Article pubs.acs.org/est
Isomer-Specific Biotransformation of Perfluorooctane Sulfonamide in Sprague−Dawley Rats Matthew S. Ross,† Charles S. Wong,†,‡ and Jonathan W. Martin*,§ †
University of Alberta, Department of Chemistry, Edmonton, Alberta T6G 2G2 Canada Environmental Studies Program and Department of Chemistry, Richardson College for the Environment, University of Winnipeg, Winnipeg, Manitoba, R3B 2E9 Canada § Division of Analytical and Environmental Toxicology, Department of Laboratory Medicine and Pathology, University of Alberta, Edmonton, Alberta T6G 2G3 Canada ‡
S Supporting Information *
ABSTRACT: Great variability exists in perfluorooctane sulfonate (PFOS) isomer patterns in human and wildlife samples, including unexpectedly high percentages (e.g., >40%) of branched isomers in human sera. Previous in vitro tests showed that branched PFOS-precursors were biotransformed faster than the corresponding linear isomer. Thus, high percentages of branched PFOS may be a biomarker of PFOS-precursor exposure in humans. We evaluated this hypothesis by examining the isomerspecific fate of perfluorooctane sulfonamide (PFOSA), a known PFOS-precursor, in male Sprague−Dawley rats exposed to commercial PFOSA via food for 77 days (83.0 ± 20.4 ng kg−1 day−1), followed by 27 days of depuration. Elimination half-lives of the two major branched PFOSA isomers (2.5 ± 1.0 days and 3.7 ± 1.2 days) were quicker than for linear PFOSA (5.9 ± 4.6 days), resulting in a depletion of branched PFOSA isomers in blood and tissues relative to the dose. A corresponding increase in the total branched isomer content of PFOS, the ultimate metabolite, in rat serum was not observed. However, a significant enrichment of 5m-PFOS and a significant depletion of 1m-PFOS were observed, relative to authentic electrochemical PFOS. The data cannot be directly extrapolated to humans, due to known differences in the toxicokinetics of PFOS in rodents and humans. However, the results confirm that in vivo exposure to commercially relevant PFOS-precursors can result in a distinct PFOS isomer profile that may be useful as a biomarker of exposure source.
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INTRODUCTION Synthetic derivatives of perfluorooctane sulfonyl fluoride (PFOSF; C8F17SO2F) were widely used in various consumer products since the early 1950s, including in firefighting foams and stain repellant coatings for carpet, paper, and textiles. Perfluorooctane sulfonate (PFOS; C8F17SO3−) is one such commercial derivative, and PFOS has garnered much regulatory and scientific scrutiny given its environmental persistence and pervasiveness, its bioaccumulation potential,1−3 and its toxicological characteristics.4 PFOS has been detected in most environmental compartments globally, including in remote regions of the Arctic,5−7 and in the blood of humans in most countries.8,9 Despite the ubiquity of PFOS, the routes by which humans and wildlife are exposed to PFOS are not well characterized. Some of the complexity around this issue arises because most PFOSF was not used to manufacture PFOS per se, but rather to make various perfluorooctane sulfonamides (C8F17SO2NRR′), which can be metabolized to PFOS in vitro10,11 and in vivo.12,13 PFOS is the ultimate metabolite of such precursors, and PFOS itself is not known to be metabolized. Thus, two general routes of exposure are involved. The first is direct exposure to PFOS, which may occur through the diet or ingestion of dust.8 The second is exposure to PFOS-precursors, and their subsequent © 2012 American Chemical Society
biotransformation to PFOS. The relative magnitude by which either of these two routes contributes to the overall PFOS body burden remains unclear, as does the variability from one population to another, and the historical variation over time. Models have estimated that PFOS-precursor exposure may account for 10% to 40% of the daily intake of PFOS for humans,8,14 however, considerable uncertainty exists in these estimates for several reasons: the lack of data on the toxicokinetics of various PFOS-precursor compounds in animals, the difficulty in extrapolating rodent data to humans, and the fact that many commercially relevant PFOS-precursors have never been analyzed in any sample.15 Recently, however, a new analytical technique utilizing the enantiomer signatures of PFOS confirmed that PFOSprecursors do contribute to human PFOS exposure.16 The historical manufacture of PFOS and its precursors occurred exclusively by electrochemical fluorination (ECF), which produces a mixture of branched and linear isomers. Historically, the 3M Co. was the predominant manufacturer of PFOS Received: Revised: Accepted: Published: 3196
