Studies of chemical reactions in desorption ionization mass spectrometry
Abstract
Desorption ionization (DI) includes a number of new methods for desorbing and ionizing nonvolatile samples. In this work, thermally labile molecules were ionized using either a pulsed laser, keV atom or ion beam, or by Joule heating in the presence of a chemical plasma. A number of chemical reactions which occur as the result of desorption ionization were studied using tandem mass spectrometry (MS/MS) with collision induced dissociation as the main method of characterization. Fast atom bombardment (FAB) of both inorganics and organics dissolved in a liquid matrix sometimes produces solvated ions and the strength of solvent-ion interactions was probed using FAB MS/MS. It was found that solvent (glycerol) molecules frequently form strong covalent bonds with the analyte. When activated using gas phase collisions with an inert target gas, the solvated product ion dissociates with elimination of a group other than the solvent molecule. In contrast, adducts which could be considered as simply solvated, fragmented with exclusive loss of the solvent molecule. Unimolecular reactions of gas phase ions generated by DI were also studied. Tandem mass spectrometry was used in these experiments to follow ion reactions pathways and to provide structural information about the ions. To summarize the findings, similarities between daughter ion and DI mass spectra support the idea that unimolecular fragmentation of the molecular ion accounts for many fragment ions observed in DI. In some cases, only molecular ions are formed during DI and collision induced dissociation combined with MS/MS provides new structural information about the ions. For example, isomeric quarternary ammonium cations could easily be distinguished and sequence ions generated from (M+H)$\sp+$ of peptides. Desorption chemical ionization (DCI) of biological cojugates separated by ion pair chromatography was investigated. The samples included metabolites of benzo(a)pyrene in a cell culture. The normal conditions used in desorption chemical ionization were found to induce nucleophilic attack by glycosidic conjugates on tetraalkylammonium ion pairing reagent present in the sample. The mechanism for production of the resulting alkyl transfer product was further studied and modified ionization conditions which avoided this degradation were found.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Analytical chemistry
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