Dichlorodiphenyltrichloroethane (DDT) is an environmental contaminant with a long lifetime and significant toxicity. Stable isotopes provide a tool to trace DDT through the environment. We present an Orbitrap Isotope Ratio Mass Spectrometry (Orbitrap-IRMS) method to measure the isotopic content of two DDT fragment ions: m/z = 235 [M+- CCl3] and m/z = 316 [M+- HCl]. With 3 nmol sample sizes and commercial 4,4'-DDT standards, we observe: for the [M+- CCl3] fragment, δ13C (0.13 ‰); δ37Cl (0.07 ‰); δ13C37Cl (0.19 ‰); δ37Cl37Cl (0.19 ‰); δ13C37Cl37Cl (0.39 ‰); and for the [M+- HCl] fragment, δ37Cl (0.22 ‰); δ37Cl37Cl (0.26 ‰); δ37Cl37Cl37Cl (0.52 ‰). We applied our method to 4,4'-DDT extracted from sediment near the White Point (WP) sewage outflow on the Palos Verdes Shelf, a site with substantial DDT contamination. Resulting precisions were worse for natural samples than standard by a factor of ≈4-5 due to their lower concentrations. Our results for [M+- CCl3] show enrichment vs our standard in δ13C (1.87 ± 0.49 ‰) and δ37Cl (1.23 ± 0.29 ‰) while δ13C37Cl and δ37Cl37Cl follow the stochastic distribution. The multiply substituted results add confidence and are useful for fingerprinting. For [M+- HCl] we see no enrichment in δ37Cl (0.31 ± 1.0 ‰) or δ37Cl37Cl (0.01 ± 1.25 ‰) and possible enrichment in δ37Cl37Cl37Cl (7.81 ± 2.11 ‰). These results are within 2σ of our stochastic error bars and are not evidence of clumped isotope enrichment but should be targeted in future study. Finally, our results place an upper bound on the amount of DDT degradation of 66 %.

Stable isotope analysis is a vital tool across chemistry, geology, and environmental science, but conventional Isotope Ratio Mass Spectrometry (IRMS) techniques have limited capabilities for site-specific or multiply substituted ("clumped") isotope analyses, and are particularly limited for analyses of complex mixtures without prior analyte purification. This study addresses this gap by employing a high-resolution Orbitrap mass spectrometer to directly measure the 13C/12C ratio in a model naphthenic acid (1,2,3,4-tetrahydro-2-naphthoic acid, THN) within complex organic matrices. We applied a "zero-enrichment" experimental design to evaluate accuracy and precision by comparing pure standards to the same compound in synthetic samples resembling natural waters. Complementary experiments using low-molecular-weight organic acids and natural rumen fluid were conducted to define the method's limits under controlled and severe ion-suppression conditions. The results demonstrated that matrix effects and ion statistics can substantially degrade both accuracy and precision under certain conditions. At very low analyte concentrations, incomplete ion accumulation led to heightened δ13C variability, a condition analogous to a "blank effect". Paradoxically, adding 1% NH4OH improved the precision of 13C/12C measurements (reducing the relative standard error from ∼0.80‰ to ∼0.63‰ at 0.1 μM THN), despite a reduced signal, by promoting more stable deprotonation and minimizing ion suppression. We also identified that coaccumulated ions, even when baseline-resolved, such as a matrix-derived fragment at m/z 177, degrade precision by perturbing the space-charge balance. Removing this interference fully restored precision, underscoring the need to control coaccumulating ions. Crucially, experiments with small organic acids demonstrated that moderate ion suppression does not lead to isotopic bias, which emerges only when severe suppression reduces analyte ion counts below a critical statistical threshold. Finally, we identified an "isotopic stability plateau"─an optimal signal range where δ13C measurements are most precise and accurate, poised between noise-dominated and space-charge-distorted regimes. This work demonstrates that Orbitrap-MS can perform reliable isotope analysis in complex organic mixtures when instrumental and chemical parameters are carefully optimized, opening new applications in petroleum geochemistry, environmental forensics, and other topics.

