Cyclotron Resonance Mass Spectrometry Research Articles

Recovering Valuable Chemicals from Polypropylene Waste via a Mild Catalyst-Free Hydrothermal Process.

Waste polypropylene (PP) presents a significant environmental challenge, owing to its refractory nature and inert C-C backbone. In this study, we introduce a practical chemical recovery strategy from PP waste using a mild catalyst-free hydrothermal treatment (HT). The treatment converts 64.1% of the processed PP into dissolved organic products within 2 h in an air atmosphere at 160 °C. Higher temperatures increase the PP conversion efficiency. Distinct electron absorption and emission characteristics of the products are identified by spectral analysis. Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) reveals the oxidative cracking of PP into shorter-chain homologues (10-50 carbon atoms) containing carboxylic and carbonyl groups. Density functional theory (DFT) calculations support a reaction pathway involving thermal C-H oxidation at the tertiary carbon sites in the polymer chain. The addition of 1% H2O2 further enhances the oxidation reaction to produce valuable short-chain acetic acids, enabling gram-scale recycling of both pure PP and disposable surgical masks from the real world. Techno-economic analysis (TEA) and environmental life cycle costing (E-LCC) analysis suggest that this hydrothermal oxidation recovery technology is financially viable, which shows significant potential in tackling the ongoing plastic pollution crisis and advancing plastic treatment methodologies toward a circular economy paradigm.

Biochar data into structure: A methodology for generating large-scale atomistic representations

A well-defined methodology for constructing appropriate atomistic representations of biochar will aid in visualizing the structural features and elucidating biochar behavior with molecular dynamics (MD) simulations. Such knowledge will facilitate engineering biochars tailored to specific applications. To achieve this goal, we adapted modeling strategies applied in coal science by employing multi-cross-polarization 13C nuclear magnetic resonance, ultimate analysis, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy to identify functional groups. Helium density, surface area, and porosity were used to assess structural features. Biochar's aromatic cluster size distribution was proposed based on data from the benzene polycarboxylic acid method. The computational framework reduces bias by incorporating chemical information derived from density functional theory, reactive MD simulations, and advanced characterization data. The construction approach was successfully applied to cellulose biochars produced at four temperatures, obtaining independent representations with a relative error on the atomic contents of <10 % for oxygen and nitrogen and <5 % for carbon and hydrogen. The atomistic representations were validated using X-ray diffraction, electron spin resonance data, and laser desorption/ionization Fourier-transform ion cyclotron resonance-mass spectrometry. The code will assist others in overcoming structural creation barriers and enable the utilization of the generated structures for further simulations.

Molecular transformation of petroleum compounds by hydroxyl radicals produced upon oxidation of reduced nontronite

At redox interfaces, the interaction between oxygen and structural Fe(II) in minerals produces hydroxyl radicals (●OH); however, the specific reactivity and mechanisms of oxidation of complex petroleum compounds by ●OH remain poorly understood. This study investigates the interaction mechanisms between ●OH generated from reduced nontronite, an Fe-rich clay mineral (NAu-2), and petroleum compounds by employing a suite of wet chemistry, gas chromatography mass spectrometry and Fourier transform-ion cyclotron resonance-mass spectrometry, as well as theoretical calculations. The results showed that the intrinsic structure, composition, solubility, and adsorption of petroleum molecules onto reduced NAu-2 significantly influenced their reactivity with ●OH. Specifically, the aromatic fraction of a crude oil sample exerted a minimal influence on the oxidation rate of structural Fe(II) in reduced NAu-2, but enhanced the production of ●OH by generating environmentally persistent free radicals. ●OH reacted more with water-soluble dissolved organic matter from oil compared to polar compounds. The newly generated molecules by ●OH exhibited a preference for the insoluble (polar) phase. The oxidation of petroleum hydrocarbons by ●OH was mediated through hydrogen atom abstraction and radical adduct formation, followed by dehydrogenation, hydroxylation, and carboxylation reactions. Consequently, the abundance of oxygen-containing compounds in polar compounds increased. However, specific oxidation pathways differed depending on the nature of petroleum compounds. Our results provide insights that oxidation of petroleum compounds by ●OH is vital for mitigating the negative impacts from hydrocarbon contamination and for evaluating the role of these processes in the carbon cycle in redox-active environments.

