TRACE ANALYSIS OF OXYTETRACYCLINE AND TETRACYCLINE IN MILK BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

A high-performance liquid chromatographic (HPLC) method for the determination of oxytetracycline (OTC) and tetracycline (TC) residues in milk at levels as low as 10 ppb has been developed. Milk was acidified at pH 2.7 and extracted with acetonitrile. The extract was partly purified by treatment with ammonium sulfate solution and concentrated into a phosphate buffer, pH 8.2. Following addition of tetrabutylammonium reagent (TBA), tetracyclines were extracted as ion pairs with TBA into dichloromethane, reextracted into acid, and analyzed on a reversed-phase C18, 5 µm, column. Overall recovery was found to be 72.7 ± 1.2% for OTC and 85.1 ± 1.3% for TC. The linearity was excellent for both compounds in the range examined (r = 0.9996, 23.7–190 ppb of OTC in milk; r = 0.09995, 26.5–212 ppb of TC in milk). Precision data based on within-day and among-days variation suggested an overall relative standard deviation of 5% for OTC and 4.2% for TC. © 1990, American Chemical Society. All rights reserved.