Sorption of PFOA onto different laboratory materials: Filter membranes and centrifuge tubes

Accepted Manuscript Sorption of PFOA onto different laboratory materials: Filter membranes and centrifuge tubes Supriya Lath, Emma R. Knight, Divina A. Navarro, Rai S. Kookana, Michael J. McLaughlin PII:

S0045-6535(19)30103-1

DOI:

https://doi.org/10.1016/j.chemosphere.2019.01.096

Reference:

CHEM 23011

To appear in:

ECSN

Received Date: 14 October 2018 Revised Date:

12 January 2019

Accepted Date: 16 January 2019

Please cite this article as: Lath, S., Knight, E.R., Navarro, D.A., Kookana, R.S., McLaughlin, M.J., Sorption of PFOA onto different laboratory materials: Filter membranes and centrifuge tubes, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.01.096. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Sorption of PFOA onto different laboratory materials: filter membranes and centrifuge

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tubes

3 Supriya Lath ([email protected]) *, a, 1

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Emma R. Knight ([email protected]) a, b

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Divina A. Navarro ([email protected]) a, b

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Rai S. Kookana ([email protected]) b

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Michael J. McLaughlin ([email protected]) a, b

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School of Agriculture Food and Wine, The University of Adelaide, PMB 1 Glen Osmond, SA 5064, Australia. b

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Present address: AECOM, Collins Square, Level 10, Tower Two 727 Collins Street, Melbourne, VIC 3008.

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CSIRO Land and Water, PMB 2 Glen Osmond, SA 5064, Australia. * Corresponding author ([email protected])

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ACCEPTED MANUSCRIPT Abstract

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Measurement and reporting of concentrations of contaminants of emerging concern such as

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per- and polyfluoroalkyl substances (PFASs), including perfluorooctanoic acid (PFOA), is an

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integral part of most investigations. Sorption losses of PFAS analytes onto particular

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laboratory-ware (e.g. glass containers) have been suggested in the published literature but

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have not been investigated in detail. We examined sorption losses from aqueous PFOA

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solutions in contact with different commonly-used materials in filter units and centrifuge

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tubes (glass and plastics). Sorption of PFOA onto different filter membrane types ranged

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from 21 to 79 % indicating that filtration can introduce a major source of error in PFOA

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analysis; pre-treatment of filter membranes with phosphate or methanol solutions did not

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improve PFOA recovery. Substantial adsorption of PFOA was also observed on tubes made

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from polypropylene (PP), polystyrene (PS), polycarbonate (PC), and glass where losses

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observed were between 32 to 45%, 27 to 35%, 16 to 31% and 14 to 24%, respectively.

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Contrary to suggestions in the literature, our results indicated the greatest sorption losses for

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PFOA occurred on PP, whereas losses on glass tubes were much lower. Variations in ionic

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strength and pH did not greatly influence PFOA recovery. When PFOA concentrations were

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increased the percent recovery of PFOA increased, indicating that binding sites on tube-

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walls were saturable. This study draws attention towards analytical bias that can occur due

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to sorption losses during routine procedures, and highlights the importance of testing the

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suitability of chosen laboratory-ware for specific PFAS analytes of interest prior to

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experimental use.

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Keywords: PFOA; filter membranes; centrifuge tubes; plastics; polypropylene; glass

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1. Introduction

Perfluorooctanoic acid (PFOA) and other related per- and polyfluoroalkyl substances

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(PFASs) have been recognised as contaminants of emerging concern due to their ubiquitous

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and persistent nature in the environment, as well as their bioaccumulative properties. As a

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result, these chemicals are being studied extensively with respect to their human and

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ecological toxicity (Hekster et al., 2003, Sundström et al., 2011), their occurrence, fate and

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transport in different environmental compartments (Ahrens et al., 2015, Hansen et al., 2002)

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as well as management and remediation strategies (Ochoa-Herrera and Sierra-Alvarez,

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2008, Ross et al., 2018). At various stages of such laboratory and field studies, the PFAS

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analytes being researched come in contact with a variety of apparatus that are usually made

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from glass, steel or plastics. The most common apparatus in any study are sampling and

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storage containers, including a range of tubes, vials and bottles. Others examples include

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disposable polystyrene (PS) well-plates used as exposure-vessels for toxicity assays, glass

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aquariums for fish toxicity tests, as well as disposable filtration membranes used for the

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separation of aqueous phases from solid or particulate matrices prior to analysis.

