Mass spectrometric investigations of structurally diagnostic ion-molecule reactions and of molecular structures of asphaltenes

James Steven Riedeman, Purdue University

Abstract

Tandem mass spectrometry (MS/MS) has proven to be a powerful tool for the molecular-level characterization of components of complex mixtures due to its ability to study ionized molecules with high sensitivity, selectivity and specificity. By combining MS/MS with liquid chromatography (LC) to enable front end separation of complex mixtures, trace level analysis of components in these mixtures is possible. Hence, LC-MS/MS has proven invaluable in the separation and characterization of saturated, alkyl aromatic, and polycyclic aromatic hydrocarbon constituents of crude oil. The most common MS/MS method involves collision-activated dissociation of mass-selected ions. Additionally, MS/MS strategies utilizing functional-group or isomer specific ion-molecule reactions have been recognized as a fast and efficient way to identify specific analytes among mixture components. However, the mechanisms of many gas-phase reactions, including dissociation and ion-molecule reactions, are not easily deciphered using MS alone. Thus, density functional theory (DFT) calculations may be used in combination with MS/MS data to derive reaction pathways leading to the formation of intermediate and product ions. Knowledge of the structures of these intermediate and product ions and pathways to their formation may be used to develop new mass spectrometry methodologies for elucidation of structures of unknown components in mixtures and to identify better reagents for functional-group and isomer specific ion-molecule reactions. This thesis focuses on the development of MS/MS strategies utilizing ion-molecule reactions, synthetic model compounds, and DFT calculations to elucidate the structures of ions and reaction pathways for a variety of gas-phase ion processes. Chemical ionization with aluminum (III) chloride followed by collision-activated dissociation (CAD) was examined as a method to distinguish epimeric 1,6-anhydrosugars which are commonly found in the pyrolysis products of cellulosic biomass. The use of dimethylamine in ion-molecule reactions with protonated N-oxides was investigated as a method to distinguish secondary and tertiary N-oxides from amines and other heteroatom-containing analytes. Additionally, MS/MS analysis was combined with Raman spectroscopy for the detailed characterization of asphaltenes derived from crude oil deposits. Crude oil deposition in oil transfer pipelines is perpetuated by the presence of polynuclear aromatic hydrocarbon structures, such as asphaltenes, whose molecular architectures are still not fully understood. The molecular structures of asphaltenes were explored by comparing the fragmentation behavior of ionized synthetic model compounds to that of genuine ionized asphaltenes isolated from Maya crude oil. Finally, rearrangement of the molecular ion of diphenylmethane leading to cleavage of methyl radical was examined by studying the fragmentation products of 2H- and 13C-labeled derivatives of diphenylmethane ions and through the use of DFT calculations since diphenylmethane is a common carbon skeleton in numerous therapeutic drugs whose dissociation mechanisms were not fully understood previously.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Chemistry

COinS