Improving the chromatographic selectivity of β-lactam residue analysis in milk using phenyl-column chemistry prior to detection by tandem mass spectrometry.

Analyte isobaric interferences can limit the development of a comprehensive analytical method for the quantitative liquid chromatography-tandem mass spectrometry profiling of an important cohort of veterinary drugs. In this work, a selective chromatographic separation was developed for the analysis of 32 β-lactam antibiotic residues (12 penicillins, 14 cephalosporins, five carbapenems and faropenem) in milk samples. A range of analytical columns with different stationary phases and mobile phases were evaluated for retention and separation of the β-lactam compounds. Results showed that, among the columns tested, only phenyl-hexyl could adequately separate ampicillin from cephalexin and amoxicillin from cefadroxil, which had shown isobaric interferences on a number of stationary phases. Chromatography was performed using a water/acetonitrile binary gradient with formic acid and ammonium acetate. The β-lactam residues were extracted from the milk samples using a water:acetonitrile solution and purified by C18 dispersive solid-phase extraction (d-SPE) clean-up, followed by concentration under nitrogen and ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) determination. Analytes were monitored in positive electrospray ionisation mode (ESI(+)). Possible interfering matrix effects were overcome by using 13 internal standards. The method was fully validated according to 2002/657/EC guidelines, showing satisfactory performance characteristics. Under within-laboratory reproducibility conditions, trueness and precision ranged from 91 to 130% and from 1.4 to 38.6%, respectively. Decision limits (CCα) were in the range 2.1-133 μg kg-1. Limits of detection (LODs) and quantitation (LOQs) ranged between 0.0090 and 1.5 μg kg-1 and from 0.030 to 5.0 μg kg-1, respectively.