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(over ∼80% of worldwide PFOS17), and generated PFOS products with a consistent isomeric composition of 70 ± 1.1% linear (mean ± standard deviation) and 30 ± 0.8% branched isomers over 8 production lots.18 Furthermore, from the limited data available, it appears that current ECF production in Asia produces a PFOS with a similar isomeric composition, with 22.8% to 30.9% branched isomers.19 Moreover, the isomeric composition of historically manufactured PFOS-precursors was similar to PFOS,20 i.e., ∼30% branched. Current PFOS-precursor manufacturing processes can thus be expected to produce a similar composition. Therefore, environmental or biological PFOS isomer profiles that differ significantly from 70% linear are likely indicative of isomer-specific fate processes in the environment and/or in organisms. By understanding such processes, the variation in PFOS isomer signatures observed in biomonitoring studies may perhaps be better interpreted to assist in identifying the many potential sources of PFOS exposure. Branching of long perfluorinated chains can have considerable effects on the toxicokinetics of perfluorinated compounds. In rats and fish, branched isomers of perfluorooctanoate (PFOA) are eliminated more quickly than the linear isomer, resulting in blood isomer profiles enriched in the linear isomer relative to the dose.21−23 Similarly, fish preferentially bioconcentrate the linear PFOS isomer,24 and rats preferentially eliminate branched isomers in urine, resulting in shorter blood half-lives for most branched PFOS isomers compared to linear.21,22 On the basis of these previous studies with PFOS, we anticipate that direct exposure to PFOS should result in isomer patterns in humans and wildlife with lower branched isomer composition than the manufactured material (i.e., ≤30%). While this principle generally holds true in wildlife biomonitoring,25−29 some instances have been reported where the branched isomer content in human blood is >40%,30,31 and in human umbilical cord blood, it can be >50%.32 The reasons for such enrichment of branched isomers remain unknown, but in vitro evidence suggests that isomer-specific metabolism of PFOS-precursors may be part of the answer. For example, in isolated human cyctochrome P450s and human liver microsomes, the branched isomers of N-ethyl perfluorooctane sulfonamide (NEtFOSA; C8F17SO2NH(C2H5)) were N-deethylated more rapidly than linear NEtFOSA, leading to an initial enrichment in the branched isomers of perfluoroctane sulfonamide (PFOSA; C8F17SO2NH2) in vitro.33 However, to date no studies have investigated the isomer-specific metabolism of a PFOS-precursor in vivo, nor the influence this might have on the resulting PFOS isomer profile in blood and tissues. This represents a significant knowledge gap, as it precludes the accurate interpretation of PFOS isomer profiles in biota and humans. The current study investigated the in vivo isomer-specific fate of PFOSA in male rats, an animal model which was previously used to elucidate the isomer-specific accumulation of PFOS.21,22 PFOSA is a known PFOS-precursor11 that can be detected in human blood,1 and which is the penultimate metabolite through which higher molecular weight PFOS-precursors are metabolized to PFOS.12 The metabolism of PFOSA to PFOS is also believed to be the rate-limiting step in PFOS precursor metabolism,11 and as such, the toxicokinetics of PFOSA metabolism will likely have a large impact on the observed PFOS isomer profile. Our hypothesis was that in vivo isomer-specific biotransformation of PFOSA would lead to preferential enrichment of the linear PFOSA at steady-state, thereby leading to a PFOS isomer profile in rat blood that should be distinct from commercially manufactured PFOS.
