Liquid chromatographic determination of tetracycline in plasma and urine

Tetracycline is extracted from plasma (0.50 ml) as an ion pair with tetrabutylammonium into chloroform-1-heptanol (9:1). After re-extraction into an acidic aqueous phase the separation is performed by reversed-phase liquid chromatography using LiChrosorb RP-2 as the support and acetonitrile-water-phosphoric acid as the mobile phase. The chromatographic system shows a high selectivity for the separation of tetracycline analogues. A high detection selectivity is obtained by the use of photometric detection at 357 nm. Analysis of urine is performed by direct injection of the sample into the liquid chromatograph. The precision in the determination of tetracycline in plasma was about 10% (relative standard deviation) at drug levels of 200 ng/ml and 200 microgram. Urine samples containing 20 and 200 microgram/ml of tetracycline were determined with a precision of 3%.