Quantitative analysis of polypeptide antibiotic residues in a variety of food matrices by liquid chromatography coupled to tandem mass spectrometry. 
A quantitative LC-MS/MS method was developed for the determination of five polypeptide antibiotics (bacitracin, colistin A, colistin B, polymyxin B1 and polymyxin B2) in a variety of food matrices (muscle, liver, kidney, egg and milk).  The described method is sufficiently sensitive, selective and provides acceptable recoveries for all compounds.  The extraction is based on acidified methanolic solvent.  This is followed by a reversed phase solid phase extraction step to clean-up and concentrate the extracts.  The use of a modern core shell column in combination with an eluent consisting of trifluoroacetic acid, formic acid and acetonitrile provides chromatographically well resolved analyte peaks The single-step clean-up is fast and produces a sufficiently clean extract in order to control matrix-related signal suppression in the electrospray interface.