Nitrogen‐Containing Functional Groups Dominate the Molecular Absorption of Water‐Soluble Humic‐Like Substances in Air From Nanjing, China Revealed by the Machine Learning Combined FT‐ICR‐MS Technique

AbstractThe light absorption capacity of water‐soluble humic‐like substances (HULISWS) at the molecular level is crucial for reducing the uncertainties in modeling the radiative forcing. This study proposed a machine learning approach to allocate the light absorption coefficient at 365 nm (Abs365) of HULISWS into 8084 Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR‐MS) detached molecular markers and their potential functional groups. The ML model showed an acceptable uncertainty (&lt;5%) to the whole Abs365 value based on the prediction errors. The results showed that five critical light‐absorbing molecules (C4H6O4NS, C8H6O4NS, C11H15O3N2, C12H15O3N2, and C19H21O6) could explain 74% (±3%) of the variation of Abs365 in the winter, whereas no crucial light‐absorbing molecules were found in the summer. Besides, the nitrogen‐containing functional groups were found to dominate (61% ± 8%) the molecular absorption near the 365 nm of the spectrum. This work illustrated how functional groups affect the absorption of HULISWS, providing critical information for future research of HULISWS on the molecular level.

New Insights into the Role of Natural Organic Matter in Fe-Cr Coprecipitation: Importance of Molecular Selectivity.

Coprecipitation of Fe/Cr hydroxides with natural organic matter (NOM) is an important pathway for Cr immobilization. However, the role of NOM in coprecipitation is still controversial due to its molecular heterogeneity and diversity. This study focused on the molecular selectivity of NOM toward Fe/Cr coprecipitates to uncover the fate of Cr via Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The results showed that the significant effects of Suwannee River NOM (SRNOM) on Cr immobilization and stability of the Fe/Cr coprecipitates did not merely depend on the adsorption of SRNOM on Fe/Cr hydroxides. FT-ICR-MS spectra suggested that two pathways of molecular selectivity of SRNOM in the coprecipitation affected Cr immobilization. Polycyclic aromatics and polyphenolic compounds in SRNOM preferentially adsorbed on the Fe/Cr hydroxide nanoparticles, which provided extra binding sites and promoted the aggregation. Notably, some specific compounds (i.e., polyphenolic compounds and highly unsaturated phenolic compounds), less unsaturated and more oxygenated than those adsorbed on Fe/Cr hydroxide nanoparticles, were preferentially incorporated into the insoluble Cr-organic complexes in the coprecipitates. Kendrick mass defect analysis revealed that the insoluble Cr-organic complexes contained fewer carbonylated homologous compounds. More importantly, the spatial distribution of insoluble Cr-organic complexes was strongly related to Cr immobilization and stability of the Fe/Cr-NOM coprecipitates. The molecular information of the Fe/Cr-NOM coprecipitates would be beneficial for a better understanding of the transport and fate of Cr and exploration of the related remediation strategy.

Quantitative and qualitative comparison of marine dissolved organic nitrogen recovery using solid phase extraction

AbstractMarine dissolved organic carbon and nitrogen (DOC and DON) are major global carbon and nutrient reservoirs, and their characterization relies on extraction methods for preconcentration and salt removal. Existing methods optimize for capturing and describing DOC. Here, we report an optimized analytical strategy to recover marine DON for subsequent molecular characterization. Retention efficiencies between 5% and 95% are reported for seven solid phase extraction (SPE) sorbents, with PPL recovering 23% of marine DON compared to 95% recovered with C18. Additional comparisons of the effect of varying sample volumes and elution speed, and the resulting molecular composition of DON extracts, were investigated using C18 and PPL sorbents. Sample volumes &gt; 200 mL decreased DON retention efficiency independent of SPE sorbent, and gravity elution recovered 1.7‐ to 4.2‐fold more DON compared to vacuum elution. Characterization of extracted DON by negative‐ion electrospray ionization Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR MS) highlights compositional differences between DON species recovered by each method. DON isolated with optimized methods includes low molecular weight (&lt; 600 Da) peptide‐like compounds with low O:C ratios (0.2 to 0.5) that are not detected by other SPE sorbents (e.g., PPL). The majority of additional DON isolated with this approach was undetectable by direct infusion negative mode FT‐ICR MS analysis.

