Position-specific hydrogen isotope equilibrium in propane

Abstract

Intramolecular isotope distributions can constrain source attribution, mechanisms of formation and destruction, and temperature-time histories of molecules. In this study, we explore the D/H fractionation between central (-CH 2 -) and terminal (-CH 3 ) positions of propane (C 3 H 8 ) -a percent level component of natural gases. The temperature dependence of position-specific D/H fractionation of propane could potentially work as a geo-thermometer for natural gas systems, and a forensic identifier of specific thermogenic sources of atmospheric or aquatic emissions. Moreover, kinetically controlled departures from temperature dependent equilibrium might constrain mechanisms of thermogenic production, or provide indicators of biological or photochemical destruction. We developed a method to measure position-specific D/H differences of propane with high-resolution gas source mass spectrometry. We performed laboratory exchange experiments to study the exchange rates for both terminal and central positions, and used catalysts to drive the hydrogen isotope distribution of propane to thermodynamic equilibrium. Experimental results demonstrate that D/H exchange between propane and water happens easily in the presence of either Pd catalyst or Ni catalyst. Exchange rates are similar between the two positions catalyzed by Pd. However, the central position exchanges 2.2 times faster than the terminal position in the presence of Ni catalyst. At 200°C in the presence of Pd catalyst, the e-folding time of propane-water exchange is 20 days and of homogeneous exchange (i.e., equilibrium between central and terminal positions) is 28 minutes. An equilibrated (bracketed and time-invariant) intramolecular hydrogen isotope distribution was attained for propane at three temperatures, 30°C, 100°C and 200°C; these data serve as an initial experimental calibration of a new position-specific thermometer with a temperature sensitivity of 0.25‰ per ˚C at 100 ˚C. We use this calibration to test the validity of prior published theoretical predictions. Comparison of data with models suggest the most sophisticated of these discrepant models (Webb and Miller, 2014) is most accurate; this conclusion implies that there is a combined experimental and theoretical foundation for an 'absolute reference frame' for positionspecific H isotope analysis of propane, following principles previously used for clumped isotope analysis of CO 2 , CH 4 and O 2 (Eiler and Schauble, 2004;Yeung et al., 2014;Stolper et al., 2014).

Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases

“…First, we evaluated the range in predictions of the site specific carbon isotope fractionation of propane at 400K and found that three different model predictions ranged by 0.9‰ at 400K (Webb and Miller, 2014;Piasecki et al, 2016; this study). We also note that Xie et al 2018 (7), experimentally tested the accuracy of these three models, demonstrating that their variance is generally similar to their average accuracy. Second, a recent study evaluated the range in predictions of the bulk carbon isotope fractionation between CO2 and methane found that the models fell within 2‰ of the measurements from 300-1200°C (8).…”

Position-specific distribution of hydrogen isotopes in natural propane: Effects of thermal cracking, equilibration and biodegradation

“…All sample measurements are made by sample/standard bracketing, referenced to an intra-laboratory working gas, CITP-1. CITP-1 has a δD of -179‰ and εD C-T of -26‰ (Xie et al, 2018). The DFS can render very high massresolution (M/∆M ~ 80-100,000), though we operate it at resolutions of 35,000 (full-width halfmaximum definition) for propane analysis.…”

“…Propane (C 3 H 8 , or CH 3 -CH 2 -CH 3 ) has two chemically nonequivalent sets of atomic sites: the central CH 2 group and the terminal CH 3 groups. The carbon and/or hydrogen isotope differences between these two positions have been analyzed by GC-pyrolysis-GC-IRMS (gas chromatography isotope ratio mass spectrometry) (Gilbert et al, 2016;Li et al, 2018), biochemical degradation with GC-IRMS , high-resolution direct molecular mass spectrometry (Piasecki et al, 2016a;Xie et al, 2018) and nuclear magnetic resonance (Liu et al, 2018). It has been shown that site-specific isotopic measurements are able to differentiate abiotic propane sources from common thermogenic propane (Suda et al, 2017), track thermal maturation Liu et al, 2019;Julien et al, 2020), and identify residues of subsurface microbial degradation (Gilbert et al, 2019).…”

“…The temperature dependence of equilibrium isotope fractionation between the central and terminal hydrogen positions has been theoretically predicted (Webb and Miller, 2014;Piasecki et al, 2016b) and experimentally calibrated (Xie et al, 2018). Therefore, position-specific hydrogen isotope distribution in propane can potentially work as a 'geothermometer' that could track the equilibration temperature at which propane has been generated and/or stored.…”

Computer-generated isotope model achieves experimental accuracy of filiation for position-specific isotope analysis

“…Propane is a good test case since it is the smallest hydrocarbon with multiple unique carbon positions and its position-specific enrichments have been widely studied both computationally (Piasecki et al, 2016b;Webb and Miller, 2014;Ni et al, 2011;Cheng and Ceriotti, 2014) and experimentally. (Xie et al, 2018;Piasecki et al, 2016aPiasecki et al, , 2018Gilbert et al, 2016a;Gao et al, 2016b;Li et al, 2018;Liu et al, 2018) This work compares a propane mechanism produced with this method against experimental PSIA data measured by Gilbert et al(Gilbert et al, 2016a)…”