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Environmental Measurements Methods
Gas-Phase Detection of Fluorotelomer Alcohols and Other Oxygenated PFAS by Chemical Ionization Mass Spectrometry Theran P. Riedel, Johnsie Lang, Mark J. Strynar, Andrew B. Lindstrom, and John H. Offenberg Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00196 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 14, 2019
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Environmental Science & Technology Letters
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Gas-Phase Detection of Fluorotelomer Alcohols and Other Oxygenated PFAS by Chemical
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Ionization Mass Spectrometry
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Theran P. Riedel1*, Johnsie R. Lang2, Mark J. Strynar1, Andrew B. Lindstrom1, John H.
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Offenberg1
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1National
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Research Triangle Park, North Carolina, 27711, United States
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2Oak
Exposure Research Laboratory, United States Environmental Protection Agency,
Ridge Institute for Science and Education, National Health and Environmental Effects
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Research Laboratory, United States Environmental Protection Agency, Research Triangle Park,
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North Carolina, 27711, United States
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*Corresponding Author
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Email:
[email protected] 16
Phone: 919-541-0877
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ABSTRACT
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Per- and polyfluoroalkyl substances (PFAS) are incorporated into an ever-increasing number of
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modern products and inevitably enter the environment and ultimately human bodies. Herein we
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show that chemical ionization mass spectrometry with iodide reagent ion chemistry is a useful
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technique for the detection of fluorotelomer alcohols (FTOHs) and other oxygenated PFAS,
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including per- and polyfluoro carboxylic acids such as hexafluoropropylene oxide dimer acid. This
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technique offers direct, high time resolution measurement capability with parts per trillion by
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volume (ng m-3) gas-phase detection limits. Measurements were made by direct volatilization of
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samples without prior processing, allowing for fast measurements and reduced sample treatment
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compared to established PFAS methods. We demonstrate the utility of this technique by sampling
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volatile and semivolatile PFAS from fluoro-additives and fluoro-products to quantify levels of
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FTOHs and identify additional fluorinated compounds for which standards were unavailable.
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INTRODUCTION
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Per- and polyfluoroalkyl substances (PFAS) are used in a wide variety of industrial
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applications including textile, materials, and chemical manufacturing due to beneficial mechanical
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properties and relative inertness they can help lend to consumer and industrial products.1 Upon
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release into the environment, these compounds are subject to slow or negligible environmental
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degradation and are thus considered an emerging class of pollutants capable of persistent global
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buildup and increasing human exposure.2-3
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PFAS are believed to number in the thousands,4 without considering the likely complex
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array of related environmental degradates. The rate with which these chemicals are produced and
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released to environments often vastly outpaces the time required for the development of traditional
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measurement approaches that typically focus on a small subset of chemicals and occurs over
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multiple years. As a consequence, human populations incur exposure and risk potentially for years
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before the presence of contamination is recognized. Advances in high-resolution mass
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spectrometry and data analytics provides a means of broadening the window of PFAS
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identification to evaluate current occurrence rather than the multiyear lag of traditional methods.5
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Fluorotelomer alcohols (FTOHs), perfluorinated carboxylic acids (PFCAs), and perfluoro
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ether carboxylic acids (PFECAs) are three environmentally relevant and related classes of PFAS.
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In this study we focus on four representative FTOHs, two PFCAs, and one PFECA: 4:2, 6:2, 8:2,
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and 10:2 FTOH; perfluorooctanoic acid (PFOA); perfluorobutanoic acid (PFBA); and
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hexafluoropropylene oxide dimer acid (HFPO-DA). FTOHs have been shown to be the dominant
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PFAS measured in landfill gas, but measurements are limited to the fraction that can be collected
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on impregnated foam disks.6 FTOHs can degrade to form certain PFCAs, which are extremely
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stable compounds, and have been implicated to play a major role in the global dissemination of
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PFCAs.7-9 PFOA has been linked to adverse health effects.10-11 It is often referred to as a “legacy”
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PFAS since it has largely been phased out of industrial use over the past decade in the United
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States and Europe but still remains environmentally relevant due to persistent levels in ground and
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surface waters and landfills. HFPO-DA (perfluoro-2-propoxypropanoic acid) is often referred to
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under the product name “GenX” and has received attention in states such as North Carolina where
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PFAS manufacturing processes take place.12
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Considering the long-chain fluorinated “tails” present on most PFAS, these compounds
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have a considerably larger vapor pressure than equivalent hydrocarbon-based ones. For example,
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dodecan-1-ol (C12H26O) has an equilibrium vapor pressure almost five hundred times less than that
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of 10:2 FTOH (C12F21H5O), an analogous FTOH.13-14 Thus, a significant fraction of PFAS can
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reasonably be expected to be present in the gas phase, which is therefore a likely exposure pathway.
