Water to air transfer of branched and linear PFOA influen 2017 emerging con

Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con Water to air transfer of branched and linear PFOA influen 2017 emerging con

Emerging Contaminants (2017) 46e53 Contents lists available at ScienceDirect Emerging Contaminants journal homepage: http://www.keaipublishing.com/en/journals/ emerging-contaminants/ Water-to-air transfer of branched and linear PFOA: Influence of pH, concentration and water type Jana H Johansson a, *, Hong Yan a, b, Urs Berger c, Ian T Cousins a €g 8, 11418 Stockholm, Sweden Department of Environmental Science and Analytical Chemistry (ACES), Stockholm University, Svante Arrhenius va State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, China c Department Analytical Chemistry, Helmholtz Centre for Environmental Research e UFZ, Leipzig, Germany a b a r t i c l e i n f o a b s t r a c t Article history: Received 15 December 2016 Received in revised form February 2017 Accepted March 2017 Available online 18 March 2017 The volatilisation of perfluorooctanoic acid (PFOA) was measured experimentally at a range of pH values using a previously published laboratory method Water-to-air transfer was studied for five structural isomers, namely: the linear isomer (n-PFOA) and the four most commonly occurring branched isomers (3-, 4-, 5- and 6-PFOA) The influence of water concentration and water type on the pH-dependent waterto-air transfer was also investigated for n-PFOA The water-to-air transfer was studied over the course of 48 h at pH values ranging from 0.2 to 5.5 Under all experimental conditions tested, the volatilisation of PFOA was negligible at pH > 2.5 In experiments performed with MilliQ water, volatilisation increased with decreasing water pH In experiments performed with tap water and lake water, maximum volatilisation was observed at pH The concentration of PFOA in water had no influence on the pH value at which water-to-air transfer was observed (i.e at pH < 2.5) for the concentration range tested (0.1e50 mg/ L PFOA in deionised water) Although the percentage of PFOA volatilised was significantly different for the four branched isomers at low pH, volatilisation was not observed above pH 2.5 for any branched isomer suggesting that all PFOA isomers have a low pKa Overall, these laboratory results demonstrate that volatilisation of any structural isomer of PFOA from water is negligible at environmentally-relevant conditions It is unlikely that PFOA isomers will be fractionated in the environment as a result of volatilisation because it is a process of negligible environmental relevance Copyright © 2017, The Authors Production and hosting by Elsevier B.V on behalf of KeAi Communications Co., Ltd This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) Keywords: PFOA Isomer Branched Acid dissociation pKa Introduction Perfluoroalkyl carboxylic acids (PFCAs) are a group of anthropogenic chemicals which have been observed in humans [1e3], biota [4] and abiotic matrices [5,6] worldwide All PFCAs are persistent [7] and some long-chain PFCAs (with ! perfluorinated carbons) have also been identified as bioaccumulative [8] Longchain PFCAs are thus considered to be very persistent and very bioaccumulative (vPvB) chemicals and are subject to regulation under REACH [9] PFCAs comprise a carboxylic acid group linked to a fully fluorinated carbon chain The dissociated anionic forms of PFCAs are water soluble [10e12], surface active [13] and assumed to be involatile [14] The neutral forms, however, are volatile [15,16] * Corresponding author E-mail address: jana.johansson@aces.su.se (J.H Johansson) Peer review under responsibility of KeAi Communications Co., Ltd and have lower solubilities in water [17] As the properties of the protonated and deprotonated forms of these acids are diametrically opposite with respect to volatility, accurate knowledge of their acid dissociation constants (herein referred to as pKa, the negative logarithm of the constant) is crucial to understand how they are released to the environment (i.e to air, water or soil), their environmental fate [18,19] and their uptake in biota [20] Because the pKa value is important to understanding the environmental cycling behaviour of perfluorinated alkyl acids (PFAAs), it has received a lot of attention in the scientific literature [21e25] It was pointed out that traditional titration methods are not suitable for determination of the pKa values of PFCAs, due to the substances' high surface activity, as well as the low water solubility of their neutral form [26] Attempts at determining the pKa values of PFCAs have therefore been made using mass spectrometry [27] and computer models [17,24,26,28], generating estimates in the range 0e1 Vierke et al [29] developed an indirect pKa estimation method which takes advantage of the fact that the neutral form of the acid is http://dx.doi.org/10.1016/j.emcon.2017.03.001 2405-6650/Copyright © 2017, The Authors