Article
MATERIALS AND METHODS
Isomer Nomenclature. The PFOS isomer nomenclature used here is based on a previously published convention.21,24,34 Monoperfluoromethyl isomers were named according to the carbon number on which the branch was situated. For example, 4m-PFOS refers to perfluoro-4-methyl-heptanesulfonate. The linear and isopropyl isomers are named as n- and iso-PFOS, respectively. All diperfluoromethyl PFOS isomers were integrated together as one chromatographic peak and are referred to collectively as Σdimethyls. The linear isomer of PFOSA (n-PFOSA) in ECF PFOSA was identified based on an authentic linear standard. We were unable to identify unambiguously the branched PFOSA isomers due to lack of authentic standards, and the absence of any isomer-specific tandem mass spectrometric multiple reaction monitoring transitions, as can be observed for branched PFOS isomers.34 Therefore, the various branched PFOSA isomers measured in this study were identified simply as Br1, Br2, and Br3 on the basis of earliest to latest elution order, respectively (Figure S1 of the Supporting Information, SI). Br1 consisted of at least three coeluting isomers, but assuming an analogous elution pattern to PFOS isomers, Br1 likely represents the dimethyl branched PFOSA isomers. Br2 may be a coelution of 3 or 4 monomethyl isomers, and Br3 is likely iso-PFOSA;34 however, these assignments could not be confirmed. Experimental Design. Prior to exposure, rats were randomly divided into control (n = 4) and experimental (n = 8) groups. For the duration of the uptake phase (77 days), experimental rats were given ad libitum access to ECF PFOSAspiked food, using ECF PFOSA provided to us by the 3 M Co., and created as noted in the SI. Control rats were given ad libitum access to unspiked food. On day 77, 3 experimental and 2 control rats were euthanized by continued exposure to CO2 for at least 15 min after the cessation of breathing. The remaining rats were switched to a diet of unspiked food for an additional 27 days (depuration phase), at which point all remaining rats were euthanized. Blood samples were collected throughout the experiment at predetermined times, and the weight of individual animals was concurrently recorded. All tissue samples were excised immediately following euthanasia. Additionally, every one to two weeks throughout the uptake and depuration phases, the same two rats from the experimental group were placed into metabolic cages for 24 h to collect urine and feces. Further details on sample collection are in the SI. All experimental procedures were approved by the University of Alberta Animal Policy and Welfare Committee. Extraction. Prior to extraction, all samples were spiked with mass-labeled n-PFOS and n-PFOSA as internal standards. All blood samples were extracted with methanol using a modification of the method of Tomy et al.,35 which has previously been shown to conserve the PFOS isomer profile.27 Tissue and feces samples were extracted similarly, with additional clean up by solid phase extraction on Oasis HLB solid phase extraction cartridges (Waters Co., Milford, MA, U.S.). Urine samples were extracted by solid phase extraction on Oasis HLB SPE cartridges. Further details on extraction are in the SI. Instrumental and Data Analysis. Isomers of PFOS and PFOSA were separated with a FluoroSep RP Octyl column (3 μm dp, 100 Å, 150 × 2.1 mm, ES Industries, West Berlin, NJ, U.S.), based on a slight modification of a previously developed method.21,34 Further details are discussed in the SI. 3197
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Body masses (g) for individual rats were fit to an exponential growth function, ln(mass)= (a × t) + ln b, where a is the growth rate (g d−1), t is time (d), and b is the initial mass (g). Concentrations of PFOS and PFOSA in blood samples were corrected for growth dilution by multiplying concentrations of individual isomers by 1 + (at/b).22 Elimination kinetics of n-PFOSA were determined by fitting the growth corrected concentrations to the first-order rate equation (C) = C0e‑kt, where C0 is the concentration (ng g−1) in the blood at the end of the uptake phase, t is the total time of the depuration phase (days), and k is the elimination rate constant. For Br2 and Br3, the relative response of each isomer relative to 13C-n-PFOSA was calculated and fit to the first-order elimination model. All data are presented as mean ±1 standard deviation unless otherwise noted. Differences among groups (e.g., between tissues) were determined by analysis of variance (ANOVA), with a Tukey Honestly Significant posthoc test. Comparisons between time points were done using repeated measures ANOVA. For all tests, statistical significance was determined with α = 0.05.