Molecular hydrogen (H2) is a promising carbon-free energy carrier and a key metabolite in microbial ecosystems. Distinguishing microbially-produced H2 from abiotic H2 could improve our ability to recognize H2 sources. The molecular average deuterium content of H2 has been used previously for identifying its sources and sinks, and recent studies suggest the multiply-substituted (‘clumped’) isotopologue, DD, could provide additional constraints on molecular formation mechanisms and temperatures. However, the clumped isotope composition of biologically produced H2 is yet to be constrained. Here, we measured the molecular average and clumped isotope composition of H2 formed during batch fermentations of Clostridium pasteurianum and examined the effects of growth phase (exponential vs. stationary) and different H2 concentrations on the isotopic compositions of the produced H2. Our results show that microbially-produced H2 expresses a consistent deviation from the equilibrium fractionation during exponential growth, for both the clumped and molecular average (D/H) isotopic compositions, whereas during stationary phase the clumped isotope composition conforms to near-equilibrium isotopic compositions at the culturing temperature. Although lower H2 concentrations during stationary phase led to a consistent deviation from the molecular-average D/H equilibrium fractionation with respect to co-existing water, they did not affect the clumped isotope composition, which remained near equilibrium. We interpreted these results by way of a quantitative isotopic model that describes isotope effects associated with the enzymatic reaction steps of the [Fe-Fe]-hydrogenase, concluding that both the relative rates of the different reaction steps and the reversibility of the rate limiting step control the isotopic composition of the produced H2. Overall, the observed deviations from equilibrium were relatively small in comparison to the large natural variations of hydrogen D/H ratios and clumped anomalies, suggesting that despite the variations, the isotopic composition of microbially-produced H2 is generally expected to approach equilibrium at low formation temperatures. Recognition of non-equilibrium biosignatures in natural settings likely requires comparison of the H2-water D/H fractionation and H2 clumped isotope anomaly.

Subsurface natural hydrogen is an emerging clean energy resource, but its origins, loss mechanisms and cycling timescales, particularly at greater depths, remain poorly constrained. Here, we report large disequilibrium in the clumped isotopes of H2 (2H2H) and hydrogen isotope fractionation between water and H2, measured in gas samples (containing 0.10%-1.59% H2) collected from deep (3000-7000 m) wells in the central Sichuan Basin. Because molecular hydrogen can equilibrate isotopes rapidly in the presence of water, such large disequilibrium indicates active kinetically controlled production and/or consumption. Analyses of geological and chemical evidence demonstrate that isotopic disequilibrium is driven by consumption by chemical reactions in host rocks coupled with partial hydrogen exchange with co-existing water. The results imply that measured compositions do not preserve the information on original sources and processes. More broadly, the dynamic cycling of hydrogen renders sediment-hosted deep gas reservoirs unfavorable for long-term or large-scale accumulation of natural hydrogen.

Simultaneous measurements of HDO, H218O, and H216O in water evolved during pyrolysis of powdered rock samples acquired by the Curiosity rover within Gale crater's clay-bearing units indicate extreme and variable heavy-isotope enrichments averaging ~4.5 times the D/H ratio and ~1.03 times the 18O/16O ratio of terrestrial seawater. These enrichments are recorded in water desorbed from mineral surfaces and evolved from poorly crystalline phases, hydrated salts, jarosite, and clays. All evolved waters are deuterium-enriched relative to common terrestrial waters, reflecting hydrogen loss to space. Because oxygen in structurally bound hydroxyl groups is least likely to exchange with other sources over geologic timescales, we focus on oxygen in water evolved during dehydroxylation of smectite clays. Several samples have 18O/16O ratios commensurate with precipitation from, or near-complete equilibration with, water moderately 18O-enriched relative to terrestrial meteoric waters-consistent with other evidence that Mars's hydrosphere is basically like Earth's in terms of oxygen isotopes. Unlike hydrogen, oxygen atmospheric escape did not lead to extreme 18O enrichments on Mars. Locally, however, most Gale smectites' 18O/16O values require a pronounced 18O-enrichment of their parental waters. On Earth, the most extreme 18O enrichments in surface waters are found in closed basins having undergone significant evaporative loss into a low-humidity atmosphere, and the 18O/16O of authigenic clay minerals formed in these environs reflect those enrichments. A similar process acting on the hydrologic reservoir local to Gale at the time of clay formation and early diagenesis is a plausible explanation for the distinctive oxygen isotopic compositions of these clays.

