Gas Chromatography-isotope Ratio Mass Spectrometry Research Articles

A novel method for tracing gasoline using GC-IRMS and Relief-Stacking fusion model

Due to China’s vast geographical expanse and the complexity of its natural environment, traditional methods are not suitable for tracing the origins of gasoline samples from different regions of China. The aim of this study is to trace the origins of gasoline samples through the analysis of carbon stable isotopic characteristics from 60 gasoline samples collected across four major regions in China. In this study, a novel method that combines gas chromatography-isotope ratio mass spectrometry (GC-IRMS) with Relief-Stacking was proposed for the first time, significantly enhancing the precision of the provenance results. Firstly, eight characteristic components were identified by Relief feature selection model for the first time and applied to the assessment of carbon stable isotope ratios across different regions. The discovery of these components effectively reduces data redundancy, thereby enhancing the accuracy of provenance analysis. Subsequently, five effective source tracing models (XGBoost, Random Forest, CatBoost, LightGBM, and KNN) were selected based on the CEI values. To further enhance the learning efficiency of the sourcing models, this study integrated the five selected source tracing models using three different methods(voting, stacking, and blending) and validated the performance of the optimal sourcing model. The results indicated that the Stacking fusion model achieves a CEI value of 0.923, significantly outperforming other fusion models. This advantage is attributed to the Stacking fusion model combining the s trengths of Boosting and Bagging ensemble methods and leveraging the advantages of individual classifiers within the model. The method proposed in this study can effectively address the limitations of previous tracing methods and offer valuable insights for the development of platforms related to stable isotope source tracing. In future research, through appropriate parameter adjustments and feature adaptations, the Relief-Stacking model can likewise be applied to source tracing studies in other countries characterized by extensive geographic ranges and variable environmental conditions.

Geochemical fingerprints of Dongpu Depression crude oils in Bohai Bay Basin, northern China: Insights from biomarkers and isotopic compositions of selected alkylnaphthalenes and alkylphenanthrenes

The Dongpu Depression is a rift lake in the Bohai Bay Basin of eastern China, where abundant oil and gas resources were discovered. Previous geochemical investigations of Dongpu Depression oils have revealed a notable lack of data regarding the isotopic compositions of individual polycyclic aromatic hydrocarbons (PAHs) from the discovered Cenozoic crude oils. Using gas chromatography-mass spectrometry (GC–MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), a novel approach combining molecular parameters and stable carbon isotope compositions of individual PAH was applied to study the depositional environment conditions, the different source inputs, the maturation stage and to identify the potential family of five crude oils selected from Dongpu Depression.A decrease of Ga/C30H together with low Pr/Ph ratios and changes in isotopic compositions indicate that the five Dongpu Depression oils were derived from OM deposited in an anoxic saline environment with a stratified water column where mixing of saline water and freshwater occurred. The low isoprenoid, tricyclic and tetracyclic terpane ratios in combination with the δ13C values of 1,6-DMN, 1,2,5-TMN, 1,3,6,7-TeMN, 9-MP and 1-MP attest that their sources have a mixed origin with high aquatic organism (planktonic) input and a lower land plant (C3 plant) contribution. The δ13C values of phenanthrene between −30.0 ‰ and −20.0 ‰ indicate that the studied Dongpu Depression oils belong to a single family generated from the peak to the late oil generation stage (Rc-TNR-2 between 0.85 and 1.15 %). A correlation with various basins indicates that oil samples featured by δ13C values of Phen between −30.0 ‰ and −20.0 ‰ correspond to crude oils derived from mixing of aquatic and high plant contributions with different degrees of mixtures, which have not yet reached their cracking stage (Rc < 2.0 %). Crude oils from the Carboniferous and Mesozoic-Cenozoic Tarim rocks, Australian petroleum systems, Termit Basin, Bongor Basin, Fushan Depression and the studied Dongpu Depression oils belong to that group of oils. Crude oils characterized by lighter phenanthrene isotopic compositions (δ13C values of Phen < −30.0 ‰) are mainly derived from marine/aquatic input and have already entered the cracking window (Rc > 2.0 %). This group of oils is represented in this study by the Cambrian-Ordovician Tarim Basin oil samples. The research shows the importance of the aromatic isotopic compositions in petroleum system characterization and could be used as a reference for a practical exploration campaign of petroleum.

