The ionic strength and pH of a measured 50-mL aliquot of sample are adjusted by addition of ammonium chloride and potassium fluoride.. The sample is poured onto an Extrelut column. ETU is eluted from the column in 400 mL of methylene chloride. A free radical scavenger is then added in excess to the eluate. The methylene chloride eluant is concentrated to a volume of 5 mL after solvent substitution with ethyl acetate. Gas chromatographic conditions are described which permit the separation and measurement of ETU with a nitrogen-phosphorus detector (NPD).