An analytical method to determine conjugated residues of ceftiofur in milk using liquid chromatography with tandem mass spectrometry Original Research Article

A new analytical method to identify and quantify ceftiofur and its metabolites in milk is described. The method uses solid phase extraction and liquid chromatography with tandem mass spectrometry (LC-MSMS). Samples are deproteinated with acetonitrile and cleaned up with a C18 SPE column to reduce matrix interference. Conjugates of desfuroylceftiofur are deconjugated and converted to desfuroylceftiofur by treatment with dithioethrytol in a borate buffer. The desfuroylceftiofur that is released is unstable, so is stabilised by derivatisation to desfuroylceftiofur acetamide. Extracts are then purified by cation exchange SPE, and the derivative determined by LC-MSMS monitoring two selected transitions from the protonated molecular ion. The performance characteristics of the method have been calculated as specified in Commission Decision 2002/657/EC.