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Regardless of the types of experiments conducted, one aspect that is commonly cited in the

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methods’ section of several published studies relates to the use of sample containers made

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of particular materials. Specifically, several studies exclude the use of equipment made from

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glass in experimental or analytical protocols involving PFASs (Ahrens et al., 2015, Hansen et

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al., 2002). The justification regularly provided for this is that glass adsorbs PFAS analytes.

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The USEPA and ISO methods, which are the most widely accepted test methods for PFAS

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analytes, also stipulate that PFAS standards, extracts and samples should not come in

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contact with any glass containers or pipettes (ISO, 2009, Shoemaker et al., 2009). They

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recommend that polypropylene (PP) containers be used for all sample, standard and

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extraction preparation and storage, and suggest that other plastics may be used if they meet

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quality control requirements (Shoemaker et al., 2009). As a result, PP has been adopted as

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ACCEPTED MANUSCRIPT the material of choice in several studies (Ahrens et al., 2010, Ahrens et al., 2011, Hellsing et

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al., 2016, Higgins et al., 2007). However, a number of studies have used materials other

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than PP for PFAS-related studies. For example, Higgins and Luthy (2006) chose PS tubes

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over PP or glass because their preliminary, unpublished data provided higher recoveries

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when using PS tubes for a range of PFASs. Other studies have used polyethylene (PE),

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polycarbonate (PC) and high-density PE containers (Johnson et al., 2007, Washington et al.,

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2010), often without any indication as to the suitability of these materials.

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One consensus in the published literature is the avoidance of equipment containing

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fluoropolymer materials like polytetrafluoroethylene (PTFE) in PFAS studies, as their

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manufacture has historically involved the use of some PFASs as a ‘polymerisation aid’

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(Prevedouros et al., 2006). Leaching of remnant PFASs from such products can cause

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contamination of the dissolved phase (Martin et al., 2004). However, in the case of glass and

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plastics, there is considerable contradiction and inconsistency in the published literature

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regarding which materials may be best suited. Losses to laboratory ware could lead to

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considerable bias in analytical data. Despite the severe implications of such routine losses

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due to adsorption onto glass and plastics, it has not been investigated in its own right.

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Apart from choice of containers, filtration is another routine consideration in sample handling

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and preparation. Filtration is often employed as a major clean-up stage for most

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environmental and laboratory samples, during which additional losses can occur (Ahmad et

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al., 2001, Carlson and Thompson, 2000). A study investigating active air-sampling of

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gaseous PFOA using glass fibre (GF) filters observed that gas-phase PFOA was

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underestimated due to its sorption onto the GF filters (Johansson et al., 2017). However, an

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investigation using a variety of PFASs from the aqueous phase, testing four different filter

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membranes, determined GF filters to be suitable for several PFASs, but recommended

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specific testing to account for unpredictable effects (Chandramouli et al., 2015).

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ACCEPTED MANUSCRIPT The aim of this study was to examine the sorption losses of PFOA on common glass and

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plastic materials – specifically, centrifuge tubes and disposable syringe filter membranes –

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during routine laboratory procedures. Sorption of PFOA from aqueous solutions onto a

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variety of filter-membranes was tested; the influence of varying concentrations, and two pre-

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treatments were investigated. Additionally, sorption onto a variety of glass and plastic (PP,

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PC, PS) tubes was tested. The influence of contact time, pH, ionic strength and PFOA-

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concentrations were examined to gain insights into the nature of PFOA-binding interactions.

