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Environmental Measurements Methods
Characterization of Nine Isomers in Commercial Samples of Perfluoroethylcyclohexane Sulfonate and of Some Minor Components Including PFOS Isomers Tomislav Stefanac, Robert McCrindle, Alan McAlees, Nicole Riddell, Allison L Brazeau, and Brock Chittim Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02369 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018
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Characterization of Nine Isomers in Commercial Samples of
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Perfluoroethylcyclohexane Sulfonate and of Some Minor Components
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Including PFOS Isomers
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Tomislav Stefanac,*,† Robert McCrindle,*,†,§ Alan J. McAlees,† Nicole Riddell,† Allison L.
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Brazeau,† and Brock C. Chittim†
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†
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§
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Wellington Laboratories Inc., Research Division, Guelph, ON, Canada, N1G 3M5 Department of Chemistry, University of Guelph, Guelph, ON, Canada, N1G 2W1
Abstract
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Electrochemical fluorination of 4-ethylbenzenesulfonyl halides produces a mixture of
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compounds that has found extensive use as an erosion inhibitor in aircraft hydraulic fluids. This
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paper reports a study of the composition of commercial samples of this material from two
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industrial scale manufacturers in terms of the structures and relative concentrations of their
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components, the major of which is perfluoroethylcyclohexane sulfonate (PFECHS).
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Fractionation of one of these mixtures by column chromatography produced fractions in which
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all significant components were of sufficient purity to allow assignment of their structures by 19F
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NMR spectroscopy. Assessment of the relevant signals in the
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commercial mixtures by integration revealed the presence of fourteen constituents at levels
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≥0.5% in terms of molar contributions. Ten of these involve five pairs of geometric isomers,
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including cis- and trans-PFECHS which accounted for between 55-60% of the components
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present. Three constituents were determined to be isomers of perfluorooctane sulfonate (PFOS),
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two branched and the other linear PFOS itself. The availability of samples of the various
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F NMR spectra of the
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components also allowed us to identify the compounds responsible for the peaks observed when
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the commercial samples were analyzed by LC/MS using either C18 or perfluorophenyl stationary
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phases.
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Introduction
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Perfluoro-4-ethylcyclohexanesulfonate (PFECHS: 1) (Figure 1) was one of the many
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products protected in patents1,2 covering the electrochemical production3 of fluorocarbon acid
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fluorides and their derivatives. Perfluoroalkane carboxylates and sulfonates have been
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extensively used in industrial processes and consumer products because of their unique
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properties and, as a result, are now found in a large variety of global environments.4-9 PFECHS
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was singled out for mention in a 1972 patent10 on the use of such surfactants as additives in
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inhibiting the erosion engendered by phosphate based hydraulic fluids. A more recent patent11
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compared the effectiveness of two 3M products, FC-95 [commercial perfluorooctane sulfonate
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(PFOS)] and FC-98, a mixture obtained by fluorination of a 4-ethylbenzenesulfonyl halide,
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specifically as components of base stocks in aircraft hydraulic fluids. Indeed, the use of PFECHS
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in aircraft hydraulic fluids was suggested as a likely source of this product in surface waters and
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fish from the Great Lakes of North America especially since the USEPA in 2005 had granted
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permission for the use of “this chemical based on the lack of alternatives, its critical role in the
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safe performance of aircraft, and anticipated minimal environmental release”.12 With global
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production volumes of 4.5–227 tonnes in 199813 it is not surprising that this compound has
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subsequently been identified in North American indoor dust samples,14 in herring gull and
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Caspian tern eggs,15,16 and liver samples from Canadian Arctic polar bears.17 PFECHS has also
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been identified in Canadian bodies of water proximal to airports in Ontario18-21 and the High
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Arctic.22 Recent studies have found that this contamination is not confined to North America but ACS Paragon Plus Environment
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is likely a worldwide problem. PFECHS has been detected in water, sediment and fish samples
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in an area surrounding Beijing International Airport, China23 and possibly in concrete and soil
