Multiresidue determination of beta-lactam antibiotics in milk and tissues with the aid of high-performance liquid chromatographic fractionation for clean up.

Screening of milk shipments for beta-lactam antibiotic residues is mandatory in the USA and is widely used in other countries. Interpretation of positive screening test results has been difficult. Only six beta-lactam antibiotics are approved for use in food-producing animals in the USA but many others are used in other countries. A multiresidue procedure was developed for identification and quantitation of unknown beta-lactam antibiotics. The residues were extracted with acetonitrile and tetraethylammonium chloride. The extract was concentrated by evaporation and filtered. The concentrated extract was then loaded onto an HPLC column in 100% 0.01 M KH2PO4 and eluted with an acetonitrile gradient. Fractions corresponding to analytes of interest were collected and tested for antibiotics using rapid milk screening tests. Fractions testing positive were analyzed by HPLC. The identity of beta-lactams was confirmed by treating a replicate with beta-lactamase.