Validation and Evaluation of High-Resolution Orbitrap Mass Spectrometry on Molecular Characterization of Dissolved Organic Matter

Enhancing Molecular Characterization of Dissolved Organic Matter by Integrative Direct Infusion and Liquid Chromatography Nontargeted Workflows

Dissolved organic matter (DOM) in aquatic systems is a highly heterogeneous mixture of water-soluble organic compounds, acting as a major carbon reservoir driving biogeochemical cycles. Understanding DOM molecular composition is thus of vital interest for the health assessment of aquatic ecosystems, yet its characterization poses challenges due to its complex and dynamic chemical profile. Here, we performed a comprehensive chemical analysis of DOM from highly urbanized river and seawater sources and compared it to drinking water. Extensive analyses by nontargeted direct infusion (DI) and liquid chromatography (LC) high-resolution mass spectrometry (HRMS) through Orbitrap were integrated with novel computational workflows to allow molecular- and structural-level characterization of DOM. Across all water samples, over 7000 molecular formulas were calculated using both methods (∼4200 in DI and ∼3600 in LC). While the DI approach was limited to molecular formula calculation, the downstream data processing of MS2 spectral information combining library matching and in silico predictions enabled a comprehensive structural-level characterization of 16% of the molecular space detected by LC-HRMS across all water samples. Both analytical methods proved complementary, covering a broad chemical space that includes more highly polar compounds with DI and more less polar ones with LC. The innovative integration of diverse analytical techniques and computational workflow introduces a robust and largely available framework in the field, providing a widely applicable approach that significantly contributes to understanding the complex molecular composition of DOM.

Metabolic responses of Euglena gracilis under photoheterotrophic and heterotrophic conditions

The protist Euglena gracilis has various trophic modes including heterotrophy and photoheterotrophy. To investigate how cultivation mode influences metabolic regulation, the chemical composition of cellular metabolites of Euglena gracilis grown under heterotrophic and photoheterotrophic conditions was monitored from the early exponential phase to the mid-stationary phase using two different techniques, i.e, nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS). The combined metabolomics approach allowed an in-depth understanding of the mechanism of photoheterotrophic and heterotrophic growth for biomolecule production. Heterotrophic conditions promoted the production of polar amino and oxygenated compounds such as proteins and polyphenol compounds, especially at the end of the exponential phase while photoheterotrophic cells enhanced the production of organoheterocyclic compounds, carbohydrates, and alkaloids.

Molecular transformation of dissolved organic matter in manganese ore-mediated constructed wetlands for fresh leachate treatment

The organic matter (OM) and nitrogen in Fresh leachate (FL) from waste compression sites pose environmental and health risks. Even though the constructed wetland (CW) can efficiently remove these pollutants, the molecular-level transformations of dissolved OM (DOM) in FL remain uncertain. This study reports the molecular dynamics of DOM and nitrogen removal during FL treatment in CWs. Two lab-scale vertical-flow CW systems were employed: one using only sand as substrates (act as a control, CW-C) and the other employing an equal mixture of manganese ore powder and sand (experimental, CW-M). Over 488 days of operation, CW-M exhibited significantly higher removal rates for chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), and dissolved organic matter (represented by dissolved organic carbon, DOC) at 98.2 ± 2.5%, 99.2 ± 1.4%, and 97.9 ± 1.9%, respectively, in contrast to CW-C (92.8 ± 6.8%, 77.1 ± 28.1%, and 74.7 ± 9.5%). The three-dimensional fluorescence excitation-emission matrix (3D-EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses unveiled that the influent DOM was predominantly composed of readily biodegradable protein-like substances with high carbon content and low unsaturation. Throughout treatment, it led to the degradation of low O/C and high H/C compounds, resulting in the formation of DOM with higher unsaturation and aromaticity, resembling humic-like substances. CW-M showcased a distinct DOM composition, characterized by lower carbon content yet higher unsaturation and aromaticity than CW-C. The study also identified the presence of Gammaproteobacteria, reported as Mn-oxidizing bacteria with significantly higher abundance in the upper and middle layers of CW-M, facilitating manganese cycling and improving DOM removal. Key pathways contributing to DOM removal encompassed adsorption, catalytic oxidation by manganese oxides, and microbial degradation. This study offers novel insights into DOM transformation and removal from FL during CW treatment, which will facilitate better design and enhanced performance.

Noise Filtering Algorithm Using Gaussian Mixture Models for High-Resolution Mass Spectra of Natural Organic Matter

High-resolution mass spectra of natural organic matter (NOM) contain a large number of noise signals. These signals interfere with the correct molecular composition estimation during nontargeted analysis because formula-assignment programs find empirical formulas for such peaks as well. Previously proposed noise filtering methods that utilize the profile of the intensity distribution of mass spectrum peaks rely on a histogram to calculate the intensity threshold value. However, the histogram profile can vary depending on the user settings. In addition, these algorithms are not automated, so they are handled manually. To overcome the mentioned drawbacks, we propose a new algorithm for noise filtering in mass spectra. This filter is based on Gaussian Mixture Models (GMMs), a machine learning method to find the intensity threshold value. The algorithm is completely data-driven and eliminates the need to work with a histogram. It has no customizable parameters and automatically determines the noise level for each individual mass spectrum. The algorithm performance was tested on mass spectra of natural organic matter obtained by averaging a different number of microscans (transients), and the results were compared with other noise filters proposed in the literature. Finally, the effect of this noise filtering approach on the fraction of peaks with assigned formulas was investigated. It was shown that there is always an increase in the identification rate, but the magnitude of the effect changes with the number of microscans averaged. The increase can be as high as 15%.

Toward a More Comprehensive Approach for Dissolved Organic Matter Chemical Characterization Using an Orbitrap Fusion Tribrid Mass Spectrometer Coupled with Ion and Liquid Chromatography Techniques

Dissolved organic matter (DOM) represents one of the largest active organic carbon pools in the global carbon cycle. Although extensively studied, only <10% of DOM has been chemically characterized into individual dissolved compounds due to its molecular complexity. This study introduced a more comprehensive DOM characterization method by coupling both ion chromatography (IC) and liquid chromatography (LC) with high mass accuracy and resolution mass spectrometry. We presented a new on-the-fly mass calibration of the Orbitrap technique by utilizing the "lock mass" function in the Orbitrap Fusion Tribrid mass spectrometer (OT-FTMS), which assures high mass accuracy at every scan by a postcolumn introduction of internal labeled standards. With both IC and LC, tested unlabeled standards of amino acids, small peptides, and organic acids were consistently below 1.0 ppm mass error, giving the OT-FTMS the potential of reaching mass accuracy of the Fourier-transform ion cyclotron resonance mass spectrometer. In addition to mass accuracy, a pooled quality control sample (QC) was used to increase reproducibility by applying systematic error removal using random forest (SERRF). Using an untargeted mass spectrometry approach, estuarine DOM samples were analyzed by OT-FTMS coupled to IC in negative mode and LC in positive mode detection to cover a wide range of highly cationic to highly anionic molecules. As a proof of concept, we focused on elucidating the structures of three distinct DOM compound classes with varied acidities and basicities. In UPLC-OT-FTMS, a total of 915 compounds were detected. We putatively elucidated 44 small peptides and 33 deaminated peptides of these compounds. With IC-OT-FTMS, a total of 1432 compounds were detected. We putatively elucidated 20 peptides, 268 deaminated peptides, and 188 organic acids. Except for five compounds, all putatively elucidated compounds were uniquely detected in their corresponding chromatography technique. These results highlight the need for combining these two techniques to provide a more comprehensive method for DOM characterization. Application of the combined IC and LC techniques is not limited to DOM chemical characterization. It can analyze other complex compound mixtures, such as metabolites, and anthropogenic pollutants, such as pesticides and endocrine-disrupting chemicals, in environmental and biological samples.

