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Characterization of Natural and Affected Environments
Occurrence and degradation potential of fluoroalkylsilane substances as precursors of perfluoroalkyl carboxylic acids Bao Zhu, Wei Jiang, Wenxing Wang, Yongfeng Lin, Ting Ruan, and Guibin Jiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00690 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019
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Submit for publication in Environmental Science & Technology
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Occurrence and degradation potential of fluoroalkylsilane
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substances as precursors of perfluoroalkyl carboxylic acids
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Bao Zhu1,2, Wei Jiang1, Wenxing Wang1, Yongfeng Lin2, Ting Ruan2*, and Guibin Jiang2
8 9 10
1
Environment Research Institute, Shandong University, Binhai Road 72, Qingdao, 266237,
China
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2
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for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center
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*Corresponding author
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Dr. Ting Ruan
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Research Center for Eco-Environmental Sciences,
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Chinese Academy of Sciences
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Tel: 8610-6284-9334
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Fax: 8610-6284-9179
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E-mail:
[email protected] 27 1
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ABSTRACT
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Polyfluoroalkylsilanes (PFASis) substances are a class of artificial chemicals with wide
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applications in surface coating, which arouse attention due to the hydrophobic/oleophobic
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properties and potential biological effects. In this study, a robust high-resolution mass
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spectrometry method through direct injection into Fourier transform ion cyclotron
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resonance instrument was established, with the aid of CF2-scaled Kendrick mass defect
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analysis and isotope fine structure elucidation. The occurrence of 8:2 polyfluoroalkyl
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trimethoxysilane (8:2 PTrMeOSi ) and 8:2 polyfluoroalkyl triethoxysilane (8:2 PTrEtOSi ),
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as well as their cationic adducts, solvent substitutions and other compound analogues, were
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identified in commercial anti-fingerprint liquid products. In the hydroxyl radical-based
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total oxidizable precursor assay, differential molar yields of products were observed in
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regard to varied PFASi carbon-chain lengths and terminal groups. The yields of
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perfluoroalkyl carboxylic acids (PFCAs) from 8:2 PTrMeOSi conversion were the highest
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(92 ± 9 %, n = 3), with the C (n-1) perfluoroheptanoic acid (PFHpA, 49 ± 11 %, n = 3) as
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the dominating product. Distinct conversion of 8:2 PTrMeOSi in the simulated solar
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exposure experiments was found that C (n) perfluorooctanoic acid (PFOA, 0.6 ± 0.04 ‰,
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n = 3) was predominant, and 8:2 fluorotelomer carboxylic acid (8:2 FTCA, 0.59 ± 0.08‰,
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n = 3), 8:2 fluorotelomer unsaturated carboxylic acid (8:2 FTUCA, 0.09 ± 0.00‰, n = 3)
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intermediates were also observed. To our knowledge, it is the first to report the occurrence
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and degradation potential of several fluoroalkylsilane substances as PFCA precursors.
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Keywords: Per- and polyfluoroalkyl substances; Total oxidizable precursor assay; Photo-
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degradation; High resolution mass spectrometry; Exposure Pathway
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1. INTRODUCTION
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Per- and polyfluoroalkyl substances (PFASs) are a group of anthropogenic chemicals,
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which possess unique physical-chemical properties including thermal resistance,
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hydrophobicity and oleophobicity. Extensive industry and household applications of
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PFASs have resulted in unintentional release through direct usage emissions and indirect
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transformation of polyfluorinated precursors. Particularly, perfluoroalkyl sulfonic acids
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and perfluoroalkyl carboxylic acids (PFCAs) have been detected in various kinds of
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environmental compartments and biological species, which have been subjected to public
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scrutiny in recent years.1-4 Restrictions on usages and voluntary phase-out of conventional
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PFASs trigger compensatory usage of fluorinated alternatives with diverse functional
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groups.5,6 Increasing attention has thus been paid on identification of novel PFAS pollutants
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in the environment.7-10 For instance, zwitterionic, cationic and anionic fluorinated
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chemicals were discovered in aqueous film forming foams (AFFFs) and impacted
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groundwater.11-13 Chlorine- and hydro-substituted polyfluorocarboxylates were found in
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wastewater from fluorochemical manufacturing processes.14
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Polyfluoroalkylsilane (PFASi) substances are a group of silicon derivatives, in which a
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fluorotelomer chain is attached with a siloxane group. Except for the diversity of
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fluoroalkyl carbon-chain lengths, a variety of PFASi analogues have been manufactured in
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regard to chlorine, methoxy and ethoxy substitutions. Fluoroalkyl chain provides
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hydrophobic and low surface-free energy characteristics, and reactive halide or alkyloxy
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substitutions in the molecular structure easily hydrolyze and combine with inorganic
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substances.15 Thus, a transparent and water repellent self-assembling film could be formed,
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which facilitates a number of applications in surface treatment for optical glass,16 electronic
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display screens, metals, wood, cotton and leather.17 Industrial synthesis of PFASi chemicals
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was reported through hydrosilylation of fluorotelomer olefins.18 Several analogues, such 3
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as 8:2 polyfluoroalkyl trichlorosilane (CAS No.: 78560-44-8), 6:2 polyfluoroalkyl
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trichlorosilane (CAS No.: 78560-45-9) and 8:2 polyfluoroalkyl trimethoxysilane (8:2
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PTrMeOSi, CAS No.: 83048-65-1), are included in the American Toxic Substances Control
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Act (TSCA)19 and European Inventory of Existing Commercial Chemical Substances
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(EINECS) lists.20 In regard to observation of emphysema, hemorrhages and inhibition of
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lung surfactant function in inbred male BALB/cA mice,21-23 usages of 8:2 polyfluoroalkyl
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triethoxysilane (8:2 PTrMeOSi ) and similar substances were banned in Denmark.15
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Nevertheless, investigations on composition profiles and potential fate of PFASi analogues
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under environmental conditions have not been conducted.
