Cold-temperature stability of five β-lactam antibiotics in bovine milk and milk extracts prepared for liquid chromatography–electrospray ionization tandem mass spectrometry analysis

The stability of five major β-lactam antibiotics (amoxicillin, ampicillin, cloxacillin, oxacillin, and penicillin G) in fortified milk and in milk extracts prepared for LC-ESIMS/MS analysis was studied at varying cold temperatures (4, −20, and −76 °C). Storage of milk samples at 4 °C resulted in measurable losses of all β-lactams after 6 days (>50% in most cases). Slow degradation of penicillin G, cloxacillin, and oxacillin was observed in milk stored at −20 °C, but no losses were recorded at −76 °C over 4 weeks. All antibiotics showed good stability at all temperature tested in milk extracts prepared for LC-ESIMS/MS analysis. The results of this study emphasize adherence to adequate sample handling and storage protocols as to reflect residue levels at the time of sample submission.