Differentiation of tetracyclines and their 4-epimers by mass spectrometry of the alkali metal adduct ions.

Tetracyclines (TCs) are a family of broad-spectrum antibiotics. During the manufacturing process or storage, epimerization of tetracyclines could occur, leading to 4-epimers which are nearly inactive. From an analytical point of view, isomers are often difficult to distinguish. Previously, four pairs of TCs (oxytetracycline, tetracycline, doxycycline, chlortetracycline and their respective 4-epimers) were differentiated by mass spectrometry (MS) through protonated ions. However, they do not follow common rules and so it is still quite difficult to differentiate between them. In order to solve this, the four pairs were differentiated in the current study by collision induced dissociation (CID) spectra of the alkali adduct ions, including lithium, sodium and potassium. In the spectra of the sodium adducts, all studied tetracyclines showed a tendency to form [M+Na-NH3]+ ions, while the 4-epimers liked to form [M+Na-NH3-H2O]+ ions. Meanwhile, energy resolved mass spectrometry (ERMS) showed that all four 4-epimers' sodium adducts had the tendency to fragment at higher energy points. In the CID spectra of lithium adducts of TCs, a similar trend was observed for three pairs, except for doxycycline. For potassium adducts, the fragmentation was found to be less discriminative. As was derived from the 3D model, the four pairs all interact with the alkali metal through the dimethyl amino group at the C-4 position. The lithium adduct species also bound through the hydroxyl group at the C-5 position. If the TCs did not have a hydroxyl group at the C-5 position, they bound with the hydroxyl group at the C-6 position. For the same TC, with an increase of the diameter of the metal ion, the loss of H2O decreased gradually. As sodium adduct ions are common during the ionization process, TCs and their 4-epimers could be differentiated rapidly by ERMS of the sodium adduct ions.