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Multiple analytical techniques are applied as powerful tools for detection and
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quantification of unidentified fluorinated precursors.24 High resolution mass spectrometry
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(HRMS) has inherent advantages of high resolving power (i.e., RP ≥ 10000) and sufficient
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mass accuracy (i.e. ≤ 5 parts-per-million), which enables characterization of novel PFAS
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compounds by accurate elemental combination assignment. For instance, by combination
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of quadrupole-time-of-flight and Fourier transform ion cyclotron resonance instruments,
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empirical structures of infrequently reported fluorinated surfactants were consecutively
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elucidated in commercial AFFF concentrates.25 Due to the unique feature that accurate
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mass of fluorine is very close to its nominal mass, Kendrick mass defect (KMD) filtering
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based on CF2 scaling algorithm could effectively separate signals of PFAS compounds
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from a majority of irrelevant interferences in mass spectrum.26 Congeners with the same
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KMD values were positioned and visualized in the horizontal direction, which resulted in
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recognition of perfluoroethane sulfonic acid and perfluopropane sulfonic acid as two new
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ultrashort-chain pollutants along with the C4 – C8 congeners in groundwater samples.27
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Meanwhile, the total oxidizable precursor (TOP) assay is an effective means to reflect
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residue levels and behaviors of PFAS precursors in representative degradation scenarios.28 4
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Organofluorine components in environmental matrixes, including runoff, surface water and
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soil, were assumed to be converted into detectable PFCA analytes by reactions with
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hydroxyl radicals generating from thermolysis of persulfate under basic conditions, and
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contributions of precursors could thus be quantified by exact measuring elevated analyte
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concentrations before and after the assay.29,30
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The aims of this study were to establish a robust high resolution mass spectrometry method
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for qualitative and quantitative analysis of PFASi analogues, to examine the presence and
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composition profiles in commercial products, and to explore degradation potentials in the
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TOP assay and photo-degradation assay. Outcomes of this study could help elucidate
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behaviors and impacts of PFASi derivatives in the environment.
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2. MATERIALS AND METHODS
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2.1 Chemicals and Reagents
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Chemical names, acronyms, structures, and other information of the target analogues are
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shown in Figure 1 and Table S1 in the Supporting Information (SI). Individual native
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fluorotelomer trimethoxysilanes (6:2, 8:2 and 10:2) and fluorotelomer triethoxysilanes (6:2,
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8:2 and 10:2) were purchased from Synquest Laboratories (Alachua, FL). A native C4 – C14
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PFCA standard mixture (PFAC-MXB, 2 µg/mL for each analyte), eight other native PFCA
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standards (6:2 FTCA, 8:2 FTCA, 10:2 FTCA, 4:3 FTCA, 5:3 FTCA, 7:3 FTCA, 8:2
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FTUCA, 5:3 FTUCA, 50 μg/mL for each analyte), an isotope-labeled standard mixture
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(MPFAC-MXA, 2 µg/mL for each analyte) and individual isotope-labeled standards (M3-
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PFBA, M3-PFPeA, M5-PFHxA, M4-PFHpA, M6-PFDA, 50 µg/mL for each analyte) were
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obtained from Wellington Laboratories (Ontario, Canada). Detailed chemical information
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is listed in Table S6. Dodecamethylcyclohexasiloxane (D6, C12H36O6Si6) and sodium
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hydroxide were acquired from Sigma-Aldrich (St. Louis, MO). Potassium persulfate, 5
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concentrated hydrochloric acid (37 %, v/v) and ammonium hydroxide (50 %, v/v) were
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from Acros Organics (Belgium, Germany), Merck (Darmstadt, Germany) and Alfa Aesar
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(Ward Hill, MA), respectively. The purities of all chemicals were 95 % or higher unless
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otherwise stated. Five commercial anti-fingerprint (AF) liquid products with trade name
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AF-1061, AF-1062, AF-A2000, AF-H1000 and AF-I2000 were purchased randomly from
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local markets. The products were used for material surface coating to enhance stain
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resistance properties for fingerprints, skin oil, sweat, etc. HPLC-grade methanol was
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supplied by Fisher Scientific (Waltham, MA), and ultrapure water (18.3 MΩ.cm) was
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generated by a Milli-Q system (Millipore, Bellerica, MA).
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2.2 FT-ICR MS Measurement
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The data were collected using a Solarix Fourier transform ion cyclotron resonance mass
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spectrometer (FT-ICR MS, Bruker Daltonik, GmbH, Bremen, Germany) equipped with a
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15.0 T superconducting magnet and a electrospray ionization (ESI) source.31 For
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instrumental analysis, sample dissolved in methanol was directly injected into the ESI
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source by a syringe pump at a flow rate of 120 μL/h. Ultrahigh resolution (540 000 FWHM,
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full width at half maximum at m/z 400) mass spectra were obtained in the positive
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ionization mode with broadband detection in the mass range of 200 – 900 m/z. Capillary
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Entrance voltage was set to -4.0 kV, End Plate electrode voltage was -500 V, and Corona
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Needle current was 3000 nA. The time of flight and ion accumulation time were set as 0.7
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ms and 0.5 s, respectively, in order to transfer target ions into the ICR cell. The size of data
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points was recorded in 4 M format, and 200 acquisitions were accumulated per spectrum.