The organic geochemistry of crude oil in the Saltpond Basin (Ghana): Organic source input, depositional environment, and thermal maturity

The Saltpond Basin, situated within the South Atlantic margin of Ghana, is a significant area for petroleum exploration but has received relatively limited research attention. Previous studies have examined source rock composition, but data on crude oil organic chemistry are lacking, hindering understanding of the basin’s petroleum system and evolution. To address this gap, we analyzed biomarkers and stable carbon-isotope ratios in Saltpond Basin crude oil using gas chromatography–mass spectrometry and gas chromatography–isotope ratio mass spectrometry to elucidate organic matter source, depositional environment, and thermal maturity. Findings were compared with oils from the West African segment of the South Atlantic margin, namely the Tano Basin and the Niger Delta Basin, to identify potential correlations and gain insights into regional variations. Molecular and isotopic results unveiled a significant prevalence of organic matter derived from lower marine organisms. Patterns of organic matter deposition and preservation in Saltpond oil samples suggested a suboxic marine transitional environment, contradicting conventional understanding of terrestrial dominance in such settings. Moreover, the potential for degradation processes to obscure differentiation between terrestrial and marine organic matter origins underscores the complex nature of organic matter dynamics in transitional marine environments. Analysis of molecular thermal maturity indices suggested Saltpond oils were expelled from source rocks exhibiting thermal maturity at the early maturity stage. Correlation analysis unveiled genetic disparities among crude oils sourced from the Saltpond Basin and those from the Tano and Niger Delta Basin, primarily due to variations in source input and depositional environment conditions. Saltpond oil exhibits lower terrestrial organic input than Tano Basin’s crude oils, which also have less terrestrial input than Niger Delta Basin crude oils. Additionally, its paleodepositional environment notably differs from oils in the Tano Basin (anoxic transitional marine-lacustrine settings) and the Niger Delta Basin (suboxic–oxic terrigenous deltaic or marine or lacustrine environments). Thermal maturity range of Saltpond oil is comparable to oils in the Tano Basin but lower than oils in the Niger Delta Basin. These findings provide valuable insights into petroleum generation history and unique organic geochemical characteristics within the Saltpond Basin, essential for exploration, production, and environmental management efforts in the region. Furthermore, correlation studies provide evidence that distinct biological, geological, and paleoenvironmental conditions shaped various oil types in the West African segment of the South Atlantic margin.

Dispersive liquid-liquid microextraction as a novel enrichment approach for compound-specific carbon isotope analysis of chlorinated phenols.

Compound-specific isotope analysis (CSIA) via gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is a potent tool to elucidate the fate of (semi-)volatile organic contaminants in technical and environmental systems. Yet, due to the comparatively low sensitivity of IRMS, an enrichment step prior to analysis often is inevitable. A promising approach for fast as well as economic analyte extraction and preconcentration prior to CSIA is dispersive liquid-liquid microextraction (DLLME) - a well-established technique in concentration analysis of contaminants from aqueous samples. Here, we present and evaluate the first DLLME method for GC-IRMS exemplified by the analysis of chlorinated phenols (4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol) as model compounds. The analytes were simultaneously acetylated with acetic anhydride and extracted from the aqueous phase using a binary solvent mixture of acetone and tetrachloroethylene. With this method, reproducible δ13C values were achieved with errors ≤ 0.6‰ (n = 3) for aqueous concentrations down to 100 μg L-1. With preconcentration factors between 130 and 220, the method outperformed conventional liquid-liquid extraction in terms of sample preparation time and resource consumption with comparable reproducibility. Furthermore, we have demonstrated the suitability of the method (i) for the extraction of the analytes from a spiked river water sample and (ii) to quantify kinetic carbon isotope effect for 2,4,6-trichlorophenol during reduction with zero-valent zinc in a laboratory batch experiment. The presented work shows for the first time the potential of DLLME for analyte enrichment prior to CSIA and paves the way for further developments, such as the extraction of other compounds or scaling up to larger sample volumes.