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2. Materials and Methods 2.1.

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Materials

Radiolabelled 14C-PFOA with a specific activity of 2.04 GBq/mmol was purchased from

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American Radiolabelled Chemicals Incorporation. Optiphase HiSafe 3 scintillation fluid for

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radiochemical analysis was purchased from PerkinElmer, Australia. All other chemicals used

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including calcium chloride dihydrate (CaCl2.2H2O), dipotassium hydrogen phosphate

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(K2HPO4), methanol, hydrochloric acid (HCl) and sodium hydroxide (NaOH) were of

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analytical grade. The specifications of filter-types and tube-types used in this study are listed

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in Table 1 and Table 2 respectively.

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2.2.

Filter sorption studies

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The sorptive losses through retention of dissolved PFOA onto different filter membrane-

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types was investigated in triplicate using syringe-filtration through disposable filter units. By

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means of single-use 10 mL PP syringes, 4 mL of a 14 µg/L PFOA solution was drawn into

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the syringes, following which filter units were attached to the end of the syringe before

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plunging the solution through the filter membrane. The initial 2 mL volume of the filtrate was

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discarded and the subsequent 2 mL was collected, of which 0.5 mL aliquots were used for

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quantitative analysis. Control samples to account for any sorption occurring on the walls of

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the syringe were prepared by taking up PFOA solution in the syringe, and dispensing the

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solutions without attaching any filter units (details in supplementary material). To determine if

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ACCEPTED MANUSCRIPT recovery of PFOA from the filters could be improved, two pre-rinsing treatments, using

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phosphate solution (100 mM K2HPO4) and methanol, were applied to the filters. For the

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phosphate treatment, 10 mL of a 100 mM K2HPO4 solution was plunged through the filter

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units, followed by 10 mL of Milli-Q water, prior to PFOA filtration. Similarly, for the methanol

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treatment, 10 mL of methanol was plunged through the filters; membrane-types that were

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incompatible with methanol were excluded. Finally, the influence of increasing

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concentrations of PFOA on sorptive losses on three filter membranes (PP, RC and GF) was

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also tested using an environmentally relevant range of concentrations (0 to 529 µg/L). The

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critical micelle concentration (CMC) of PFOA has been reported to be in the range of 1 to 1.6

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x 107 µg/L (i.e., 25 to 38 mM), at which PFOA molecules can agglomerate to form micelles

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and hemi-micelles; such phase separation can affect interfacial activity like sorption (Harada

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et al., 2005, Rattanaoudom et al., 2012). The concentrations used in our studies were

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around 104 to 106 times lower than the CMC, so the impact of micelle-formation on sorption

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losses is expected to be negligible.

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Tube sorption studies

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To measure sorption of PFOA onto different centrifuge tube-types (PP, PS, PC, G1 and G2,

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Table 2), tests were performed in triplicate under different conditions by varying the contact

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times, solution pH, ionic strength as well as PFOA concentrations. When not being varied,

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the standard test parameters included the use of 8 mL volumes of PFOA solutions of

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concentrations of 20.5 ± 1 µg/L, prepared in a 10 mM CaCl2 background electrolyte solution

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or Milli-Q water, and a contact time of 24 to 48 hrs. To investigate the influence of contact

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time, tubes were subjected to shaking times of 1, 2 and 7 days. The influence of pH was

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examined across a pH range from 4 to 9 where 0.1M HCl or 0.1M NaOH was used to adjust

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solution pH. Background electrolyte solutions of varying concentrations (0 to 100 mM CaCl2)

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were used to determine the effect of ionic strength on retention of PFOA. Sorptive losses as

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a function of concentration were also examined by testing initial PFOA concentrations

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ranging from 0 to 415 µg/L. All tubes with test PFOA solutions were placed on an end-over-

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end shaker for the duration of the test, after which subsamples (0.5 mL) were taken for

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quantitative analysis. 2.4.