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The FT-ICR mass spectra were externally calibrated using a sodium formate solution (10
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mM in 50 % isopropyl alcohol), and further internally recalibrated with a home-built
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reference masses of known siloxane homologues over the entire mass range.32 After 6
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internal calibration, a mass accuracy less than 0.4 ppm was further acquired. Elemental
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composition assignment was conducted according to the identified m/z peaks with Bruker
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Data Analysis software (version 4.0) based on the requirements of absolute signal response
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over 1x106, S/N ratio greater than or equal to 3. C, H, O, F, Si, and Na elements were
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considered in the composition assignment with limitation of molecular formulas consisting
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of up to 50 12C, 50 1H, 10 16O, 50 19F, 10 28Si and 1 23Na. A chemical criteria of elemental
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ratios of (H+F)/C < 3.0 must be meet for all assigned formulas, which ensures that the
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achieved formulas can be chemically feasible. Molecular matching scores and isotope
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information are taken into consideration if there were multiple candidates. For visual
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graphic displays of the obtained data, molecule description tools were further applied, such
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as Kendrick mass (KM) and adjusted Kendrick mass defect (AKMD) for identification of
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homologous series. CF2-scaled AKMD was calculated on the basis of the following
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equations, as described in previous studies:26
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KM (CF2) = Mmeasured × [(Mnominal of CF2)/ (Mexact of CF2)]
(1)
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KMD (CF2) = Nominal KM (CF2)-KM (CF2)
(2)
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AKMD (CF2) = 1+ KMD (CF2)
(3)
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HRMS instrumental performance was checked daily using the sodium formate solution.
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For qualitative analysis, samples of anti-fingerprint liquid formulations were prepared by
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dilution between 1×102 and 1×106 folds in methanol according to the actual concentration
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needed to fit the peak intensity of each analyte fall within the region of instrumental
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linearity. A 1 mL aliquot of the diluted liquid was transferred directly into the instrument.
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FT-ICR MS was generally used for qualitative analysis of compounds in complex matrix.
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Nevertheless, for semiquantitative and quantitative analysis of PFASi analogues in this
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research, individual native fluorotelomer trimethoxysilane mixed solution (6:2, 8:2 and
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10:2, 3MeO-Mix) and fluorotelomer triethoxysilane mixed solution (6:2, 8:2 and 10:2, 7
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3EtO-Mix) were diluted into nine-point calibration curves with the concentrations ranging
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from 0.1 to 50 ng/mL in 1.5 mL polypropylene centrifugal tubes. Dodecamethyl-
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cyclohexasiloxane (D6, 10 ng/mL) was spiked as an injection standard, and all samples
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were analyzed in triplicate. Peak intensity of each analyte was adjusted by dividing the D6
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peak intensity in order to partially eliminate instrument signal fluctuation (82 – 153 %),
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and good linear correlation coefficients (R2 = 0.992 – 0.999) were achieved (Figure S9).
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The relative standard deviations (RSDs) of all points were less than 20 %. Method detection
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limits (MDLs, Table S3) of PFASis calculated by a signal/noise ratio of three (S/N = 3)
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ranged from 0.13 ± 0.01 pg on column (6:2 PTrEtOSi, n = 3) to 0.42 ± 0.07 pg on column
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(8:2 PTrEtOSi, n = 3).
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2.3. Total Oxidizable Precursor Assay
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The TOP Assay was prepared according to literature with minor modifications.28-30 In brief,
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the oxidation assays were performed in sealed 30 mL narrow-mouth high density
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polyethylene bottles (Thermo Scientific, NY). Each bottle was filled with 10 μL of 1.0
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mg/mL PFASi stock solution, 0.324 g of potassium persulfate, 2 ml of 1.5 M NaOH, and
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pure water to reach a total solution volume of 20 mL. Individual samples were heated
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immediately in a temperature-controlled water bath at 85 °C for different periods of time
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(0, 5, 10, 30, 60 120, 240, and 360 min). At each designated time, three parallel samples
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were removed from the water bath, cooled to room temperature in an ice bath, and
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neutralized to pH = 4 – 6 using concentrated HCl (37 %, v/v) prior to dilution. Procedure
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blank (solvent only) samples and persulfate-free thermalized control samples for a reaction
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time of 360 mins were also performed, with the other experimental details kept in
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consistency. All experiments were performed in triplicate. After oxidation assay, a 100 μL
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aliquot was vortexed thoroughly, diluted with 50 mL ultrapure water, and spiked with 2 ng 8
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of each isotopically labeled surrogate standards (Table S6) for subsequent extraction
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experiments. It was loaded onto an Oasis WAX SPE cartridge by the previous method33
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summarized in the SI CONTENT2. Analytes resuspended in 1 mL of 1:1 water/methanol
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were centrifuged at 10000 revolutions per minute (rpm) for 5 min, and were spiked with 2
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ng of isotope-labeled injection standards (Table S6). An extraction blank consisting of
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ultrapure water amended with the isotope-labeled surrogate standards was included in
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every 12 samples.
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PFCA quantification was performed on an API 5500 triple-quadrupole mass spectrometer
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(AB SCIEX Inc., Framingham, MA) coupled with an Ultimate 3000 ultrahigh performance
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liquid chromatograph (Thermo Fisher Scientific Inc., Waltham, MA) operating in the
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negative electrospray ionization as described previously.34 The details of analyte separation
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and MS parameters were summarized in Table S5.
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Solvent blanks were injected at an interval of every three samples, and no carryover effect
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was observed. Quantification of all PFCAs was conducted using an internal standard
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calibration curve with the linearity (R2) > 0.99 for all target analytes. Method quantification
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limits (MQLs, S/N=10) of PFCAs were in the range of 0.09 to 28.7 pg on column,
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respectively (Table S7). Arithmetic mean concentrations of procedure blank, extraction
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blank and thermalized control in the TOP assay are shown in Table S8. No obvious
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background interference or pyrolysis product of PFASis was observed. Average recoveries
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of isotope-labeled surrogate standards (2 ng for each standard, Table S9) ranged from 76 %
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(MPFUdA) to 107 % (M3PFPeA). Quantified results of isotope-labeled injection standards
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based on external calibration curves (2 ng for each standard, Table S9) were 100 %, 121 %
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and 105 % for M3PFBA, M5PFHxA and M6PFDA respectively, suggesting minor
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ionization enhancement in the TOP assay.
