(1) ANALYSIS OF ORGANIC COMPOUNDS USING SECONDARY ION MASS SPECTROMETRY. (2) ENERGY PARTITIONING IN UNIMOLECULAR IONIC DISSOCIATIONS

ROBERT JEFFREY DAY, Purdue University

Abstract

Studies of gas phase ions generated by a variety of methods have made substantial contributions in both the fundamental and the applied sciences. This research has focussed on two areas of interest in mass spectrometry. While investigating the ionization of organic compounds in secondary ion mass spectrometry (SIMS) three distinct mechanisms of ionization were uncovered, cationization, electron transfer and direct sputtering. The second field of study explored the use of molecular orbital calculations in elucidating those factors that favor kinetic energy release in unimolecular ionic dissociations. Cationization was shown to be a general process, one that occurs with a wide variety of both metals and organic compounds. Cationized molecules are suggested to form by ion/molecule reactions between desorbed neutral molecules and sputtered metal ions. In addition to the typical adduct species composed of one metal and one molecule, singly charged ions were detected corresponding to two molecules attached to a single metal and to two metals attached to a single molecule. Ionization by electron transfer was observed to dominate the SIMS spectra of polycyclic aromatic hydrocarbons. Ion bombardment of these materials resulted in the emission of radical cations, which may be formed under bombardment by secondary electrons also emitted during sputtering. Electron transfer processes also gave rise to the emission of radical anions. When organic salts were subjected to ion bombardment, intense signals were recorded for the intact cation or anion as well as characteristic fragment species. This process is postulated to proceed by momentum transfer from the primary ion directly to the solid matrix, resulting in emission of surface species. In some cases ion impact initiated intermolecular reactions as evidenced by the emitted secondary ions. In addition, sputtering of amine salts resulted in the generation of halide bound dimers at high intensity. A number of binary organic mixtures were examined with the preliminary conclusion that similar compounds can be sputtered from a mixture, but some combinations of organic materials are subject to matrix effects that cause selective ionization. It was apparent that the sample matrix dramatically affected the SIMS results for mixtures. The semi-empirical molecular orbital method MINDO/3 was used to calculate reaction paths and transition state geometries for hydrogen eliminations from several cations and water loss from protonated methanol. The results indicate that there are two types of transition states that lead to large partitioning coefficients, tight complexes for which the energy surfaces are repulsive, and loose complexes with less repulsive energy surfaces but for which the interaction between the nascent fragments in the transition state is too weak for transfer of energy to occur. In both cases, partitioning as translation is necessitated. The results also indicated that collision induced dissociations, for which an increase in kinetic energy release was expected, can lead to reduced values of energy release if excitation accesses a different portion of the energy surface and leads to a higher degree of vibrational excitation.

Degree

Ph.D.

Subject Area

Analytical chemistry

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