Graph-Based Method for Calibration of High-Resolution Mass Spectra of Natural Organic Matter

Inaccuracies in ion detection and signal processing can undermine confidence in the molecular formula assignment of high-resolution mass spectrometry, which relies on precise matching of the mass-to-charge ratio (m/z). This study proposes a novel graph-based spectra calibration method, MSCMcalib, which implements coordinate transformation and pattern detection. MSCMcalib maps uncalibrated m/z data onto a modified 2D mass defect plot, facilitating the automatic calibration of detected lines, i.e., the calibration of uncalibrated peaks aligned with these lines. The "propagation" method is subsequently employed to accurately and automatically calibrate 605 m/z values across multiple lines, encompassing 98% of the m/z range. The calibrated m/z values divide the m/z range of the spectrum into multiple subintervals, with each subinterval undergoing a process of "scaling" calibration. The utilization of narrower partitions effectively mitigates divergence issues at both ends that arise from the polynomial fitting of errors against m/z. The effectiveness of MSCMcalib is validated through the calibration of SRFA data with m/z error ranges spanning from -10 to -6 ppm, resulting in an additional assignment of 11%-30% more molecular formulas compared to the quadratic fitting calibration.

Purification of dredged water by magnetic coagulation: Response surface optimization and dissolved organic matter removal characteristics

In the present study, magnetic coagulation was used to treat dredged water and the response surface method was used to optimize process parameters. The dissolved organic matter (DOM) removal characteristics were characterized by three-dimensional fluorescence spectrometry and ultra-high resolution mass spectrometry. During the magnetic coagulation process, the suspended solids (SS) removal rate increased initially and then decreased under conditions of increasing magnetic powder dosage and stirring rate. After magnetic coagulation and precipitation for 20 min, the contents of SS, ammonia nitrogen, chemical oxygen demand, and total phosphorus in the treated dredged water met the requirements of the discharge standard (GB8978-1996, China). Three-dimensional fluorescence results showed that magnetic coagulation selectively removed fulvic acids and humic acid substances. After magnetic coagulation with precipitation for 10 min and 20 min, the total relative content of lignins, tannins, proteins, lipids, aminosugars, unsaturated hydrocarbons, condensed aromatic structures, and carbohydrates decreased by 26.3% and 39.4%, respectively. After magnetic coagulation, the distribution range of small molecule DOM shifted to the low H/C and high O/C regions. This study provides a novel perspective for studies on the removal of DOM in dredged water by magnetic coagulation. PRACTITIONER POINTS: SS and DOM removal were significantly enhanced by the use of magnetic coagulation. SS removal efficiency was affected by stirring rate and magnetic powder dosage. Magnetic coagulation selectively removed fulvic acids and humic acid substances. DOM molecule shifted to low H/C and high O/C regions after magnetic coagulation.

Effect of salinity on molecular characterization of dissolved organic matter using ESI FT-ICR MS

Solid-phase extraction (SPE) coupled with negative-ion Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for molecular characterization of dissolved organic matter (DOM). However, little attention has paid to test whether the salinity of the sample and the presence of chloride ions in water samples affect the molecular composition of DOM extracted by SPE (SPE-DOM). In this study, one natural organic matter standard and several natural waters were selected to investigate how salinity affects the molecular composition of SPE-DOM and the selectivity of chloride ion adducts formation with respect to the molecular structure of SPE-DOM in negative ion ESI FT-ICR MS analysis. The results show that the molecular composition of SPE-DOM varied in a sample made by different salinity; and the variation pattern of DOM composition was different among different water samples under the treatment of consistent salinity gradients. The chloride ions can't be completely removed from cartridges in conventional SPE, thus leading organic compounds in SPE-DOM to form [M+Cl]‾ adducts when performing ESI FT-ICR MS analysis. In addition, the molecules with high H/C and low O/C ratios were likely to form [M+Cl]‾ ions. The relative abundance of [M+Cl]‾ ions could increase with the increase of salinity. These results are instructive to guide the pretreatment and molecular characterization of DOM in water samples with different salinity. Overall, we proposed a modification to the SPE to minimum reduce the formation of chloride ion adducts during the isolation of DOM.

Molecular comparison of organic matter removal from shale gas flowback wastewater: Ozonation versus Fenton process

Shale gas extraction process generates a large amount of shale gas flowback wastewater (SGFW) containing refractory organic compounds, which can pose serious environmental threats if not properly treated. However, the extremely complex compositions of organics in SGFW are still unknown and their transformation pathways in O3- and •OH-dominated systems are not well recognized, which restrain the selection of treatment technology and optimization of operational parameters. The removal characteristics and reaction mechanism of dissolved organic matter (DOM) in SGFW treated by ozonation and Fenton processes were comparatively investigated using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results showed that both processes could degrade low-oxygen highly unsaturated and phenolic organics, polyphenolics and polycyclic aromatics, and transform them into aliphatic organics and high-oxygen highly unsaturated and phenolic organics. With increasing action of reactive oxygen species (O3 for ozonation and •OH for Fenton process), the degradation products (mainly aliphatic organics) increased during ozonation. However, in Fenton process, a wider range of DOM was removed without aliphatic organics accumulation. The degradation mechanisms of DOM during ozonation and Fenton processes included oxygen addition reactions (+3O, +H2O2, and +2O) as dominant pathways. However, ozonation showed more violent oxygenation, hydroxylation, and carboxylation, while Fenton process presented more violent chain-breaking reactions. These results revealed the selective oxidation of ozone and nonselective oxidation of •OH during SGFW treatment, and provided theoretical support for selecting SGFW treatment approaches.

