Electrospray mass spectrometry in the structural characterization of cephalosporins

Six cephalosporins of pharmacological interest, cephalexin, cephuroxime, cephazolin, cephoperazone sodium salt, cephatrizin free acid and cephonicid disodium salt, were analysed by electrospray mass spectrometry. [M - Na]- anions were produced in high yield in the case of cephalexin, cephuroxime, cephazolin and cephoperazone, leading to signals at least two orders of magnitude more intense than those related to [M + Na]+ cations observed in the positive ion mode. In cephatrizin, [M - H]- represented the most abundant species, whereas in cephonicid the [M - 2Na + H]- anions were easily produced. No fragment ions were detectable in the electrospray spectra of any of the compounds, and MSn turned out to be essential to draw the fragmentation patterns. Most of these patterns were related to the substituent of the 7-aminocephalosporin nucleus, suggesting that the nucleus itself is highly stable.