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Environmental Processes
Facet-Mediated Adsorption and Molecular Fractionation of Humic Substances on Hematite Surface Jitao Lv, Yuexia Miao, Zaoquan Huang, Ruixia Han, and Shuzhen Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03940 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018
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Facet-Mediated Adsorption and Molecular Fractionation of Humic Substances
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on Hematite Surfaces
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Jitao Lv a, Yuexia Miao a, b, Zaoquan Huang a, b, Ruixia Han a, b, Shuzhen Zhang a, b *
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a
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Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing
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100085, China
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b
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research
University of the Chinese Academy of Sciences, Beijing 100049, China
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ABSTRACT
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Interactions between dissolved organic matter (DOM) and iron oxyhydroxides
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have important environmental and geochemical implications. The present study
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employed two hematite nanocrystals to investigate the adsorption and molecular
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fractionation of two typical humic substances (HSs) using electrospray ionization
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coupled with Fourier-transform ion cyclotron resonance mass spectrometry
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(ESI-FT-ICR-MS). Hematite with a predominant exposure of {100} facets induced
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more pronounced adsorption and molecular fractionation of HSs than {001} facets,
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indicating that the interfacial adsorptive fractionation process of HSs was mediated by
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exposed facets of hematite. Further exploration of the surface OH groups of the two
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hematite nanocrystals confirms that the facet-mediated molecular fractionation of HSs
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was attributable to the abundance of singly iron-atom coordinated -OH sites on the
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hematite surfaces. Molecules with a high oxidation state and high aromaticity such as
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oxidized black carbon, polyphenol- and tannic-like compounds preferentially formed
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ligand-exchange complexes with singly-coordinated -OH groups on the hematite
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surfaces, inducing the selective binding and molecular fractionation of HSs at the
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mineral-water interface. These results demonstrate that singly iron-atom coordinated
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-OH sites determine DOM adsorption and mediate molecular fractionation on
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hematite surfaces and this contributes substantially to our understanding of the
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molecular mechanisms of iron oxyhydroxide-mediated molecular exchange of DOM
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in soils and/or sediments.
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Keywords: adsorption, crystal facets, fractionation, ESI-FT-ICR-MS, hematite,
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humic substances, surface hydroxyl groups
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INTRODUCTION
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Dissolved organic matter (DOM) is the most mobile and active organic matter
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fraction and therefore plays a major role in organic carbon dynamics in aquatic and
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terrestrial ecosystems. It also affects the solubility, mobility and bioavailability of a
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wide range of substances of environmental concern including nutrients, metals and
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organic pollutants in the environment.1-4 On the other hand, most of the DOM
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components including polycarboxylic acids, polyphenols and lignins in soils or
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sediments are bound to minerals and form organic-mineral associations via a myriad
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of interactions that are critically important for the long-term stabilization of organic
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carbon.5-8
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Iron-containing minerals are among the most important geological sorbents of
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DOM and retain large amounts of soil organic matter.9-12 There are various types of
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iron-containing minerals in nature including well-crystallized and poorly crystalline
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minerals,
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affinity toward DOM, and therefore act as a major contributor to the adsorption of
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organic matter in soils and sediments.9, 10, 14 This is generally ascribed to their high
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surface area and small particle size.12,
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determining the behavior of DOM in the environment, considerable efforts have been
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made to understand the binding mechanisms between DOM and iron-containing
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minerals such as various iron oxyhydroxides.
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complexation between the carboxyl/hydroxyl moieties of DOM and iron oxides is
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considered to be the dominant binding mechanism based on spectroscopic analysis
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such as Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (NMR)
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and surface complexation models such as the charge distribution multi-site
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complexation (CD-MUSIC) and ligand charge distribution (LCD) models.