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In comparison to condensed-phase measurements of PFAS, gas-phase assessments are rare,
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largely due to the lack of established methods for reliable sample collection and quantification, but
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such measurements are needed considering recent increases in awareness that gas-phase releases
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may contribute significantly to overall PFAS from landfills and industrial sources. For example,
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attempts to constrain HFPO-DA in industrial stack emissions at one facility resulted in vastly
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different estimates of annual emissions: early estimates of 66.6 lb year-1 were adjusted to 2,758 lb
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year-1, a 40-fold difference, when alternative methods were used.15
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Previous publications regarding gas-phase PFAS field sampling techniques have largely
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been limited to offline analyses of liquid-phase extracts (i.e. capture of PFAS from ambient air on
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sorbent impregnated disks, followed by extraction and concentration for detection by gas
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chromatography-mass spectrometry (GC-MS) and/or liquid chromatography-mass spectrometry
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(LC-MS)).6, 16 Given the collection times, these approaches are rarely able to capture valuable
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temporal information such as diurnal variations or emission plume intercepts which inevitably are
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averaged and/or diluted over the entire collection period. Clearly, a real-time continuous
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measurement technique for the detection of these compounds would be valuable.
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In addition to field sampling, there is a need to accurately and directly quantify PFAS in
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products like aqueous firefighting foams, industrial dispersions, and cleaners in order to better
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constrain the contributions of such materials to the environmental PFAS burden from product
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manufacturing and landfills. Previous publications that reported PFAS content of products used
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extraction and cleanup steps where volatile or semivolatile PFAS could potentially be lost. For
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example, Kotthoff et al.1 reported 6:2 FTOH concentrations obtained using extractions procedures
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that included product evaporation to dryness followed by reconstitution prior to analysis.
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Our objective here is to demonstrate that an iodide-adduct chemical ionization mass
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spectrometer shows potential as a reliable method for direct detection of FTOHs and other PFAS
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that have previously been difficult to detect in the gas phase at appropriate time resolution. We
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attempt to illustrate that utility through instrument calibrations of FTOHs, PFCAs, and a PFECA,
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and by directly sampling semivolatile PFAS emitted from four commercially available fluoro-
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products: three fluorosurfactants and an aqueous film-forming firefighting foam. PFAS present in
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those products for which corresponding standards were available were quantified, and additional
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PFAS signals were identified by probable molecular compositions.
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MATERIALS AND METHODS
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Chemical Ionization Mass Spectrometry. A high-resolution time-of-flight chemical
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ionization mass spectrometer (ToF-CIMS, Aerodyne Research Inc/TOFWERK AG) operated in
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negative ion mode with iodide reagent ion chemistry was used for this study (see Table S1 and
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Figure S1 for instrument details). This technique has been used previously to quantify inorganic
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and polar organic atmospheric trace gases and offers online, direct, and fast measurements without
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the need for lengthy collection, extraction, and/or derivatization prior to detection.17-19 Sample air
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is drawn directly into the inlet where analyte species are ionized by forming molecular adducts
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with the iodide reagent ion, a soft ionization process that typically preserves parent ions with little
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fragmentation. Analyte ion detection occurs at the parent + iodide mass-to-charge ratio (m/Q), and
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all signals were normalized to the total reagent ion signal (I- + I(H2O)-) and reported per million
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counts per second (cps) of total reagent ion. As in other studies utilizing an iodide-adduct CIMS,
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the ion optics were tuned to maximize both the total ion current and the ratio of the iodide-water
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cluster to iodide (I(H2O)-/I-).17 The latter was used as an indication of the collision-induced
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molecular ion declustering potential, which we hoped to minimize thereby preserving parent ions.
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Air containing the gas-phase analytes was sampled directly into the instrument at 2.1 L
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min-1. Though the CIMS is capable of faster data acquisition (>1 Hz), full mass spectra (3 – 1132
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Da) measured at 0.33 Hz were sufficient for the investigations presented here. Preliminary proof-
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of-concept tests in which the CIMS sampled the headspace above neat FTOH, per-, and polyfluoro
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carboxylic acid standards indicated that the instrument effectively detected these classes of
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compounds, and that the volatility was such that they were present in significant gas-phase
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quantities. Instrument calibrations were then performed on 4:2, 6:2, 8:2, and 10:2 FTOH; PFOA;
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PFBA; and HFPO-DA. During calibrations, dilute solutions of the standards were prepared in ethyl
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acetate with concentrations ranging from 1 – 18 ng µL-1, depending on the standard. During
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calibrations, 1 – 10 µL of standard solution was injected onto a 47 mm diameter, 2.0 µm pore size
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polytetrafluoroethylene (PTFE) filter (Pall Corporation) within a perfluoroalkoxy (PFA) polymer
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filter holder (Savillex) placed immediately upstream of the CIMS inlet. Humidified clean air (50%
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RH) was used as the dilution/carrier gas. Upon contact with the filter, analytes were continuously
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volatilized into the CIMS, and signals were integrated until
