Chemical-ionization mass spectrometry of beta-lactam antibiotics.

Chemical-ionization (CI) mass spectra are described for methyl esters of eight clinically significant penicillins and their breakdown products. The substances give spectra with very few fragment ions and contain easily discernible protonated molecule ions. The main cleavage reaction is postulated to involve a retro 2+2 Diels-Alder-type fragmentation of the beta-lactam ring liberating one fragment (m/e=174) that is characteristic of the penicillin nucleus and a second fragment that is molecule specific, as it contains the elements of the side chain. The other fragment ions, though interesting, are of minor intensity. The free acids, on the other hand, fragment more extensively because of their relative instability and lack of volatility. These spectra resemble electron impact spectra more closely and, though they encode more structural information, are less reproducible from run to run. The ease with which the esters can be made and the relative simplicity of their CI mass spectra make this method significant for the identification and characterization of beta-lactam antibiotics.