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2.4. Photo-degradation Assay
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Photo-degradation assay (detailed experimental workflow shown in SI CONTENT3) was
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conducted in 40 mL quartz glass bottles with quartz caps (30 mm diameter, 35 mm height,
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0.5 mm thickness). Glassware were washed thoroughly with methanol, and were dried with
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nitrogen stream. An dosage of 200 µg 8:2 PTrMeOSi in methanolic solution spiking into
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the bottom of the quartz glass bottles (1.76 mM, ~ 50 mg/m2) was in accord with actual
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commercial product applications on surface coating (e.g. 1 wt% in nanofilm products, ~
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100 – 400 mg/m2)21. The quartz glass bottles were rotated gently to make the solution
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distributed evenly on the bottle surface, and then dried with a gentle stream of N2 to remove
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remaining solvent. The prepared samples with lid seal were wrapped with aluminum foil,
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and heated in an oven at 150 °C for 3 hours in order to form an layer of self-assembled
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hydrophobic coating by the widely used method in industry.35 For analysis of those
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physically adsorbed or not covalently linked 8:2 PTrMeOSi, the bottles were washed three
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times with methanol after cooling to room temperature, and the combined eluent was
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infused into FT-ICR MS with dilution to make analyte concentrations fall within the range
242
of instrumental linearity. Subsequently, isotope-labeled surrogate standards (MPFAC-
243
MXA, M3PFPeA, M4PFHpA, 2 ng for each analyte, Table S6) were spiked onto the
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coating, solvent-dried with gentle nitrogen stream, and the bottles were placed upside down
245
in a solar simulator, integrated with one circulating water cooling system and three 2500
246
W xenon lamps through air cooling (SN-500, Beifang Lihui, Beijing). The parameter of
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light intensity was set at 550 W/m2, and the simulator inner temperature was maintained at
248
35 ± 2 °C. Vertical distance from the bottom of the bottles to lamps was approximately 30
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cm. Dark controls were performed by wrapping the quartz glass bottles with three layers
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of aluminum foil to avoid light exposure, which were also placed in the same ambient. All
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experiments were conducted in triplicate, including procedure blank samples analyzed in 10
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parallel. The photo-degradation samples were taken out from the simulator at 0.25, 1, 2, 3,
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4, 7 and 10 days. At each appointed time, the samples were extracted three times with 4
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mL methanol, and the combined solution was concentrated to dryness, then re-suspended
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in 1 mL of 1:1 water/methanol spiking with 2 ng of isotope-labeled injection standards
256
(Table S6). Afterwards, another in-situ TOP assay treatment was further applied to recover
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residue amount of fluoroalkylsilane coating on the surface of quartz glass bottles.
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Instrumental analysis of PFCAs was the same as mentioned above.
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The PFCA quantification results of procedure blanks in photo-degradation assay (shown
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in Table S10) suggested that no obvious laboratory contamination or interferences was
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found. Averaged recoveries of isotope-labeled surrogate standards (n = 30, 2 ng for each
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standard, Table S11) ranged from 73 % (M4PFHpA) to 100 % (MPFHxA). Minor
263
ionization enhancement was observed, as matrix interferences of the isotope-labeled
264
injection standards (n = 30, spiked at 2 ng for each standard, Table S11) were 105 %, 125 %
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and 105 % for M3PFBA, M5PFHxA and M6PFDA in the photo-degradation assay. The
266
PFCA concentrations of procedure blanks in the TOP assay after photo-degradation are
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provided in Table S12. Negligible ionization enhancement was observed, as matrix
268
interferences of the isotope-labeled injection standards (n = 30, spiked at 2 ng for each
269
standard, Table S13) were 93 %, 110 % and 105 % for M3PFBA, M5PFHxA and M6PFDA
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in the TOP assay after photo-degradation.
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3. RESULTS AND DISCUSSION
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3.1. Analysis of PFASi Substances by FT-ICR MS
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Active terminal groups (i.e. chlorine, methoxy and ethoxy) attached to silicon in the PFASi
275
molecular structures are easily lost by hydrolysis to form silanol (Si-OH) groups with the
276
leaving tendency chlorine > methoxy > ethoxy. Mono, double or triple functional Si-OH 11
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groups can either react with the hydroxyl groups on surface of substrates (e.g. SiO2/Si
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surface) to form the siloxane (Si-O-Si) linkages, or Si-OH groups can form intermolecular
279
polymerization in solution.36 Therefore, the stock solutions of PFASis were freshly
280
prepared in methanol before all experiments, and kept under water-free environment. The
281
characteristic suggests low efficiency using gas/liquid chromatography separation
282
procedure. Thus, PFASi analogues were infused directly into the electrospray ion source
283
of FT-ICR MS in the positive ionization mode. Owing to ultrahigh resolution (> 500 000
284
at m/z 400) and mass accuracy (< 2 ppm) of the HRMS instrument, measurement of
285
compound accurate mass and isotope fine structures could be achieved especially in
286
complex mixtures.
