Intramolecular Isotope Research Articles

Deciphering origins of hydrocarbon deposits by means of intramolecular carbon isotopes of propane adsorbed on sediments

Hydrocarbons are one of the important fluids within the Earth’s crust, and different biotic and abitoic processes can generate hydrocarbon during geological periods. Tracing the sources and sinks of hydrocarbons can help us better understand the carbon cycle of the earth. In this study, an improved approach of adsorbed hydrocarbons extraction from sediments was established. The improved thermal desorption approach, compound-specific isotope analysis and position-specific isotope analysis were integrated to investigate the molecular and intramolecular isotope fractionation between trace hydrocarbon gases within sediments and geological hydrocarbon deposits. The isotopic compositions of the terminal position carbon of propane (δ13Cterminal) serves as a correlation indicator between trace hydrocarbon gases within sediments and geological hydrocarbon deposits. The tight sandstone gas from the Turpan-Hami Basin is a first case study for the application of this novel method to trace hydrocarbon origins. The results showed that the hydrocarbons in the tight sandstone gases in the study area most likely originated from humic organic matter (type III kerogen) at an early mature stage. δ13Cterminal values of the thermally desorbed propane gases from different source rocks were distinguishable and the values of the tight sandstone gases significantly overlap with those of the Lower Jurassic Sangonghe source rocks, suggesting their genetic relationship. Overall, the results provided novel position-specific carbon isotopic constraints on origins of hydrocarbons.

The influence of intramolecular isotope effects on the reaction mechanisms of Ca+ + HD

The state-to-state quantum dynamics of the Ca+ + HD reaction is investigated at collision energies ranging from 2.0 to 4.0 eV based on a non-adiabatic potential energy surface. The integral cross sections are calculated and compared with previous experimental results. The integral cross section of CaH+ is significantly larger than that of CaD+. Additionally, the differential cross sections for CaH+ and CaD+ exhibit distinct trends. Rovibrationally state resolved differential cross sections reveal that the reaction for CaH+ is dominated by the ‘knockout’ mechanism, while the reaction for CaD+ is primarily governed by the stripping mechanism.

New insights into the mechanisms of plant isotope fractionation from combined analysis of intramolecular 13C and deuterium abundances in Pinus nigra tree-ring glucose.

Understanding isotope fractionation mechanisms is fundamental for analyses of plant ecophysiology and paleoclimate based on tree-ring isotope data. To gain new insights into isotope fractionation, we analysed intramolecular 13C discrimination in tree-ring glucose (Δi', i = C-1 to C-6) and metabolic deuterium fractionation at H1 and H2 (εmet) combinedly. This dual-isotope approach was used for isotope-signal deconvolution. We found evidence for metabolic processes affecting Δ1' and Δ3', which respond to air vapour pressure deficit (VPD), and processes affecting Δ1', Δ2', and εmet, which respond to precipitation but not VPD. These relationships exhibit change points dividing a period of homeostasis (1961-1980) from a period of metabolic adjustment (1983-1995). Homeostasis may result from sufficient groundwater availability. Additionally, we found Δ5' and Δ6' relationships with radiation and temperature, which are temporally stable and consistent with previously proposed isotope fractionation mechanisms. Based on the multitude of climate covariables, intramolecular carbon isotope analysis has a remarkable potential for climate reconstruction. While isotope fractionation beyond leaves is currently considered to be constant, we propose significant parts of the carbon and hydrogen isotope variation in tree-ring glucose originate in stems (precipitation-dependent signals). Asbasis for follow-up studies, we propose mechanisms introducing Δ1', Δ2', Δ3', and εmet variability.

Deuterium Isotope Effect in Single Molecule Photophysics and Photochemistry of Hypericin.