The isotopic composition of glucose carries the signature of the environmental and metabolic processes that act on it, but most conventional isotope analytical methods cannot resolve its intramolecular isotopic structure. Here, we present a new method for position-specific isotope analysis (PSIA) of carbon in glucose using electrospray ionization-Orbitrap (ESI-Orbitrap) mass spectrometry. This method measures δ13C values at five unique intramolecular sites in glucose at natural isotope abundance and requires <50 μg of glucose per sample, over 3 orders of magnitude less than similar measurements by nuclear magnetic resonance (NMR). By oxidizing glucose to gluconate to improve both ionization yield and fragmentation behavior and measuring with ESI-Orbitrap, we resolve the isotopic composition of the molecular ion and four fragment ions with analytical precision of 0.5-0.8‰ (2 SE). Using a positionally labeled glucose standard, we demonstrate the accuracy of the measurement for both molecular average and position-specific carbon isotope composition. Our method reproduces intramolecular δ13C patterns previously demonstrated for natural sugars formed through C3 and C4 photosynthetic pathways while enabling substantially higher throughput and sensitivity. This is the first application of Orbitrap-PSIA to a carbohydrate, and it enables the tracing of sugar fluxes in environmental, biomedical, and ecological systems. Future developments could extend the method to include oxygen and hydrogen isotopes, further enhancing its value for investigating glucose dynamics across many natural settings.

Quantification of the stable isotopes within a compound aids forensic investigations as it provides a fingerprint which can determine that compound's source substrates, synthetic route, and possible mechanisms of degradation. Previous stable isotope studies have explored C-13 and H-2 measurements of the sarin precursors methylphosphonic dichloride (DC) and methylphosphonic difluoride (DF) as forensic signatures. However, these measurements required different sample preparations and measurement techniques. Orbitrap isotope ratio mass spectrometry (Orbitrap-IRMS) is a developing technique which can characterize multiple stable isotopes simultaneously. Here, we apply Orbitrap-IRMS to simultaneously observe the C-13 and H-2 content of methylphosphonic acid (MPA), the hydrolysis product of DC and DF, which can be used as a proxy for the isotopic content of DC and DF. Our method requires 20 min analyses and consumes approximate to 60 nmol of sample, with precisions of approximate to 0.9 parts per thousand (C-13) and approximate to 3.6 parts per thousand (H-2). We apply our method to both commercially acquired MPA and MPA obtained from the hydrolysis of commercially acquired DC. We validate our methods via comparison to elemental-analyzer isotope ratio mass spectrometry (EA-IRMS). The combined C-13 and H-2 measurement creates a more robust forensic tool than either isotope individually. Our results demonstrate the viability of Orbitrap-IRMS for chemical forensic measurements.

Mg-carbonate minerals form in and help constrain our understanding of low-temperature surface processes in terrestrial environments and on Mars and the parent bodies of some primitive meteorites. Knowledge of isotopic fractionations among Mg-carbonates, water, and CO2 can contribute to interpretation of the environmental significance of natural magnesium carbonate minerals, but we presently lack a comprehensive body of experimentally calibrated fractionations among these phases. In this study, we use quantum mechanics (density functional theory) to evaluate equilibrium mineral-fluid fractionation factors for C, O, and Mg isotopes, including selected site-specific fractionations, for a series of hydrous and anhydrous Mg-carbonate minerals and amorphous magnesium carbonate (AMC). We also revisit the existing calibrations for the temperature-dependent abundance of multiply-substituted ('clumped') species for hydrous Mg-carbonates. Because magnesite (anhydrous MgCO3) precipitation is kinetically hindered under laboratory conditions in the temperature range of interest, we use measurements of the isotope effects associated with AMC precipitation to assess the accuracy of our calculations. Using these fractionation factors, we predict Mg-26/Mg-24 of magnesite detected on Mars. These results can be used to investigate the formation of Mg-carbonates and the compositions of their coexisting fluids in numerous natural settings.

