Fragment ions. SSIMS

Surface distribution of drugs. The positive ion SSIMS spectrum of pure indomethacin film up to 400 amu is shown in Figure 4. The protonated molecular ion, M+l, is clearly evident at 359D with an intense signal at 139D which probably arises from the molecular fragmentation of indomethacin as follows ... 

There the SSIMS (Fig. 93 A) and the FABMS (Fig. 93 B) secondary positive ion spectra are compared for PTFE the surface of the PTFE has to be coated with a gold film for SSIMS measurement but not for FABMS. More fragment ions occur in the SSIMS spectrum than in FABMS, and in the FABMS spectrum the size of the peak at m/e=12 (C ) relative to the principal peak at m/e = 69 (CI ) is much smaller than in SSIMS. indicating much less damage of the polymer surface during FABMS analysis. 

In general. SNMS spectra do not differ much from SSIMS spectra i.e., quasi-molecular ions, fragment ions and elemental ions can be observed [31]. Of course the primary ionization process is different and results always in positively charged ions. Typical quasimolecular ions are therefore M+. Quasimolecular ions can easily be generated from aromatic ring systems (due to re.sonance excitation). 

In conclusion, SSIMS spectra provide not only evidence of all the elements present, but also detailed insight into molecular composition. Quasimolecular ions can be desorbed intact up to 15000 amu, depending on the particular molecule [3.17] and on whether an effective mechanism of ionization is present. Larger molecules can be identified from fragment peak patterns which are characteristic of the particular molecules. If the identity of the material being analyzed is completely unknown, spectral interpretation can be accomplished by comparing the major peaks in the spectrum with those in a library of standard spectra. 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Figure 6 SSIMS normalized intensicias o molecular ions containing both a1 atom and pet fragments as a function of Che aluminum integrated atomic flux. The normalization factors and the possible chemical compositions of the ions with the corresponding molecular masses are given in Table l a.

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