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Instrumental analysis

Aliquots (0.5 mL) of all samples, blanks (Milli-Q water) and stock solutions were combined

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with 4 mL of scintillation fluid in a scintillation vial and analysed radio-chemically using a β-

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liquid scintillation counter (Perkin Elmer Tri-Carb 3110 RT). Self-normalisation and

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calibration (SNC) protocol was performed and the 14C counting efficiency was determined to

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be >90%. The activity of 14C in the samples was measured as disintegrations per minute;

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each sample was measured three times, for 2 min each. At least 3 background vials

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(containing 0.5mL of Milli-Q water instead of the sample) were run with each batch of

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samples to determine the background count, which was subtracted from each sample count.

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Concentrations of PFOA were then calculated using the specific activity and the measured

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to be 0.11 µg/L.

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Data and statistical analysis

All experiments were carried out at least in triplicate. The coefficient of variations for the

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measurement of all stock solutions used in the experiments ranged from 0.35 to 4.83%.

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Losses of PFOA through adsorption onto the filters and tubes were calculated using the

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difference between the amounts of PFOA in the samples before and after the experimental

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steps (i.e. filtration or shaking). Results are reported as a percentage recovery of PFOA

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compared to the controls. A mass balance was not attempted as each additional handling,

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extraction and transfer of every sample would increase the chances for loss of analyte,

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introducing additional error to the method.

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Statistical software, IBM SPSS (v. 24), was used to determine if there were significant

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differences (p < 0.05) between treatments. After using analysis of variance test (ANOVA), a

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Tukey’s honestly significant difference test was used to compare treatment means.

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3. Results and Discussion

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3.1.

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3.1.1. Sorption losses observed on different filter membrane-types

Filter sorption studies

The sorption of dissolved PFOA onto filter membranes was determined using syringe-

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filtration. As no sorption of PFOA analyte was identified as occurring on the syringe-surfaces

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(Table 1, supplementary material) likely due to short residence time (< 10 seconds) in the

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syringes, the sorption losses were calculated using the difference between the amounts of

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PFOA in the unfiltered and the filtered solutions. Irrespective of the type of membrane

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tested, recovery of PFOA was < 100% in all cases (Figure 1). This implies that at least a

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certain proportion of PFOA was retained onto the filter membrane or housing material due to

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adsorption, leading to underestimation of dissolved PFOA concentrations. As it was not

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possible to make a distinction between the amount of PFOA retained on the membrane and

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the housing material separately, sorption was considered for the filter unit as a whole.

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Specifically, recovery ranged from 76.0% at best to 21.2% at worst, depending on the type of

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membrane used (Figure 1). The highest recoveries were achieved when using PVDF

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(76.0%), glass-fibre (74.2%), RC (74.0%) and PP (72.3%) membranes. The lowest recovery

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due to adsorption was displayed by the NY membrane filter (21.2%). Overall, the percentage

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recoveries after filtration were in the order PVDF ≈ GF ≈ RC ≈ PP > CA > PES ≈ PTFE > NY.

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While PVDF showed a high recovery, it is a fluoropolymer, and like PTFE, may cause

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contamination of analytical blanks, or over-estimation of dissolved PFAS concentrations

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(Martin et al., 2004), so it should generally be avoided in PFAS-based analysis. We did not

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test for PFAS contamination issues during filtration as a radiolabelled PFOA solution was

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used. Interestingly, despite both PVDF and PTFE being fluoropolymers, PFOA-sorption on

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PTFE was more than 2 times greater than on PVDF; this may be attributed to the differences

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in their chemical properties. The carbon-backbone of the polymeric structure in PVDF

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(chemical formula [–CH2–CF2–]n) is partially fluorinated, whereas that in PTFE (chemical

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ACCEPTED MANUSCRIPT formula [–CF2–CF2–]n) is fully fluorinated, making PTFE more hydrophobic than PVDF. It is

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likely that the PFOA tail, being hydrophobic, is able to undergo greater hydrophobic

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interactions with PTFE, leading to greater PFOA sorption.