Separation, speciation and quantification of both chromium (VI) and chromium (III) in tanned leather samples: a comparative study and validation of analytical methods

In the present work, a comparative study of analytical methods for the simultaneous and quantitative determination of trivalent and hexavalent chromium is presented. For the analysis by ion chromatography-inductively coupled plasma-mass spectrometry, two different columns were tested, as well as different mobile phases and different pH of the samples. The optimized analytical method permitted the separation of Cr(III) and Cr(VI) using 75 mmol/L NH4NO3 pH 3 as chromatographic eluent. The method was validated and applied to real samples, allowing the determination of both species simultaneously, even when there is a huge difference of concentration between Cr(III) and Cr(VI). Limit of detection and limit of quantification for Cr(III) were found to be 0.016 and 0.054 upmug/L (0.3 and 1.1 upmug/kg), respectively, and for Cr(VI) 0.13 and 0.43 upmug/L (7 and 22 upmug/kg), respectively. Possible species interconversions were monitored through the use of chromium isotopic standards, which confirmed that the optimized methodology preserves chromium speciation during extraction and analysis. Fourier-transform ion cyclotron resonance-mass spectrometry permitted the structure elucidation of the complex formed during ethylenediaminetetraacetic acid extraction.Graphical

One thousand soils for molecular understanding of belowground carbon cycling

While significant progress has been made in understanding global carbon (C) cycling, the mechanisms regulating belowground C fluxes and storage are still uncertain. New molecular technologies have the power to elucidate these processes, yet we have no widespread standardized implementation of molecular techniques. To address this gap, we introduce the Molecular Observation Network (MONet), a decadal vision from the Environmental Molecular Sciences Laboratory (EMSL), to develop a national network for understanding the molecular composition, physical structure, and hydraulic and biological properties of soil and water. These data are essential for advancing the next generation of multiscale Earth systems models. In this paper, we discuss the 1000 Soils Pilot for MONet, including a description of standardized sampling materials and protocols and a use case to highlight the utility of molecular-level and microstructural measurements for assessing the impacts of wildfire on soil. While the 1000 Soils Pilot generated a plethora of data, we focus on assessments of soil organic matter (SOM) chemistry via Fourier-transform ion cyclotron resonance-mass spectrometry and microstructural properties via X-ray computed tomography to highlight the effects of recent fire history in forested ecosystems on belowground C cycling. We observed decreases in soil respiration, microbial biomass, and potential enzyme activity in soils with high frequency burns. Additionally, the nominal oxidation state of carbon in SOM increased with burn frequency in surface soils. This results in a quantifiable shift in the molecular signature of SOM and shows that wildfire may result in oxidation of SOM and structural changes to soil pore networks that persist into deeper soils.

Accumulation of dissolved organic matter in the transition from fresh to aged seasonal snow in an industrial city in NE China.

Dissolved organic matter (DOM) plays a significant role in the reduction of snow albedo and the acceleration of snowmelt, but its accumulation in snow remains poorly understood. This study investigated the accumulation of DOM in seasonal snow including its accumulation rate, molecular characteristics, and biological and chemical processing. Sixteen snow samples of both fresh and aged snow were collected at one-day interval in Changchun, a typical industrial city in NE China. The snow DOM contents increased linearly with accumulation time at a rate of 30.3 μg L-1 d-1. The optical properties, including fluorescence intensity and optical absorption coefficient, of snowmelt increased exponentially with time owing to the rapid accumulation of terrestrial humic-like fluorophores through snow-soil exchange and deposition of soil-derived substances. Fourier transform-ion cyclotron resonance-mass spectrometry highlighted the properties of DOM at a molecular level, indicating that compounds derived from underlying soil and vascular plants make the largest contribution to DOM. Microbe-derived compounds contribute 35.5 % to the DOM pool. Degrees of saturation and oxidation increase slightly after accumulation, with the impacts of photo- and bio-chemistry on DOM molecules being non-negligible. This study provides a new perspective concerning the accumulation and fate of organic contaminants in snow ecosystems.

