Structural elucidation of deprotonated analytes via tandem mass spectrometry based on ion-molecule reactions and collision-activated dissociation
Mass Spectrometry has emerged as a powerful analytical tool for the characterization of unknown molecules. Molecular weight information and chemical formulas of the unknowns can be derived by measuring the m/z value of the ionized analyte. In addition, structural information can be obtained via tandem mass spectrometry methods such as collision-activated dissociation (CAD). However, CAD does not always guarantee unambiguously assignment of chemical structures, therefore, additional tandem mass spectrometric methods such as ion-molecule reactions were developed. In this dissertation, both CAD and ion-molecule reactions were used to elucidate the structure of deprotonated analytes. Currently, most existing neutral regents are designed to react with protonated analytes, making them suboptimal for the acidic analytes that are more sensitive when detected under negative ion mode. The identification of analyte ions containing phenoxide, carboxylate, and phosphate functionality was achieved by using a novel ion-molecule reaction setup that allowed two neutral reagents, diethylmethoxyborane (DEMB) and water, to be introduced into the ion trap. Using DEMB as the sole reagent, reactions between phenoxide, carboxylate, sulfate, and phosphate containing analytes and DEMB were studied. By pulsing in water, a new reagent, diethylhydroxyborane (DEHB), was generated inside the ion trap, allowing the reactions between phenoxide, carboxylate, sulfate, and phosphate containing analytes and DEHB/H2O to be studied as well. Reaction mechanisms were explored via isotope labeling experiments and quantum chemical calculations. The above mass spectrometry method allowed the differentiation of phenoxide, carboxylate, phosphate, and sulfate containing analyte and was successfully coupled with high-performance liquid chromatography for the analyses of a biomass conversion product mixture and drug metabolite mixture. In addition, ion-molecule reactions with thionyl chloride were explored as a mean to differentiate deprotonated benzendiol isomers that were not discernable via CAD. Finally, HPLC-MSn with CAD were used to analyze complex mixtures related to lignin, including organosolv lignin and microbial converted biomass.
Kenttamaa, Purdue University.
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