Electron ionization structural elucidations

XPS studies of Group 15 organometallic compounds with the intention of structure and bonding elucidation, reporting certain core-electron ionization energy data of the metal atom (e.g. As 3s or Sb 3d). 

After observation of a molecular or pseudomolecular ion, the most important feature of a mass spectrum is the fragmentation behavior, i.e., the differences between the observed ion mass-to-charge ratios and their relative abundances. The rules for structural elucidations for electron ionization (El) are well known and accessible. The reader is referred to McLafferty and Turacek (1993) either as an introduction or as a refresher.30... 

Mass spectrometry (MS) has not been applied extensively to the study of naturally occurring xanthones, but the mass spectral data provide valuable information about the structure elucidation of xanthones. As well as electron impact MS, which is a routine technique for the structure elucidation of xanthones, recently developed soft ionization techniques, such as desorption-chemical ionization MS (D/CI-MS) and fast atom bombardment MS (FAB-MS), are of great interest for the analysis of glycosides. Molecular ion peaks can be observed without derivatization. Tandem MS/MS can be extensively employed in directly characterizing constituents of complex mixtures. Recently, xanthone profiles of H. perforatum cell cultures were identified by HPLC-MS/MS analysis [106]. 

There are two limitations of electron impact ionization, which in other ways is nearly an ideal ion source for identification of organic compounds. The sample must be stable as neutral molecules in the gas phase prior to ionization. There are many compounds, however, that are simply not stable at the temperatures required for vaporization. Radical molecular ions are formed with excess energy that may be sufficient for complete elimination of molecular mass information from the mass spectrum, even at low electron energies (e.g. 15-20 eV). Molecular mass information is almost always required for structure elucidation of unknown compounds. A softer ionization technique, such as chemical ionization, may be used in these circumstances to establish the compound molecular mass. 

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