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RESULTS AND DISCUSSION Spiked Food Concentrations. Due to the large quantities of spiked food required, several individual batches had to be generated throughout the exposure period, leading to batchto-batch variability in PFOSA concentrations. The mean spiked food concentration was 2.1 ± 0.5 ng g−1 total PFOSA, with concentrations ranging from 1.4 to 2.6 ng g−1 (Table S2 of the SI). However, the percentage of branched PFOSA isomers was similar among all batches (22 ± 1%, Table S2 of the SI). The variability between batches and discrepancy between spiked food and NMR analysis of the PFOSA standard (Table S1 of the SI) is likely due to volatilization of PFOSA during the evaporation of the spiking solvent. In all control food, PFOSA was below the limit of quantification ( 0.05). Disposition of Total PFOSA. Exposure to ECF PFOSA in spiked food led to the detection of Br2-, Br3-, and n-PFOSA in blood and tissues. Despite the apparent variability in PFOSA blood concentrations, mean blood concentrations reached steady state (Figure 1A), with no statistically significant concentration change from day 22 to day 62. By traditional pharmacokinetic theory, 94% of the steady state concentration is reached within 4 half-lives. Thus, the time to steady state was consistent with the blood half-life determined during the depuration phase for total PFOSA (i.e., 6.1 ± 4.1 d−1). On the last day of sampling during the uptake phase (day 77), however, concentrations of total PFOSA had decreased to 18.8 ± 12.0 ng g−1, likely attributable to the significantly lower concentration of PFOSA in the batch of food administered prior to this sampling point (Table S2 of the SI), and the rapid elimination half-life for total PFOSA discussed above. On the basis of visual comparison, there was a strong relationship between total PFOSA concentration in blood and variability in PFOSA food concentration. To identify if food concentrations of PFOSA explained much of the variation in total PFOSA blood concentrations, dietary accumulation factors (AF) for total PFOSA in blood were calculated at each sampling
Figure 1. Growth-corrected whole blood concentrations ΣPFOSA and ΣPFOS (A) and percentage of total branched isomers of PFOS and PFOSA in whole blood (B) during uptake (days 0 to 77) and depuration (days 78−104) phases. ⧫ PFOSA; ■ PFOS. Each point represents the mean ±1 standard error for n = 8 up to day 77 and n = 5 between days 78 and 104. Vertical dashed line delineates the end of the uptake phase. Horizontal dashed line in (B) represents the mean percentage of branched PFOSA isomers in food.
point by dividing the blood concentration by the mean total PFOSA concentration in food over each ten day period prior to that sampling point. Mean AFs on individual sample days ranged from 10.2 ± 4.5 to 14.6 ± 5.2, with an average across all sampling points of 13.2 ± 6.9. No significant change in the AF was observed from day 22 to day 77 (Figure S2 of the SI), consistent with steady state conditions. Furthermore, this consistency indicated that the variability in PFOSA food concentration was responsible for the decreases in total PFOSA blood concentrations, due to the decreased dosages during these time periods coupled with the rapid elimination rate of PFOSA. It should be noted that AFs were calculated only for the purposes of normalizing for variation in food concentrations, and therefore should not be considered as bioaccumulation factors because no other tissues were considered. Sparse toxicokinetic data for PFOSA are available in the literature for comparison. In rats given a single gavage dose of PFOSA, the apparent half-life in the liver was 5.2 days, whereas in the blood plasma, it was less than 4 days,12 consistent with the results found in this study. Most other toxicokinetic data for PFOSA in the literature are based on exposure to NEtFOSA, a higher molecular weight PFOS-precursor that is rapidly N-deethylated to yield PFOSA. PFOSA serum half-lives in rats given a single gavage dose of NEtFOSA, or via the food for 35 days, ranged from 4.2 to 10.8 days, respectively.36,37 The whole blood half-life of total PFOSA reported here is also similar to half-lives reported for total PFOSA in sheep administered an intravenous or intraruminal bolus dose of NEtFOSA (2.1−3.1 days).38 Furthermore, the time to reach steady state is consistent with previous studies, where apparent steady state concentrations of 3198
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depuration phase and blood elimination half-lives for Br2-, Br3-, and n-PFOSA of 2.5 ± 1.0, 3.7 ± 1.2, and 5.9 ± 4.6 days, respectively. Although Br1-PFOSA (presumably diperfluoromethyl branched isomers) was detected in the spiked food, it was only intermittently detected at low concentrations in the blood. We therefore were unable to calculate an elimination rate constant for Br1-PFOSA. This observation may be due to a combination of factors, including the low concentrations of these isomers in the spiked food, rapid excretion (Br1-PFOSA was detectable in feces), and/or possible rapid biotransformation. It is unlikely, however, this was result of the disappearance of these minor branched isomers from the chromatogram as concentrations of total PFOSA decreased; all time points had total PFOSA concentrations greater than the lower limit of linearity (0.25 ng g−1).19,20 Similarly, all tissues contained a lower percentage of branched PFOSA isomers than the food (Figure S3B of the SI). The mean PFOSA branched isomer composition in tissues ranged from 6.5 ± 1.5% in the heart to 14 ± 10% in fat, although there were no statistically significant differences among the tissues. The preferential elimination of branched PFOSA isomers from blood and tissues is likely due to the more rapid metabolism of these isomers, as opposed to preferential excretion in the urine or feces. This is supported by a lack of detection ( 5m- > 4m- ≈ Σdimethyls >3m- ≈ 1m-PFOS. However, when the relative proportions of individual isomers were examined, significant differences between the patterns of individual isomers in blood and ECF PFOS emerged (Figure 3).