Application of gas chromatography-stable isotope mass spectrometry to determine the oxygen isotope ratio of water in concentrated fruit juice

In this study, we developed a method to determine the authenticity of concentrated fruit juice using the δ18O ratio of water. We utilized the gas chromatography-isotope ratio mass spectrometry (GC-IRMS) technique along with the offline isotope exchange equilibrium method to pretreat and measure the oxygen isotopes in water in concentrated apple juice. This innovative approach uses anhydrous ethanol as a solvent to dissolve concentrated juices, thereby facilitating a more comprehensive reaction between the water in the sample and the CO2 standard gas. Additionally, the reaction vessel exhibits excellent air tightness, ensuring rapid equilibrium time and enabling accurate determination of δ18O in the water content of concentrated juice. The reaction vessel had excellent air tightness and rapid equilibration time, enabling accurate measurement of δ18O of water in concentrated fruit juice. The reaction vessel used had excellent air tightness and rapid equilibration time, ensuring accurate measurement of the δ18O of water in concentrated fruit juice. We analyzed 40 samples of concentrated apple juice and found that the range of δ18O values in the water of concentrated apple juice was between 1.29 ‰ and 7.59 ‰. Additionally, we used prepared sucrose syrup as a simulated sample and found that the δ18O distribution range was between −10.43 ‰ and −10.53 ‰. Furthermore, we conducted adulteration simulation experiments on randomly selected real samples and discovered a linear negative correlation between the adulteration ratio and the δ18O value of water in the samples.

Single-species dinoflagellate cyst carbon isotope fractionation in core-top sediments: environmental controls, CO2 dependency and proxy potential

Abstract. Sedimentary bulk organic matter and various molecular organic components exhibit strong CO2-dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (εp) has been employed as a proxy for paleo-pCO2. Yet, culture experiments indicate that CO2-dependent εp is highly specific at genus and even species level, potentially hampering the use of bulk organic matter and non-species-specific organic compounds. In recent years, significant progress has been made towards a CO2 proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific εp values. These values were, however, based on motile specimens, and it remains unknown whether these relations also hold for the organic-walled resting cysts (dinocysts) produced by these dinoflagellate species in their natural environment. We here analyze dinocysts isolated from core tops from the Atlantic Ocean and Mediterranean Sea, representing several species (Spiniferites elongatus, S. (cf.) ramosus, S. mirabilis, Operculodinium centrocarpum sensu Wall and Dale (1966) (hereafter referred to as O. centrocarpum) and Impagidinium aculeatum) using laser ablation–nano-combustion–gas-chromatography–isotope ratio mass spectrometry (LA/nC/GC-IRMS). We find that the dinocysts produced in the natural environment are all appreciably more 13C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall εp values, and, moreover, exhibit large isotope variability. Where several species could be analyzed from a single location, we often record significant differences in isotopic variance and offsets in mean δ13C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on O. centrocarpum, shows that εp correlates significantly with various environmental parameters. Importantly, O. centrocarpum shows a CO2-dependent εp above ∼ 240 µatm pCO2. Similar to other marine autotrophs, relative insensitivity at low pCO2 is in line with active carbon-concentrating mechanisms at low pCO2, although we here cannot fully exclude that we partly underestimated εp sensitivity at low pCO2 values due to the relatively sparse sampling in that range. Finally, we use the relation between εp and pCO2 in O. centrocarpum to propose a first pCO2 proxy based on a single dinocyst species.

Determination of intermolecular and intramolecular isotopic compositions of low-abundance gaseous hydrocarbons using an online hydrocarbon gas concentration method

The stable isotopic composition of natural gas can be used to identify its origin and source. However, low concentrations of gaseous hydrocarbons in high-mature natural and shale gases hinder accurate determination of their compound- and position-specific isotopic compositions. In this study, an online C2+ hydrocarbon gas concentration system combined with gas chromatography–isotope ratio mass spectrometry (GC–IRMS) or gas chromatography–pyrolysis–gas chromatography–isotope ratio mass spectrometry (GC–Py–GC–IRMS) was developed to determine compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons. The lower limit of the gas concentration required for isotope ratio determination using the online concentration system is 0.001% (0.003%) for compound-specific carbon (hydrogen) isotopes and 0.005% for position-specific carbon isotopes and is thus applicable to most natural gas samples. The online concentration technique does not cause significant isotopic fractionation effects, and the combination with GC–IRMS and GC–Py–GC–IRMS can accurately and precisely determine the compound-specific δ13C and δD values of low-content C2+ gaseous hydrocarbons and the position-specific δ13C values (δ13Ca, δ13Cb, and SP values) of propane in low-content propane samples, respectively. The application of our method to two natural gas samples from the Ordos and Sichuan basins further confirms that the online concentration method allows simple and rapid determination of the compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons.