287
Different cationic adducts of 8:2 PTrMeOSi ions could be seen in the mass spectrum
288
(Figure S1), and the peak intensity of Na+ adduct was the most abundant compared with
289
those for NH4+, K+ or H+ for all trimethoxy and triethoxy PFASis. Generally, [M+H]+
290
adduct is formed with high abundance in the positive ESI mode, nevertheless, universal
291
and stable Na+ adducts as ions of siloxane in mass spectrum37 were observed. Compared
292
with the [M+H]+ ions, [M+Na]+ ions selected as monitoring ions could significantly
293
increase analyte instrumental responses in the range of 11-fold (8:2PTrEtOSi ) to 351-fold
294
(10:2 PTrMeOSi) enhancement, the MDLs of which reached to 0.13 ± 0.01 pg (6:2
295
PTrEtOSi, n = 3). Thus, following discussion was all based on the Na+ adducts of PFASis
296
if not specially stated.
297
After accurate mass measurement and elemental composition assignment, isotope fine
298
structures could be used as an effective means to further confirm existence of Si element
299
in the molecule.38 For instance, m/z = 591.02562 is measured monoisotopic mass of the
300
[M+Na]+ ion of 8:2 PTrMeOSi native standard (Figure 2 and Figure S2-S5), with a mass
301
error of 0.25 ppm compared with its theoretical mass. [M+1+Na]+ and [M+2+Na]+ ions 12
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were actually mass clusters with major contributions from 29Si, 13C and 30Si, the relative
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isotopic natural abundance of which is 1.08 %, 5.08 % and 3.35 %, respectively. It is
304
obvious that individual
305
signals of [M+1+Na]+ ions with an mass difference of only 3.79 mDa could be successfully
306
separated under current instrumental resolution status, and distinct 30Si (m/z = 593.02258),
307
29
308
also noticed. It facilitated calculation the number of Si and C atoms in the formula on the
309
basis of isotopic natural abundance and relative intensity of 29Si and 13C peaks in both the
310
[M+1+Na]+ and [M+2+Na]+ ions, which is useful in the exclusion of false assignment
311
results.39 The isotope fine structures were considered for accurate molecular formula
312
confirmation if more than one elemental assignment appeared in the following research.
313
Due to active terminal groups, solvent substitution effects of PFASis were also observed.
314
For PTrMeOSi analogues, only mono Si-OH groups were observed mainly resulting from
315
the replacement of terminal methoxyls by alcoholic hydroxyls. Meanwhile, the terminal
316
ethoxyls of PTrEtOSis could be substituted by both the methoxyls and hydroxyls in the
317
methanolic solvent. The solvent-substituted instrumental signals of PTrEtOSis could be
318
divided into 7 analogues (A0 – A6, Table S14) according to the terminal carbon numbers.
319
In general, the solvent substitution effects of PFASis (Σaveraged relative intensity < 2 %
320
for each analyst) were negligible, except for 8:2 PTrEtOSi with averaged relative intensity
321
accounting for 23 % (A2 3 %, A3 4 %, A4 14 %, A5 2 %), which might lead to underestimate
322
of the analyte amounts in actual experiments.
323
Kendrick mass analysis is effective in recognizing various kinds of compounds containing
324
the same base core structure or homopolymers with common repeating unit in complex
325
samples.40 Per- and polyfluorinated compounds differed by CF2 units would fall in
326
horizontal lines with fixed mass spacing (e.g. m/z 50 corresponding to a CF2 unit), and the
29
Si (m/z = 592.02546) and
13
C (m/z = 592.02916) containing
Si+13C (m/z = 593.02841), 13C+13C (m/z = 593.03252) signals of [M+2+Na]+ ions were
13
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method was helpful for visualization of mass spectrum by reducing interferences.26 The
328
research targets in this study were mainly limited to elements with negative mass defects,
329
including 28Si (-23.073 mDa), 23Na (-10.230 mDa), 16O (-5.085 mDa) and 19F (-1.597 mDa)
330
except for 12C (0.000 mDa) and 1H (+7.825 mDa), which have KMD values less than zero.
331
CF2-scaled model AKMD plot (AKMD-CF2) was constructed by adding 1 to the negative
332
values of KMD in Figure S6a, the values of known PFASis were in the range of 0.86 – 1.00.
333
Seven classes of model PFASis (Na+ adducts) were presented as black dots in lines within
334
2 ppm mass error thresholds. Parallel-lined dots positioned in red arrow direction illustrated
335
the difference of carbon-chain lengths in molecules along with varied terminal groups, blue
336
arrow represented one F atom replaced by one H atom in the molecular structures, and
337
green arrow showed a series of homologous compounds with a CH2 repeating unit.
338 339
3.2. Telomer-based PFASis in Commercial Anti-Fingerprint Liquid Products.
340
The PFASi analogues in two commercial anti-fingerprint (AF) liquid products with trade
341
name AF-1061 and AF-1062 were further investigated by 1×106 folds dilution in methanol
342
and instrumental analysis by ESI FT-ICR MS. The most responsive component of AF-1061
343
in mass spectrum was identified as 8:2 PTrMeOSi (Figure S7a, A3, m/z = 591.02479, mass
344
error = 1.15 ppm, relative intensity (RI) = 100 %). The green diamond (A2, m/z =
345
577.00919, mass error = 1.10 ppm, RI = 1 %) positioned in the green arrow direction with
346
8:2 PTrMeOSi was reasonably attributed to the solvent-substitution analogue as one
347
terminal silicon methoxyl group replaced by one alcoholic hydroxyl group. The other three
348
points in this direction (A4 – A6, m/z = 605.04043, 619.05604, 633.07175, with mass errors
349
< 1.18 ppm, RIs < 0.9 %) were 8:2 PTrEtOSi and its solvent-substitution analogues, which
350
may be manufacture impurities. Longer-chain 10:2 FTO-TMeOSi (A3, m/z = 691.01828,
351
mass error = 1.17 ppm, RI = 0.3 %) was also observed. The major component of AF-1062 14
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was recognized as 8:2 PTrEtOSi (Figure S7b, A6, m/z = 633.07158, mass error = 1.32 ppm,
353
RI = 100%) along with its three solvent substitution analogues (A3 – A5, m/z = 619.05603,
354
605.04043, 591.02481 with mass errors < 1.20 ppm, RIs < 37 %). Additionally, two kinds
355
of shorter analogues 7:2 PTrEtOSi (A6, m/z = 583.07495, mass error = 1.13 ppm, RI =
356
0.2 %) and 6:2 PTrEtOSi (A6, m/z = 533.07828, mass error = 0.99 ppm, RI = 1 %) were
357
identified as well as their relevant mono methoxy substitution products (A5, m/z =
358
569.05919, 519.06264 with mass errors < 1.36 ppm, RIs < 0.6 %). Isotope fine structures
359
of all identified PFASis in measured spectrum were checked and confirmed, which fit well
360
with those in theoretical spectrum.
