Date of Award
Doctor of Philosophy
The motivation of this dissertation is to provide insight towards primary ionization mechanisms within MALDI mass spectrometry. Albeit MALDI-MS is an extensively used analytical technique, the mechanism in which primary ions are created is still under scrutiny. Two current models of primary ionization exist which claim to elucidate the ion formation mechanisms within MALDI. In this work, excited state dynamics of MALDI matrices are shown to play an important role in the ionization mechanism. Upon inspection of the thermodynamic properties of commonly used MALDI matrices, no correlation was observed when plotted against their respective analyte ion yields. However, the excited state singlet lifetimes of these matrices seem to correlate well with their respective analyte ion yields. In the broadest sense, this correlation further supports the fact that photophysical properties of the matrix should be included in current UV-MALDI models. Investigation of a claim which stated singlet energy pooling reactions were absent in the MALDI matrix 2,4,6-trihydroxyacetophenone (THAP) resulted in the discovery of a new energy pooling mechanisms. Characteristic of aromatic ketones such as THAP, intersystem crossing is an efficient process in solution, which gives way to fluorescence in the solid state. Triplet pooling mechanisms from two neighboring THAP molecules are proposed and appear to be dependent on the preparation solvent used. These triplet pooling reactions are thought to play an important role in the primary ion formation mechanism within MALDI. To further investigate the theory of triplet species playing a vital role in MALDI ionization, the internal heavy-atom effect was employed to determine the effect of the triplet species. MALDI mass spectra and excited state decays of these heavy-atom substituted matrices were collected to demonstrate the relationship between triplet species and analyte ionization efficiency. Gas-phase thermodynamics and absorption at 337 nm were also examined to determine if these properties affected the analyte ion signal observed in the MALDI mass spectrum. Using the information collected from the previous study, an advanced MALDI matrix is synthesized. Addition of covalently bound iodine to the gold standard matrix, α-cyano-hydroxycinnamic acid, should drastically improve the performance of the non-substituted matrix due to the increase in triplet species present for pooling reactions. Sample preparation methods in MALDI are examined as are the effects of crystal morphology on the overall signal observed in the mass spectrum. Exciton hopping and pooling rates are highly dependent on intermolecular interactions, so it is expected that crystal packing will affect MALDI. As noted for THAP, preparation solvent plays a significant role in not only crystal morphology, but also the excited state dynamics for all matrices studied.
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