Molecular secondary ion mass spectra of kanamycins.

Sir: In our laboratories two strategies for obtaining mass spectra of involatile and/or thermally unstable natural products have been used. One is an optimization of sample vaporization, which involves rapid sample heating followed by thermal desorption and ionization under chemical ionization (CI) conditions, that is emitter CI mass spectrometry (emitter CIMS). Recently, we have reported a preliminary study on aminoglycosidic antibiotics, kanamycins, by this technique.') Another technique is a direct ionization of the sample from a surface. Several techniques such as field desorption (FD),') 252Cf plasma desorption (PD, fission fragment induced),') laser desorption (LD)'> and secondary ion (SI) mass spectrometry5,6) have been devised for this purpose. We adopt molecular SIMS because of its simple operation and good reproducibility. Moreover, it should be emphasized that SIMS does not require heating of the sample for its desorption and ionization. Although several reports on application of SIMS to organic compounds have appeared, no spectra of biologically important naturally occurring products with a mass greater than 350 have hitherto been reported except for our studies.7,8) In this study a preliminary investigation of aminoglycosidic antibiotics has been carried out using kanamycins as typical examples. A double focusing mass spectrometer from Hitachi (M-80) equipped with an additional electron impact (EI) ion source for producing primary ions (Ar+) was used in this study. A direct sample inlet probe was modified to support a metal substrate on which samples were loaded.'' Silver was used as the metal substrate and 3 kV was applied for acceleration of secondary ions. Fig. 1 shows the secondary ion mass spectra of kanamycin A (1), B(2) and C(3). The three cationized molecular species, protonated molecule, MH+, (M+Na)+ and (M+Ag)+, are clearly present in the high mass region and they indicate exactly the molecular weights. In particular, the argentated molecules are most important feature in the mass spectra supported on silver, since Ag-containing ions appear as doublet peaks due to the presence of Ag-isotopes (107, 109). The principal fragmentations are considered to be restricted to the cleavage of the glycosidic bonds. The resulting fragment ions are summarized in Fig. 2. The pseudodisaccharide derived ions are observed as protonated