Avoiding Interferences in Advance: Cyclodextrin Polymers to Enhance Selectivity in Extraction of Organic Micropollutants for Carbon Isotope Analysis

Compound-specific isotope analysis (CSIA) of organic water contaminants can provide important information about their sources and fate in the environment. Analyte enrichment from water remains nonetheless a critical yet inevitable step before measurement. Commercially available solid-phase extraction (SPE) sorbents are inherently nonselective leading to co-extraction of concurrent dissolved organic matter (DOM) and in turn to analytical interferences, especially for low-occurring contaminants. Here, we (i) increased extraction selectivity by synthesizing cyclodextrin polymers (α-, β-, γ-CDP) as SPE sorbents, (ii) assessed their applicability to carbon isotope analysis for a selection of pesticides, and (iii) compared them with commonly used commercial sorbents. Extraction with β-CDP significantly reduced backgrounds in gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and enhanced sensitivity by a factor of 7.5, which was further confirmed by lower carbon-normalized CDOM/Canalyte ratios in corresponding extracts as derived from dissolved organic carbon (DOC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Gibbs free energies of adsorption demonstrated weak competition between DOM and analyte on the three CDPs. No isotopic fractionation (Δδ13C within ± 0.3‰) was observed for the investigated pesticides after using β-CDP as an SPE sorbent covering a range of concentrations (5-500 μg L-1), flow velocities (5-40 cm min-1), and sorbent regeneration (up to six times). The present study highlights the benefit of selecting innovative extraction sorbents to avoid interferences in advance. This strategy in combination with existing cleanup approaches offers new prospects for CSIA at field concentrations of tens to hundreds of nanograms per liter.

Two-dimensional gas chromatography coupled to isotope ratio mass spectrometry for determining high molecular weight polycyclic aromatic hydrocarbons in sediments

The accuracy of compound-specific isotope analysis (CSIA) of trace-level pollutants in complex environmental samples has always been limited by two main challenges: poor chromatographic separation and insufficient amounts of analytes. In this study, a two-dimensional gas chromatography-isotope ratio mass spectrometry (2DGC-IRMS) system was constructed for compound-specific δ13C analysis of high molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) in estuarine/marine sediments. This construction occurred through hyphenating an extra gas chromatography system (GC) to a conventional GC-IRMS using a commercially available multi-column switching-cryogenic trapping system (MCS-CTS). Compared with the previous 2DGC-IRMS strategy, which utilizes a Deans Switch device, the newly implemented 2DGC-IRMS scheme resulted in online purification of target analytes as well as enriched them online via duplicate injection and cryogenic trapping in CTS; this resultingly lowered the limits of detection (LOD) of CSIA. To improve the sample transfer efficiency to the IRMS, a broader-bore and longer fused-silica capillary was utilized to replace the original sample capillary running from the sample open split to the IRMS. A ẟ13C analysis of PAH standards showed accurate ẟ13C values, and high precisions (standard deviations 0.13–0.37%) were achieved, with the LOD of HMW-PAHs reduced to at least 1.0 mg/L (i.e., 0.07 to 0.09 nmol carbon per compound on-column). The successful application of this newly developed 2DGC-IRMS scheme provides a practical solution for the reliable CSIA of trace-level pollutants in complex environmental samples that cannot be measured using the conventional GC-IRMS system.

Validation of gas chromatographic methods for lavender essential oil authentication based on volatile organic compounds and stable isotope ratios