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detail, the surface species of iron oxyhydroxides, for example goethite, are assumed to
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among which amorphous iron or reactive iron phases display the highest
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Because of their important role in
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The ligand exchange and surface
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be composed of singly, doubly and triply coordinated hydroxyl groups according to
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their protonation behavior,19, 21 and they are considered to be the specific adsorptive
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sites for DOM on iron oxyhydroxide surfaces.8, 22 Using the CD-MUSIC model, Fujii
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et al. further described the adsorption mechanism of fulvic acid (FA) on goethite
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surfaces.19 They found that the carboxylic groups of FA were bound as inner sphere
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complexes with singly coordinated OH groups or as outer-sphere complexes with
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both singly and triply coordinated OH groups.19 Unfortunately, investigation of
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binding mechanisms between DOM and iron oxyhydroxide was performed only on
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irregular particles and the descriptions of the adsorption sites are largely based on the
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surface complexation model but lacking experimental evidence. Mineral surfaces with
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different properties display particular affinities for specific functional moieties of
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DOM due to different binding mechanisms such as electrostatic interaction,
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hydrophobic interaction, hydrogen bonding, and ligand exchange.17-18 Using
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ambiguous surface structure greatly limits our understanding of the intrinsic
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mechanisms on how the iron oxyhydroxide surface structure determines DOM
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adsorption.
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In recent decades numerous studies have provided evidence confirming that DOM,
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especially humic substances, comprise a super-molecular association of small
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molecules self-assembled by hydrophobic interactions and hydrogen bonds.23,
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According to this concept, adsorption on minerals will induce molecular fractionation
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of DOM at the mineral-water interface.25, 26 This process can be evidenced from the
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changes in the molecular formulas of DOM after its adsorption on minerals. In a
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recent study we investigated the molecular fractionation of DOM induced by its
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adsorption on three iron oxyhydroxides (ferrihydrite, goethite and lepidocrocite). The
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results indicate that amorphous ferrihydrite induced clear molecular fractionation of
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DOM at the mineral-water surface, whereas molecular fractionation of DOM at the 4
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surfaces of goethite and lepidocrocite was equivocal.26 It was expected that the
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differences in DOM molecular fractionation on the three iron oxyhydroxides would
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be derived from their differences in crystalline phase and surface structure. However,
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even for the same crystalline phase, for example hematite, the structures of different
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exposed surfaces are distinct.27, 28 In order to explain the mechanisms behind the
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selective adsorption and molecular fractionation of DOM on mineral-water interfaces,
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a priority strategy is to use iron oxyhydroxides with well-defined crystalline
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architectures to clarify the facet-selective adsorption and molecular fractionation of
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DOM on iron oxyhydroxides.
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Two architectural hematite nanocrystals with different exposed facets were
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therefore synthesized in the current study. The interfacial adsorption mechanisms of
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typical humic substances (HSs) on the two hematite nanocrystals were investigated
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using electrospray ionization coupled with Fourier-transform ion cyclotron resonance
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mass spectrometry (ESI-FT-ICR-MS) analysis. The facet-mediated molecular
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fractionation of HSs on hematite surfaces was further explored. The results of this
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study will advance our understanding of the intrinsic interfacial mechanisms
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determining the adsorption and molecular fractionation of DOM on iron
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oxyhydroxides.
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MATERIALS AND METHODS
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Materials. All reagents used for hematite synthesis were of analytical grade, were
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purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, and were
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used without further purification. Ultrapure water was used throughout the
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experiments. Suwannee River Fulvic Acid Standard II (SRFA, 2S101F) and
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Suwannee River Humic Acid Standard II (SRHA, 2S101H) were obtained from the
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International Humic Substances Society (IHSS). 5
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Synthesis, characterization and structural models of hematite nanocrystals.
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Hexagonal plate- and rod-like hematite (-Fe2O3) nanocrystals were synthesized by
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previously reported solvothermal methods.
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described in detail in the SI. The powder X-ray diffraction (XRD) patterns of the
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samples were recorded with an X’ Pert Pro Multipurpose Diffractometer (MPD, Cu
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Kα radiation, PANalytical, Almelo, Netherlands), operating at 30 mA and 40 kV. Field
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emission scanning electron microscopy (FESEM) images were obtained with a
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JSM-6700. Transmission electron microscopy (TEM) images, high-resolution
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transmission electron microscopy (HRTEM) images and selected-area electron
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diffraction (SAED) patterns were recorded with an FEI Tecnai F20 TEM (Thermo
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Fisher Scientific, Waltham, MA) operating at an acceleration voltage of 200 kV. After
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identifying the crystal structure of the hematite nanocrystals, 3D models of
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nanoplates and nanorods were built using WinXMorph software,31 and the visualized
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models of the atomic arrangement on different exposed facets were generated with
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the crystal-building software CrystalMaker 2.3.2 based on the structural parameters
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of rhombohedral phase hematite -Fe2O3 (a = b = 5.0143 Å, c = 13.6733 Å, and
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= 60°, γ = 120°).32 Procedures for the calculation of the density of surface hydroxyl
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sites on hematite nanocrystals are provided in detail in the SI.
