Improved high-performance liquid chromatographic procedure for the determination of tetracyclines in plasma, urine and tissues.

An improved extraction procedure for the determination of oxytetracycline, tetracycline and chlortetracycline in urine, plasma and tissues by high-performance liquid chromatography is described. The addition of phenylbutazone (3,5-dioxo-1,2-diphenyl-4-n-butylpyrazolidine) to water, urine or plasma enhances the extraction of these compounds by ethyl acetate. The recovery of oxytetracycline from plasma is increased six fold and the need for two separate extractions of urine and plasma is eliminated. The formation of phenylbutazone-tetracycline ion pairs and their role in the extraction process is discussed.