Reservoir oil geochemistry in the northern Western Desert of Egypt – High-resolution mass spectrometry and stable carbon isotope characterization

Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) with negative ion electrospray ionization (ESI-N) and positive ion atmospheric pressure photoionization (APPI-P), and compound-specific carbon isotope measurements together with conventional biomarker analyses were applied on 13 crude oil samples from Cretaceous reservoirs in the Shushan, Dahab/Mireir, and Abu Gharadig basins of the northern Western Desert of Egypt to identify their origin, maturity, alteration level, and petroleum relationships. Five oil families were identified, and the FT-ICR-MS data enabled a differentiation between crude oils with terrigenous, marine, and mixed organic matter input based on polar compounds. The dominance of acidic (ESI-N) and low-polarity (APPI-P) oxygen-containing compounds in the Shushan oils reflects land plant material input and oxic depositional conditions for their source rocks. The abundance of oxygen-containing compounds gradually decreases in the NE-Abu Gharadig and Dahab/Mireir oils (source rocks: mixed Type III/II organic matter (OM)) to reach minimum values in the marine-dominated Abu Gharadig oils. Based on compound-specific carbon isotopes and hydrocarbon biomarkers, the Shushan Basin hosts two different oil families from source rocks with Type III OM that likely reveal different thermal maturities and ages. The detected APPI-P aromatic hydrocarbons together with other molecular maturity ratios suggest a higher thermal maturity for the Shushan family A and Abu Gharadig oils than for oils of Shushan family B and from the Dahab/Mireir Basins, respectively.

Matrix-assisted laser desorption/ionization-Fourier-transform ion cyclotron resonance-mass spectrometry analysis of exosomal lipids from human serum.

Exosomes contain biomarkers such as proteins and lipids that help in understanding normal physiology and diseases. Lipids, in particular, are infrequently studied using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) for biomarker discovery. In this study, MALDI was equipped with a high-resolution MS to investigate exosomal lipids from human serum. Exosomal lipids were profiled using MALDI with Fourier-transform ion cyclotron resonance (FTICR)-MS. Four matrices (i.e., α-cyano-4-hydroxycinnamic acid [CHCA], 2,5-dihydroxybenzoic acid, sinapinic acid, and graphene oxide [GO]) and three sample preparation methods (i.e., dried droplet, thin layer, and two layer) were compared for the number of lipid species detected and the relative abundance of each lipid from human serum and human serum exosomes. In sum, 172 and 89 lipid species were identified from human serum and human serum exosomes, respectively, using all the methods. The highest number of exosome lipid species, 69, was detected using the CHCA matrix, whereas only 8 exosome lipid species were identified using the GO matrix. Among the identified lipid species, phosphatidylcholine was identified most frequently, probably due to the use of a positive ion mode. Exosomes and human serum showed comparable lipid profiles as determined using MALDI-FTICR-MS. These findings provide a new perspective on exosomal lipidomics analysis and may serve as a foundation for future lipidomics-based biomarker research using MALDI-FTICR-MS.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter.

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O2, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

Comparison of Sample Preparation Techniques for the (−)ESI-FT-ICR-MS Analysis of Humic and Fulvic Acids

Soil organic matter (SOM) plays a key role in the global carbon and nitrogen cycles. Soil biogeochemistry is regularly studied by extracting the base-soluble fractions of SOM: acid-insoluble humic acid (HA) and acid-soluble fulvic acid (FA). Electrospray ionization-Fourier transform-ion cyclotron resonance-mass spectrometry (ESI-FT-ICR-MS) is commonly utilized for molecularly characterizing these fractions. Different sample preparation techniques exist for the analysis of HA and FA though questions remain regarding data comparability following different preparations. Comparisons of different sample preparation techniques here revealed that the negative-mode ESI-FT-ICR-MS analytical window can be skewed to detect different groups of molecules, with primary differences in oxygenation, aromaticity, and molecular weight. It was also observed that HA and FA from soils versus an aquatic matrix behaved very differently. Thus, we conclude that sample preparation techniques determined to be "most optimal" in our study are in no way universal. We recommend that future studies of HA and FA involve similar comparative studies for determining the most suitable sample preparation technique for their particular type of HA or FA matrices. This will enhance data comparability among different studies and environmental systems and ultimately allow us to better understand the complex composition of environmental matrices.