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When testing a range of filter media with PFAS-spiked water, Chandramouli et al. (2015)

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reported that the poorest recoveries were observed with PTFE (2 to 24%), followed by NY

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(62 to 80%) filters, while GF filters displayed the best recovery (> 85%) overall. For a range

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of perfluorinated carboxylate and sulphonate compounds, Labadie and Chevreuil (2011)

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also reported better recoveries using GF (70 to 98%), as opposed to NY (40 to 98%);

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however, specifically in the case of PFOA, GF and NY were reported to perform equally.

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Like in the case of GF membrane, the recoveries obtained from RC as well as PP

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membranes in our study were greater compared to other filters, however accounts of the use

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of these membranes in the published literature are rare. Chain-length of the PFASs were an

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important factor controlling sorption losses reported in these studies (Chandramouli et al.,

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2015, Labadie and Chevreuil, 2011), where greater sorption occurred as chain-length

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increased. This has been reported in studies related to sorption in environmental media such

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as soil and sediments, as well as adsorbents such as activated carbon (Du et al., 2014).

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While the literature focussing on PFAS-investigations in water, soil and sediments commonly

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use GF filters (Ahrens et al., 2015, Kwadijk et al., 2010, Lein et al., 2008), the literature

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dealing with air-sampling of PFAS is exploring ways to reduce the sorption of PFAS onto

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GF-filters (Arp and Goss, 2008, Johansson et al., 2017). This contrast may be attributed to

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the differences in sampling and filtration equipment set-up for different environmental media.

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Moreover, it appears that membrane-types, apart from GF and quartz-fibre filters, have not

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been tested for PFAS sorption.

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While GF filters have been reported to show the best recoveries for aqueous phases

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(Chandramouli et al., 2015, Labadie and Chevreuil, 2011), it is important to note that glass

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fibres used in membranes usually have variable structural integrity. As a result, they are

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ACCEPTED MANUSCRIPT often ascribed with a particle retention rating, covering a range of sizes, and in our case it

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was 0.8 to 8 µm rather than a specific pore-size. The GF membranes used in this study were

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stated to be of 20 µm pore size, but the manufacturer cannot guarantee an actual pore size.

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An interesting observation was that the inclusion of a GF membrane as a pre-filter in the

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case of PES and CA membranes, caused recovery to improve by up to 18% and 11%,

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respectively, when compared to the PES and CA membranes where no pre-filter was

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included. Usually, pre-filters are effective at improving recovery in samples with high

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particulate load. The solutions used in this study were devoid of any particulate matter, thus

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it is unclear why an improvement was observed.

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One approach to reduce sorption of PFOA onto the filters could be through pre-treatment of

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the filters to pre-saturate or block the available binding sites for PFOA on the membrane – a

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strategy that has been demonstrated for other organic compounds (Bin et al., 2000, Hupfeld

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et al., 2009). However, on pre-treating the membranes with either a phosphate solution (100

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mM), or methanol (99% purity), no significant improvements in PFOA-recovery were

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observed (Figure S1 and Figure S2; supplementary material). In the case of the methanol

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treatment, in fact, a decrease in recovery of 7 to 28%, was observed for some membranes

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(RC, PP and PES; Figure S2). The exact mechanisms controlling the binding of PFOA to the

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different filter membranes are not known. In the study on active air-sampling of PFOA using

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GF filters, pre-treatment with a siliconizing reagent led to an appreciable reduction in

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sorption of gaseous PFOA onto the GF filters (Johansson et al., 2017), particularly when

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atmospheric concentrations of PFOA were high. While no specific mechanism was identified,

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it was attributed to deactivation of surface active sites on the GF filter. Given that no benefits

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in terms of reduction in sorption losses were observed in our study, combined with the added

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inconvenience, time and expense associated with the process, pre-treatment of disposable

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filter-units is not recommended for improving recovery from solution. However, pre-rinsing

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remains a suitable strategy to reduce contamination of the dissolved phase from certain

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filters (e.g. PTFE) (Labadie and Chevreuil, 2011).