Molecular Characteristics and Formation Mechanisms of Unknown Ozonation Byproducts during the Treatment of Flocculated Nanofiltration Leachate Concentrates Using O3 and UV/O3 Processes

Both ozone (O3) and UV/O3 treatment processes can effectively remove organic matter in the flocculated membrane filtration concentrate from landfill leachate, but the ozonation byproducts (OBPs) generated in the processes remain unknown. Using electrospray ionization-coupled Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS), this study investigated the molecular characteristics of unknown OBPs and their formation mechanisms during the treatment of flocculated nanofiltration concentrate (FNFC) using the O3 and UV/O3 processes. The results showed that after being treated by the O3 and UV/O3 processes, the average value of the oxygen-to-carbon ratio (O/Cavg) in the FNFC organic matter increased substantially from 0.49 to 0.61-0.64 and 0.63-0.71, respectively, with an O3 dosage of 13.4-54.4 mg/min. The main OBPs were CHO and CHON compounds, which were mainly produced through oxygenation (+O2/+O3 and -H2+O2), oxidative deamination (-NH3+O2), decyclopropyl (-C3H4), and deisopropyl (-C3H6) reactions. The hydroxyl radical (•OH) can intensify these reactions, resulting in an abundance of OBPs with a high oxidation degree and low molecular weight. OBPs at five m/z values were fragmented and analyzed with tandem mass spectrometry, and abundant hydroxyl groups, carboxyl groups, and carbonyl groups were tentatively identified, presenting a potential toxicity to aquatic organisms. Due to the high molecular diversity of the OBPs in FNFC, their lower ΔGCoxo compared to natural fulvic acid, and potential toxicity, their impact on the water environment should be given more attention.

Role of pretreatment type and microbial mechanisms on enhancing volatile fatty acids production during anaerobic fermentation of refinery waste activated sludge

This study compared the effects of alkaline, thermal, thermal-peroxymonosulfate (PMS), and alkyl polyglucose (APG) pretreatments on volatile fatty acids (VFAs) production from refinery waste activated sludge (RWAS), including VFAs yield, composition, organics components, microbial communities, and the potential improvement of mechanisms. All pretreatments effectively enhanced the bioconversion of RWAS and consequently promoted the hydrolysis process, which inhibited the methanogenesis process. However, the release of lignin/carboxyl-rich alicyclic molecules (CRAM)-like compounds and tannin substances in thermal-PMS and APG groups significantly influenced the acidogenesis and acetogenesis processes. Among all pretreatments, alkaline pretreatment showed the highest VFAs yield of 95.06 mg/g volatile solids (VS) and VS removal of 17%. This result could be associated with the enrichment of functional hydrolytic-acidification bacteria, such as Planococcus and Soehngenia, and increased metabolism of amino acids, carbohydrates, and nucleotides. By considering an economical and efficient perspective, this study recommended the alkaline pretreatment for the anaerobic fermentation of RWAS.

Mass spectrometry-based metabolomics for discovering active ingredients and exploring action mechanism of herbal medicine

Natural products derived from herbal medicine are a fruitful source of lead compounds because of their structural diversity and potent bioactivities. However, despite the success of active compounds derived from herbal medicine in drug discovery, some approaches cannot effectively elucidate the overall effect and action mechanism due to their multi-component complexity. Fortunately, mass spectrometry-based metabolomics has been recognized as an effective strategy for revealing the effect and discovering active components, detailed molecular mechanisms, and multiple targets of natural products. Rapid identification of lead compounds and isolation of active components from natural products would facilitate new drug development. In this context, mass spectrometry-based metabolomics has established an integrated pharmacology framework for the discovery of bioactivity-correlated constituents, target identification, and the action mechanism of herbal medicine and natural products. High-throughput functional metabolomics techniques could be used to identify natural product structure, biological activity, efficacy mechanisms, and their mode of action on biological processes, assisting bioactive lead discovery, quality control, and accelerating discovery of novel drugs. These techniques are increasingly being developed in the era of big data and use scientific language to clarify the detailed action mechanism of herbal medicine. In this paper, the analytical characteristics and application fields of several commonly used mass spectrometers are introduced, and the application of mass spectrometry in the metabolomics of traditional Chinese medicines in recent years and its active components as well as mechanism of action are also discussed.

Machine-learning assisted molecular formula assignment to high-resolution mass spectrometry data of dissolved organic matter

High-resolution mass spectrometry (HRMS) provides molecular compositional information of dissolved organic matter (DOM) through isotopic assignment from the molecular mass. However, due to the inevitable deviation of molecular mass measurement and the limitation of resolving power, multiple possible solutions frequently occur for a given molecular mass. Lowering the mass deviation threshold and adding assignment restriction rules are often applied to exclude the incorrect solutions, which generally involves time-consuming manual post-processing of mass data. To improve the result accuracy in an automated manner, we developed a molecular formula assignment algorithm based on machine-learning technology. The method integrated a logistic regression model using manually corrected isotopic composition and the peak features of HRMS data (m/z, signal-to-noise ratio, isotope type, and number, etc.) as training data. The developed model can evaluate the correctness of a candidate formula for the given mass peak based on the peak features. The method was verified by various DOM samples FT-ICR MS data (direct infusion negative mode electrospray), achieving a ∼90% accuracy (compared to the traditional approach) for formula assignment. The method was applied to a series of NOM samples and showed a significant improvement in formula assignment compared with the mass matching method.

Transformation of dissolved organic matter at a full-scale petrochemical wastewater treatment plant

Transformation of dissolved organic matter (DOM) in petrochemical wastewater (PCW) treatment has rarely been studied. In this work, low- and high-salinity PCW were collected from a treatment plant and the transformations of DOM at molecular level along the treatment processes of both PCW were comparatively investigated. By using Orbitrap MS, the polar DOM constituents were categorized into five molecular classes namely saturated compounds, aliphatics, highly unsaturated and phenolic compounds (Huph), polyphenols and condensed polycyclic aromatics (Cpla). Aliphatics (58.62%) with low molecular weight (150-250 Da) and O/C (0-0.2) were dominant in raw low-salinity PCW; while Huph (65.03%) with O/C at 0.2-0.8 were rich in raw high-salinity PCW. After full-scale treatment, differentiated DOM constituents in both raw PCWs were transformed into aliphatics and Huph with O/C at 0.3-0.5. Anoxic/Oxic treatment of low-salinity system (L-A/O) removed a high fraction of aliphatics (53.05%); while Huph with low O/C (0.1-0.3) (65.68%) in the effluent of L-A/O were further mineralized by ozonation of low-salinity system (L-ozonation). In comparison, anoxic/oxic treatment of high-salinity system (H-A/O) mainly removed unsaturated Huph (34.10%) and aliphatics (30.86%). This resulted in a decrease of dissolved organic carbon as indicated via Spearman correlation. Different from L-ozonation, ozonation of high-salinity system (H-ozonation) degraded aliphatics (26.09%) and Huph (41.85%) with a relatively high O/C (0.2-1.2). After L-A/O and L-ozonation treatments, remaining saturated compounds that were originated from raw low-salinity PCW, were removed by subsequent biological aerated filter. Comparatively, after H-A/O and H-ozonation treatments, residual Huph and aliphatics which were mainly bio-derivates and ozonated intermediates, were further removed by air flotation filter. Hence, DOM transformation of different PCWs along similar treatments varied significantly. This study provides in-depth insights on DOM transformation along a full-scale PCW treatment process.