A combination of Gas Chromatography-tandem Mass Spectrometry (GC-MS/MS) and Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS) methods is a powerful tool to control false declaration of various food matrices including lavender essential oil. However, the methods need to be validated to demonstrate data reliability.In the present study, we developed a GC-MS/MS method for determining volatile organic compounds in lavender essential oil. For this, a single chromatographic run was optimised to separate thirty-eight pure reference standards in<17 min using a suitable capillary column, a faster injection system, temperature ramp gradient, and acquiring mass spectra through dynamic monitoring reaction and full scan mode. The GC-MS/MS method was validated according to the “Harmonised guidelines of IUPAC” considering parameters such as linearity range, limit of detection, limit of quantification, regression coefficient, repeatability and reproducibility. The validation parameters suggested that the method was sufficiently sensitive, precise and accurate for the determination of VOCs.Moreover, we developed a GC-IRMS method to characterize the isotope ratios of carbon (δ13C) and hydrogen (δ2H) of a large selection of the VOCs present in the lavender oil. For this, different matrices of various essential oil compounds of known δ13C and δ2H values (obtained from bulk isotope ratio analysis) were analysed in the GC-IRMS. Similarly to the GC-MS/MS method, the GC-IRMS method was validated according to the recommendation by the Forensic Isotope Ratio Mass Spectrometry Network (FIRMS). In particular, the linearity test suggested that concentrations of at least 25 mg L−1 and 200 mg L−1 were required to produce reliable δ13C and δ2H values, respectively. Both δ13C and δ2H values were satisfactorily repeatable, reproducible and accurate. As expected, precision and accuracy of the δ2H were lower than those of the δ13C.Finally, the practical applicability of the methods was confirmed by the analysis of 3 lavender and 3 lavandin essential oil samples. We quantified the concentrations of 71 VOCs and the δ13C value of 32 and δ2H value of 30 compounds in real essential oil samples. These preliminary results showed that the combination of GC-MS/MS and GC-IRMS is potentially useful for botanical classification between lavender and lavandin essential oils thus representing an additional powerful tool for assessing the authenticity of commercial essential oils.

Essential Oil Composition and Stable Isotope Profile of Osmorhiza occidentalis Torr. (Apiaceae) from Utah.

Osmorhiza occidentalis Torr. is an essential-oil-bearing plant in the Apiaceae family. Volatile oil was produced through steam distillation (n = 3) of the above ground plant parts and was analyzed by gas chromatography (GC/FID, GC/MS), and gas chromatography/isotope ratio mass spectrometry (GC/IRMS) to establish the essential oil composition and stable isotope profile. The resulting essential oils were found to be comprised of 33 volatile compounds. Prominent volatile compounds include methyl chavicol (avg. 61.6%), (Z)-β-ocimene (avg. 14.7%), sabinene (avg. 10.5%), and γ-terpinene (avg. 2.8%). Stable isotope values were determined for prominent volatile compounds, including methyl chavicol, (Z)-β-ocimene, sabinene, and γ-terpinene. Values for δ2H range from −393.479 (avg. sabinene) to −171.516 (avg. methyl chavicol). Those for δ13C range from −35.957 (avg. methyl chavicol) to −30.820 (avg. (Z)-β-ocimene). The essential oil yield was 0.12% (w/w). The current study establishes for the first time, to the best knowledge of the authors, the essential oil yield, essential oil composition, and stable isotope profile of prominent volatile compounds extracted from the above-ground portions of O. occidentalis. These results provide insight into the volatile chemical composition produced by the plant and provide fundamental data for substantiation of ethnobotanical applications.

State-of-the-Art Analytical Approaches for Illicit Drug Profiling in Forensic Investigations.

In forensic chemistry, when investigating seized illicit drugs, the profiling or chemical fingerprinting of drugs is considered fundamental. This involves the identification, quantitation and categorization of drug samples into groups, providing investigative leads such as a common or different origin of seized samples. Further goals of drug profiling include the elucidation of synthetic pathways, identification of adulterants and impurities, as well as identification of a drug’s geographic origin, specifically for plant-derived exhibits. The aim of this state-of-art-review is to present the traditional and advanced analytical approaches commonly followed by forensic chemists worldwide for illicit drug profiling. We discussed numerous methodologies for the physical and chemical profiling of organic and inorganic impurities found in illicit drug. Applications of powerful spectroscopic and chromatographic tools for illicit drug profiling including isotope-Ratio mass spectrometry (IRMS), gas chromatography–mass spectrometry (GC-MS), gas chromatography–isotope ratio mass spectrometry (GC-IRMS), ultra-high-performance liquid chromatography (UHPLC), thin layer chromatography (TLC), liquid chromatography–mass spectrometry (LC-MS) and inductively coupled plasma-mass spectrometry (ICP-MS) were discussed. Altogether, the techniques covered in this paper to profile seized illicit drugs could aid forensic chemists in selecting and applying a suitable method to extract valuable profiling data.