The peripherical protons of the dye molecule hypericin can undergo structural interconversion (tautomerization) between different isomers separated by a low energy barrier with rates that depends sensitively on the interaction with local chemical environment defined by the nature of host material. We investigate the deuterium (D) isotope effect of hypericin tautomerism at the single-molecule level to avoid ensemble averaging in different polymer matrices by a combined spectroscopic and computational approach. In the 'innocent' PMMA matrix only intramolecular isotope effects on the internal conversion channel and tautomerization are observed; while PVA specifically interacts with the probe via H- and D-bonding. This establishes a single molecular picture on intra- and intermolecular nano-environment effects to control chromophore photophysics and -chemistry.

Fingerprinting Organofluorine Molecules via Position-Specific Isotope Analysis.

Organofluorine substances are found in a wide range of materials and solvents commonly used in industry and homes, as well as pharmaceuticals and pesticides. In the environment, organofluorine molecules are now recognized as an important class of anthropogenic pollutants. Fingerprinting organofluorine compounds via their carbon isotope ratios (13C/12C) is crucial for correlating molecules with their source. Here we apply a 19F nuclear magnetic resonance spectroscopy (NMR) technique to obtain the first position-specific carbon isotope ratios for a diverse set of organofluorine molecules. In contrast to traditional isotope ratio mass spectrometry, the 19F NMR method provides 13C/12C isotope ratios at each carbon position where a C-F bond is present, and does not require fragmentation or combustion to CO2, overcoming challenges posed by the robust C-F covalent bonds. The method was validated with 2,2,2-trifluoroethanol, and applied to analyze heptafluorobutanoic acid, 5-fluorouracil and fipronil. Results reveal distinct intramolecular carbon isotope distributions, enabling differentiation of chemically identical molecules. Notably, the NMR method accurately analyzes carbon isotopes within target molecules despite impurities. Potential applications include the detection of counterfeit products and drugs, and ultimately pollution tracking in the environment.

The marriage between stable isotope ecology and plant metabolomics - new perspectives for metabolic flux analysis and the interpretation of ecological archives.

Even though they share many thematical overlaps, plant metabolomics and stable isotope ecology have been rather separate fields mainly due to different mass spectrometry demands. New high-resolution bioanalytical mass spectrometers are now not only offering high-throughput metabolite identification but are also suitable for compound- and intramolecular position-specific isotope analysis in the natural isotope abundance range. In plant metabolomics, label-free metabolic pathway and metabolic flux analysis might become possible when applying this new technology. This is because changes in the commitment of substrates to particular metabolic pathways and the activation or deactivation of others alter enzyme-specific isotope effects. This leads to differences in intramolecular and compound-specific isotope compositions. In plant isotope ecology, position-specific isotope analysis in plant archives informed by metabolic pathway analysis could be used to reconstruct and separate environmental impacts on complex metabolic processes. A technology-driven linkage between the two disciplines could allow us to extract information on environment-metabolism interaction from plant archives such as tree rings but also within ecosystems. This would contribute to a holistic understanding of how plants react to environmental drivers, thus also providing helpful information on the trajectories of the vegetation under the conditions to come.

Contribution of abiotic methane polymerization of C2+ hydrocarbons in highly mature natural gas reservoirs

The formation and evolution of thermogenic gases were investigated using a combination of intermolecular and intramolecular isotope analyses of 32 natural gas samples collected from the Sichuan and Tarim basins (China) and the Arkoma Basin (USA). Three evolution stages (I–III) were identified: In stage I, hydrocarbon gases are produced through thermal decomposition of organic matter, and kinetic isotope effects in C–C bond breakage control their isotopic distributions; In stage II, C2–C5 hydrocarbons crack with increasing thermal maturity, with their formation and decomposition tending toward thermodynamic equilibrium, and at the end of this stage, the intermolecular and intramolecular isotopic compositions of gaseous hydrocarbons are in thermodynamic equilibrium; A remarkable feature of stage III is the surface-catalyzed abiotic polymerization of methane, which provides a critical origin of C2+ hydrocarbons in this stages and leads to isotopic anomalies in C2+ hydrocarbons, including the reversal of δ13C distributions of C1–C3 and the reverse evolution trend of SP value of propane (i.e., tending to be positive). The contribution of C2+ hydrocarbons from the abiotic polymerization of methane can be determined based on a two-end member model. C2+ hydrocarbons in the Changning shale gases are all generated from abiotic methane polymerization, and the contribution ratio in the Weiyuan shale gases is about 85 %, while, the contribution of C2+ hydrocarbons in dry gases from the Tarim Basin formed by this way is no more than 55 %. High methane abundance, high temperature, and abundant catalyst are beneficial to abiotic methane polymerization.

