[Rapid simultaneous determination of 53 beta-lactam antibiotics and their metabolites in milk by ultra-performance liquid chromatography coupled with triple quadrupole mass spectrometry]. 
A rapid method for the determination of 53 beta-lactams and their metabolites residues in milk was developed by ultra-performance liquid chromatography-triple quadrupole mass spectrometry (UPLC-MS/MS).  The method was based on protein precipitation by adding equal quantity of acetonitrile, followed by the ultra filtration of the extracts.  The analysis of the residues was achieved on an ACQUITY BEH C18 column with gradient elution using mobile phases of 0.1% formic acid in water and acetonitrile.  The analytes were detected by positive electrospray ionization tandem mass spectrometry in the multiple reaction monitoring (MRM) mode, and quantified by matrix-matched internal standard calibration.  The cycle time of each analysis was 10 min.  Under the optimum operation conditions, excellent linear dynamic range was observed from the quantification limits to 200 microg/kg with the correlation coefficients better than 0.991 1 for all the compounds.  The average recoveries for all the beta-lactams and their metabolites ranged from 71% to 121% with the relative standard deviations of 1.7%-19% (n = 6).  This method enabled the screening for more than 50 samples per day by one person and also showed good performance for quantitation, and allowed the determination of the beta-lactams and their metabolites residues in milk below the maximum residue levels (MRLs) according to the Bulletin No.  235 (2002) of Ministry of Agriculture, P. R. China and EU Regulation 2377/90/EC (2008 Revised).