Effects of cellulose-lignin interaction on the evolution of biomass pyrolysis bio-oil heavy components

Cellulose, lignin and their mixture were pyrolyzed at 500–700 °C for 60 s and 90 s to investigate the influence of cellulose-lignin interaction on the evolution of biomass pyrolysis bio-oil heavy components with temperature and reaction time. Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the composition and main functional groups of the heavy oil. Strong interaction among cellulose and lignin was revealed, which promoted the generation of lipids and unsaturated hydrocarbons while inhibiting the generation of the condensed aromatic hydrocarbons, phenolic species and sugars in the heavy components. With the use of van Krevelen diagrams, three reaction pathways are proposed to explain the evolution of heavy species under the influence of interaction, which is strongly affected by the removal of oxygen containing functional groups.

Molecular characterization of dissolved organic nitrogen and phosphorus in agricultural runoff and surface waters

Agricultural runoff is a significant contributor to nitrogen (N) and phosphorus (P) pollution in water bodies. Limited information is available about the molecular characteristics of the dissolved organic N (DON) and P (DOP) species in the agricultural runoff and surface waters. We employed Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometry (FT-ICR-MS) to investigate the changes in the molecular characteristics of DON and DOP at three watershed positions (upstream water, runoff from agricultural fields, and downstream waters). Across three watershed locations, more-bioavailable compounds (such as amino sugars, carbohydrates, lipids, and proteins) accounted for <5% of DON and 4–31% of DOP molecules, whereas less-bioavailable compounds (such as lignin, tannins, condensed hydrocarbons, and unsaturated hydrocarbons) were >95% of DON and 69–96% of DOP. Of the dissolved organic matter, runoff waters from agricultural fields contained the greatest proportion of DON formulas (20–25%) than upstream (18%) and downstream (13–14%) waters, indicating the presence of a greater diversity of DON species in the runoff. Various nutrient sources present in agricultural fields such as crop residues, soil organic matter, and transformed fertilizers likely contributed to the diverse composition of DON and DOP in the runoff, which were likely altered as the surface water traversed along the flow pathways in the watershed. The presence of more-bioavailable molecules detected in upstream compared to agricultural runoff and downstream waters suggests that photochemical and/or microbial processes likely altered the characteristics of DON and DOP compounds. The findings of this study increase our understanding of DON and DOP compounds lability and transformations in runoff and surface waters , which may be useful in quantifying the contribution of organic N and P sources to water quality impairment in aquatic ecosystems.

Trapped Under Ice: Spatial and Seasonal Dynamics of Dissolved Organic Matter Composition in Tundra Lakes

AbstractArctic lakes store, modify, and transport large quantities of carbon from terrestrial environments to the atmosphere; however, the spatial and temporal relationships between quantity and composition of dissolved organic matter (DOM) have not been well characterized across broad arctic regions. Moreover, most arctic lake DOM compositions have been examined during the ice‐free summer, whereas DOM cycling between the ice‐covered winter months and summer have not been addressed. To resolve these spatial and seasonal uncertainties in DOM cycling, we sampled a series of arctic lakes from the North Slope of Alaska across a latitudinal gradient in the winter and summer over 3 years. Samples were analyzed for dissolved organic carbon concentration and DOM composition was characterized using optical and fluorescence properties combined with molecular‐level analysis using Fourier transform‐ion cyclotron resonance mass spectrometry. Tundra lake DOM properties including aromaticity and molecular stoichiometries were similar to other northern high‐latitude lakes, but optical parameters related to aromaticity and molecular weight were greater in major arctic rivers and in coastal lakes in the North Slope region. DOM composition was highly seasonal, with ice exclusion concentrating microbially processed DOM in the winter water columns, potentially influencing DOM cycling the following summer. However, the greatest variations in DOM composition were related to lake depth and likely other physical features including morphology and bathymetry. As the Arctic warms, we expect changes in hydrology and ice cover to enhance under‐ice microbial DOM processing, early summer photodegradation, and ultimately carbon fluxes to the atmosphere after ice‐out.

