(1) Position-specific hydrogen isotope distribution of propane

This study investigates hydrogen isotope distribution between the two non-equivalent positions in propane, –CH2– and –CH3. We analyze propane with a reverse-geometry double focusing sector mass spectrometer. Via electron impact ionization, a fraction of propane molecules dissociates to ethyl fragment ion. We measure D/H ratio for the ethyl fragment and the molecular ion to obtain D/H ratio of each position. We calibrated equilibrium isotope effect between central and terminal positions. This study can be found in this paper.

We applied the mass spectrometric method to analyze propane samples from hydrous pyrolysis experiments, hydrogen exchange experiments and natural gas fields. Our results show that hydrogen isotope structure of propane is largely controlled by irreversible formation processes, expressing kinetic isotope effects, but thermally activated hydrogen exchange could equilibrate propane at sufficient temperature and depth. A paper associated with this study has been published at GCA.

(2) Multiply substituted isotopologues of methane

The relative abundances of doubly-substituted isotopologues of methane (aka. 'clumped isotopes') have been used to track the formation of thermogenic and microbial methane, as well as consumption of methane via methanotrophs and atmospheric oxidants. Previous studies have generally indicated that thermogenic methane conform to intramolecular isotope equilibrium — such that clumped isotopes could be used to reconstruct methane formation temperature. We made resolved measurements of two distinct doubly-substituted isotopologues, ¹²CH₂D₂ and ¹³CH₃D, for natural gas samples from shale gas formations. It was found that methane at early maturation is characterized by a pronounced deficit in ∆¹²CH₂D₂. This departure from equilibrium could be diminished by hydrogen exchange with increased burial and maturation. The evolution of methane clumped isotope signature reveals fundamental mechanisms governing gas formation and provides new constraints to thermal maturation of gas. This study was published in this paper.

(3) Statistical modeling of thermal cracking

This study aims at creating a generic statistical simulation method for predicting isotopomer abundances of products from thermal cracking. Conventional models of thermal cracking do not output isotopomer information because they don't track elementary reactions and explicit reactants and intermediates, which is ultimately due to the complexity of reaction pathways and chemical structures. In this study, we use the kinetic Monte Carlo method to simulate the thermal breakdown of different organic substrates. We obtain abundances of all isotopomers of interest for C1-C7 compounds that could be converted into compositional ratios and intermolecular and intramolecular stable isotope compositions. We show that molecular structure of the parent material exerts an important influence over the intramolecular isotope distribution of its products. Refer to my presentation at Goldschmidt 2020 for more information.