Oxygen isotopic composition of organic compounds is an important tracer of processes and environmental conditions, with applications ranging from prebiotic organic synthesis in the early solar system to paleoclimate reconstruction. To meaningfully interpret the oxygen isotopic signature of such molecules, we need to consider their rate of oxygen exchange with water, e.g., during aqueous alteration on asteroidal parent bodies or residence on the Earth. In this study, we experimentally constrained the kinetics of oxygen isotope exchange between water and ketones (acetone, cyclopentanone) or carboxylates (acetate, butyrate) near neutral pH between 274.7-373 K. We incubated them in 18O rich water, and measured their 18O/16O ratios after incubation using an electrospray ionization Orbitrap mass spectrometer. While ketones completely exchanged oxygen with water within hours, carboxylates achieved only up to 8 % oxygen exchange after days of incubation. The time evolution of their 18O/16O were consistent with first order kinetics, with exchange rate constants of 1.99 (+0.1) x 10-4 s- 1 (acetone), 6.55 (+0.23) x 10- 5 s- 1 (cyclopentanone), 3.9 (+0.1) x 10-8 s- 1 (acetate), 3.9 (+0.2) x 10-8 s- 1 (butyrate) at 298 + 1 K and activation energies of 14.47 + 0.43 kcal/mol (acetone), 20.35 + 1.47 kcal/mol (cyclopentanone), 3.53 + 0.26 kcal/mol (acetate), 2.81 + 0.43 kcal/mol (butyrate). Rapid exchange for ketones is explained by their hydration in aqueous media, and implies that ketones extracted from meteorites should have completely re-equilibrated with the last water they were in contact with over periods of hours or more (possibly at asteroidal parent bodies, or by exposure to terrestrial meteoric or laboratory waters). Carboxylic acids are resistant to exchange due to electrostatic repulsion between the hydroxyl ion and carboxylate ion in basic media. Furthermore, we predict that significant oxygen isotopic exchange between meteoritic insoluble organic matter (IOM) and aqueous fluid is likely during aqueous alteration, based on rate constants reported in this study and previous studies and constraints on the bonding environments of oxygen in IOM.

The isotope anomalies of noncarbonaceous (NC) and carbonaceous (CC) extraterrestrial materials provide a framework for tracing the distribution and accretion of matter in the early solar system. Here, we extend this framework to sulfur (S)—one of six “life-essential” volatile elements [T C ~ 664 K]—via the mass-independent S-isotope compositions of differentiated meteorites. We observe that on average, NC and CC iron meteorites are characterized by distinct Δ 33 S (Δ 33 S NC = 0.013 ± 0.003‰; Δ 33 S CC = −0.021 ± 0.009‰; 2 SE). The average Δ 36 S of NC and CC irons are less well resolved (Δ 36 S NC = −0.006 ± 0.039‰; Δ 36 S CC = −0.101 ± 0.114‰; 2 SE), but the Δ 36 S values of the CC irons are concentrated in the lower half of the range of those observed for iron meteorites. A lack of CC achondrite S-isotope analyses prevents direct comparison of the Δ 33 S and Δ 36 S of NC and CC achondrites, but the average Δ 33 S and Δ 36 S of NC achondrites (Δ 33 S = 0.02 ± 0.008; Δ 36 S = −0.019 ± 0.064‰; 2 SE) overlap with those of the NC irons. The average Δ 33 S values of NC achondrite groups also correlate with nucleosynthetic anomalies of other elements (e.g., Cr) previously used to define isotopic heterogeneity within the NC reservoir. The position of the Earth in Δ 33 S-Δ 36 S composition space implies that ~24% of terrestrial S derives from CC materials, while the majority (~76%) was delivered by NC materials.

