Aspects of ion chemistry in mass spectrometry
Abstract
Both analytical applications and fundamental aspects of gas phase ion chemistry were investigated using mass spectrometry. Tandem mass spectrometry was used to characterize hydroxyaromatic compounds in coal and identify metalloporphyrin series in petroleum samples. A membrane probe was designed which allowed direct introduction of aqueous solutions into a mass spectrometer. The probe was used in an on-line mode to detect trace organic compounds in water and to monitor in real time the product formation and reactant depletion of an organic reaction, the methylation of cyclohexanone. A modified probe allowed in vivo blood analysis for toxic compounds such as dichloromethane and styrene administered nasally. The operational characteristics of a quadrupole ion trap operated in the mass-selective instability mode were examined and its capabilities were utilized for studies of ion/molecule chemistry and ion activation. For initial studies of ion/molecule reactions, chemical ionization was implemented, demonstrated for analytical applications, and was used as a means of characterizing pressure, time, and temperature effects of ion/molecule reactions in an ion trap. More complex ion/molecule reactions, such as the methylation and halomethylation of substituted aromatic compounds, were studied to elucidate the reaction mechanisms, determine product ion structures, examine substituent effects on reactivity, and probe the existence of loosely-bound pi complexes among the ion/molecule reaction products. For a more fundamental study of thermochemical aspects of ion/molecule reactions in an ion trap, two methods of determining gas phase basicities were compared, the kinetic method and the proton-transfer equilibrium technique. Relative basicities could be determined for pairs of bases differing by 0.8-17.1 kJ/mole, with an accuracy of $\pm$0.8 kJ/mole. Aspects of ion activation were also examined in an ion trap mass spectrometer. The activation conditions were characterized by studying the collision activated dissociation behavior of phosphate isomers. Activation was found to occur in a step-wise fashion with low energy deposition in each collision. Finally, an unusual rearrangement induced by collisional activation of protonated 2,2,6-trimethylcyclohexanone was examined with respect to the enthalpic and entropic requirements of the process.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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