CONTROLLED COLLISIONAL-EXCITATION OF GAS-PHASE IONS AND ITS APPLICATION TO STRUCTURAL PROBLEMS (INTERNAL, ENERGY, RESOLVED MS, PTO)

SUNITA VERMA, Purdue University

Abstract

This thesis takes as its subject a discussion of the role of various experimental parameters in controlling the amount of energy deposited into an ion during ion-neutral collisions. In particular, the role of the scattering angle in determining collision endothermicity is examined. Energy resolved and angle resolved mass spectrometry techniques are applied to distinguish isomeric structures which in a fixed energy, fixed scattering angle experiment give very similar or indistinguishable spectra. Fragmentations caused by inelastic ion-neutral gas collisions have often been used to characterize the original ion. Because of the sensitivity of the relative abundances of the fragment ions to the internal energy of the parent ion, development of methods permitting the control and/or determination of the energy transfer in an ion-neutral collision is highly desirable. Energy transfer in an ion-neutral collision is determined by their relative motion, the angle at which the two particles approach each other and the polarizability of the neutral. Experimental variables related to these parameters are ion-kinetic energy, scattering angle and the nature of the target gas. In the particular case of energy control via scattering angle, it is not clear whether the laboratory scattering angle is predominantly determined by the scattering of the precursor during the collision or by the scattering of the fragments in the dissociation step. A linear relationship is observed between the scattering angle and the internal energy deposited during collision. This observation is made by comparing the angle resolved mass spectra of methanol, nitromethane, n-butylbenzene, n-pentylbenzene and 2-phenylethanol to the corresponding breakdown curves obtained by either photoionization-photoelectron coicidence techniques or photodissociation mass spectrometry/mass spectrometry. Application of energy resolved and angle resolved mass spectrometry to C(,3)H(,6)O('+(.)), C(,2)H(,4)O('+(.)), C(,5)H(,8)('+(.)), C(,2)H(,3)S('+(.)) isomeric ions, as well as n-butylbenzene shows the usefulness of these new techniques in distinguishing isomeric ions where conventional collision-activated dissociation (CAD) fails. New insights into the structures and interconversions in these systems have been achieved through the application of these methods. A comparison of the energy resolved MS and angle resolved MS suggest that it is possible to monitor fragmentation from relatively high energy states in low energy CAD process compared to high energy CAD. Furthermore, the energy deposited in high energy collisions falls in a region where distinction between isomeric structures is rather difficult in some cases.

Degree

Ph.D.

Subject Area

Analytical chemistry

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