The Earth's surface undergoes continuous changes due to the redistribution of surface mass through erosion, sediment transport, and deposition. Quantifying these mass fluxes is essential for understanding the patterns and rates of landscape evolution. Established approaches for estimating these fluxes in drainage basins often fail to distinguish among bedrock sources and suffer from transport-related biases. This problem is emphasized in carbonate terrains that typically lack distinct mineral compositions indicative of sediment sources.Here, we develop a novel approach that combines established mineral proxies, together with oxygen, carbon and ‘clumped’ isotope analyses of detrital carbonates, to evaluate the provenance and relative fluxes of sediment in carbonate-dominated drainage systems. The new approach is applied to the Morag catchment in the Hatrurim Syncline, southern Israel, a well-suited and illustrative field case for its large variability in isotope compositions between marine and metamorphosed carbonate rock sources. In this setting, the clumped isotope analysis is a sensitive tool, enabling us to distinguish among potential source units by their thermal history.The analysis reveals that the variations in mineral and isotope compositions of sediment samples collected from various locations in the Morag catchment are consistent with mixing between two end-member carbonate bedrock sources. We developed an inverse mixing model that infers the compositions of these sources and predicts the mixing-ratio based on the measured mineral and isotope compositions from sediment samples. Optimal sources found by the model are consistent with: 1) Marine carbonates; and 2) marine carbonates altered by post-metamorphic re-crystallization.Model-predicted mixing ratios of end member components of the sediment samples correlate with relative exposure areas of relevant source units. This consistency suggests spatially uniform erosion conditions, such that relief is neither created nor destroyed within at least one fluvial response time. Consequently, a prominent increase in steepness across a lithologic and structural boundary at the upper reaches of the catchment is interpreted to reflect a signature of lithology-dependent erodibility, rather than faster erosion of the steeper terrain.

The chirality of amino acids in extraterrestrial materials may provide an insight into the origin of the essential l-enantiopure amino acids in the terrestrial biosphere. In 2020, the Hayabusa2 mission succeeded in bringing back surface materials from the C-type asteroid (162173) Ryugu to the Earth. Amino acids were one of the targeted organic molecules to be studied in the Ryugu samples. To analyze the various structural isomers of amino acids, which were expected to be present, from the limited amount of the returned samples, the development of a highly-sensitive and selective analytical method was necessary. In the present study, a three-dimensional high-performance liquid chromatography (3D-HPLC) system has been developed for the enantioselective determination of five proteinogenic and three non-proteinogenic amino acids in the Ryugu samples, in which amino acids in the sample were separated by reversed-phase, anion-exchange and enantioselective columns after the fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiozole. The applicability of the analytical system to the extraterrestrial samples was evaluated by analyzing several types of carbonaceous meteorites before applying the system to the Ryugu samples. In the analysis of the Ryugu samples, all of the target amino acids were successfully determined quantitatively. Non-proteinogenic amino acids including 2-amino-n-butyric acid, isovaline and norvaline, rarely present in the terrestrial environment, were found as almost racemic mixtures with 47.1 to 55.2%l.

Carbon capture and storage (CCS) of CO2 is a key technology for substantially mitigating global greenhouse gas emissions. Determining the biogeochemical processes in host rocks after CO2 injection informs the viability of carbon storage as a long -term sink for CO2, the complexity of reservoir CH4 cycling, as well as the direct and indirect environmental impacts of this strategy. The doubly substituted ('clumped') isotopologues of methane ((CH3D)-C-13 and (CH2D2)-C-12) provide novel insights into methane origins and post -generation processing. Here, we report the chemical compositions of hydrocarbons (C-1/C2+ molecular ratios), and methane bulk and clumped isotopes (delta C-13, delta D, Delta(CH3D)-C-13 and Delta(CH2D2)-C-12) of a CO2 enhanced coal bed methane recovery (CO2-ECBM) area in Qinshui basin, China and is an analogue for carbon capture and storage. The clumped isotopologue compositions observed in the study area are generally consistent with a range of temperatures spanning 73 to 193 C-degrees. The range in apparent temperature and correlations among clumped and bulk isotopic indices are best explained by mixing between a high maturity thermogenic methane (high in delta C-13 and delta D, with a clumped isotope composition equilibrated near similar to 249 C-degrees) and biogenic methane formed or processed in the reservoir (low in delta C-13 and delta D, with a clumped isotope composition equilibrated near 16-27 C-degrees). We hypothesize that the biogenic endmember may result from slow methanogenesis and/or anaerobic oxidation of methane (AOM). This study demonstrates that the potential of methane clumped isotope approach to identify in situ microbial metabolic processes and their association with carbon cycling in CO2-ECBM area, improving our understanding of biogeochemical mechanisms in analogous geological reservoirs.