Speciation and molecular characterization of thiophenic and sulfide compounds in petroleum by sulfonation and methylation followed by electrospray mass spectrometry

Thiophenes and sulfides are the dominant sulfur-containing compounds in petroleum and have been widely of concern in the fields of petroleum refining and geochemistry. In this study, a novel approach was developed for selective separation and characterization of petroleum-derived thiophenic and sulfide compounds. Thiophenic compounds were selectively converted to sulfonates in the presence of vitriolic acid and can be characterized by negative ion electrospray mass spectrometry. Thiophenic sulfonates were further separated from the oil by silica chromatography and enabled the molecular characterization of sulfides in the residual oil. Various model sulfur compounds and a vacuum gas oil were used to validate the method; thiophenic and sulfide biomarker compounds in a well-documented crude oil were selectively characterized. The results indicate that the approach is feasible for molecular characterization of thiophenic and sulfide compounds, which is complementary to recently developed methods for separation and/or ionization of sulfur compounds in petroleum.

Description of Dissolved Organic Matter Transformational Networks at the Molecular Level

Dissolved Organic Matter (DOM) is an important component of the global carbon cycle. Unscrambling the structural footprint of DOM is key to understand its biogeochemical transformations at the mechanistic level. Although numerous studies have improved our knowledge of DOM chemical makeup, its three-dimensional picture remains largely unrevealed. In this work, we compare four solid phase extracted (SPE) DOM samples from three different freshwater ecosystems using high resolution mobility and ultrahigh-resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FT-ICR MS/MS). Structural families were identified based on neutral losses at the level of nominal mass using continuous accumulation of selected ions-collision induced dissociation (CASI-CID)FT-ICR MS/MS. Comparison of the structural families indicated dissimilarities in the structural footprint of this sample set. The structural family representation using Cytoscape software revealed characteristic clustering patterns among the DOM samples, thus confirming clear differences at the structural level (Only 10% is common across the four samples.). The analysis at the level of neutral loss-based functionalities suggests that hydration and carboxylation are ubiquitous transformational processes across the three ecosystems. In contrast, transformation mechanisms involving methoxy moieties may be constrained in estuarine systems due to extensive upstream lignin biodegradation. The inclusion of the isomeric content (mobility measurements at the level of chemical formula) in the structural family description suggests that additional transformation pathways and/or source variations are possible and account for the dissimilarities observed. While the structural character of more and diverse types of DOM samples needs to be assessed and added to this database, the results presented here demonstrate that Graph-DOM is a powerful tool capable of providing novel information on the DOM chemical footprint, based on structural interconnections of precursor molecules generated by fragmentation pathways and collisional cross sections.

Formation of halogenated macromolecular organics induced by Br- and I- during plasma oxidation/chlorination of DOM: Highlighting competitive mechanisms

Understanding the effects of halogens on the production of macromolecular disinfection byproducts (DBPs) is critical for drinking water safety. The effects of Br^- and I^- on the chemical diversity of dissolved organic matter (DOM) during plasma preoxidation and the subsequent formation of macromolecular halogenated DBPs after chlorination were deciphered. Plasma preoxidation changed DOM diversity from aromatic component-oriented to lignin and tannin component-oriented, resulting in 62.0% and 21.2% decreases in N-DBPs (C_kH_nO_mN_zCl_x formulas) and C-DBPs (C_kH_nO_mCl_x formulas) after chlorination, respectively. Br^- could induce the formation of organobromine compounds (OBrCs) during plasma oxidation; however, the intensities of OBrCs decreased by 56.3% (CHO formulas) and 75.2% (CHON formulas) after further chlorination. OBrCs still accounted for 79.8% of the total organohalogen compounds (OXCs, X=Cl or Br) due to the higher substitutability of bromine. I^-promoted OIC production in the DOM preoxidation process, and OICs acted as intermediates to form OClCs during chlorination. When Br^-and I^-coexisted, Br^- promoted OIC production in the DOM preoxidation process; therefore, more OBrCs and OClCs were generated due to intermediates of OICs in subsequent chlorination. Connections between OXCs and their precursors were established using network computation. The precursors of OClCs were located in the aromatic structure region (0.2 < H/C ≤ 0.7; O/C ≤ 0.67); those of OBrCs and OICs were located in the lignin (0.7 < H/C ≤ 1.5; 0.1 < O/C < 0.67) and tannin (0.6 ≤ H/C ≤ 1.5, 0.67 < O/C < 1.0) regions with relatively greater H/C and O/C ratios, respectively.

Transformation of dissolved organic matter in landfill leachate during a membrane bioreactor treatment

In this study, a cutting-edge mass spectrometry (MS) technique, Orbitrap fusion MS with ultrahigh resolution, was used to analyze the molecular composition, chemical properties, formation mechanism, and environmental impact of refractory dissolved organic matter (rDOM) in leachate. The results showed that the bioavailable DOM (bDOM) and rDOM constituents varied substantially during the biological treatment of landfill leachate. Compared with bDOM, the rDOM in leachate had a higher degree of unsaturation, aromaticity, and oxidation, and a larger molecular weight, and contained more organic matter with benzene ring and biphenyl structures. Using high-throughput 16S rRNA sequencing, metagenomics, the Kendrick mass defect (KMD), and a mass difference network (MDiN), it was found that rDOM in leachate is generated through carboxylation (+COO), dehydro-oligomerization (-H₂), and chain scission (-CH₂) pathways due to the activity of microbes such as Patescibacteria, Chloroflexi, and Proteobacteria. Compared with Suwannee River fulvic acid (SRFA), the rDOM in leachate contained more organics with nitrogen, sulfur, benzene rings, and biphenyls. If the rDOM in leachate enters the environment it will affect the composition of the original organic matter, and its biogeochemical transformation and environmental fate will then need to be monitored and may require special attention.

Changes in light absorption and composition of chromophoric marine-dissolved organic matter across a microbial bloom

Marine chromophoric dissolved organic matter (m-CDOM) mediates many vital photochemical processes at the ocean's surface. Isolating m-CDOM within the chemical complexity of marine dissolved organic matter has remained an analytical challenge. The SeaSCAPE campaign, a large-scale mesocosm experiment, provided a unique opportunity to probe the in situ production of m-CDOM across phytoplankton and microbial blooms. Results from mass spectrometry coupled with UV-VIS spectroscopy reveal production of a chemodiverse set of compounds well-correlated with increases in absorbance after a bacterial bloom, indicative of autochthonous m-CDOM production. Notably, many of the absorbing compounds were found to be enriched in nitrogen, which may be essential to chromophore function. From these results, quinoids, porphyrins, flavones, and amide-like compounds were identified via structural analysis and may serve as important photosensitizers in the marine boundary layer. Overall, this study demonstrates a step forward in identifying and characterizing m-CDOM using temporal mesocosm data and integrated UV-VIS spectroscopy and mass spectrometry analyses.