Fractionation of hydrocarbons and NSO-compounds during primary oil migration revealed by high resolution mass spectrometry: Insights from oil trapped in fluid inclusions

The composition of oil trapped in fluid inclusions (FI), occurring in mineral cements, can provide valuable insights into oil migration. Here, FI oils in a calcite vein (representing expelled fluids) and source rock (SR) extracts (representing retained bitumen) from the Hosszúhetény Calcareous Marl Formation (HCMF) in the Mecsek Mountains area of Hungary were investigated to assess how organic compounds are fractionated during primary migration. Biomarkers analyzed by gas chromatography–mass spectrometry and gas chromatography-stable carbon isotope ratio mass spectrometry were used to demonstrate that the FI oils had been expelled from the HCMF marl (calculated vitrinite reflectance of ~0.74% Rc). Fourier transform-ion cyclotron resonance-mass spectrometry then provided insights into polar compound geochemistry, showing that O1, N1, N1O1 and S1O1 compound classes are preferentially retained in the source rock bitumen, while less polar compounds like aromatic hydrocarbons (HCs) and S1 compounds are assumed to be preferentially expelled. Independent of the compound class, compounds with higher double bond equivalents (DBEs) are enriched in the retained source rock bitumen. Thus, besides the molecular polarity determined by the functional groups, the molecular size and the degree of aromaticity appear to be the most important factors affecting the migration and retention behavior of the petroleum HCs and NSO-compounds in the HCMF. Moreover, the enrichment of high DBE compounds with shorter alkyl chains in the SR extracts infers that shielding effects could have played a major role for compound retention and expulsion during primary oil migration.

Molecular Characterization of Polar Compounds in Crude Oil Affecting Sandstone Wettability Revealed by Fourier Transform-Ion Cyclotron Resonance-Mass Spectrometry

Summary Knowing the composition, molecular size, and structure of polar compounds in crude oil that affect sandstone wettability is a prerequisite for a better understanding of oil/rock/brine interactions and for better application of enhanced oil recovery (EOR) techniques to increase recovery factors and improve the economic efficiency of field development. The nitrogen-, sulfur-, and oxygen-containing polar compounds in crude oil are key factors for sandstone wettability changes. In this study, an outcrop sandstone core selected from Jurassic formation in Sichuan Basin, China, was aged by crude oil to restore the wettability after being cleaned by hot Soxhlet extraction with an azeotropic solvent mixture of methanol and dichloromethane (MDC, vol/vol = 7:93). Then, Amott-Harvey experiments that were conducted by combining spontaneous imbibition and forced displacement steps of coreflooding were performed to characterize sandstone wettability after it was cut into four core blocks. The wettability index (IA-H) of four core blocks from the inlet to outlet of oil flooding were −0.523, 0.214, −0.087, and −0.861, respectively, which illustrated different degrees of sandstone wettability. The extracts of three sequential hot Soxhlet extraction steps of four core blocks were analyzed by gas chromatography-mass spectrometry (GC-MS) and high-resolution Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) combined with electrospray ionization (ESI) in negative ion mode. Almost no polar compounds were detected in the n-hexane extracts, and a total of seven classes of different polar molecular compounds—namely, N1, N1O1, N1S1, O1, O2, O3, and O4—were detected in dichloromethane (DCM) and MDC extracts. The relative abundances of the N1S1 and O1 classes in the extracts of DCM and MDC were too low to be ignored. Compared to those of polar compounds in DCM extracts, the relative abundance of neutral nitrogen compounds (N1 and N1O1 classes) in the MDC extracts decreased significantly. In contrast, the acidic compounds (O2, O3, and O4 classes) all showed an obvious increase in the MDC extracts compared to the DCM extracts. It was notable that most of the polar compounds in MDC extracts were O2 and O3 compounds with double bond equivalent (DBE) values = 1 from the perspective of DBE distribution. The proportion of these two compound classes was much higher than that of all other polar compounds. Therefore, we believe that these two compound classes are the decisive factors for changing sandstone wettability combined with previous studies. In addition, based on the number of oxygen atoms and DBE values, we inferred that the O2 (DBE = 1) class was the long-chain saturated fatty acids and that the O3 (DBE = 1) class was the hydroxyl acids containing both one carboxyl and one hydroxyl group. Furthermore, the final determination of the wettability degree of the sandstone surface was the amount of all polar compounds, not only the relative abundance of these two types of acids. These two types of acids in crude oil were equivalent to anchor molecules on the surface of sandstone, and other polar compounds were adsorbed onto their surface to make the sandstone preferentially oil-wet.