361
It is interesting to find that a series of additional compounds were observed in three
362
commercial anti-fingerprint (AF) liquid products with trade name AF-A2000, AF-H1000
363
and AF-I2000 (Figure S6b-S6d). For instance, the most intensive peak with m/z =
364
463.00041 was speculated as the [M+Na]+ ion of F(CF2)6(CH2)2Si(OH)2(OCH3)1 (A1, mass
365
error = 0.32 ppm). Nevertheless, no corresponding 29Si [M+1+Na]+ and 30Si [M+2+Na]+
366
isotope peaks were further found in the mass spectrum, suggesting Si element was not
367
existent in the molecule. According to relative instrumental response of less-abundant 13C
368
(m/z = 464.00425, 12.3%) and 18O (m/z = 465.00575, 0.11%) isotope peaks based on the
369
monoisotopic peak intensity (m/z = 463.00041, 100%), as well as combining
370
corresponding natural abundance (1.08% for 13C and 0.21 % for 18O), maximal number of
371
element in the chemical was further limited to C0-14F0-50H0-50O0-2N0-3Na0-1K0-1 (Figure S8).
372
The unique formula [C10H4F17O]+ was assigned within the mass error of 5 ppm, which was
373
tentatively identified as perfluorooctyl vinyl ether through database search (e.g. PubChem
374
CID 54096250, C8F17OCH=CH2). Perfluorooctyl vinyl ether as well as its analogues were
375
another important surface coating material used for generating a highly ordered copolymer
376
thin films.41 15
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3.3. TOP Assay
379
In general, PFASis (n:2 PTrMeOSi, n = 6, 8 and 10, and 8: 2 PTrEtOSi) were found to be
380
easily oxidized into PFCAs in this study, suggesting degradation potentials of the analytes
381
as PFCA precursors. There was no detection of PFCAs in thermalized control samples,
382
indicating no PFCA could be generated by thermal degradation without the addition of
383
persulfate. Radical oxidation dynamics and molar yields of detectable C4 – C12 PFCAs in
384
the TOP assay at reaction time of 0 – 360 min were summarized in Figure 3.
385
Taking 8:2 PTrMeOSi as an example, conversion of the spiked analytes into PFCAs was
386
mainly completed in an reaction time of 30 min, and the maximal ΣPFCAs molar yields of
387
92 ± 9 % (n = 3) were reached at 240 min. The total molar recoveries were closed to
388
reported results of 8:2 fluorotelomer sulfonic acid (95 ± 9 %),28 implying a majority of
389
products in the TOP assay were recovered. With increase of reaction time, the molar yields
390
of ΣPFCAs came to a downward trend. As illustrated in Figure S10b, C (n-1)
391
perfluoroheptanoic acid (PFHpA, 49 ± 11 %, n = 3) was the dominant product analogue at
392
the reaction time of 240 min, which was followed by C (n) perfluorooctanoic acid (PFOA,
393
14 ± 3 %), C (n-2) perfluorohexanoic acid (PFHxA, 12 ± 3 %), C (n+1) perfluorononanoic
394
acid (PFNA, 2 ± 0.2 % ) and the other shorter-chain analogues with decreasing molar yields.
395
The molar ratios between the four main PFCAs (C (n-2): C (n-1): C (n): C (n+1)) were
396
approximately 0.24: 1: 0.29: 0.04, respectively. At the reaction time of 360 min, both the
397
molar yields of C (n-1) PFHpA (31 ± 14 %, n = 3) and ΣPFCAs (71 ± 15 %) were relatively
398
lower, with the molar ratios of C (n-2): C (n-1): C (n): C (n+1) changed to 0.48: 1: 0.35:
399
0.05. The change of analogue profiles suggested longer-chain PFCAs might break up into
400
shorter-chain ones through F cleavage mechanism with decreasing pH valves during TOP
401
assay period.30 16
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It should be noted that, presence of plausible non-detectable analytes (i.e. C2 and C3 PFCA)
403
as well as inorganic F- ions would lead to underestimate of total conversion recovery. For
404
instance, yields of the maximal ΣPFCAs for 6:2 PTrMeOSi (Figure 3a) at 240 min fell to
405
68 ± 7 % ( n = 3), due to degradation products shifted to shorter-chained analogues. With
406
regard to the 10:2 PTrMeOSi, equilibrium of reaction was not achieved within the exposed
407
period of 360 min, and the molar yields of ΣPFCAs increased slowly up to 54 ± 15 % (n =
408
3, Figure 3c). In this case, a low analyte reaction degree of the 10:2 PTrMeOSi analyte, or
409
the loss of long-chain PFCA products (e.g. adsorption onto surface, low water solubility)
410
might be key factors resulting in low molar yields of ΣPFCAs.42
411
For the ethoxy-substituted analogue 8:2 PTrEtOSi, a different composition profile of PFCA
412
products was observed. The maximal molar yields of ΣPFCAs were 38 ± 4 % (n = 3, Figure
413
3d) with the C (n) PFOA as the most abundant analogue, which were significantly lower
414
than that of the 8:2 PTrMeOSi (92 ± 9 %, n = 3). Terminal triethoxy groups bounded to Si
415
could influence the hydroxyl radical (•OH) reactive site in molecular structure and result in
416
excess radicals scavenging, which may be responsible for the distinct distribution of PFCA
417
products and decrease of product molar yield.