Methane is an economic energy resource and potent greenhouse gas. Distinguishing secondary microbial methane from thermogenic gas is important for natural gas exploration and consideration of subsurface microbial activity in the global carbon cycle, but remains challenging. To understand controls on methane origins in natural gas systems, we investigated the methane clumped isotopologue distributions in the Qinshui Basin high-thermal maturity coal bed methane (CBM) reservoir. Here, near-equilibrium clumped isotopologues distribution (Delta(CH3D)-C-13 and Delta(CH2D2)-C-12) inferred a temperature interval of 21.6-252.3 degrees C. The high-temperature thermodynamic equilibrium most likely represents original thermogenic CBM characteristics during coalification. The low-temperature equilibrium clumped isotopologue distributions suggest microbial alteration to CH4 isotopic bond ordering by increased enzymatically catalyzed isotopic exchange. The independent constraints from clumped isotopes, integrated with other geochemical and genomic evidence, confirm notable secondary microbial methane from biodegradation in the highly mature reservoir. Thus, methane clumped isotopes can be used as unequivocal tracers to distinguish secondary microbial methane from thermogenic gases and hence provide the ability to incorporate them separately into global methane budgets.

The Archean rock record is limited and there is minimal organic matter available to understand the origin and evolution of life on early Earth. Low carbon isotope ratios have been measured in organic and reduced carbon phases in Archean rocks and have been invoked as biosignatures. However, it can be challenging to distinguish whether these low values reflect biotic formation, abiotic reactions, or post-depositional processes. To re-address this long-standing question, we compiled a comprehensive dataset of carbon isotope ratio measurements from organic carbon phases from Archean units that were analyzed using a variety of geochemical techniques. Our compilation also includes available descriptions and measurements of the stratigraphy, mineralogy, elemental ratios, and metamorphic grade for each data point. Our statistical analyses re-enforce a result that has been noted by prior compilations, that the carbon isotopic composition of Archean organic matter (OM) is broadly more 13C-deplete than the composition of Phanerozoic OM: The median δ13C values ( ±SD) of Archean total organic carbon and kerogen were −30.5±8‰ (n=2421) and −33.7±11.3‰ (n=556; Phanerozoic OM δ13C ±SD = −26.7±4.6‰ with n=449 from a prior compilation). Our study also identifies a previously unrecognized bimodality within the δ13C values of Archean OM that is observed even with subsampling of the data to account for geographic and stratigraphic sampling bias. We describe and model the isotopic and structural changes associated with the transformation of marine Type II kerogen from formation through diagenesis, catagenesis and metagenesis, and metamorphism, as described by trends on a van Krevelin diagram. Empirically, early maturation of organic matter during diagenesis results in shifts up to a few per-mille, which can occur in either direction depending on selective preservation and degradation of compounds. Thermal cracking that occurs during catagenesis can drive increases in δ13C of 5–12‰. At temperatures above greenschist metamorphism, carbon atoms exchange with other reactive carbon pools, driving increases in δ13C of up to 20‰. Together, our analyses suggest that the most metamorphosed graphitic samples from the earliest Archean are likely signatures of alteration, while low and multimodal ranges of δ13C values may preserve records of Archean ecology.

