LIQUID-CHROMATOGRAPHIC DETERMINATION OF BETA-LACTAM ANTIBIOTICS IN MILK - A MULTIRESIDUE APPROACH

In the United States, testing of all milk for residues of beta-lactam antibiotics is now mandatory, Although a number of screening tests for determination of beta-lactam antibiotic residues have been proposed, few reference methods of the required sensitivity (<10 ppb) are availa ble, Methods for determination of several beta-lactam antibiotics usin g an automated liquid chromatography (LC) cleanup have been described recently. This paper describes the integration of these methods into a single extraction and cleanup procedure. Milk was deproteinized with 0.2M Et(4)NCl and acetonitrile, The resulting filtrate was evaporated to about 1 mL, made to 4 mL, and filtered through a disposable filter cartridge, For cleanup, 2 mL filtrate was loaded onto a bonded C-18 LC column in 0.01M KH2PO4 (A) and eluted with an acetonitrile (B) gradie nt using a program of 100 A:0 B (0-3 min) to 60 A:40 B (30 min), The b eta-lactams were concentrated into narrow bands and separated from eac h other. A fraction corresponding to each compound of interest was col lected and rechromatographed for analysis, The procedure has been appl ied successfully to determination of ampicillin, amoxicillin, cephapir in, penicillin G, penicillin V, ceftiofur, and cloxacillin. In princip le it can be applied to the determination of any beta-lactam antibioti c or metabolite thereof by collecting the appropriate fractions.