Photo-Reactivity and Photo-Transformation of Algal Dissolved Organic Matter Unraveled by Optical Spectroscopy and High-Resolution Mass Spectrometry Analysis

The rapid proliferation of planktonic algae induced by eutrophication and climate warming make algae dissolved organic matter (AOM) an important source of dissolved organic matter (DOM) in surface waters, but the understanding of the link between AOM composition and photo-reactivity/photo-transformation of DOM in aquatic systems is limited. Here, intracellular organic matter (IOM) from Microcystis aeruginosa was extracted and subjected to molecular weight (MW) fractionation. Results indicated that IOM had lower aromaticity and higher photosensitive activity compared to Suwannee River fulvic acid (SRFA). The photosensitive activity of IOM relied on both its molecular weight distribution and fluorescence components. The IOM fraction with the highest MW proteins had the lowest quantum yields of reactive intermediates (Φ_RIs), which increased with the decrease of MW, while the fractions with more low-excitation tyrosine-like components had relatively higher Φ_RIs. Parallel factor analysis and high-resolution mass spectrometry revealed that light radiation of IOM resulted in the composition transformation from tryptophan-like and tyrosine-like components to humic-like components, forming less aromatic and more saturated recalcitrant dissolved organic carbon. Our findings provide new insights into the photo-reactivity and photo-transformation of algae-derived organic matters and help to predict DOM formation involved in carbon cycling in water environment.

Coupled variations of dissolved organic matter distribution and iron (oxyhydr)oxides transformation: Effects on the kinetics of uranium adsorption and desorption

The interactions between dissolved organic matter (DOM) molecules and minerals play significant roles in affecting the fate of carbon and contaminants in soil environment. However, the mechanisms controlling the variations of DOM molecules distribution during the transformation of Fe (oxyhydr)oxides, and the effects of these variations on contaminant behaviors are still largely unknown. In this study, the dynamic variations of DOM properties and distributions, and the kinetics of uranium adsorption on and desorption from Fe (oxyhydr)oxides during the transformation were investigated, employing a combination of Orbitrap mass spectrometry (MS), high-resolution transmission electron microscopy (HR-TEM), and kinetic experiments. Orbitrap MS results indicated that aliphatic molecules and phenolic and polyphenolic molecules with lower O/C values were preferentially released to solution. HR-TEM results indicated that the coprecipitated DOM molecules by ferrihydrite were mainly released to solution rather than sorbed on the newly formed lepidocrocite or goethite during the transformation. Furthermore, the stirred-flow experiment results suggested that soil DOM significantly reduced the adsorption of uranium on, and accelerated the release of uranium from Fe (oxyhydr)oxides, which was ascribed to the changed distribution of DOM molecules and the structure and composition of Fe (oxyhydr)oxides. Our results contribute to predicting contaminant behaviors in soils.

Effect of biochar amendment on organic matter and dissolved organic matter composition of agricultural soils from a two-year field experiment

Dissolved organic matter (DOM) is an important organic matter fraction that plays a key role in many biological and chemical processes in soil. The effect of biochar addition on the content and composition of soil organic matter (SOM) and DOM in an agricultural soil in Italy was investigated within a two-year period. UV-Vis spectroscopy and analytical pyrolysis have been applied to study complex components in DOM soil samples. Additionally, analytical pyrolysis was used to provide qualitative information of SOM at molecular level and the properties of biochar before and one year after amendment. A method was developed to quantify biochar levels by thermogravimetric analysis that enabled to identify deviations from the amendment rate. The water-soluble organic carbon (WSOC) concentrations in the amended soils were significantly lower than those in the control soils, indicating that biochar decreased the leaching of DOM. DOM in treated soils was characterized by a higher aromatic character according to analytical pyrolysis and UV-Vis spectroscopy. Moreover, a relatively high abundance of compounds with N was observed in pyrolysates of treated soils, suggesting that biochar increased the proportion of microbial DOM. The results from thermal and spectroscopy techniques are consistent in highlighting significant changes in DOM levels and composition due to biochar application with important effects on soil carbon storage and cycling.

Changes in molecular dissolved organic matter and disinfection by-product formation during granular activated carbon filtration by unknown screening analysis with Orbitrap mass spectrometry

The minimization of disinfection by-product (DBP) formation by the removal of its precursors before water disinfection is a highly effective approach. Granular activated carbon (GAC) filtration is widely used for water treatment, but our understanding of molecular dissolved organic matter (DOM) remains insufficient. This research investigates the removal of DOM and the minimization of DBP formation by pilot-scale coal- and coconut-based granular activated carbon filtrations (coAC and ccAC, respectively) using unknown screening analysis with Orbitrap mass spectrometry. DOM adsorption rates by both GACs were fitted with pseudo-second order models with initial adsorption rates of 0.005 mg g^-1 min^-1 and 0.022 mg g^-1 min^-1 for ccAC and coAC, respectively. Based on observations, ccAC was more effective in the removal of dissolved organic carbon and prolonged adsorption longer than coAC, as the breakthrough of coAC was found on Day 10. ccAC removed compounds with carbon, hydrogen, and oxygen (CHO features) with a wide range of oxidation states, as indicated by the carbon oxidation state (C_os), and a wide range of unsaturation, as indicated by oxygen subtracted double bond equivalent per carbon ([DBE-O]/C), while coAC selectively removed only those CHO features with less oxidized characters. Less oxidized compounds (low C_os) were preferentially removed with less contact time, while more oxidized compounds needed more contact time to adsorb on the GACs. A biofilm was developed on Day 60, and many CHO features were found to have increased after GAC treatment on Day 60, indicating the formation of microbial products. Chlorination resulted in a decrease in many CHO and CHO with Cl atom (CHOCl) features and the formation of CHOCl DBPs more than CHO DBP features. ccAC was effective in the minimization of trihalomethane (THM) and CHOCl DBP feature formations on Day 10 and Day 60, while coAC was found to be much less effective.

Unsupervised Structural Classification of Dissolved Organic Matter Based on Fragmentation Pathways

Dissolved organic matter (DOM) is considered an essential component of the Earth's ecological and biogeochemical processes. Structural information of DOM components at the molecular level remains one of the most extraordinary analytical challenges. Advances in determination of chemical formulas from the molecular studies of DOM have provided limited indications on structural signatures and potential reaction pathways. In this work, we extend the structural characterization of a wetland DOM sample using precursor and fragment molecular ions obtained by a sequential electrospray ionization-Fourier transform-ion cyclotron resonance tandem mass spectrometry (ESI-FT-ICR CASI-CID MS/MS) approach. The DOM chemical complexity resulted in near 900 precursors (P) and 24 000 fragment (F) molecular ions over a small m/z 261-477 range. The DOM structural content was dissected into families of structurally connected precursors based on neutral mass loss patterns (P_n-1 + F_1:n + C) across the two-dimensional (2D) MS/MS space. This workflow identified over 1900 structural families of DOM compounds based on a precursor and neutral loss (H₂O, CH₄O, and CO₂). The inspection of structural families showed a high degree of isomeric content (numerous identical fragmentation pathways), not discriminable with sole precursor ion analysis. The connectivity map of structural families allows for the visualization of potential biogeochemical processes that DOM undergoes throughout its lifetime. This study illustrates that integrating effective computational tools on a comprehensive high-resolution mass fragmentation strategy further enables the DOM structural characterization.