Position-specific carbon stable isotope analysis of glyphosate: isotope fingerprinting of molecules within a mixture.

Glyphosate [N-(phosphonomethyl) glycine] is a widely used herbicide and a molecule of interest in the environmental sciences, due to its global use in agriculture and its potential impact on ecosystems. This study presents the first position-specific carbon isotope (13C/12C) analyses of glyphosates from multiple sources. In contrast to traditional isotope ratio mass spectrometry (IRMS), position-specific analysis provides 13C/12C ratios at individual carbon atom positions within a molecule, rather than an average carbon isotope ratio across a mixture or a specific compound. In this work, glyphosate in commercial herbicides was analyzed with only minimal purification, using a nuclear magnetic resonance (NMR) spectroscopy method that detects 1H nuclei with bonds to either 13C or 12C, and isolates the signals of interest from other signals in the mixture. Results demonstrate that glyphosate from different sources can have significantly different intramolecular 13C/12C distributions, which were found to be spread over a wide range, with δ13C Vienna Peedee Belemnite (VPDB) values of -28.7 to -57.9‰. In each glyphosate, the carbon with a bond to the phosphorus atom was found to be depleted in 13C compared to the carbon at the C2 position, by 4 to 10‰. Aminomethylphosphonic acid (AMPA) was analyzed for method validation; AMPA contains only a single carbon position, so the 13C/12C results provided by the NMR method could be directly compared with traditional isotope ratio mass spectrometry. The glyphosate mixtures were also analyzed by IRMS to obtain their average 13C/12C ratios, for comparison with our position-specific results. This comparison revealed that the IRMS results significantly disguise the intramolecular isotope distribution. Finally, we introduce a 31P NMR method that can provide a position-specific 13C/12C ratio for carbon positions with a C-P chemical bond, and the results obtained by 1H and 31P for C3 carbon agree with one another within their analytical uncertainty. These analytical tools for position-specific carbon isotope analysis permit the isotopic fingerprinting of target molecules within a mixture, with potential applications in a range of fields, including the environmental sciences and chemical forensics.

A guide to precise measurements of isotope abundance by ESI-Orbitrap MS.

Stable isotopes of carbon, hydrogen, nitrogen, oxygen and sulfur are widespread in nature. Nevertheless, their relative abundance is not the same everywhere. This is due to kinetic isotope effects in enzymes and other physical principles such as equilibrium thermodynamics. Variations in isotope ratios offer unique insights into environmental pollution, trophic relationships in ecology, metabolic disorders and Earth history including climate history. Although classical isotope ratio mass spectrometry (IRMS) techniques still struggle to access intramolecular information like site-specific isotope abundance, electrospray ionization-Orbitrap mass spectrometry can be used to achieve precise and accurate intramolecular quantification of isotopically substituted molecules ('isotopocules'). This protocol describes two procedures. In the first one, we provide a step-by-step beginner's guide for performing multi-elemental, intramolecular and site-specific stable isotope analysis in unlabeled polar solutes by direct infusion. Using a widely available calibration solution, isotopocules of trifluoroacetic acid and immonium ions from the model peptide MRFA are quantified. In the second approach, nitrate is used as a simple model for a flow injection routine that enables access to a diverse range of naturally occurring isotopic signatures in inorganic oxyanions. Each procedure takes 2-3 h to complete and requires expertise only in general mass spectrometry. The workflows use optimized Orbitrap IRMS data-extraction and -processing software and are transferable to various analytes amenable to soft ionization, including metabolites, peptides, drugs and environmental pollutants. Optimized mass spectrometry systems will enable intramolecular isotope research in many areas of biology.

