Structural elucidation of organic molecules directly in complex mixtures by laser-induced acoustic desorption and collision-activated dissociation mass spectrometry

Lucas Mark Amundson, Purdue University

Abstract

Mass spectrometry (MS) has proven invaluable in the field of complex mixture analysis due to its high sensitivity, selectivity, and speed, as well as its reliability, reproducibility, and its capability of rapidly providing information-rich data for mixtures without prior separation. In the past couple of decades, mostly due to the advent of diverse atmospheric pressure ionization techniques, mass spectrometry has revolutionized the way complex mixture analysis is performed. Tandem mass spectrometry (MS n) using collision-activated dissociation (CAD) is the technique of choice for structural elucidation of analytes in mixtures after atmospheric pressure ionization. The coupling of MS with high performance liquid chromatography (HPLC) has further facilitated the analysis of components in complex mixtures. However, these techniques don't always allow for the unambiguous identification of unknown compounds. The first part of this thesis (Chapters 3-6) focuses on different ways of obtaining structural information for mixture components that is difficult by established methods. Several tandem mass spectrometry methods based on CAD were developed to identify dissimilar oxygen-containing functional groups in aromatic analytes within complex mixtures. The novel methods discussed combine HPLC separation, either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI), and CAD. These methodologies were successfully implemented on commercially available linear quadrupole ion trap (LQIT) mass spectrometers. Moreover, the analysis of complex mixtures can be complicated by unwanted side reactions either within the ionization source or within the mass spectrometer. Therefore, APCI source conditions in LQIT mass spectrometers play a significant role with respect to the occurrence of interfering gas-phase aggregation and ion-molecule reactions. These effects are discussed in Chapter 7 with respect to the analysis of asphaltenes and asphaltene model compounds. Since its introduction as an evaporation method for mass spectrometric analysis of nonvolatile organic compounds, laser-induced acoustic desorption (LIAD), coupled with electron ionization (EI) or chemical ionization (CI), has been demonstrated to facilitate the analysis of a wide variety of nonvolatile, thermally labile compounds. Over the years, LIAD has been typically employed in Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers, and was only recently coupled with an LQIT mass spectrometer. The last part of this thesis focuses on the development of LIAD instrumentation and methodology for complex mixture analysis. The coupling of this technique orthogonally to the ion beam in a triple quadrupole (QqQ) mass spectrometer is discussed in Chapter 8. The design, construction, and characterization of a novel high-power LIAD probe coupled to an APCI source of LQIT mass spectrometers is discussed in Chapter 9. This probe, when fitted to a novel rastering assembly, provides significantly greater power densities and desorption efficiencies, and allows for more efficient sample use.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry

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