Production and hosting by Elsevier B.V on behalf of KeAi Communications Co., Ltd This is an open access article under the CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) J.H Johansson et al / Emerging Contaminants (2017) 46e53 volatile and can partition from a water solution to the overlying air From their observations, Vierke et al drew the conclusion that the pKa of C6-C14 PFCAs must be < 1.6 Furthermore, they constrained the pKa of PFOA to 0.5 by fitting measured data to an air-water exchange model The PFOA isomer pattern observed in environmental samples has been used as an indicator of the type of sources which gave rise to the contamination [30] PFOA has been manufactured using two different production processes: electrochemical fluorination (ECF) generates a mix of branched and linear isomers, whereas telomerisation generates the linear isomer only The relative amount of ECF- and telomer-derived PFOA found in a specific environment can therefore be determined by investigating the PFOA isomer pattern The two production processes dominated the market at different points in time and have been used to manufacture different types of chemical formulations in which PFOA or precursors of PFOA can occur Study of the distribution between ECF- and telomer-derived PFOA in environmental samples therefore contributes to knowledge about the types of perfluoroalkyl acid sources influencing the environment However, it is not known to what extent PFOA isomers are subject to abiotic fractionation, which could confound the determination of manufacturing origin McMurdo et al [31] hypothesised that substantial differences in the pKa values of branched and linear PFOA isomers lead to a significantly elevated long-range atmospheric transport potential of linear PFOA (nPFOA) in relation to branched isomers To our knowledge, pKa values of branched PFOA isomers have not previously been reported However, the fact that PFOA isomers and their derivatives can be separated by liquid and gas chromatography [32e35], respectively, implies differences in their hydrophobicity and vapour pressure It was hypothesised that electron withdrawing trifluoromethyl (CF3-) groups stabilise the deprotonated species, making branched isomers stronger acids than the corresponding linear isomer This inductive effect is negligible when the separation between the trifluoromethyl group and acid group exceeds four alkyl units, meaning that among monomethyl isomers it is only expected for 2- and 3-PFOA [36] As long as pKa data is largely missing, it will not be known whether the branched and linear PFOA isomers have similar environmental fates, and thus the isomer patterns cannot be used to unequivocally trace sources Here, we use the experimental setup developed by Vierke et al [29] and test the method over a range of PFOA concentrations We hypothesise that PFOA concentration does not influence the apparent pKa in the tested range, as it is well below the critical micelle concentration (12.4 g/L) [37] and the concentration (828 mg/ L) at which formation of dimers has been reported at low pH [27] However, as the method of Vierke et al has previously only been tested at one PFOA water concentration, we set out to demonstrate its robustness over a range of PFOA concentrations We also perform experiments with tap and lake water to investigate whether presence of counter ions and organic matter influences the water-to-air transfer of PFOA Furthermore, we use the novel approach developed by Vierke et al to study the water-to-air transfer of four branched PFOA isomers A thorough explanation of the theory behind the approach is given by Vierke et al [29] Methods and materials 2.1 Terminology Herein, the PFOA isomers are described using nomenclature suggested by Benskin et al [35], i.e linear perfluorooctanoic acid (n-PFOA), perfluoro-3-methylheptanoic acid (3-PFOA), perfluoro-4methylheptanoic acid (4-PFOA), perfluoro-5-methylheptanoic acid (5-PFOA), perfluoro-6-methylheptanoic acid (6-PFOA) 47 Furthermore, perfluorooctanoate (PFO) and its conjugate acid perfluorooctanoic acid (PFOA) are collectively described by the acronym PFO(A), as suggested by Buck et al [38] 2.2 Experiments Water-to-air transfer experiments were carried out at nominal n-PFOA concentrations of 0.1, 1, 10 and 50 mg/L in MilliQ water (referred to as experiments IeIV in Table S1 in the Supplementary Data) Experiments were also performed with tap water and lake water spiked with n-PFOA at mg/L (experiments V-VI) Experiments IeVI were carried out at nominal pH values of 0.2, 0.6, 1.0, 1.5, 2.5 and 3.5 Experiment IV (50 mg/L) was also performed at pH 4.5 and 5.5 Further experiments were performed for four branched PFOA isomers (3-, 4-, 5-, 6-PFOA), at pH 0.5, 0.7, 1.5, 2.5 and 3.0 and a nominal analyte concentration of mg/L (experiments VII-X) These were the four perfluoromonomethyl-branched isomers which were commercially available at the time of the study Perfluorodimethyl-branched isomers were not