Position-specific and clumped isotope equilibria in propane: Ab initio calculations beyond the harmonic and Born-Oppenheimer approximations

Position-specific and clumped isotope compositions that reveal intramolecular isotope distributions can offer novel insights into the physical and chemical properties of substances. In particular, the intramolecular isotope effects observed in propane have demonstrated significant potential for constraining the formation and evolution of hydrocarbons. To calibrate measurements and interpret observations, a comprehensive understanding of equilibrium isotope fractionation within propane is required. However, previous studies yielded inconsistent calculations of position-specific isotope equilibria, and lacked any prediction of clumped isotope equilibria in the methyl group of propane. Here, we employ quantum chemical calculations beyond the harmonic approximation and the Born-Oppenheimer approximation to study the intramolecular isotope equilibria in propane. Benchmark coupled cluster calculations (CCSD(T)) were used to produce reliable frequencies of propane isotopologues. By integrating the CCSD(T) outcomes with effects beyond the harmonic and Born-Oppenheimer approximations, we present high-accuracy equilibrium carbon and hydrogen isotope fractionation between the methylene and methyl sites, and report for the first time the equilibrium Δ13CH2D and Δ12CHD2 signatures of the methyl group in propane. Our results of position-specific isotope equilibria can serve as a reference frame for calibrating both theoretical calculations and experimental measurements. The equilibrium Δ13CH2D and Δ12CHD2 values provide a theoretical framework for further studies of 13CD and DD clumping in methyl groups of alkanes, and are valuable for tracing the sources and sinks of methyl groups as well as processes related to methylation.

Iodine(III)-Catalyzed Oxidative Cyclization of Aryl Amines to Construct N-Alkylbenzimidazoles.

An I(III)-catalyzed oxidative cyclization reaction using selectfluor as the oxidant was developed that converts ortho-substituted anilines to benzimidazoles. The mild reaction requires as little as 0.5 mol % of iodobenzene, and its scope is broad: electron-withdrawing or electron-releasing groups on the aniline portion are tolerated, and cyclic or acyclic N-alkylamines are permitted as ortho-substituents. Preliminary mechanistic investigations suggest that benzimidazole formation occurs via cationic reactive intermediates, and an intramolecular kinetic isotope effect of 1.98 ± 0.05 was measured.

Transition State Analysis of Key Steps in Dual Photoredox-Cobalt-Catalyzed Elimination of Alkyl Bromides.

A combination of inter- and intramolecular 13C kinetic isotope effects and density functional theory analysis is used to evaluate the key mechanistic events of sequentially operating catalytic cycles in the dual photoredox-cobalt-catalyzed elimination of alkyl bromides. The results point to a mechanism proceeding via irreversible halogen-atom transfer (XAT) from the alkyl halide, resulting in an alkyl radical, which undergoes hydrogen-atom transfer (HAT) to a Co(II) intermediate to deliver the product olefin. Alternative pathways involving nucleophilic substitution by a Co(I) species and by β-hydride elimination are discounted based on the poor agreement of experimental and predicted 13C KIEs. This mechanistic understanding is used to evaluate the origins of regioselectivity in the elimination step for an unsymmetrical alkyl halide catalyzed by electronically and sterically distinct cobaloxime catalysts. This study represents the experimental validation of the key features of the transition state structure of XAT by α-aminoalkyl radicals, an important class of atom transfer reactions that generate carbon-centered radicals from alkyl and aryl halides. Furthermore, it illustrates the power of 13C KIEs in probing complex mechanisms in metallaphotoredox catalysis.

State-to-State Quantum Dynamics Study of Intramolecular Isotope Effects on Be(1S) + HD (v0 = 2, j0 = 0) → BeH/BeD + H/D Reaction.

