The study of gas phase ion-molecule chemistry using a quadrupole ion trap mass spectrometer

Bobette Darlene Nourse, Purdue University

Abstract

The unique capabilities of the quadrupole ion trap mass spectrometer (ITMS) were employed to investigate gas phase ion-molecule chemistry, to answer fundamental chemical questions and to more fully characterize the ability of the ITMS to yield quantitative information. Rate constant measurements were made, using both the ITMS and the simpler ion trap detector (ITD), and compare well with values reported in the literature. Differences in rate constants for four $\rm C\sb5H\sb8$ isomers, determined using the ITD, allowed partial isomer distinction to be achieved. The rate constant and product distribution for the reaction of ionized oxygen with methane were examined using the ITMS and then utilized to estimate an effective ion temperature for ions contained within this trapping device. The ion temperature was determined to be approximately 650K under standard operating conditions. The nature of methyl cation bonding to the three dihydroxybenzene isomers was examined using the ITMS. From comparisons of energy resolved mass spectra (ERMS) and collision activated dissociation (CAD) data, the amounts of ring versus heteroatom methylation could be estimated for each isomer. Influences of experimental conditions on the reproducibility of relative ion abundances in ERMS and CAD data were also examined. The ability of the ITMS, using the kinetic method, to measure very small differences in proton affinities (PA) was explored. Differences in PA $\leq$ 0.1 kcal/mole were measurable and allowed for effects of ring size and strain, methyl substitution and deuterium labeling (isotope effects) on PA to be determined. In addition, previously unknown PA values were determined for a series of alicyclic carboxylic acids and methyl substituted benzoic acids. The multiple isolation and activation capabilities of the ITMS were employed to characterize the pyrene system. Delineation of fragmentation pathways was performed and energy deposition studies suggested that tens of eV of internal energy could be deposited using multiple activation steps. Under extreme activation conditions, energies up to 17 eV could be deposited in a single activation event. Ion-molecule reactions of the M$\sp{+.}$, (M-H)$\sp+$ and (M-H$\sb2)\sp{+.}$ ions of pyrene were also investigated, employing cycloaddition and alkyl halide addition reactions. Only the molecular radical cation of pyrene showed no reactivity.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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