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3.1.2. Effect of PFOA concentration on PFOA recovery

To test the influence of increasing PFOA concentrations on recovery, three of the filters

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exhibiting the lowest sorptive losses – PP, RC and GF – were used; PVDF was excluded on

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account of being a fluoropolymer. In terms of percentage of PFOA recovered (Figure 2),

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there was no evidence for consistent effect of PFOA concentration on recovery. A similar

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outcome was reported for PFOA by Chandramouli et al. (2015) when tested on NY filters;

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PFOA concentrations used ranged from 0.02 to 1 µg/L, which were 2 to 3 orders of

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magnitude lower than the concentrations tested in our study. We used higher concentrations

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with the expectation that sorption might be saturable, but this was not observed.

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As all filter-membranes displayed some losses of PFOA through adsorption, we recommend

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that it is better to avoid filtration, wherever possible. Other studies have also advised against

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the use of filtration in sample-preparation steps for the same reasons (Schultz et al., 2006,

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Voogt and Sáez, 2006). For instance, Schultz et al. (2006) elected centrifugation as their

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only viable sample clean-up step when analysing PFAS-contaminated municipal wastewater

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samples due to such losses. If, however, filtration is considered necessary in any

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procedures, we advise that specific testing of analyte-sorption onto filter media is undertaken

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to account for potential underestimation of dissolved PFAS concentrations due to such

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losses.

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3.2.

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3.2.1. Effect of contact time on recovery of PFOA

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Tube sorption studies

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Recovery of PFOA decreased in the order G1 ≈ G2 > PC > PS > PP (Figure 3), with greater

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recoveries from glass tubes, compared to the plastic tubes. Specifically, PFOA-recovery

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observed from glass (G1 and G2) tubes ranged from 77 to 86%, whereas that in PP tubes

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ranged from 55 to 68%, which is contrary to what is widely implicit in the published literature.

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Amongst the plastics tested, the recoveries from PC (69 to 78%) and PS tubes (65 to 73%) 11

ACCEPTED MANUSCRIPT were greater than from PP tubes. No significant changes were observed over time (1, 2 and

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7 days) in the case of G1, G2 and PC tubes (Figure 3).

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In the case of PP and PS tubes, however, some small time effects were observed. Losses at

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day 7 were similar to losses observed at day 1. In the case of PP1, PP2 and PS tubes, in

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fact, losses at day 2 were greater (by 8 to 13%) than those at days 1 and 7 (Figure 3).

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Theoretically, at a fixed analyte concentration, recovery was expected to decrease with time

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and reach equilibrium at a stage where no more PFOA was being sorbed onto the tube

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walls. As this trend was not observed in our data, it is possible that PFOA sorption onto the

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glass and plastic surfaces occurred in a much shorter time frame (within hours) than was

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investigated in this study. In a way, this idea is supported by our observations during

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syringe-filtration, where despite using polypropylene syringes, no sorption of PFOA was

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observed on the syringe-surfaces (Table 1, supplementary material). The PFOA solutions

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were in contact with the syringes for no more than 5 – 10 seconds, which may have been too

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short a time for any PFOA analyte to be retained on the surface via sorption. Further studies

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using shorter time-frames (e.g. < 6hrs) would be required to corroborate this.

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3.2.2. Effect of solution chemistry on recovery of PFOA

The influence of solution chemistry on sorption of PFOA in different tubes was determined

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by measuring PFOA-recovery under varying pH (Figure 4a) and ionic strength (Figure 4b)

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conditions. Both tests corroborated our previous observations that greater recoveries were

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obtained from the glass tubes, compared to the plastic tubes (G1 ≈ G2 > PC > PS > PP).