The dynamic mechanisms and intramolecular isotope effects of the Be(1S) + HD (v0 = 2, j0 = 0) → BeH/BeD + H/D reaction are studied at the state-to-state level using the time-dependent wave packet method on a high-quality potential energy surface. This reaction can proceed along the indirect pathway that features a barrier and a deep well or the smooth direct pathway. The reaction probabilities, total and state-resolved integral cross sections, and differential cross sections are analyzed in detail. The calculated dynamics results show that both of the products are mainly formed by the dissociation of a collinear HBeD intermediate when the collision energy is slightly larger than the threshold. As the collision energy increases, the BeH + D channel is dominated by the direct abstraction process, whereas the BeD + H channel mainly follows the complex-forming mechanism.

The Conductance Isotope Effect in Oligophenylene Imine Molecular Wires Depends on the Number and Spacing of 13C-Labeled Phenylene Rings.

We report a strong and structurally sensitive 13C intramolecular conductance isotope effect (CIE) for oligophenyleneimine (OPI) molecular wires connected to Au electrodes. Wires were built from Au surfaces beginning with the formation of 4-aminothiophenol self-assembled monolayers (SAMs) followed by subsequent condensation reactions with 13C-labeled terephthalaldehyde and phenylenediamine; in these monomers the phenylene rings were either completely 13C-labeled or the naturally abundant 12C isotopologues. Alternatively, perdeuterated versions of terephthalaldehyde and phenylenediamine were employed to make 2H(D)-labeled OPI wires. For 13C-isotopologues of short OPI wires (<4 nm) in length where the charge transport mechanism is tunneling, there was no measurable effect, i.e., 13C CIE ≈ 1, where CIE is defined as the ratio of labeled and unlabeled wire resistances, i.e., CIE = Rheavy/Rlight. However, for long OPI wires >4 nm, in which the transport mechanism is polaron hopping, a strong 13C CIE = 4-5 was observed. A much weaker inverse CIE < 1 was evident for the longest D-labeled wires. Importantly, the magnitude of the 13C CIE was sensitive to the number and spacing of 13C-labeled rings, i.e., the CIE was structurally sensitive. The structural sensitivity is intriguing because it may be employed to understand polaron hopping mechanisms and charge localization/delocalization in molecular wires. A preliminary theoretical analysis explored several possible explanations for the CIE, but so far a fully satisfactory explanation has not been identified. Nevertheless, the latest results unambiguously demonstrate structural sensitivity of the heavy atom CIE, offering directions for further utilization of this interesting effect.

Intramolecular carbon isotope of propane from coal-derived gas reservoirs of sedimentary basins: Implications for source, generation and post-generation of hydrocarbons

An intramolecular isotopic study was conducted on natural gases collected from coal-derived gas reservoirs in sedimentary basins of China to determine their position-specific isotope distributions. The propane from the Turpan-Hami Basin exhibited negative ΔC-T (δ13Ccentral-δ13Cterminal) values ranging from −3.9‰ to −0.3‰, with an average of −2.1‰. Propane from the Ordos Basin, Sichuan Basin, and Tarim Basin showed positive ΔC-T values, with averages of 1.3‰, 5.4‰ and 7.6‰, respectively. Position-specific carbon isotope compositions reveal the precursors and the propane generation pathways in the petroliferous basins. Propane formed from the thermal cracking of Type III kerogen has larger δ13Ccentral and δ13Cterminal values than propane from Type I/II kerogen. The precursor for natural gases collected in this study is identified to be Type III kerogen. Comparing our data to calculated results for thermal cracking of Type III kerogen, we found that propane from the low-maturity gas reservoir in the Turpan Basin was generated via the i-propyl radical pathway, whereas propane from the Sulige tight gas reservoir in the Ordos Basin was formed via the n-propyl radical pathway. δ13Cterminal values covered a narrow range across basins, in contrast to δ13Ccentral. The terminal carbon position in propane is less impacted by microbial oxidation and more relevant to maturity levels and precursors. Thus, δ13Cterminal has a good potential to infer the origin and maturity level of natural gas. In examining post-generation processes, we proposed an improved identification strategy for microbial oxidation of natural gases, based on the position-specific carbon isotope distributions of propane. Samples from the Liaohe Depression of the Bohai Bay Basin and the Sichuan Basin were detected of post-generation microbial oxidation. Overall, position-specific carbon isotope composition of propane provides new insights into the generation mechanism and post-generation processes of natural gas in the geological period at the atomic level.