included, as each of these only make up a negligible contribution to the technical product produced using ECF and the global emissions of PFOA Differences in the water-to-air transfer of branched PFOA isomers were tested by two-way analysis of variance (ANOVA: Two Factor with Replication) using the Microsoft Excel Data Analysis Tool The two different categorical independent variables were (a) the branched isomers and (b) the various pH values at which volatilisation was measured The continuous dependent variable (% PFOA sorbed to top wall at t2) was measured in duplicate for each isomer and for each pH The data used in the statistical analysis is displayed in Table S4 2.3 Chemicals, reagents and solvents Methanol (MeOH, LiChrosolv grade) was supplied by Merck ProAnalysis ammonium acetate was purchased from Merck Sodium hydroxide (98.6% J T Baker) solutions were prepared in water (17 mol/L) and in methanol (2.5 mol/L) If not otherwise stated, the water used in the study was generated in a MilliQ integral water purification system (Millipore) Lake water was taken from Norra € n, Sweden Details on sampling and storage are given Bergundasjo by Zou et al [39] Tap water was taken directly from a normal tap (drinking water system) at Stockholm University The water used in the water-to-air transfer experiments was adjusted to a certain pH with concentrated sulfuric acid (95e97% Sigma Aldrich) The actual pH was determined with a pH meter (PHM210 Radiometer Analytical, pH À9e23, ±0.2 pH units) Actual pH values for each data point are given in Table S2 An aqueous stock solution of native n-PFOA was prepared by dissolution of a crystalline standard (supplied by DuPont) in 60 mL methanol and subsequent addition of 500 mL water The solution was blown down under a stream of nitrogen at 37  C until 120 mL of solvent had evaporated (determined by weighing) As water and methanol not form an azeotrope and methanol has a higher vapour pressure than water, it is expected that all methanol was removed from the final standard The stock solution was further diluted with water to four concentrations (2, 20, 200 and 1000 mg/L) for use in the concentration dependence experiments Standard solutions of single native PFOA isomers were purchased from Wellington Laboratories The standards were obtained as 200 mL methanolic solutions at concentrations ranging from 1.90 to 3.10 mg/mL These were mixed with 500 mL water Thereafter the solutions were blown down with a stream of nitrogen at 37  C until 300 mL of solvent had evaporated The solutions were then diluted by a factor of 100 with water to approximately 20 mg/L The isotope labelled internal standard 13C4-PFOA was purchased from 48 J.H Johansson et al / Emerging Contaminants (2017) 46e53 Wellington Laboratories as a mg/mL solution in methanol and was further diluted to 50 mg/L This standard was certified to contain < 0.5% of its native analogue expected to be low and introduce only a small error to the mass balance calculations A detailed description of the sample preparation and extraction method is given in Vierke et al [29] 2.4 Experimental setup 2.6 Instrumental analysis and quantification All experiments (I-X), discussed in section 2.3 and listed in Table S1, comprise to data points Each data point represents one pH value and is generated from the preparation of four individual air-water partitioning experiments (Fig S1) The experiments were carried out in closed polypropylene vessels, whose top walls acted as passive samplers for gaseous PFOA We used the same type of 100 mL polypropylene vessels as Vierke et al [29] Each experiment was prepared by filling a vessel with mL of PFOA stock solution and 19 mL of the studied water type (pH-adjusted, tap or lake water) The transfer of PFOA from the water to the overlying gas phase was monitored after 48 h by analysis of the mass of PFOA on the top wall of each vessel, as well as the loss of PFOA from the bulk water To enable calculation of the overall mass balance in the system, sorption was also monitored for walls in contact with water The mass of PFOA observed in different compartments of the system after two days (t2) was compared to the measured mass of PFOA present in the water at the start of the experiment (t0) All experiments were performed in duplicate Thus, four vessels were set up for each PFOA concentration and pH value or water type Two of these were harvested for extraction and analysis right after setting up the experiments (t0) and two were harvested at the termination of the experiment (t2) An additional setup was used to monitor pH during the course of the experiment Samples were analyzed on a UPLC/MS/MS system (Waters Acquity and Xevo TQ-S) using negative electrospray ionization The column was a BEH C18 (1.7 mm particles, 50 mm  2.1 mm, Waters), operated at 40  C and with a flow rate of 0.4 mL/min The mobile phases were 10/90 methanol/water and methanol, both containing mM ammonium acetate The injection volume was 10 mL