Stable isotope ratio analysis combined with likelihood ratio as a new tool for establishing ethanol origin

Blood alcohol concentration is an important objective basis in traffic accidents and accounts for the majority of forensic toxicology worldwide each year. The origin of ethanol detected in bio-samples has three possible origins: consumption, contamination or decomposition, which are frequently at the center of legal disputes. The stable carbon isotope is a value that can characterize the origin of a substance and has received increasing attention from forensic chemistry researchers in recent years. In a previous study, the stable carbon isotope of ethanol showed its potential for traceability; however, the overlap between the δ13C values of different ethanol origins gradually appeared as the sample size increased. In this study, 324 alcohol consumption samples (group A), 97 contamination samples (group B), and 26 decomposition samples (group C) were analyzed by gas chromatography-isotope ratio mass spectrometry. The δ13C values of the three groups ranged from –33.05 ‰ to −2.85 ‰ with different densities. The overlap interval of the groups ranged from 48.64 % to 56.69 % of the total. Based on the corresponding probability density curve, the likelihood ratio was used to describe the indication of ethanol origin. This study analyzed ten samples in real cases. Innovative pathway (stable carbon isotope -likelihood system) and traditional pathway (ethyl glucuronide-n-propanol system) were used to indicate the ethanol origin in the blood. This study demonstrates the feasibility and potential of combining stable isotope analysis with chemometrics to discriminate ethanol origin in alcohol-related cases and also provides new ideas to discriminate ethanol origin.

Authenticity Assessment of (E)-Cinnamic Acid, Vanillin, and Benzoic Acid from Sumatra Benzoin Balsam by Gas Chromatography Combustion/Pyrolysis Isotope Ratio Mass Spectrometry.

Authenticity assessment of (E)-cinnamic acid, vanillin, and benzoic acid from various origins (n = 26) was performed using gas chromatography-isotope ratio mass spectrometry coupled with combustion and pyrolysis mode (GC-C/P-IRMS). For that reason, the above three compounds (1–3) from synthetic, natural, and Sumatra benzoin balsam (laboratory prepared, adulterated, and commercial) were investigated. The δ13CV-PDB and δ2HV-SMOW values for compounds 1–3 from synthetic samples (S1–S5) ranging from −26.9 to −31.1‰ and 42 to 83‰, respectively, were clearly different from those of authentic samples (N1–N4, L1–L9) varying from −29.8 to −41.6‰ and −19 to −156‰. In adulteration verification testing, for compounds 1 and 3, both δ13CV-PDB and δ2HV-SMOW data of A1 (5.0% added) and A2 (2.5% added) except A3 (0.5% added) fell into the synthetic region, whereas for compound 2, the δ2HV-SMOW data of adulterated samples (A1–A3) fell into the synthetic region, and even the lowest adulterated sample A3 is no exception. With this conclusion, some commercial Sumatra benzoin balsam samples were identified to be adulterated with synthetic 1 (C1, C3, and C5) and synthetic 2 (C3, C4, and C5) but not with synthetic 3. GC-C/P-IRMS allowed clear-cut differentiation of the synthetic and natural origin of 1, 2, and 3 and definite identification of whether a Sumatra benzoin balsam was adulterated or not.

A GC-IRMS method for measuring sulfur isotope ratios of carbonyl sulfide from small air samples.