418 419
3.4. Photo-degradation Assay
420
The reaction of PFASis with hydroxyl radicals (•OH) in the TOP assay revealed plausible
421
degradation capacity of the chemicals in the environment. As PFASis were mainly used as
422
hydrophobic self-assembled monolayers (SAMs) through silane coupling to form -Si-O-
423
Si- covalent linkage on material surfaces like optical glasses, exposure to light during daily
424
usage is inevitable. Considering the difficulties of maintaining adequate and stable
425
illumination intensity under sunlight, Xenon lamps were conducted as simulating light
426
source, of which the energy distribution was similar across the entire visible spectrum. 17
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Quartz glass has an excellent optical performance in continuous wavelength range from
428
ultraviolet to infrared radiation. Transparency of the quartz glass bottles used in this
429
research was about 90 %. Hence, native 8:2 PTrMeOSi was applied to the inner wall of the
430
quartz glass bottles to form fluoroalkylsilane SAMs43 prior to simulated photo exposure.
431
A mixture of PFCAs were produced after Xenon light irritation, and the yields of ΣPFCAs
432
were almost linear with the increase of exposure time from 6 h to 10 days (Figure 4a). After
433
the 10-day exposure, the molar yields of ΣPFCAs reached to 2 ± 0.1 ‰ (n = 3). Distinct
434
composition profile of the transformation products was found, compared with that of the
435
TOP assay, that C (n) PFOA (0.6 ± 0.04 ‰, n = 3) was the most abundant product. Two
436
polyfluorinated intermediates including fluorotelomer carboxylic acid (8:2 FTCA, 0.6 ±
437
0.08 ‰, n = 3) and fluorotelomer unsaturation carboxylic acid (8:2 FTUCA, 0.09 ± 0.00 ‰,
438
n = 3) (Figure 4b) were also detected. A negligible amount of ΣPFCAs was also found in
439
the 10-day dark control samples (n = 3), which were less than 2 % compared with the molar
440
yields of ΣPFCAs generated from the photo-degradation assay. It indicates almost all of
441
the observed PFASi degradation products were originated from the photochemical reaction.
442
The energy of short wave in sunlight especially the ultraviolet region (UVA, 320 ~ 420 nm,
443
UVB, 275 ~ 320 nm) was very similar to the bond energy in the chemical formula of
444
PFASis, e.g., Si-O (242 KJ/mol), C-C (376 KJ/mol), and C-Si (414 KJ/mol),44 which is
445
sufficient to cause generation of carbon-based radicals or decomposition of fluorinated
446
carbon backbone. Nevertheless, it might not be sufficient to cleavage C-F (485 KJ/mol)
447
leading to the main formation of C (n-1) degradation products as observed in the TOP assay.
448
It implies different degradation pathways might take place in the simulated solar exposure
449
experiment under the less rigorous condition.
450
8:2 PTrMeOSi residues and photo-degradation products could be recovered from three
451
parts, with workflow given in the SI (SI CONTENT3, Figure S11). After the 18
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fluoroalkylsilane SAM formation, all quartz glass bottles were washed with methanol to
453
measure residual amounts of the analyte physically adsorbed onto surfaces. Combined
454
eluent was diluted appropriately and quantified by FT-ICR MS. The averaged molar ratios
455
in the methanol wash fraction were in the range of 5 % to 7 % (Figure S12). After Xenon
456
light irradiation, the remained firm fluoroalkylsilane coating on the bottle surface was
457
assessed through TOP assay. In this section, the average molar ratios (namely the whole
458
detectable PFCAs) were in the range of 14 % to 17 %. Taking the minor proportion of
459
degradation products induced by photo exposure into consideration, the total mass fractions
460
from the three parts were in the range of 19 % to 24 %. It was hypothesized that volatile
461
intermediates existed in the gas phase of quartz bottles, and a large amount of 8:2
462
PTrMeOSi might stay firmly in the form of highly cross-linked fluoroalkylsilane
463
polymers.43,45
464 465
ENVIRONMENTAL IMPLICATIONS
466
In this study, a robust high resolution mass spectrometry method was established for
467
qualitative and quantitative analysis of PFASi analogues. To our knowledge, this is the first
468
study to confirm occurrence of several kinds of novel PFASi analogues in commercial
469
products. Degradation potentials of fluoroalkylsilane substances as PFCA precursors were
470
further explored by the TOP assay and photo-degradation assay. A mixture of PFCAs and
471
fluoroalkyl intermediates were found with distinct composition profiles during the radical
472
oxidation or photo irradiation processes. PFASi treatment on material surface (e.g. glass,
473
touch screens, carpets, bathroom floor, and the like)22,23,46 might constantly release PFCAs
474
in the living environment. External factors including friction, humidity, temperature or
475
biological action could promote this process, which would give rise to potential health risks.