Characterizing the Chemical Landscape in Commercial E-Cigarette Liquids and Aerosols by Liquid Chromatography-High-Resolution Mass Spectrometry

The surge in electronic cigarette (e-cig) use in recent years has raised questions on chemical exposures that may result from vaping. Previous studies have focused on measuring known toxicants, particularly those present in traditional cigarettes, while fewer have investigated unknown compounds and transformation products formed during the vaping process in these diverse and constantly evolving products. The primary aim of this work was to apply liquid chromatography-high-resolution mass spectrometry (LC-HRMS) and chemical fingerprinting techniques for the characterization of e-liquids and aerosols from a selection of popular e-cig products. We conducted nontarget and quantitative analyses of tobacco-flavored e-liquids and aerosols generated using four popular e-cig products: one disposable, two pod, and one tank/mod. Aerosols were collected using a condensation device and analyzed in solution alongside e-liquids by LC-HRMS. The number of compounds detected increased from e-liquids to aerosols in three of four commercial products, as did the proportion of condensed-hydrocarbon-like compounds, associated with combustion. Kendrick mass defect analysis suggested that some of the additional compounds detected in aerosols belonged to homologous series resulting from decomposition of high-molecular-weight compounds during vaping. Lipids in inhalable aerosols have been associated with severe respiratory effects, and lipid-like compounds were observed in aerosols as well as e-liquids analyzed. Six potentially hazardous additives and contaminants, including the industrial chemical tributylphosphine oxide and the stimulant caffeine, were identified and quantified in the e-cig liquids and aerosols analyzed. The obtained findings demonstrate the potential of nontarget LC-HRMS to identify previously unknown compounds and compound classes in e-cig liquids and aerosols, which is critical for the assessment of chemical exposures resulting from vaping.

Laboratory In-Situ Production of Autochthonous and Allochthonous Fluorescent Organic Matter by Freshwater Bacteria

This work investigates the origin and range of fluorescent organic matter (FOM) produced in-situ by environmentally sourced freshwater bacteria. Aquatic FOM is an essential component in global carbon cycling and is generally classified as either autochthonous, produced in-situ via microbial processes, or allochthonous, transported into aquatic systems from external sources. We have demonstrated that, within laboratory model systems, environmentally sourced mixed microbial communities and bacterial isolates can produce and/or export FOM associated with both autochthonous and allochthonous material. This study focuses on fluorescence peak B, T, M, C and C+, exploring (1) the cellular nature of FOM produced, (2) FOM exported as extracellular material into the water column and (3) the impact of physical cell lysis on FOM signature. For the laboratory model systems studied, Peak T fluorescence is retained within bacterial cells (>68%), while Peak C fluorescence is mainly observed as extracellular material (>80%). Peak M is identified as both cellular and extracellular FOM, produced by all isolated freshwater microorganisms investigated. The origin of Peak C+ is postulated to originate from functional metabolites associated with specific microorganisms, seen specifically within the Pseudomonas sp. monoculture here. This work challenges the binary classification of FOM as either allochthonous or autochthonous, suggesting that FOM processing and production occurs along a dynamic continuum. Within this study, fluorescence intensity data for the environmental bacteria isolate monocultures are presented as enumeration corrected data, for the first time providing quantitative fluorescence data per bacterial colony forming unit (cfu). From this, we are able to assess the relative contribution of different bacteria to the autochthonous FOM pool and if this material is cellular or extracellular.

Diacylglycerols ions as novel marker indicators for the classification of edible oils using ultrahigh resolution mass spectrometry

Diacylglycerols (DAGs) ions, instead of triacylglycerols (TAGs) ions, were established as marker indicators for an improved classification of edible oils using ultrahigh resolution mass spectrometry (UHRMS). DAGs ions can be used not only to identify triacylglycerols (TAGs) and their embedded fatty acids (FAs), but also to distinguish positional isomers of TAGs. In this work, DAGs ions were determined in edible oils by direct infusion atmospheric pressure chemical ionization-ultrahigh resolution mass spectrometry (APCI-UHRMS), where the ultrahigh resolving power up to 500,000 FWHM (full width at half maximum) can provide accurate molecular compositions and detailed fingerprints MS spectra in a minute. A total of 146 samples belonging to 22 species of plant oils and animal fats, were characterized. Chemometric analyses were performed using principal component analysis, partial least square-discriminant analysis and orthogonal partial least squares-discriminant analysis. DAGs ions were proved to be better than TAGs ions as marker indicators in the chemometric analyses. An overall correct rate of 93.40% was achieved for the classification of tested samples. In addition, blend oils and gutter oils were also characterized by this developed method.

Unraveling roles of dissolved organic matter in high arsenic groundwater based on molecular and optical signatures

Dissolved organic matter (DOM) is a crucial controlling factor in mobilizing arsenic. However, direct delineations of DOM regarding both optical properties and molecular signatures were rarely conducted in high-arsenic groundwater. Here, both groundwater and surface water were taken from the Hetao Basin, China, to decipher DOM properties with both optical spectrophotometer and Fourier transform ion cyclotron resonance mass spectrometry. The tryptophan-like component (C4) was averagely less than 30% in groundwater DOM, being positively associated with high H/C-ratio molecules (H/C > 1.2) and mainly grouped as highly unsaturated and phenolic compounds and aliphatic compounds. Other three humic-like components (C1, C2, C3) had positive associations with low H/C-ratio molecules (H/C < 1.2), which mainly consisted of highly unsaturated and phenolic compounds, polyphenols, and polycyclic aromatics. Groundwater arsenic concentrations were positively correlated with humic-like, low H/C-ratio, and recalcitrant organic compounds, which may be the consequence of labile organic matter degradation. The degradation caused Fe(III) oxide reduction and mobilized the solid arsenic. In addition, high abundances of these recalcitrant organic compounds in high-arsenic groundwater may contribute to arsenic enrichment via electron shuttling, competition for surface sites, and complexation process. It suggested that groundwater proxies would be either the result or the cause of biogeochemical processes in aquifers.