A new system was developed for measuring sulfur isotopes δ 33S and δ 34S from atmospheric carbonyl sulfide (COS) on small air samples of several liters, using pre-concentration and gas chromatography - isotope ratio mass spectrometry (GC-IRMS). Measurements of COS isotopes provide a tool for quantifying the COS budget, which will help towards better understanding climate feedback mechanisms. For a 4 liter sample at ambient COS mixing ratio, ~500 parts per trillion (ppt), we obtain a reproducibility error of 2.1 ‰ for δ 33S and 0.4 ‰ for δ 34S. After applying corrections, the uncertainty for an individual ambient air sample measurement is 2.5 ‰ for δ 33S and 0.9 ‰ for δ 34S. The ability to measure small samples allows application to a global-scale sampling program with limited logistical effort. To illustrate the application of this newly developed system, we present a timeseries of ambient air measurements, during the fall and winter of 2020 and 2021 in Utrecht, the Netherlands. The observed background values were δ 33S = 1.0 ± 3.4 ‰ and δ 34S = 15.5 ± 0.8 ‰ (VCDT). The maximum observed COS mixing ratios was only 620 ppt. This, in combination with the relatively high δ 34S suggests that the Netherlands receives little COS-containing anthropogenic emissions. We observed a change in COS mixing ratio and δ 34S with different air mass origin, as modelled with HYSPLIT backward trajectory analyses. An increase of 40 ppt in mean COS mixing ratio was observed between fall and winter, which is consistent with the expected seasonal cycle in the Netherlands. Additionally, we present the results of samples from a highway tunnel to characterize vehicle COS emissions and isotopic composition. The vehicle emissions were small, with COS/CO 2 being 0.4 ppt/ppm; the isotopic signatures are depleted relatively to background atmospheric COS.

Stable isotopic signatures of methane from waste sources through atmospheric measurements

This study aimed to characterize the carbon isotopic signatures (δ13C-CH4) of several methane waste sources, predominantly in the UK, and during field campaigns in the Netherlands and Turkey. CH4 plumes emitted from waste sources were detected during mobile surveys using a cavity ring-down spectroscopy (CRDS) analyser. Air samples were collected in the plumes for subsequent isotope analysis by gas chromatography isotope ratio mass spectrometry (GC-IRMS) to characterize δ13C-CH4. The isotopic signatures were determined through a Keeling plot approach and the bivariate correlated errors and intrinsic scatter (BCES) fitting method. The δ13C-CH4 and δ2H-CH4 signatures were identified from biogas plants (−54.6 ± 5.6‰, n = 34; −314.4 ± 23‰ n = 3), landfills (−56.8 ± 2.3‰, n = 43; −268.2 ± 2.1‰, n = 2), sewage treatment plants (−51.6 ± 2.2‰, n = 15; −303.9 ± 22‰, n = 6), composting facilities (−54.7 ± 3.9‰, n = 6), a landfill leachate treatment plant (−57.1 ± 1.8‰, n = 2), one water treatment plant (−53.7 ± 0.1‰) and a waste recycling facility (−53.2 ± 0.2‰). The overall signature of 71 waste sources ranged from −64.4 to −44.3‰, with an average of −55.1 ± 4.1‰ (n = 102) for δ13C, −341 to −267‰, with an average of −300.3 ± 25‰ (n = 11) for δ2H, which can be distinguished from other source types in the UK such as gas leaks and ruminants. The study also demonstrates that δ2H-CH4 signatures, in addition to δ13C-CH4, can aid in better waste source apportionment and increase the granularity of isotope data required to improve regional modelling.

Position-specific carbon isotopic composition of thermogenic propane: Insights from pyrolysis experiments

Position-specific isotope analysis (PSIA) of propane provides novel insights into formation mechanisms and thermal maturity of natural gas. The evolution of thermogenic propane in natural gas was simulated with pyrolysis experiments using types I, IIA, III kerogen and an oil. PSIA of propane involved on-line pyrolysis with gas chromatography–isotope ratio mass spectrometry to explore evolution characteristics and fractionation mechanisms of propane position-specific δ13C values. Results indicate the formation of propane can be considered in three stages. Stage I (<1.3% EasyRo) involves propane production mainly from primary kerogen cracking, with its precursors having homogeneous isotopic distributions and with position-specific 13C variation patterns depending on the kerogen molecular structure. In Stage II (1.3% EasyRo to maturity of peak propane yield), propane is derived mainly from secondary cracking of bitumen/oil, resulting in 13C enrichment of terminal (δ13Ca) and central (δ13Cb) C atoms with maturity and a strong linear relationship between δ13Ca and δ13Cb. A suggested new pattern is explained by a Rayleigh model for the evolution of propane position-specific δ13C values in this stage. In Stage III (maturity greater than the peak propane yield), propane destruction mainly controls its amount and isotopic signature. Propane site-preference (SP) value (δ13Ca–δ13Cb) can be used to determine its formation mechanism and maturity, with positive and negative values respectively indicating primary kerogen cracking and secondary oil/bitumen cracking. Increasing maturity causes decreasing values. The application of propane PSIA to the Tarim Basin gas samples demonstrates its potential for determining the mechanism of formation and maturity of natural gas.