In contrast to the rapid steady-state reached for PFOSA, steadystate concentrations of PFOS were not attained over the time frame of this experiment, consistent with the much longer halflife of PFOS. Rather, a slow increase in individual PFOS isomer concentrations was observed throughout the uptake phase (Figure 1A). Total PFOS concentrations peaked at 43.4 ± 12.8 ng g−1 on day 78, with other isomers reaching maximum concentrations on day 77 or 78. It should be noted, however, that total PFOS concentrations in control rats also increased slightly throughout the course of the study, with maximum concentrations of 15.4 ± 8.6 ng g−1 on day 92. The detection of PFOS in controls was likely due to accumulation from background levels in unspiked food, as noted earlier. Nonetheless, the accumulation of PFOS from the food unfortunately obfuscated the PFOS isomer profiles and would lead to erroneous calculations of total PFOS concentrations in exposed rats. Nonetheless, over the course of the study, little change was observed in the PFOS isomer profile in control rats (mean = 13 ± 3%), similar to the isomer profile detected in pre-exposure blood samples (mean = 12 ± 2%,), indicating that the contribution of background PFOS to the observed PFOS isomer profiles was near constant throughout the experiment. Furthermore, despite the batch-to-batch variability in PFOS concentrations in food, there were no differences in PFOS concentration between control and spiked food. Therefore, we felt that contributions of background PFOS to the overall PFOS body burden would likely be similar between control and exposed rats. Thus, the mean concentration of n-PFOS and individual branched isomers in control animals were used to correct the PFOS profile of exposed animals by subtracting the mean relative response of individual PFOS isomers in control rats from the relative response in individual exposed rats. Although this adds an element of variability and increases the possible error associated with the isomer profile and concentrations of PFOS reported herein, not doing so would have led to an overestimate of the contribution of PFOSA to PFOS concentrations, as well as an erroneous underestimate of the branched isomer composition (due to exposure to the high percentage of n-PFOS in the food). The measured percentage of branched PFOS isomers in blood was always greater than for branched PFOSA in food (albeit not significantly), and the PFOS isomer profile remained relatively constant throughout the experiment (Figure 1B). No statistically significant changes in the percentage of total branched PFOS isomers in blood were observed after day 22, with the exception of a slight increase (28 ± 3% branched PFOS) on day 64, and despite variability in PFOSA dosage and blood PFOSA branched isomer composition. At the end of the uptake phase, the liver had a significantly enriched branched PFOS isomer profile (32 ± 3% branched isomers), relative to PFOSA in food (Figure S5 of the SI). However, this enrichment is likely influenced by isomer-specific PFOS toxicokinetics, rather than only by isomer-specific metabolism of PFOSA. For example, in ECF-PFOS exposures, the preferential elimination of n-PFOS in bile and feces, and the selective retention of the branched isomers in the liver was previously observed.21 In fact, in all other tissues (other than blood) the isomer composition was actually enriched in n-PFOS, relative to the blood and food (Figure S5 of the SI), similar to previous findings when ECF PFOS was administered directly.21,22 Therefore, caution should be exercised in interpreting isomer distributions of PFOS in liver tissues, as they may not necessarily accurately reflect the isomer composition of the blood, other tissues, or the exposure source.
Figure 3. Percentage of individual PFOS isomers in the whole blood of rats at the end of the uptake phase (day 77) and in a 3M manufactured electrochemically fluorinated PFOS. ΣDM; sum dimethyl-PFOS. The asterisk indicates significant differences between blood and standard. n-PFOS was omitted for clarity.
