Mass spectrometric studies of asphaltenes' molecular structures and the development of atmospheric pressure chemical ionization laser-induced acoustic desorption

David Jesse Borton, Purdue University

Abstract

Mass spectrometry is a powerful analytical tool that has proven useful for the analysis of complex mixtures and elucidating the molecular structures of the compounds present in the mixtures through tandem mass spectrometry experiments. In spite of the utility of mass spectrometry in complex mixture analysis, it does have its limitations. For example, analysis of mixtures of hydrocarbons, such as crude oil and its nonvolatile fractions, is still a problematic area for mass spectrometry. Hence, very little is known about the heaviest fractions of crude oil, arguably one of the most complex mixtures in nature. These heavy fractions of crude oil, such as asphaltenes, create many problems for the oil industry. Hence, elucidating the structures of the compounds within these mixtures is important in remediating these problems. Experiments described in this thesis employ tandem mass spectrometry to achieve a better understanding of asphaltenes and their molecular structures. Chapter 3 discusses the development of methodology for the analysis of asphaltenes and the comparison of the CAD mass spectra of several ionized model compounds to those of ionized asphaltenes to gain insight into the structures of asphaltenes. The results indicate that asphaltenes are predominately comprised of compounds with a single aromatic core (containing several fused benzene rings) and multiple alkyl chains of varying lengths attached to the core, as opposed to multiple aromatic cores connected by alkyl bridges. Chapter 4 examines changes to asphaltenes' molecular structures when they are subjected to hydrocracking, one of the main processes in crude oil refinement. Hydrocracking was found to shorten the length of the alkyl chains attached to the aromatic core of asphaltene molecules. Chapter 8 compares field deposit asphaltenes, which have been removed from a clogged pipeline, to heptane precipitated asphaltenes that have been precipitated from crude oil in a laboratory setting. The latter asphaltenes are predominately studied to understand asphaltenes' behavior. This study was carried out to explore whether the precipitated asphaltenes are structurally relevant to those that actually create issues in the field during oil production. Chapters 5, 6, and 7 focus on further development of laser-induced acoustic desorption (LIAD), a technique which has greatly aided the mass spectrometric analysis of petroleum fractions and other thermally labile nonvolatile compounds. LIAD had been traditionally performed under high vacuum in obsolete dual cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers. Chapter 5 discusses the development of LIAD/atmospheric pressure chemical ionization (LIAD/APCI) technique, which couples LIAD to modern mass spectrometers which utilize atmospheric pressure ionization sources. Chapter 6 discusses advances to LIAD/APCI, namely, the development of a high laser power probe for the reproducible evaporation of high-mass compounds into the gas phase, and the development of a rastering assembly which greatly increases the surface area of the LIAD foil that can be sampled, thus increasing sensitivity. Chapter 7 discusses a novel chamber for preparing sample foils for LIAD, which uses a drying gas to prepare foils with a more uniform sample layer than possible previously, and can be used with nonpolar analytes. The ability to prepare more uniform foils improves the reproducibility of LIAD.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry

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