DETERMINATION OF SOME MONOBASIC PENICILLINS IN MILK BY ION-PAIR LIQUID-CHROMATOGRAPHY

An ion-pair liquid chromatographic method for the determination of four commonly used monobasic penicillins (penicillin G, penicillin V, oxacillin, and cloxacillin) in milk has been developed. According to this method, which provides quantitation down to 3-4 ppb, milk is acidified at pH 3 and extracted with dichloromethane. The extracted penicillins are partitioned into a phosphate buffer, pH 7, and following addition of ammonium sulfate, the extracts are purified by treatment with diethyl ether and repartitioned into acetonitrile. The acetonitrile extracts are concentrated into phosphate buffer, pH 7, and after addition of tetrabutylammonium hydrogen sulfate, the formed penicillin ion pairs are extracted into chloroform. Liquid chromatography was performed on a reversed-phase C18, 5-mu-m, column using an aqueous acetonitrile, pH 6, mobile phase containing 5 mM tetrabutylammonium hydrogen sulfate. Overall recoveries were found to be better than 82.9%, where as overall relative standard deviation for all penicillins ranged between 3.8% and 6.6%.