Self-chemical ionization(SCI)/quadrupole mass spectrometry was developed for characterization of five erythromycin antibiotics. The protonated molecular ion peaks(MH^+) were obtained with higher abundance. Much more fragment ions were obtained by SCI than by electron impact ionization (EI). The fragmentation patterns are discussed. The result shows that SCI/quadrupole mass spectrometry can not only reveal molecular weight but also offer much structural information of unstable compounds in ionization. In SCI, the reacting gas was not used, and the MH^+ was produced by the collision of the molecular and the fragment ions that can supply H^+. The absence of the ions produced by the reacting gas makes spectrum clean and simple, and thus facilitates structural elucidation.

Key words: ERYTHROMYCINUM, ANTIBIOTICS, CHEMICAL IONIZATION SOURCE MASS SPECTROMETRY, QUADRUPOLE MASS SPECTROMETRY, STRUCTURAL ANALYSIS