Azithromycin "ghost peak": A solution degradation product of azithromycin via leaching from borosilicate glass volumetric flasks and vials.

An interference peak was found while detecting related substances of azithromycin. It is impressive that the degradation peak occurred at about 70 min in the next injection of the test solution (4 mg/mL or higher). Once the degradation peak was observed, it would keep growing. By using a strategy that Q-TOF high resolution mass spectrometry with mechanism-based stress studies, followed by preparative subsequent structure characterization by 1D and 2D NMR, the unknown peak was identified as azithromycin hydrogen borate. It apparently results from azithromycin and residual boron leaching out of the inner surface of the glass volumetric flasks and vials used in the sample preparation. By simulating the above chemical process, boric acid and azithromycin were dissolved in the same extraction diluent and a big interference peak occurred. It was found that boron-free flasks and vials, such as PMP or PP flasks and PTFE or PP vials could be used for the detection of azithromycin related substances to avoid the production of azithromycin hydrogen borate.