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Light--a source of error in the fluorometric determination of tryptophan.

Light was found to be destructive on the assay mixture prepared for the fluorometric determination of tryptophan according to the method of Denckla & Dewey. The effect was restricted to the procedures preceding the incubation of the assay mixture. The fluorophore produced, norharman, was rather insensitive to light. The effect was dependent on the presence of the ferric chloride reagent and operated at wave-lengths shorter than 400 nm with increasing effect for shorter wave-lengths. By excluding daylight and preparing the assay mixture under tungsten lighting, the error introduced by light was omitted. The decomposition of tryptophan was followed by the changes in the UV spectrum of the assay mixture and by paper chromatography. The use of ferric ions in combination with UV light in obtaining column blanks for the determination of tryptophan metabolites is discussed.