Several mechanisms could produce the biorelevant compounds in carbonaceous meteorites. These include radiation-driven reactions in the interstellar medium, gas-phase mineral-catalyzed reactions in the solar nebula, and aqueous chemistry in meteorite parent bodies. The ratio of heavy-to-light isotopes in a compound can constrains its formation history: a reaction’s substrates, mechanisms, and physiochemical conditions impact isotope ratios. Studies of the stable isotope compositions of meteoritic organic compounds have focused on sample- and molecular-average isotope measurements and have interpreted those data via qualitative or semi-quantitative models. Here we create quantitative models (i.e., explicitly fit to measurements) for hydrogen and carbon isotope compositions of organic compounds in primitive carbonaceous meteorites and use these models to reach broader conclusions regarding the environments, substrates, and chemical processes that contributed to pre- and early-solar-system organic synthesis. The hydrogen model fits measured molecular-average deuterium concentrations in a compound class (e.g., amines, carboxylic acids) as linear combinations of hydrogens with similar chemical environments. In the chondrites studied, methyl hydrogens are amongst the most deuterium-enriched moiety and hydrogens attached to α-carbons are the least. Deuterium enrichment is inversely related to both a compound class’s water solubility and a meteorite sample’s degree of aqueous alteration and terrestrial weathering. These values suggest that ISM-sourced compounds reacted to form deuterium-enriched molecules on meteorites’ parent bodies and the enrichments were attenuated through exchange with water during aqueous alteration on the parent body and subsequent terrestrial processing. The carbon model fits the δ13CVPDB of products from various reaction mechanisms by applying isotope effects to reactant δ13C measurements. The model with the most accurate δ13C fits of the compounds in the Murchison meteorite (62 % of previous measurements fit by model) and the lowest average residuals (5 ‰) uses the integrated aldehyde network (oxidation, reductive amination, and Strecker synthesis on aldehydes and ketones) to produce straight-chain compounds that undergo formaldehyde addition to create branched-chain compounds. Formaldehyde addition has not been previously considered in prebiotic chemical reaction networks, but the best-fit network’s ability to fit compounds that span over 100 ‰ in carbon isotope abundances makes it an attractive chemistry to explore.

Stable isotope ratios of C1-C5 alkanes, the major constituents of subsurface gaseous hydrocarbons, can provide valuable insights on their origins, transport, and fates. Equilibrium isotope effects are fundamental to interpreting stable isotope signatures, as recognition of them in natural materials indicates reversible processes and constrains the temperatures of equilibrated systems. Hydrogen isotope equilibrium of C1-C5 alkanes is of particular interest because evidence shows that alkyl H can undergo isotopic exchange with coexisting compounds under subsurface conditions. We present the results of a combined experimental and theoretical effort to determine equilibrium hydrogen isotope distributions in mixtures of these hydrocarbon compounds. We created two mixtures: one with C1, C2 and C3 (where C1 indicates methane, C2 ethane, etc., hereinafter called 'C1-C3 mixture') and another one with C2, C3, iC4, nC4, iC5 and nC5 (hereinafter called 'C2-C5 mixture'); in both cases, the mixtures were created to start out of hydrogen isotope equilibrium. We tested the performance of several metal catalysts as aids to H isotopic exchange by exposing these mixtures to different metal catalysts at 100 or 200 degrees C and analyzing the compound-specific hydrogen isotope ratios of the product gases. The C1-C3 mixture exchanged hydrogen isotopes among the starting gas components rapidly in the presence of Ru/Al2O3 at 200 degrees C. The isotope ratios reach a steady-state (presumably equilibrium) after 72 h of heating (up to 120 h). The hydrogen isotopic ratios of alkanes in the C2-C5 mixture shifted significantly in the presence of Rh/Al2O3 at 100 degrees C and C3-C5 compounds approached steady state after 70 h, whereas C2 had not yet reached a steady state D/H ratio after 216 h of heating. We evaluated the reaction progress of isotope exchange for each compound as a function of time with a reaction-network model informed by constraints on chemical kinetics. The model indicates that C3, iC4, nC4, iC5 and nC5 in the C2-C5 mixture reached internal isotope equilibrium after 70 h, hence we used their values to calculate experimental equilibrium results. We also calculated equilibrium isotope fractionations with the Bigeleisen-Mayer theorem using vibrational frequencies computed from the density functional theory (B3LYP/aug-cc-pVTZ). We included torsional conformers and explicit H positions in the calculations. We found that the experimental results from both the 200 & ring;C C1-C3 experiment and 100 & ring;C C2-C5 experiment are consistent with theoretical equilibrium results within experimental uncertainty. Additionally, our reaction-network model for catalyzed hydrogen isotope exchange between alkanes succeeded in fitting our experimental results. This modeling framework can be adjusted to simulate exchange in natural settings for constraining temperature histories and sources of natural gases.