Determination of carbon isotopic composition of crocetane in sediments by heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry

Compound-specific isotope analysis of crocetane in sediments can be used to determine its origin. However, the co-elution of crocetane with phytane and complex substrates hinders the accurate determination of its isotopic composition. A heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry (2D-GC–IRMS) method was developed to determine δ13C values of crocetane. Full chromatographic separation of crocetane was achieved with a 30 m CycloSil-B chiral capillary first column and a 60 m Chiraldex B-pH capillary second column (two series-connected 30 m Chiraldex B-pH capillary columns). Analyses of standard mixtures confirmed that the new method has satisfactory accuracy (deviations from the authentic values <0.5‰) and precision (RSDs <5%) for carbon isotope analyses of crocetane and phytane. The application of the method to two sediment samples indicates that baseline resolution is achieved (R >1.5) for crocetane and phytane, with 13C-depleted crocetane (δ13C values of −119.5‰ and −111.5‰ in the two natural samples, respectively) being an indicator of anaerobic oxidation of methane in cold seep areas. The new 2D-GC–IRMS method allows simple and efficient determination of the compound-specific carbon isotopic compositions of crocetane.

Geochemical characteristics of inclusion oils from the Skarv field A segment and their implications for the oil charge and leakage history

The oil-bearing fluid inclusions (FIs) in reservoir sandstones in the Skarv field in the Haltenbanken region offshore Norway recorded a complex oil charge and leakage history. Inclusion oils, hosted in quartz cements, and corresponding adsorbed residual oils on mineral surfaces from the Jurassic Garn and Tilje Formation (Fm) sandstones have been investigated using coupled gas chromatography-mass spectrometry, coupled gas chromatography-isotope ratio mass spectrometry and, as a new technique for FI oil analysis, Fourier transform-ion cyclotron resonance-mass spectrometry. The biomarker, stable carbon isotope and nitrogen, sulfur and oxygen-containing compound results suggest that FI oils from both the Garn and Tilje Fms and adsorbed (Ad) oil from the Tilje Fm were derived from the same source, whereas Ad oil from the Garn Fm shows a lower maturity level and differences in facies indicators. The similar geochemical signals for the FI oils from the Garn and Tilje Fms point to a first charge from the same source rock i.e. the late-Jurassic/earliest-Cretaceous Spekk Fm (Oxfordian to Ryazanian age). The fact that the Ad oil in the Tilje Fm is also similar to the respective FI oil indicates that the Tilje Fm was charged only once. In contrast, differences between the FI and Ad oils in the Garn Fm suggest that first charged oil has leaked and was replenished by a later charge sourced from a spatially different and less mature kitchen area of the Spekk Fm.

Analysis of Carbon and Hydrogen Stable Isotope Ratios of Phenolic Compounds: Method Development and Biodegradation Applications

Phenolic compounds are priority water pollutants of both natural and anthropogenic origins. Distinguishing the sources and degradation pathways of phenolic compounds in the environment is essential for sustainable water management. In this study, we have developed and validated methods based on gas chromatography–isotope ratio mass spectrometry for analysis of hydrogen and carbon isotope ratios of phenol and cresols. For carbon stable isotope analysis, solid-phase extraction was applied, whereas liquid–liquid extraction, followed by derivatization with trifluoroacetic anhydride, was used for hydrogen stable isotope analysis. The methods were verified in biodegradation experiments using p-cresol and phenol as substrates. Methyl group oxidation of p-cresol by Pseudomonas pseudoalcaligenes and Aromatoleum aromaticum resulted in the strong hydrogen (−76 ± 2 and −103 ± 2 mUr, respectively) and varying carbon isotopic fractionation (−2.1 ± 0.1 and −1.0 ± 0.1 mUr, respectively), leading to distinguishable two-dimensional plots for hydrogen versus carbon isotope ratios, indicating slightly different enzymatic reactions. For anaerobic phenol degradation by Desulfosarcina cetonica, the absence of hydrogen isotopic fractionation indicates adenosine triphosphate-dependent carboxylation as the initial step of the biodegradation pathway. The methods developed in this study are useful to detect biological degradation of phenolic compounds in highly contaminated aqueous environmental systems.