Identifying carbon processing based on molecular differences between groundwater and water-extractable aquifer sediment dissolved organic matter in a Quaternary alluvial-lacustrine aquifer

Dissolved organic matter in groundwater (GDOM) plays an important role in many biogeochemical processes. The molecular composition of GDOM varies with the source and processing pathways, and also affects its reactivity. Sedimentary organic matter (SOM) is a key carbon source in groundwater, but its association with GDOM remains unclear. In this study, Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) and comprehensive data analysis were used to identify the molecular characteristics of SOM and the fate of DOM in groundwater by comparing the differences between the GDOM with depth-matched water-extractable aquifer SOM (WEOM; the most bioavailable and dynamic pool of organic carbon in SOM) in a Quaternary alluvial-lacustrine aquifer with abundant SOM in Jianghan Plain, central China. The results show that WEOM is a key carbon source in groundwater as reflected by the strong correlation (rs = 0.76, P = 0.028) of the DOC content between GDOM and WEOM, and the large proportion (70.64 ± 14.09%) of common compounds present both in GDOM and WEOM. However, the molecular compositions of GDOM and WEOM are significantly different even for common compounds when considering the relative abundance. WEOM contains relatively high abundance of low molecular weight and thermodynamically more favorable components for microbial oxidation (aminosugars and carbohydrates), making WEOM an energetic carbon source for groundwater that can fuel reduction half-reactions such as iron oxide reduction. Although the hydrochemical compositions and the DOC content of groundwater are quite different, the molecular composition of GDOM is relatively similar and enriched in oxygen-poor highly unsaturated and phenolic compounds compared with WEOM. This indicates that organic matter is gradually processed into stable components with relatively medium aromaticity, low average nominal carbon oxidation state, low O/C and medium H/C; these transformations occur through adsorption to minerals and biodegradation after being mobilized from aquifer sediment into groundwater. We identified the processing pathways of GDOM based on comprehensive comparisons of molecular compositions between aquifer WEOM and GDOM, and highlight the combined influence of aquifer SOM and processing pathways on the GDOM molecular compositions in the alluvial-lacustrine aquifer with abundant aquifer SOM.

Spatial and Temporal Variability of Dissolved Organic Matter Molecular Composition in a Stratified Eutrophic Lake

AbstractDissolved organic matter (DOM) is an intermediate between organic carbon formed by primary producers and carbon dioxide (CO2) produced through respiration, making it a key component of the carbon cycle in aquatic ecosystems. Its composition influences the routes of mineralization. Here, we evaluate DOM composition as a function of time and depth in Lake Mendota, a highly productive eutrophic lake that stratifies in warm months and is located in Madison, Wisconsin, USA. Dissolved organic carbon concentrations and optical properties are presented for 73 samples collected at a single location at varying depths within the water column from June to November. A subset of samples is analyzed by Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICR MS) to investigate DOM composition at the molecular level. Temporally, increases in more oxidized formulas are observed in both the epilimnion and hypolimnion. At the surface, correlations between DOM formulas and both chlorophyll concentrations and light intensity show that photochemical reactions contribute to DOM oxidation. In the hypolimnion, redox conditions and interactions with sediments likely influence temporal compositional change. Our results show DOM composition varies with depth with more highly oxidized formulas identified deeper in the water column. However, DOM composition varies more temporally than by location within the water column. This work has implications for climate change as DOM photooxidation in lakes represents an understudied flux of CO2 to the atmosphere. Additionally, lake eutrophication is increasing due to warming temperatures and this data set yields detailed molecular information about DOM composition and processing in such lakes.