COLLISIONAL EXCITATION OF GAS-PHASE IONS AND THERMOCHEMICAL INFORMATION FROM ION-BOUND DIMERS (MS/MS, POLYATOMIC)

SCOTT ALEXANDER MCLUCKEY, Purdue University

Abstract

Fundamental aspects of high velocity ion/neutral collisions are discussed with regard to their use in exciting polyatomic ions in a mass spectrometer, and a mass spectrometric technique is described which allows gas-phase acidities and basicities to be ranked via the relative dissociation rates from ion-bound dimers. The use of high velocity ion/neutral collisions as a means for exciting gas-phase ions is of particular value in mass spectrometry/mass spectrometry (MS/MS). Currently, two distinct collision energy regions are utilized in MS/MS; viz. 1-150 eV and greater than 1 keV. Comparisons between MS/MS spectra obtained using low and high energy collisions are made for a variety of molecular ions including allene, propane, n-butylbenzene, perfluoropropylene, n-propylbenzene, cyclohexene, and others. Variation of the collision energy in the low collision energy region and angular selection of the ionic products from high energy collisions allow MS/MS spectra to be obtained as a function of ion internal energy. Comparisons are drawn between results obtained from these techniques and others used to vary or select ion internal energies in mass spectrometry including photoion-photoelectron coincidence spectroscopy and field ionization kinetics. Low energy collisions transfer a higher percentage of the collision energy into ion internal energy than do high energy collisions but the absolute magnitude of energy transfer is usually higher in the high energy collisions. Significant differences are observed between some MS/MS spectra (e.g. those of ionized perfluoropropylene) acquired using low and high collision energies which indicate that the form in which energy is deposited differs in the two collision energy regions. The relative rates of fragmentation of ion-bound dimers reflect the difference in gas-phase basicities between two ligands bound to a central ion. For a series of chemically related compounds quantitative gas-phase acidities and basicities can be obtained. The method is illustrated for proton-bound dimers of anilines, pyridines, alkylamines, and alkylcarboxylic acids. Extension of the method to metal cation basicities include studies of Ag('+)-bound dimers of alcohols and alkenes, Cu('+)-bound dimers of alcohols and ketones, and Na('+)-bound dimers of alcohols. Comparisons are drawn between basicities for the different cations.

Degree

Ph.D.

Subject Area

Analytical chemistry

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