Determination of Polypeptide Antibiotics in Milk by Simple LLE Technique and LC–MS/MS Analysis

Antibiotics are widely employed worldwide in animal production, both for promoting cattle growth and for therapeutic purposes. When these drugs are used without adhering to withdrawal periods, they can be excreted unchanged in the milk of dairy animals, posing potential risks to human health. A simple extraction method and liquid chromatography coupled with mass spectrometry (LC-MS/MS) was developed for the determination of residual peptide antibiotics (bacitracin A, B1, B2, and B3, colistin A and B, and virginiamycin S1) in bovine milk. Milk samples were deproteinized and extracted with trichloroacetic acid in acetonitrile. After clean-up with C18, the target antibiotics were separated on a reversed-phase column, using a mobile phase of 0.5% formic acid in water and 0.5% formic acid in acetonitrile:water (90:10). The detection limits of the method for residues of peptide antibiotics in milk were between 2.5 and 25 μg L−1, reaching the limits established by current legislation.