476
Degradation capability, including possible biodegradability of PFASis as well as other 19
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477
PFCAs precursors, make up for a significant knowledge gap of the unaccounted source of
478
PFCAs.47 Moreover, direct exposure to PFASis might occur through the routes of their skin
479
touching anti-fingerprint coatings on the surface of materials or the inhalation of volatile
480
analytes in atmosphere/on small particle surface. Attention is also warranted because of not
481
only the intrinsic toxicity of PFASis15, 21-23 but also the possible safety issues of plausible
482
degradation products (e.g. fluorinated carboxylic acids, alcohols, and aldehyde
483
compounds).48
484 485
SUPPORTING INFORMATION
486
Preparation of TOP Assay; SPE method; Photo-degradation assay workflow; (Table S1)
487
Detailed information of PFASi substances; (Table S2) FT-ICR MS instrumental parameters;
488
(Table S3) MDLs of PFASi analytes; (Table S4) Peak intensity adjustment for analytes by
489
D6; (Table S5) LC-MS/MS instrumental parameters; (Table S6) Detailed information of
490
PFCA analytes; (Table S7) MDLs, MQLs and linear regression coefficients of PFCAs;
491
(Table S8-S13) PFCA concentrations (ng/ml) and recoveries of isotope-labeled surrogate
492
standards and injection standards in procedure blanks, extraction blanks and thermalized
493
controls in the TOP assay, photo-degradation assay and the TOP assay after photo-
494
degradation, respectively; (Table S14) analogues of solvent-substituted PTrEtOSis; (Figure
495
S1) Cationic adducts of 8:2 PTrMeOSi; (Figure S2-S6) isotopic ions of 6:2 PTrMeOSi,
496
10:2 PTrMeOSi, 6:2 PTrEtOSi, 8:2 PTrEtOSi, 10:2 PTrEtOSi and D6; (Figure S7) AKMD-
497
CF2 of PFASi analogues in AF-1061 and AF-1062; (Figure S8) The most intensive peak in
498
AF-A2000; (Figure S9) Linear calibration curves of PFASi analytes; (Figure S10) PFCA
499
yields from PFASi conversion in the TOP assay; (Figure S11) Diagrammatic sketch of the
500
photo-degradation assay; (Figure S12) Transformation dynamics of 8:2 PTrMeOSi and
501
PFCA products in the photo-degradation assay. The material is available free of charge via 20
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the Internet at http://pubs.asc.org.
503 504
ACKNOWLEDGMENTS
505
We thank the National Natural Science Foundation of China (91843301, 21622705,
506
21577151, and 91743101) and the Youth Innovation Promotion Association CAS projects
507
for joint financial support.
21
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FIGURE LEGENDS Figure 1. Chemical names, acronyms and molecular structures of polyfluoroalkylsilane (PFASi) substances. Figure 2. Isotopic peaks and isotope fine structures of 8:2 PTrMeOSi by FT-ICR MS analysis. (a) [M+Na]+, [M+1+Na]+ and [M+2+Na]+ isotopic ions of 8:2 PTrMeOSi in AF1061 commercial product diluted 106 times by methanol (upper), native standard (middle), and theoretical spectrum (bottom); (b) expanded [M+1+Na]+ isotope fine structures of 8:2 PTrMeOSi (blue dotted border); (c) expanded [M+2+Na]+ isotope fine structures of 8:2 PTrMeOSi (yellow dotted border). In the equations in panel b, MIM and RI are abbreviations of monoisotopic mass and relative intensity, respectively. RI 29Si and RI 13C represent instrumental signals of 29Si and 13C isotopic peak. nSi and nC represent the number of Si and C in the formula. The numbers of 5.08% and 1.08% refer to the natural abundance of 29Si and 13C isotopes, respectively. Figure 3. Formation dynamics of detectable C4-C12 PFCA products from PFASi (a. 6:2 PTrMeOSi; b. 8:2 PTrMeOSi; c. 10:2 PTrMeOSi; d. 8:2 PTrEtOSi) conversion in the TOP assay at reaction time of 5 – 360 min. Figure 4. Photo-degradation of 8:2 PTrMeOSi at exposure time of 6 h – 10 days (a), and comparison of PFAS molar ratios between photo-degradation and dark control at the exposure time of 10 days (b). TOC Art (image created by the authors in the original manuscript submission)
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Figure 1. Chemical names, acronyms and molecular structures of polyfluoroalkylsilane (PFASi) substances. 459x143mm (300 x 300 DPI)
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Figure 2. Isotopic peaks and isotope fine structures of 8:2 PTrMeOSi by FT-ICR MS analysis. (a) [M+Na]+, [M+1+Na]+ and [M+2+Na]+ isotopic ions of 8:2 PTrMeOSi in AF-1061 commercial product diluted 106 times by methanol (upper), native standard (middle), and theoretical spectrum (bottom); (b) expanded [M+1+Na]+ isotope fine structures of 8:2 PTrMeOSi (blue dotted border); (c) expanded [M+2+Na]+ isotope fine structures of 8:2 PTrMeOSi (yellow dotted border). In the equations in panel b, MIM and RI are abbreviations of monoisotopic mass and relative intensity, respectively. RI 29Si and RI 13C represent instrumental signals of 29Si and 13C isotopic peak. nSi and nC represent the number of Si and C in the formula. The numbers of 5.08% and 1.08% refer to the natural abundance of 29Si and 13C isotopes, respectively. 229x72mm (300 x 300 DPI)
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Figure 3. Formation dynamics of detectable C4-C12 PFCA products from PFASi (a. 6:2 PTrMeOSi; b. 8:2 PTrMeOSi; c. 10:2 PTrMeOSi; d. 8:2 PTrEtOSi) conversion in the TOP assay at reaction time of 5 – 360 min. 189x134mm (300 x 300 DPI)
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Figure 4. Photo-degradation of 8:2 PTrMeOSi at exposure time of 6 h – 10 days (a), and comparison of PFAS molar ratios between photo-degradation and dark control at the exposure time of 10 days (b). 189x76mm (300 x 300 DPI)
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TOC Art (image created by the authors in the original manuscript submission) 84x47mm (600 x 600 DPI)
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