Position-specific carbon isotopes of propane in coal systems in China

Evaluating the origin and fate of hydrocarbons holds significance in many fields, such as energy, geology, astrobiology, biosphere, and environment. However, challenges arise in many cases owing to the limitations of conventional methods. Intramolecular isotope analysis of propane is a new technique that offers the potential to provide insights into gas formation mechanisms; however, a comprehensive understanding of the method remains limited. Specifically, there is little knowledge about its ability to distinguish gases generated by source rocks deposited in similar sedimentary environments such as coal and coaly mudstone. Therefore, this study was undertaken to investigate position-specific carbon isotopes of 40 propane samples from 15 coal-type gas accumulations across four basins in China (Sichuan, Ordos, Qaidam, and Songliao); additionally, clumped isotopologues of methane samples were also analyzed. These coal systems cover a wide range of thermal maturity (from marginally mature to over-mature) at various age strata from the Permian, Triassic, and Jurassic periods, to the Cretaceous period. Our results revealed that propane generated from coal and coaly mudstone differed greatly in intramolecular isotope compositions despite having similar bulk δ13C3 values. Propane generated from coaly mudstone at a wide range of maturity had relatively more stable δ13C values in both the central (δ13Ccental) and terminal carbons (δ13Cterminal), near the theory generation line of chain-alkane cracking in the plot of δ13Cterminal vs. δ13Ccental. However, propane generated from coal had higher stable δ13Cterminal values and lower δ13Ccentral values that progressively increased with maturity. Under cooling-down conditions, propane from over-mature natural gases became extremely 13C-depleted in both the terminal and central positions, with Δ13Ccentral (δ13Ccentral minus δ13Cterminal) values being as low as –10‰, suggesting a partial origin of methane polymerization. Two propane samples from Cretaceous brown sandstone in the Songliao Basin exhibited an increase in both central and terminal carbons, suggesting that they may have surpassed chemical oxidation owing to high-valence Fe(Mn) oxides. These results indicate that intramolecular isotopes of propane can effectively differentiate dominant gas sources in a coal system with coal or coaly mudstone, as well as easily identify isotopic fractionation caused by post-generation processes, such as methane polymerization, mixing and chemical oxidation.

The dynamic studies of the C+ + HD reaction using the time-dependent wave packet method

The dynamic calculations of the C+ + HD (v 0 = 0, j 0 = 0) → D/H + CH+/CD+ reaction in the collision energy range of 0.001–1.80 eV are carried out by using a time-dependent wave packet. The intramolecular isotope effect of the C+ + HD reaction is discussed in detail. Some meaningful dynamic properties of the C+ + HD reaction are reported and compared with available theoretical and experimental results. The results indicated that present values are, in general, in better agreement with experimental data than previous theoretical reports. However, due to the lack of consideration of rotational excitation of reactant and non-adiabatic effect, the present results are always smaller than the experimental results. Differential cross-sections of the D + CH+ and H + CD+ product channels exhibit forward–backward symmetric behaviour, which indicates that the ‘complex-forming’ mechanism plays a dominant role in the reaction. Highlights The dynamic calculations of the C+ + HD (v 0 = 0, j 0 = 0) → D/H + CH+/CD+ reaction are performed using a time-dependent wave packet method based on the newly reported potential energy surface. The dynamicl properties, such as integral cross-sections, rate constants, etc., are calculated and compared with previous theoretical and experimental results. The rovibrational state distributions of the product are reported.

