenzo-p-dioxins and dibenzofurans, 40 pg g-1 hexachlorodibenzo-p-dioxins and dibenzofurans, 40 pg g-1 heptachlorodibenzo-p-dioxins and dibenzofurans, and 100 pg g-1 octachlorodibenzo-p-dioxin and dibenzofuran. Above the limits of detection, the method gave results for polychlorinated dibenzo-p-dioxin and dibenzofuran classes in shellfish tissue comparable to those obtained by gas chromatography-high resolution mass spectrometry.