Exploring Nature’s Fingerprints with Isotopic Distributions

Author: Csernica, Timothy Andrew

Year: 2024

Degree: Dissertation (Ph.D.)

Advisor: Eiler, John M.

Committee Members: Blake, Geoffrey A.; Stoltz, Brian M.; Marcus, Rudolph A.; Eiler, John M.

Option: Chemistry

DOI: 10.7907/zt08-jh39

Abstract

This thesis examines the measurement and interpretation of isotopic distributions and the application of these techniques to forensic questions. Stable isotope abundances are a powerful tool for examining a compound’s history. However, their use is complicated by the fact that 1) isotope substitutions can occur at many positions of a molecule, resulting in a combinatorial increase in possible combinations of isotopes (or isotopologues) with molecule size, and 2) it is difficult to experimentally distinguish between isotopologues, so observational data averages over many isotopologues with distinct properties and histories. We here develop experimental and theoretical strategies to address these questions via observations of the isotopic distributions of small organic compounds obtained by Orbitrap mass spectrometry. In Chapter II, we develop mathematical procedures for manipulating and tracking isotopologues through various experimental designs, allowing us to make precise statements about how observable quantities are affected by underlying physical and chemical processes. Chapters III and IV explore corresponding experimental methods: Chapter III presents a sample introduction technique for the long duration observations required to measure rare, multiply substituted isotopologues, while Chapter IV applies these to observe 146 isotopic properties of methionine, a model analyte. We then explore the use of these Orbitrap methods to applied science problems. In Chapter V, we characterize the 13C and 2H enrichment of methylphosphonic acid, a breakdown product of sarin precursors, and examine the signatures of its synthesis methods. In Chapter VI, we apply these techniques to extraterrestrial, abiotic syntheses of nucleobases, focusing on the chemistry of adenine. These results are interpreted in the context of proposed extraterrestrial syntheses of adenine and other purine nucleobases and used to predict the isotopic distributions of these compounds.

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