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A method is described for the gas-liquid chromatographic determination of traces of selenium in marine biological materials. The method is based on the reaction of Se(IV) with bromo- and chloro-substituted 1,2-diaminobenzenes. The benzoselenadiazoles so formed are sensitive to electron capture detection. The sample is digested in a nitric-perchloric acid mixture and selenium is reduced to the IV oxidation state. Different aliquots of the digest solution are reacted with either 4-bromo- or 4-chloro-l,2-diaminobenzene to quantitatively form the corresponding 2,1,3-benzoseIenadiazole. Recovery of added selenite to a fish meal sample was 95% for the bromo derivative and 101% for the chloro derivative. Different portions of a well mixed fish meal sample were analyzed in independent laboratories by the fluorometric method and by atomic absorption spectrophotometry (hydride generation). The following mean values (μg/g) were found: present method 1.89, fluorometric method 1.91, atomic absorption method 2.1. The lower limit of detection for the method described was 13 ng, using the bromo derivative, and 27 ng, using the chloro derivative.