FUNDAMENTAL STUDIES IN MOLECULAR SECONDARY ION MASS SPECTROMETRY AND TANDEM MASS SPECTROMETRY (POLYCYCLIC AROMATIC HYDROCARBONS, CHARGE STRIPPING, RAMAN SCATTERING)
Abstract
Secondary ion mass spectrometry (SIMS) and tandem mass spectrometry (MS/MS) are useful techniques for the ionization of nonvolatile molecules and the study of gas-phase ionic processes, respectively. The mechanism of molecular desorption and ionization has frequently been debated. Current theories and experimental results which led insight into the processes underlying molecular SIMS are reviewed here. A case is made for the contribution of both high-energy (collision cascade) and low-energy (thermal spike) phenomena to the overall mechanism. Molecular SIMS studies of model catalysts are reported, as are occurrences of unusually extensive fragmentation behavior in SIMS. An attempt is made for the first time to quantitatively describe the internal energy distributions of molecular ions in SIMS by considering SIMS spectra of several different classes of compounds. Vacuum-induced decomposition of some analytes in SIMS is discussed. Sputtering of materials from one surface onto another surface is reported as a means for the modification and preparation of molecular species on surfaces. Experiments in which metal ions are accelerated into molecules apparently result in surfaces with unique characteristics. Coating substrates with silver films, which are subsequently laser-irradiated, produces robust surfaces for use in surface-enhanced Raman scattering (SERS) experiments. These materials possess advantages over surfaces prepared electrochemically. Application of a number of different MS/MS techniques to the analysis of polycyclic aromatic hydrocarbons (PAH) is described. The difficulty in using mass spectrometry to distinguish isomers of these compounds is attributed to their unusually high activation energies for fragmentation, which results in extensive isomerization prior to dissociation. Ions with a high degree of linearity represent likely candidates for the isomeric structures. Charge stripping in high-energy MS/MS correlates with the site of cationization of some substituted benzenes, but not with others. Large stripping peaks are observed for ring-cationized N- and O-substituted compounds, but not for the substituent-cationized compounds.
Degree
Ph.D.
Subject Area
Analytical chemistry
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