Molecular dissolved organic matter removal by cotton-based adsorbents and characterization using high-resolution mass spectrometry

This research investigates the characteristics of dissolved organic matter (DOM) removal by synthesized cotton-fiber adsorbents using unknown screening analysis with high resolution and accurate mass spectrometry. Molecular characteristics of DOM removed by adsorbents were investigated semiquantitatively and unknown disinfection byproduct (DBP) formation potentials were also investigated. Adsorbents were modified using ferric nitrate to increase the magnetic property. The XRD pattern showed Fe-containing crystalline structures in the modified adsorbent (M-CF). The M-CF possessed higher mesopore volume, which enhanced the dissolved organic carbon (DOC) removal efficiency to 74.50% (compared to 32.12% in the unmodified CF adsorbent). The kinetics experiment showed that both adsorbents were better fitted to pseudo-second orders than pseudo-first orders. The initial rate constant was higher in M-CF (1.40 mg/g min) than in CF (0.02 mg/g min) treatments due to the higher mesopore volume in M-CF. M-CF removed almost 700 carbon‑hydrogen‑oxygen based DOMs (CHO features), 300 more CHO features than CF. CF selectively adsorbed only higher-molecular-weight (MW) CHO features (more CH₂ groups), while the mesopores in M-CF removed DOM with lower MW (fewer CH₂ groups) that were refractory to CF. The low MW DOM removed only by M-CF mesopore exhibited more oxidized (positive carbon oxidation state, C_os) and saturated characters (negative oxygen-subtracted double bond equivalent per carbon, (DBE-O)/C). After chlorination, over 50 unknown DBPs were detected, 33 of which were commonly found in all samples. M-CF decreased unknown formation potential more than CF. However, adsorption of M-CF and CF before chlorination resulted in different remaining precursors to water chlorination and formed unique DBPs from those precursors.

Characterization of lower Phong river dissolved organic matters and formations of unknown chlorine dioxide and chlorine disinfection by-products by Orbitrap mass spectrometry

Dissolved organic matter (DOM) have been reported as precursors of disinfection byproducts (DBPs) and its molecular characteristics are rarely investigated due to its complexity. In this study, changes in the characteristics of DOM were investigated in the lower Phong River in Thailand in dry season and after the first rain in rainy season, using a non-targeted analysis with Orbitrap mass spectrometry. The river was rich with CHO features dominated by lignin-like molecules, while lipid-like molecules increase after domestic wastewater discharges. Wastewater discharge released DOM with higher molecular weight (MW) that was less oxygenated (low O/C) and less oxidized (low carbon oxidation state [C_os]). A lake affected by anthropogenic activities contributed more oxidized DOM into the river, while surface runoff carried DOM that is more oxygenated (high O/C), less hydrogenated (low H/C), and more oxidized (high C_os) to the stream. Water treatment further modified DOM to be lower MW. Approximately three hundred Cl-containing features (CHOCl) detected upstream were also found downstream. Disinfection by chlorine (Cl₂) or chlorine dioxide (ClO₂) formed both CHO and CHOCl DBPs. Low chlorine dosage applied to upstream and downstream samples resulted in many common unknown DBPs while increasing chlorine dosage resulted in more unique DBPs. At the same dosage, Cl₂ reacted with DOM more than ClO₂, including more oxidized molecules that are refractory to ClO₂. Both Cl₂ and ClO₂ produced chlorinated and non-chlorinated DBPs, and some DBPs were commonly found by both disinfections_. Cl₂-produced DBPs were more unsaturated (higher [DBE-O]/C) and oxidized (higher C_os) than ClO₂-DBPs.

Changes in dissolved organic matter during water treatment by sequential solid-phase extraction and unknown screening analysis

Isolation of complex dissolved organic matter (DOM) from environmental water is a major challenge for unknown screening analysis by high-resolution mass spectrometry. In this study, DOM in process water during advanced drinking water treatment was fractionated sequentially by three solid-phase extraction (SPE) cartridges based on the polarity and charge of DOM molecules. By sequential SPE with unknown screening analysis, over 3000 DOM features were found in raw water, whereas around 2000 were obtained by a single SPE. The hydrophobic neutral (HPON) fraction contained CHO features with highest averaged molecular weight followed by hydrophobic acid (HPOA) and then hydrophilic acid (HPIA). The average degree of carbon double bond equivalents and carbon oxidation states indicated that the HPON fraction contained molecules that were more unsaturated and less oxidized than those of the HPOA and HPIA fractions. Ozone selectively decomposed (1) more unsaturated and less oxidized HPON features, (2) more unsaturated HPOA compounds, and (3) less oxidized HPIA molecules. Oxidation by-products were mostly HPON and HPIA compounds that were more oxidized than the decomposed molecules. During biological activated carbon (BAC) filtration, less oxidized HPON were preferentially removed, whereas HPOA were removed without selectivity. HPON and HPIA molecules with more oxidized character were found to be refractory to BAC treatment. HPON with more unsaturated and HPIA with more oxidized characters were decomposed by chlorine. Many types of HPIA decomposed during chlorination were the oxidation by-products of ozonation that were refractory to BAC treatment. Sequential SPE with unknown screening analysis provided previously unknown details of the molecular characteristics of DOM and its changes during advanced water treatment.

Characterizing changes of dissolved organic matter composition with the use of distinct feeds in recirculating aquaculture systems via high-resolution mass spectrometry

Recirculating aquaculture systems (RAS) are a new alternative to traditional aquaculture approaches, allowing full control over the fish production conditions, while reducing the water demand. The reduction of water exchange leads to an accumulation of dissolved organic matter (DOM) that can have potential effects on water quality, fish welfare and system performance. Despite the growing awareness of DOM in aquaculture, scarce scientific information exists for understanding the composition and transformation of DOM in RAS. In this study, a non-targeted approach using ultra-performance liquid chromatography coupled to a hybrid quadrupole-time of flight mass spectrometer (UPLC-QTOF-MS) was used to characterize compositional changes of low molecular weight (LMW) DOM in RAS, when operated under two different feed types. A total of 1823 chemicals were identified and the majority of those contained a CHON chemical group in their structure. Changes in the composition of LMW-DOM in RAS waters were observed when the standard feed was switched to RAS feed. The DOM with the use of standard feed, consisted mainly of lignin/CRAM-like, CHO and CHOS chemical groups, while the DOM that used RAS feed, was mainly composed by unsaturated hydrocarbon, CHNO and CHNOS chemical groups. The Bray-Curtis dissimilarity cluster demonstrated differences in the composition of DOM from RAS and was associated to the type of feed used. When the RAS feed was used, the Kendrick mass defect plots of -CH₂- homologous units in the pump-sump (after the water treatment) showed a high removal capacity for CHNO, CHNOS and halogenated chemicals with high Kendrick mass defect, KMD > 0.7. To our knowledge, this is the first report of LMW-DOM characterization of RAS by high-resolution mass spectrometry (HRMS).

Source identification and component characterization of dissolved organic matter in an acid mine drainage reservoir

Acid mine drainage (AMD) is one of the most serious environmental problems and extreme environments on the earth, with high concentrations of sulphate and dissolved metals. A comprehensive description of dissolved organic matter (DOM) in these reservoirs is lacking, and it can play an important role in AMD pollution treatment and ecosystem. Thus, the source, composition and property of DOM in an AMD reservoir in Ma'an shan, China were studied using Fourier transform ion cyclotron resonance mass spectrometry and three-dimension excitation emission matrix fluorescence spectroscopy. The results suggested that the autochthonous algal metabolites significantly contributed to the DOM pool in the AMD reservoir. Bioavailable substances with lower oxidation, unsaturation and aromaticity such as lipids and carbohydrates were lacking in the AMD reservoir especially in the deeper layers. In addition, the proportion of sulfur compounds was significantly higher than that in other waters, suggesting the potential formation of organic matter with sulfur atom in a sulfur-rich environment. These findings underscore that the investigation of DOM in AMD reservoirs may offer references for the AMD treatment with addition of organic matter and broaden the understanding of special carbon cycling in the extreme environment of AMD.