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cores close to Brisbane Airport, Australia.24 The recent detection of PFECHS in the snow of the
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Devon Ice Cap in the High Arctic,25 and other remote Arctic26 sites, prompted the authors to
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suggest that one possibility for its presence in such a remote location is emission from the
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pressurized hydraulic systems of aircraft, which are used at all stages of flight. This possibility
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warrants further investigation of a much wider range of sites, for example some in Europe and in
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the USA. The use of PFECHS in aircraft hydraulic fluids is apparently unique, leading to the
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possibility of using it as a marker for the source of other contaminants found in environmental
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samples that potentially have been distributed by aircraft, e.g. phosphate esters and other
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components of engine lubricants and hydraulic fluids.27-29
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The Material Safety Data Sheet provided by 3M for FC-98 listed the following ingredients
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PFECHS (1) (66-70%), perfluoro-4-methylcyclohexane sulfonate (PF4MeCHS: 2) (18-22%), a
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perfluoro-dimethylcyclohexane sulfonate (identified herein as the 4,4-dimethyl isomer,
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PF44diMeCHS: 3) (9-13%) and perfluorocyclohexane sulfonate (PFCHS: 4) (1-3%) along with
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other “residual organic fluorochemicals” (0.1-0.5%). Structures of these compounds, and of
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others identified in this study, are shown in Figure 1. De Silva et al.12 detected 1 and 2 in their
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PFECHS standard. Using liquid chromatography (LC) tandem mass spectrometry (MS) and a
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pentafluorophenyl (PFP) stationary phase, Wang et al.23 were able to demonstrate that
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constituents in their “standard” solution generated five major peaks, all of which showed the
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same mass for the parent ion. They attributed four of these peaks to positional isomers of
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perfluoroethylcyclohexane sulfonate and the fifth to one or more positional isomers of
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perfluoropropylcyclopentane sulfonate. Also,
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compounds were found in airport runoff water than those of PFOS.23 Importantly, isomer-
much higher concentrations of this group of
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specific differences in tissue distributions and bioaccumulation factors of these compounds were
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noted for fish from a pond impacted by the runoff.23
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Figure 1. Structures of compounds identified in this study. The open chain structures (8-10) are
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drawn in the manner shown to allow the visualization of single bond cleavages in potential cyclic
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intermediates for the formation of these products (see below).
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Although interest in analyzing environmental matrices for the presence of PFECHS is steadily
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increasing, the work on identification is hampered by the fact that these technical products
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contain a complex mixture of isomeric and related compounds which leads to an increased
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uncertainty associated with the reported concentrations. Early workers in this area used18 a
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commercial mixture of PFECHS for a qualitative comparison while later workers purchased a
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“standard” which, although claimed to have 97% purity, seems to contain a relatively complex
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mixture of components similar to those present in commercial products A and B (Table 1).23
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Unfortunately, it appears that the acute toxicity of PFECHS on the zooplankton Daphnia magna
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was assessed using this same mixture.20 In this context, we report on the results of a study aimed
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at elucidating the composition of commercial samples of PFECHS from two major industrial
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scale manufacturers (Table 1: samples A and B) in terms of the structures and relative
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concentrations of their components.
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Table 1. Summary of the characterized components of PFECHS commercial mixtures A and B. Structure Number 1 2 3 4 5 6 7 8 9 10
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IUPAC Name
Acronym
cis-perfluoro-4-ethylcyclohexane sulfonate trans-perfluoro-4-ethylcyclohexane sulfonate cis-perfluoro-4-methylcyclohexane sulfonate trans-perfluoro-4-methylcyclohexane sulfonate perfluoro-4,4-dimethylcyclohexane sulfonate perfluorocyclohexane sulfonate syn-perfluoro-3-ethyl-3-methylcyclopentane sulfonate anti-perfluoro-3-ethyl-3-methylcyclopentane sulfonate cis-perfluoro-3-ethylcyclohexane sulfonate trans-perfluoro-3-ethylcyclohexane sulfonate cis-perfluoro-3-propylcyclopentane sulfonate trans-perfluoro-3-propylcyclopentane sulfonate perfluoro-6-methylheptane-3-sulfonate perfluoro-5,5-dimethylhexane-2-sulfonate perfluorooctane-1-sulfonate
cis-PFECHS trans-PFECHS cis-PF4MeCHS trans-PF4MeCHS PF44diMeCHS PFCHS syn-PF3Et3MeCPS anti-PF3Et3MeCPS cis-PF3EtCHS trans-PF3EtCHS cis-PF3PrCPS trans-PF3PrCPS PF6MeHp3S PF55DiMeHx2S linear-PFOS
Molecular Formulae† C8F15O3S¯ C7F13O3S¯ C8F15O3S¯ C6F11O3S¯ C8F15O3S¯ C8F15O3S¯ C8F15O3S¯ C8F17O3S¯
Percentage‡ A B 31.2 30.5 26.7 27.7 1.8 2.0 1.4 1.7 16.9 18.6 2.1 0.9 5.2 5.0 4.0 3.9 1.1 0.6 0.5 0.2 3.6 4.0 2.9 2.9 2.2 1.7 0.5 0.2 0.2