Harnessing triaryloxonium ions for aryne generation

Arynes are highly reactive and versatile intermediates for the functionalization of aromatic rings that are often generated using strong bases or fluoride sources, which, in some cases, can limit functional group tolerance. Here we demonstrate that triaryloxonium ions can be transformed into arynes through treatment with solid potassium phosphate at room temperature. A substantial range of functional group-bearing arynes, including 4,5-pyrimidynes, may be generated and trapped using cycloaddition reactions with high yields. Other arynophiles including nitrones, alkenes and azides are compatible with these conditions. Quantum computation in conjunction with an intramolecular kinetic isotope study is consistent with an elimination, unimolecular, conjugate base-like mechanism of elimination to form the aryne. These investigations demonstrate that the oxonium ion is a powerful electron-withdrawing group and a particularly effective leaving group. We anticipate that this study will stimulate further investigations into the synthetic utility of aryl oxonium ions.

Intramolecular carbon isotope geochemistry of butane isomers from laboratory maturation and Monte-Carlo simulations of kerogen types I, II, and III

Position-specific (PS) carbon isotope compositions of light hydrocarbons such as propane and butane isomers (n-butane and i-butane) can provide a wealth of information on the history of natural gases in the subsurface reservoirs and other environments. For PS carbon isotope analysis of butane isomers, we have established a GC-pyrolysis-GC-isotope ratio mass spectrometry method with demonstrated accuracy. With this method, we analyzed PS δ13C values of butane isomers generated from the systematic laboratory pyrolysis experiments of three different kerogen types (I, II, and III) at temperatures of 310–430 °C with corresponding thermal maturity (Easy %Ro) ranging from 0.7 to 3.3. The observed evolution in the abundances of butane isomers can be interpreted and semi-quantitatively modeled based on the abundances of different CC bonds within the kerogens at low maturity and thermal degradation of butane isomers at high maturity. The δ13C values at the central sites of both nC4 and iC4 were heavier than those at the terminal positions, similar to our previous observations of propane. Their isotopic evolution with the maturity were controlled largely by kinetic isotope effects associated with breaking of different CC bonds during the generation and degradation of butane isomers. Kinetic Monte Carlo (kMC) simulations of n-butane generated from thermal cracking of model kerogens (I, II, and III) and an oil with a series of reactions (homolytic cleavage, β-scission, radical isomerization, H-abstraction, and termination by radical recombination) provided generally consistent results with the experimental observations, although the difference in PS δ13C values between the central and terminal positions are somewhat overestimated. On the other hand, the kMC simulation with homolytic cleavage and capping reactions alone produced significant deviations from the experimental results. Re-assessment of very limited data of PS δ13C values of natural butanes with our experimental and simulation results show that biodegradation significantly increased δ13C values at the central positions, not only of propane, but also of both butane isomers. This study lays a foundation and demonstrates the potential of PS isotope geochemistry of butane isomers to further improve our understanding of the sources, and geochemical and microbial processes of light hydrocarbons in the subsurface and other natural environments.

Analysis of intramolecular carbon isotope distributions in alanine by electrospray ionization Orbitrap mass spectrometry

The ability to detect intramolecular isotopic differences within a single molecule can answer questions about molecule synthesis and alteration across numerous scientific fields. Until recently, intramolecular (i.e., position-specific isotope analysis, PSIA) isotope measurements were laborious, requiring large amounts of analyte or specialized instrumentation. Orbitrap™ mass spectrometers are capable of fragmenting molecules and have the high mass resolution needed to constrain position-specific isotopic differences among the resulting fragment ions. Orbitrap mass spectrometers with electrospray ionization accurately measured the molecular average isotope composition of acetate, nitrate, sulfate, phosphate, and the amino acid methionine, as well as the position-specific isotopic structure of methionine. Here, we document the ability of this method to measure the position-specific carbon isotope structure of the amino acid alanine. Data include measurements of 13C-enriched materials to assign specific atoms in fragments to the original molecular structure and detect any recombination of atoms in resultant fragments. We further demonstrate high-precision intramolecular isotope analyses for standards with independently determined position-specific carbon isotope compositions. Isotope data from ESI-Orbitrap-MS agrees with values obtained using gas source isotope ratio mass spectrometry, giving further confidence to this novel approach to PSIA. The carbon isotope analyses by Orbitrap-MS were rapid and required ∼5 μg of analyte to obtain both molecular average and position-specific values in triplicate with precision ≤1‰.