Occurrence and degradation potential of fluoroalkylsilane substances

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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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Environmental Science & Technology

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

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Environment Research Institute, Shandong University, Binhai Road 72, Qingdao, 266237,

China

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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]

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ACS Paragon Plus Environment


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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

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of instrumental linearity. Subsequently, isotope-labeled surrogate standards (MPFAC-

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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

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in a solar simulator, integrated with one circulating water cooling system and three 2500

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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

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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

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(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

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ionization enhancement was observed, as matrix interferences of the isotope-labeled

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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

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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

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interferences of the isotope-labeled injection standards (n = 30, spiked at 2 ng for each

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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

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molecular structures are easily lost by hydrolysis to form silanol (Si-OH) groups with the

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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

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polymerization in solution.36 Therefore, the stock solutions of PFASis were freshly

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prepared in methanol before all experiments, and kept under water-free environment. The

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characteristic suggests low efficiency using gas/liquid chromatography separation

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procedure. Thus, PFASi analogues were infused directly into the electrospray ion source

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of FT-ICR MS in the positive ionization mode. Owing to ultrahigh resolution (> 500 000

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at m/z 400) and mass accuracy (< 2 ppm) of the HRMS instrument, measurement of

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compound accurate mass and isotope fine structures could be achieved especially in

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complex mixtures.

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Different cationic adducts of 8:2 PTrMeOSi ions could be seen in the mass spectrum

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(Figure S1), and the peak intensity of Na+ adduct was the most abundant compared with

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those for NH4+, K+ or H+ for all trimethoxy and triethoxy PFASis. Generally, [M+H]+

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adduct is formed with high abundance in the positive ESI mode, nevertheless, universal

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and stable Na+ adducts as ions of siloxane in mass spectrum37 were observed. Compared

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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

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(10:2 PTrMeOSi) enhancement, the MDLs of which reached to 0.13 ± 0.01 pg (6:2

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PTrEtOSi, n = 3). Thus, following discussion was all based on the Na+ adducts of PFASis

296

if not specially stated.

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After accurate mass measurement and elemental composition assignment, isotope fine

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structures could be used as an effective means to further confirm existence of Si element

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in the molecule.38 For instance, m/z = 591.02562 is measured monoisotopic mass of the

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[M+Na]+ ion of 8:2 PTrMeOSi native standard (Figure 2 and Figure S2-S5), with a mass

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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

303

isotopic natural abundance of which is 1.08 %, 5.08 % and 3.35 %, respectively. It is

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obvious that individual

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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

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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

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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.

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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

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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



327

method was helpful for visualization of mass spectrum by reducing interferences.26 The

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research targets in this study were mainly limited to elements with negative mass defects,

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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


Page 14 of 33

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352

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



377 378

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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402

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



427

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


Page 18 of 33

Page 19 of 33


452

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



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


Page 20 of 33

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502


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



508

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509

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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)

28


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Figure 1. Chemical names, acronyms and molecular structures of polyfluoroalkylsilane (PFASi) substances. 459x143mm (300 x 300 DPI)



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)



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)


Occurrence and degradation potential of fluoroalkylsilane substances

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