The instrumental method was previously described in detail by Vestergren et al [41] For quantification of n-PFOA, a six point external linear calibration curve was established covering a concentration range from 0.1 to 40 mg/L in methanol:water 1:1 (r2 > 0.99 for all curves) Branched PFOA isomers were quantified using a one-point external linear calibration (approximately 20 mg/L, three injections) A separate calibration solution in 1:1 methanol:water (containing mM ammonium acetate) was prepared for each individual PFOA isomer The linearity of the instrument response was tested for each of the analyzed PFOA isomers up to 20 mg/L (R2 > 0.99 for 6-PFOA and R2 > 0.96 for 3-, 4- and 5-PFOA) The concentration of internal standard was the same in all calibration solutions and corresponded to that in the sample extracts Quantification was undertaken using the isotope dilution method Response factors relative to the internal standard were derived from the calibration curve (1/X weighted linear regression), which was forced though the origin 2.5 Sample extraction 2.7 Quality assurance At the termination of each experiment, water was carefully decanted into a 50 mL polypropylene tube and spiked with mL internal standard solution To prevent further sorption or volatilisation of the analytes, the pH was immediately elevated to >10 by addition of NaOH Thereafter, the water samples were mixed with 20 mL of methanol and ultrasonicated for 30 at room temperature Due to the formation of a white precipitate in samples corresponding to pH 0.2 and 0.6, it was necessary to centrifuge these before an aliquot was taken for instrumental analysis In order to achieve quantifiable concentrations (i.e exceeding the instrumental quantification limit) in the final extracts of water samples from experiments carried out at 0.1 mg/L, these samples were enriched by a factor of 10 on a solid phase extraction (SPE) cartridge, using a method previously described by Ullah et al [40] Briefly, these samples were extracted on pre-washed mixed mode C8 quaternary amine solid phase extraction cartridges (CUQAX256, 500 mg, mL, UTC) Target compounds were eluted with mL of 2% 1-methyl piperidine in methanol/acetonitrile (80/20) After evaporation to 500 mL under a gentle stream of nitrogen, the extracts were mixed with 500 mL MilliQ water containing mM ammonium acetate After cutting the vessels open, the walls of the top and bottom parts were extracted separately by rinsing with mL of methanol containing 2.5 mol/L NaOH in addition to 50 mL internal standard solution Before instrumental analysis, 100 mL of the extract was mixed with 100 mL MilliQ water containing mM ammonium acetate To ensure that the PFOA observed on the top wall only represented sorbed gaseous species, the cut was made approximately cm above the water surface Consequently, the mass of PFOA reported on the walls of the lower half of the vessel can represent sorption of both gaseous and aquatic species, while the mass observed on the top wall may underestimate the total mass of PFOA volatilised However, the contribution from gaseous PFOA is Three sets of blank samples (i.e three duplicate experiments at t0 and t2) were prepared at pH 3.5 These were treated in the same way as the other set-ups apart from the fact that they were not spiked with PFOA No blank contamination was observed Therefore, PFOA was considered detected in a sample if its signal to noise ratio exceeded All signals above the detection limit were quantified After addition of internal standard no further sample cleanup was performed Thus, sample preparation recovery of the internal standard was by design of the experiment around 100% To get an idea of potential matrix effects, apparent recoveries (i.e the combination of matrix effects and recovery losses) of the internal standard in the samples was calculated by comparing peak areas observed for the internal standard in sample extracts and in the calibration curve (Table S2) These values ranged from 27% to 257% in water samples and from 32% to 184% in samples representing the top and bottom walls (Table S3) The fact that several of the values exceeded 100% shows that the analysis was influenced by matrix effects, likely caused by the presence of high concentrations of NaOH and H2SO4 in the sample extracts However, the use of 13C4PFOA as internal standard ensured reliable quantification despite matrix effects A majority of the duplicate water samples deviated from the mean by less than 10% The overall deviance from the mean (i.e including water and vessel wall extracts) is displayed graphically for each data point in Figs 4e6 The bottom walls were extracted at t0 for experiments III-IV and VI-X PFOA was detected in some of these samples However, the detected masses generally represented less than 0.3% of the total mass of PFOA present in the system at t0 Results and discussion Mass balances of the experiments showed that for pH > 1.5 close J.H Johansson et al / Emerging Contaminants (2017) 46e53 49 Fig Mass balance for concentration dependence water experiments (IeIV) Percentage of the total mass of n-PFOA remaining in different compartments of the system (water, top and bottom wall) at t2 relative to t0 for all pH values tested Fig Mass balance for tap and lake water experiments (V and VI) Percentage of the total mass of n-PFOA remaining in different compartments of the system (water, top and bottom wall) at t2 relative to t0 for all pH values tested to 100% of the total mass of PFOA spiked to the water at t0 was recovered at t2 (Figs 1e3) Consequently, the interpretation of the data rests on a valid assumption, namely that volatilisation is negligible at pH values for which PFOA cannot be detected on the top wall of the vessel The mass of PFOA observed in the water and on the walls decreased at lower pH values For some experiments 50 J.H Johansson et al / Emerging Contaminants (2017) 46e53 Fig Mass balance for branched isomer experiments (VIIeX) Percentage of the total mass of branched PFOA isomers remaining in different compartments of the system (water, top and bottom wall) at t2 relative to t0 for all pH values tested Fig Concentration dependence experiment (IeIV) Percentage of the total mass of PFOA in the system at t2 sorbed to the top of the vessel, as a function of water pH Each point represents the average of a duplicate sample Error bars represent the upper and lower bound of measured data Values < LOD are not plotted J.H Johansson et al / Emerging Contaminants (2017) 46e53 51 Fig Water characteristics experiment e comparison of MilliQ (experiment II), tap (III) and lake water (IV) Percentage of the total mass of n-PFOA in the system at t2 sorbed to the top of the vessel, as a function of water pH Each point represents the average of a duplicate sample Error bars represent the upper and lower bound of measured data Values < LOD are not plotted performed at pH 0.2, the mass balances could only account for around 60% of the spiked mass of PFO(A) Vierke et al [29] also observed this behaviour and stated that it is likely due to the fact that PFO(A) sorbs strongly to the vessel walls and cannot be extracted efficiently by rinsing with alkaline methanol Increased sorption to walls can be expected at low pH as neutral PFOA has a lower water solubility than its corresponding anion [17] The trend is not likely due to degradation, as PFOA is stable in water at low pH [42] The trend was not as pronounced in the tap and lake water experiments (Fig 2) as in those performed with MilliQ water (Fig 1) Sorption to walls in contact with water (bottom walls) was below 6% for all experiments except those performed with lake water, for which sorption of up to 12% was observed PFOA was observed on the top wall in all experiments carried out at pH 0.2e1.5 The percentage of the total mass of PFO(A) at t2 sorbed to the top of the vessel increased with decreasing water pH At pH > 2.5 the mass of PFOA observed on the top wall was either not detectable or represented less than 1% of the total mass in the system This suggests that the pKa of PFOA is approximately two pH units below 2.5, i.e approximately 0.5 The experiments performed with concentrations ranging from 0.1 to 50 mg/L (Fig 4) all display the same trend, which is in line with the observation made by Vierke et al [29] at mg/L and supports their conclusion that the pKa of n-PFOA is approximately 0.5 We therefore conclude that the PFOA concentration does not influence the water-to-air transfer at different water pH and thus the apparent pKa in the tested range Presence of PFOA aggregates and dimeric (PFO)2HÀ clusters result in higher observed apparent pKa values than those observed for monomeric PFOA [27,37,43] The fact that we did not observe any differences in the experiments performed at different PFOA concentrations suggests that at environmentally relevant PFOA concentrations (0.1e1 mg/L), and up to 50 mg/L, aggregates and dimers either are not formed or have a negligible effect on the airwater transfer of PFOA The results of the experiments performed with tap and lake water displayed maximum sorption to the top of the vessels at approximately pH (Fig 5) Potentially the phenomenon is caused by partitioning of neutral PFOA to organic matter or formation of involatile ion pairs between PFOÀ and cations present in solution This hypothesis is supported by the fact that at the two lowest pH values, 60e72% of the added PFO(A) remained in tap and lake water (Fig 2), while only 34e43% remained in MilliQ water (1 mg/L experiment, Fig 1) Sorption of PFOA to dissolved organic matter has been demonstrated previously [44,45] The statistical analysis using ANOVA (Two Factor with Duplication; see results in Table S5 of the Supplementary Data) showed statistically significant differences (p < 0.05) in PFOA volatilised (percentage of total mass sorbed to top at t2) for the four branched isomers and a statistically significant difference (p < 0.05) between