Negative ion electrospray ionization tandem mass spectrometry in the structural characterization of penicillins

The mass spectrometry (MS) behaviour of ten commercially available penicillins has been studied by means of electrospray and multiple-stage MS/MS experiments performed using an ion trap instrument. For all the examined compounds negative ions are produced under ESI conditions, with a yield two or three orders of magnitude higher than that observed for positive ions. MSn experiments indicate the occurrence of a fragmentation pathway related to the beta-lactam ring, different from that usually described for positive ions of these compounds, and provide structural information on both the beta-lactam ring and the side chains.