The most notable difference was a relative deficiency of 1mPFOS, and relative enrichment of 5m-PFOS in blood (Figure 3), relative to ECF PFOS. Given the known slow elimination of 1m-PFOS in Sprague− Dawley rats,21,22 relatively high levels of 1m-PFOS would be expected to accumulate in blood relative to other PFOS isomers (assuming isomer specific metabolism rates to be equal), as the α-branched PFOSA isomer constitutes 3.2% of the ECF PFOSA used in this study (Table S1 of the SI). Consistent with previous studies,21,22 1m-PFOS was the most slowly eliminated PFOS isomer in the current study, thus it had the highest propensity to accumulate relative to the other isomers. Elimination processes, therefore, cannot explain the observed relative depletion of 1m-PFOS in the current work. Instead, the observed relative depletion of 1m-PFOS is most likely due to the reduced absorption or metabolism of the α-branched PFOSA isomer. In contrast to 1m-PFOS, a significant relative enrichment in the proportion of 5m-PFOS was observed in the blood. The enrichment was significantly greater than the enrichment of 3200
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3m-PFOS, which, although the percentage of 3m- or 5mPFOSA in our spiked food is unknown, is present in ECF PFOS in similar percentages as 5m-PFOS, and is therefore likely present in ECF PFOSA in similar percentages. Furthermore, both 3m- and 5m-PFOS are excreted at a similar rate.21,22 This enrichment is therefore either a reflection of the preferential metabolism of the 5m-PFOSA isomer, or preferential absorption of 5m-PFOSA and its subsequent metabolism. Few studies to date have investigated the profile of individual PFOS isomers in the blood of humans or wildlife. Gebbink et al.26 recently showed that the proportion of individual PFOS isomers in glaucous gull eggs varied among breeding colonies in the Great Lakes region. These differences, as the authors noted, may be due in part to differences in PFOS-precursor exposure, among other possible factors. While differences in the percentage of total branched isomers differentiated a single egg collection location from the remainder, a number of sites could be differentiated from one another based on percentages of individual isomers. Such differences cannot yet be linked to definite sources, but the ability to distinguish separate populations of the same species based on PFOS isomer patterns is an important first step in the use of isomer-specific analysis as a source tracking technique. In humans, the percentage of branched PFOS isomers in serum is quite variable. The branched content of archived serum samples from Norway ranged from 22 to 47%, and increased in recent years.45 Likewise, pooled serum samples from the United States contained 29−41% branched isomers.31 However, a significant difference in the percentage of branched isomers has been reported among studies in different countries. Serum and plasma samples from the U.K. and Australia were composed of a significantly greater proportion of branched isomers than serum samples from Sweden,30 which may indicate differences in exposure sources between these locations. While the cause(s) of branched PFOS isomer enrichment observed in human serum remains unclear, the lack of a conclusive finding of any enrichment in total branched isomer content in the current study does not preclude the possibility that metabolism of precursor compounds are playing a role. The salient result from the current investigation was that the branched PFOS-precursors were most rapidly eliminated, consistent with results from in vitro metabolism of a related PFOS-precursor.33 The large interspecies differences in the biological handling of PFOS isomers46,47 indicates that current rodent models may not accurately reflect PFOS accumulation processes for humans. For instance, the reported half-life of PFOS in humans is 5.6−5.9 years,48 considerably greater than the half-life in rats of 30 to 100 days.21,22 Although there is currently not enough evidence to support the immediate utility of using isomer compositions for exposure assessment, we remain optimistic of the utility of isomer profiling for PFOS source apportionment. More information is needed on the in vivo metabolism of other PFOS precursors that may be metabolized to PFOSA, as the observed PFOS isomer pattern in humans and wildlife is dependent, not only on the metabolism of PFOSA to PFOS, but also on the metabolism of higher molecular weight precursors to PFOSA; therefore, the isomer-specific metabolism of other precursors will also influence the isomer composition of PFOS in blood. Furthermore, more information is needed on the abiotic fate and isomeric fractionation of PFOS and precursors in the environment, as this too will contribute to the isomer profile of PFOS and PFOS precursor exposed biota.
Article
ASSOCIATED CONTENT
S Supporting Information *
Further details on the animal husbandry, dose preparation, experimental procedure, and instrumental analysis, along with tables of control and spiked food concentrations, figures describing PFOSA and PFOS concentrations in individual tissues, urine, and feces. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding was provided through the Natural Sciences and Engineering Research Council (Martin and Wong), an Alberta Ingenuity New Faculty Grant (Martin), and the Canada Research Chairs Program (Wong). We thank Alberta Health and Wellness for support of laboratory activities and the 3M Company for generous donation of ECF PFOS and PFOSA standards. Jonathan Benskin and Brian Asher are gratefully acknowledged for sampling assistance.
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REFERENCES
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