Long-term biochar addition alters the characteristics but not the chlorine reactivity of soil-derived dissolved organic matter

Biochar is widely and increasingly applied to farmlands. However, it remains unclear how long-term biochar addition alters the characteristics and chlorine reactivity of soil-derived dissolved organic matter (DOM), an important terrestrial disinfection byproduct (DBP) precursor in watersheds. Here, we analyzed the spectroscopic and molecular-level characteristics of soil-derived DOM and the formation and toxicity of DBP mixtures from DOM chlorination for two long-term (5 and 11 years) biochar addition experimental farmlands. As indicated by spectroscopic indices and Fourier transform ion cyclotron resonance mass spectrometry analyses, 11 years of biochar addition could increase the humic-like and aromatic and condensed aromatic DOM and decrease the microbial-derived DOM, while 5 years of biochar addition at the other site did not. The response of condensed aromatic dissolved black carbon did not increase with increasing cumulative biochar dose but appeared to be affected by biochar aging time. Despite the possible increase in aromatic DOM, biochar addition neither increased the reactivity of DOM in forming trihalomethanes, haloacetonitriles, chloral hydrates, or haloketones nor significantly increased the microtoxicity or genotoxicity of the DBP mixture. This study indicates that biochar addition in watersheds may not deteriorate the drinking water quality via the export of terrestrial DBP precursors like wildfire events.

Molecular investigation into the transformation of dissolved organic matter in mature landfill leachate during treatment in a combined membrane bioreactor-reverse osmosis process

This study investigated the effectiveness of a combined membrane bioreactor (MBR) and reverse osmosis (RO) process for treating leachate produced by a large-scale anaerobic landfill. The MBR process had limited treatment efficiency for removing organic pollutants, but when combined with RO, the integrated system completely removed macromolecular compounds (i.e., humic- and fulvic-like substances) and produced effluent that satisfied the applicable discharge standard. The landfill leachate contained many types of DOM that had high molecular weight and were highly unsaturated. Although the MBR process removed some DOM that had a relatively low saturated degree (mainly aliphatic compounds (2.0 ≥ H/C ≥ 1.5) with relatively high bioavailability), many bio-refractory compounds were not removed. The RO system greatly reduced the content of residual DOM in MBR effluent and was effective for removing heteroatom DOM, especially polycyclic aromatics (AI > 0.66) and polyphenols (0.66 ≥ AI > 0.50). The effluent from the combined process of MBR and RO treatment mainly contained a small number of aliphatic compounds and phenolic compounds (AI ≤ 0.50 and H/C < 1.5) that had higher bioavailability than DOM in the raw leachate and posed little environmental risk.

Structural Characterization of Dissolved Organic Matter at the Chemical Formula Level Using TIMS-FT-ICR MS/MS

TIMS-FT-ICR MS is an important alternative to study the isomeric diversity and elemental composition of complex mixtures. While the chemical structure of many compounds in the dissolved organic matter (DOM) remains largely unknown, the high structural diversity has been described at the molecular level using chemical formulas. In this study, we further push the boundaries of TIMS-FT-ICR MS by performing chemical formula-based ion mobility and tandem MS analysis for the structural characterization of DOM. The workflow described is capable to mobility select (R ∼ 100) and isolate molecular ion signals (Δm/z = 0.036) in the ICR cell, using single-shot ejections after broadband ejections and MS/MS based on sustained off-resonance irradiation collision-induced dissociation (SORI-CID). The workflow results are compared to alternative TIMS-q-FT-ICR MS/MS experiments with quadrupole isolation at nominal mass (∼1 Da). The technology is demonstrated with isomeric and isobaric mixtures (e.g., 4-methoxy-1-naphthoic acid, 2-methoxy-1-naphthoic acid, decanedioic acid) and applied to the characterization of DOM. The application of this new methodology to the analysis of a DOM is illustrated by the isolation of the molecular ion [C₁₈H₁₈O₁₀-H]^- in the presence of other isobars at nominal mass 393. Five IMS bands were assigned to the heterogeneous ion mobility profile of [C₁₈H₁₈O₁₀-H]^-, and candidate structures from the PubChem database were screened based on their ion mobility and the MS/MS matching score. This approach overcomes traditional challenges associated with the similarity of fragmentation patterns of DOM samples (e.g., common neutral losses of H₂O, CO₂, and CH₂-H₂O) by narrowing down the isomeric candidate structures using the mobility domain.

Molecular transformation of dissolved organic matter in refinery wastewater

Dissolved organic matter (DOM) has an important impact on the water treatment and reuse of petroleum refinery wastewater. In order to improve the treatment efficiency, it is necessary to understand the chemical composition of the DOM in the treatment processes. In this paper, the molecular composition of DOM in wastewater samples from a representative refinery were characterized. The transformation of various compounds along the wastewater treatment processes was investigated. A total of 61 heteroatomic class species were detected from the DOM extracts, in which CHO (molecules composed of carbon, hydrogen, and oxygen atoms) and CHOS (CHO molecules that also contained sulfur) class species were the most abundant and account for 78.43% in relative mass peak abundance. The solid phase extraction DOM from the dichloromethane unextractable fraction exhibited a more complex molecular composition and contained more oxygen atoms than in the dichloromethane extract. During wastewater treatment processes, the chemical oxygen demand (COD) and ammonia-nitrogen were reduced by more than 90%. Volatile organic compounds (VOCs) accounted for about 30% of the total COD, in which benzene and toluene were dominant. After biochemical treatment, the VOCs were effectively removed but the molecular diversity of the DOM was increased and new compounds were generated. Sulfur-containing class species were more recalcitrant to biodegradation, so the origin and transformation of these compounds should be the subject of further research.

In-House Standard Method for Molecular Characterization of Dissolved Organic Matter by FT-ICR Mass Spectrometry

Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely used for molecular characterization of dissolved organic matter (DOM). However, ESI FT-ICR MS generally has poor repeatability and reproducibility because of its inherent ionization mechanism and structural characteristics, which severely hindered its application in quantitative analysis of complex mixtures. In this article, we developed an in-house standard method for molecular characterization of DOM by ESI FT-ICR MS. Instead of obtaining reproducible results by determining the instrument parameters, we adopted an approach of object control on the mass spectrum to solve the problem of poor reproducibility. The mass peak shape, resolution, and relative intensity distribution of a natural organic matter standard were adjusted by optimizing the operating conditions to obtain a repeatable result. The quality control sample was run 26 times by the different operators in a 6-month-long period to evaluate the reproducibility. Results showed that the relative standard deviation (%) of repeatability and reproducibility are 1.02 and 2.35 for average H/C, respectively. The in-house standard method has been validated and successfully used for the characterization of more than 4000 DOM samples, which is transferable to other laboratories.