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The synthesis procedures are
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Sorption experiments. Batch experiments for the sorption of the HSs (5-102 mgC L-1)
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onto hematite (2 g L-1) without background electrolyte were conducted in glass
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centrifuge tubes. All containers and ultrapure water were sterilized before use and no
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microbial inhibitor was added to the system. Blanks without HS and controls without
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hematite were prepared in the same way and were involved in the adsorption
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experiment. The initial pH was adjusted to 6.5 because the original pH when hematite 6
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and HS were mixed approached 6.5, and no further pH adjustment was made during
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the experiment. The suspensions were equilibrated on a reciprocating shaker at 120
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rpm at 25 °C for 48 h in the dark. After equilibration the samples were centrifuged for
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40 min at 5000 rpm and filtered through 0.22 μm Nylon filters. The concentration of
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organic carbon in the filtered supernatants was determined with a TOC analyzer (vario
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TOC, Elementar, Hanau, Germany). HSs retained by the hematite were calculated by
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the difference between the concentrations in the initial and equilibrium solutions. All
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experiments were carried out in triplicate. Controls showed unobservable changes in
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HSs, indicating negligible adsorption by glass centrifuge tubes or degradation of HSs
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during the incubation period.
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ESI-FT-ICR-MS Analysis. Initial HSs at a concentration of 20 mg C L-1 and s filtered
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supernatants of their sorption were analyzed by ESI-FT-ICR-MS. Solid phase
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extraction (SPE) was performed on all samples using Varian Bond Elute PPL
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cartridges (0.2 g (3 mL)-1) to desalinate and concentrate them according to a previous
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study33. Samples were adjusted to an equal mass concentration before analysis in
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order to eliminate the influence of concentration on the ionization efficiency of
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molecules detected. Ultrahigh resolution mass spectra were acquired using a Bruker
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SolariX FT-ICR-MS (Bruker, Billerica, MA) equipped with a 15.0 T superconducting
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magnet and an ESI ion source. Samples for ESI FT-ICR-MS analysis were
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continuously infused into the ESI unit by syringe infusion at a flow rate of 120 μL h -1.
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The ESI needle voltage was set to -3.8 kV. All samples were analyzed in negative
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ionization mode with broadband detection. Ions accumulated in a hexapole ion trap
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for 0.2 s before being introduced into the ICR cell. 4M words of data were recorded
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per broadband mass scan. The lower mass limit was set to m/z 200 Da and the upper
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mass limit was set to m/z 1000 Da.26, 34-37 A total of 100 scans were summed for each 7
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mass spectrum. Two replicates of each sample were examined. The spectra were
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externally calibrated with 10 mM of sodium formate solution in 50% isopropyl
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alcohol using a linear calibration and then internally recalibrated using an in-house
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reference mass list. After internal calibration, the mass error was < 500 ppb over the
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entire mass range. Peaks were identified with Bruker Data Analysis software.
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Molecular formula assignment. All possible formulas were calculated with
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Formula Calculator software based on the requirement that the mass error for a given
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chemical formula between the measured mass and calculated mass was < 0.5 ppm.
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Molecular formulas with elemental combinations of C5–80H10–200O0–40N0-3 were
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assigned to peaks with signal-to-noise ratio (S/N) ≥ 4 according to stringent criteria.
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Only molecular formulae identified in both duplicates were considered to ensure the
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reproducibility and reliability of the detection.34 The elements P and S were excluded
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because of the low levels found in all HSs. The elemental ratios H/C < 2.0 and O/C
