Determination of plasma ethambutol with liquid chromatography and ultraviolet spectrophotometry

: The method presented here is based on the ability of ethambutol to give chelates with divalent cationic metals. With copper salts, the chelate presents a characteristic ultraviolet absorbance at 270 nm, making it possible to titrate ethambutol extracted from plasma with chloroform in alkaline medium. The column which was used contained silica (LiChrosorb Si 60, 5 microns) and the mobile phase was a mixture of equal parts of water and acetonitrile, containing copper sulphate and ammonia. The internal standard was a lower homologue of ethambutol with the same chelating ability. The detection limit was 0.15 mg/l. No interference with other antitubercular agents was observed.