PFOA volatilised at the various pH values studied No statistically significant (p < 0.05) interaction effects were observed between the two independent variables (pH and structural isomer) Although the percentage of PFOA sorbed to the top was significantly different for the branched isomers, volatilisation was not observed above pH 2.5 for any branched isomer suggesting that all PFOA isomers have low pKa values (Fig 6) We cannot, however, accurately derive pKa values for the branched PFOA isomers, as that would require knowledge of their air-water partitioning constants, which are not available for these substances To our knowledge, no measured pKa values have been reported for branched PFOA isomers Using the 52 J.H Johansson et al / Emerging Contaminants (2017) 46e53 Fig Percentage of the total mass of PFOA isomers in the system at t2 sorbed to the top of the vessel, as a function of water pH (experiments II and VII-X) Each point represents the average of a duplicate sample Error bars represent the upper and lower bound of measured data Values < LOD are not plotted All experiments using branched PFOA isomers were performed at the same pH-values The data points have been offset slightly to increase visibility At pH 3, spiking with 6-PFOA was neglected Therefore, data for this isomer is only available for the three lowest pH values computer model COSMOtherm, Wang et al [17] predicted the pKa of 6-PFOA to be 0.9, similar to that of n-PFOA, while the pKa values for 3-, 4- and 5-PFOA were predicted to be 0.7e1.0 units higher However, Wang et al stated that their estimates are highly uncertain as it was not possible to accurately model all molecular conformations of the anionic species, which have a strong influence on the modelled values Modelled estimates reported by Rayne and Forest [24] suggest that no substantial deviations from the pKa of the straight chain analogue are expected for the branched isomers tested here Monomethyl branching at the a-carbon (i.e the carbon atom closest to the carbonyl group) is expected to give rise to a 0.5 log unit decrease in pKa in relation to n-PFOA [36] This isomer (2PFOA) could not be included in our study, as it was not retailed at the time However, as it only makes up 0.1% of the technical PFOA produced using ECF [30], it is not expected to substantially influence the distribution between branched and linear PFOA isomers observed in the environment The isomers studied here (3-, 4-, 5and 6-PFOA) made up 21.6% of M ECF PFOA, in which the total content of branched isomers was estimated to 22.4% [30] Thus, our results not support the hypothesis put forward by McMurdo et al [31] that a unit difference in the pKa values of branched and linear PFOA isomers would result in a significant increase in the long-range atmospheric transport potential of n-PFOA in relation to branched isomers Instead, our data suggests that PFOA isomers not undergo abiotic fractionation via volatilisation Fractionation of PFOA isomers has been observed in soil [46] and has been hypothesised to occur in the formation of sea spray aerosols [31] However, it was not observed when investigated in a Chinese river [47] Thus, more research is needed to establish whether and in what matrices PFOA isomer patterns perform well as markers of manufacturing origin This information is crucial in the interpretation of field data, when seeking to understand the influence of different sources of PFOA to the environment Conclusions Our results show that the results of Vierke et al [29] can be reproduced successfully and that the performance of the method is not sensitive to PFOA concentration in the tested range (0.1e50 mg/ L) Presence of counter ions and dissolved organic carbon did not enhance the partitioning of PFOA from water to air On the contrary, volatilisation was suppressed at low pH values in these experiments Furthermore, our results suggest that none of the most ubiquitous PFOA isomers volatilise at environmentally relevant pH (>2.5) in surface waters This means that PFOA isomers are not likely to be fractionated via volatilisation from water because it is a process of negligible environmental relevance Acknowledgements This research was funded by the Swedish Research Council Formas (project number 2011-1345) Appendix A Supplementary data Supplementary data related to this article can be found at http:// J.H Johansson et al / Emerging Contaminants (2017) 46e53 dx.doi.org/10.1016/j.emcon.2017.03.001 References [24] [1] K Kato, L.-Y Wong, L.T Jia, Z Kuklenyik, A.M Calafat, Trends in exposure to polyfluoroalkyl chemicals in the U.S population: 1999À2008, Environ Sci Technol 45 (2011) 8037e8045 [2] L.S Haug, C Thomsen, G Becher, Time trends and the influence of age and gender on serum concentrations of perfluorinated compounds in archived human samples, Environ Sci Technol 43 (2009) 2131e2136 [3] T Zhang, Q Wu, H.W Sun, X.Z Zhang, S.H Yun, K Kannan, Perfluorinated 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