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Specifically, PFOA-recovery observed from glass (G1 and G2) tubes ranged from 93 to

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103%, whereas that in PP tubes ranged from 74 to 81%. Recoveries from PC and PS tubes

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were 85 to 89% and 81 to 86%, respectively.

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On increasing the pH from 4 to 8, the PFOA-recovery from the glass tubes (G1 and G2)

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remained largely unaffected. In the PC, PS and PP, a slight increase in recovery, by 4.1%,

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ACCEPTED MANUSCRIPT 4.7% and 6.5%, respectively, was apparent. Glass tubes, G1 and G2 displayed the least

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fluctuations and PP the most. Despite some of these pH effects being statistically significant

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(p < 0.05), on the whole, the sorption losses due to variations in pH (i.e., 1.4 to 6.5%) were

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minor when compared to the underlying losses due to the inherent nature of the different

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materials being tested (i.e., 5 to 25%).

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By using PFOA solutions prepared in background CaCl2 electrolyte solutions of varying

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concentrations, effects of ionic strength on PFOA recovery were examined. No significant

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effects of increasing CaCl2 concentrations were observed on recovery of PFOA in PP, PS

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and PC tubes (p > 0.05). Compared to the control PFOA solution prepared in Milli-Q water,

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recovery of PFOA from G1 tubes decreased by 5.3% and 6.2%, respectively, in the

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presence of 25 mM and 100 mM CaCl2 (p = 0.043 and 0.015), but no effects were observed

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for other ionic strength treatments (1 mM, 10 mM and 50 mM CaCl2). In the case of G2

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tubes, compared to the control, recovery decreased by 3.5 to 6.4% (p < 0.025) for all ionic

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strength treatments, except for the 50 mM CaCl2 treatment (p = 0.132).

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Ionic strength usually affects the electrostatic nature of a surface as well the solubility of the

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compound, thus controlling interactions occurring at that surface. Glass is known to have a

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negative surface charge above pH 2.6 (Gu and Li, 2000), whereas a variety of plastics

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(including PP) have been reported to carry a negative surface charge above pH 3.5 to 4

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(Lameiras et al., 2008, Leininger et al., 1964). In the experimental conditions used in this

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study (pH 5.6 ± 0.2), all tubes may be expected to carry a slight negative charge. Divalent

320

cations like Ca2+ have been found to act as a bridge between the negatively charged

321

functional head groups of PFASs and the negatively charged surfaces of a variety of

322

adsorbents (Higgins and Luthy, 2006). It is thus possible that as ionic strength increased, the

323

Ca2+-induced bridging effect caused more PFOA to be retained on the negative surfaces of

324

the glass tubes, thereby slightly decreasing recovery. However, as in the case of solution

325

pH, these variations were relatively minor compared to the considerably greater underlying

326

losses resulting due to the physiochemical nature of the materials themselves.

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ACCEPTED MANUSCRIPT Glass, which is composed of several polar inorganic components like silica (silicon dioxide),

328

boron trioxide and sodium oxide is known to be hydrophilic in nature (DeRosa et al., 2003).

329

On the other hand, plastic polymers like PP and PS are known to be hydrophobic materials

330

as they are both solid hydrocarbons composed entirely of carbon and hydrogen (Kovalchuk

331

et al., 2014). In comparison, PC may be expected to have a lower hydrophobicity than PP

332

and PS (given the presence of polar oxygen functional groups in its polymeric chemical

333

structure), but greater hydrophobicity than glass. While sorption mechanisms need

334

investigation, greater sorption of PFOA onto the plastics compared to glass may be related

335

to potential hydrophobic interactions of PFOA with the hydrophobic components of the

336

polymeric plastics. Further investigation is required at a molecular level to identify the

337

mechanisms controlling the interactions between PFOA and the different materials it may be

338

in contact with.

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3.2.3. Effect of PFOA concentration on recovery of PFOA

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On increasing the concentrations of PFOA in the test solutions, the percentage of PFOA

342

recovered from the solutions within each tube-type increased (Figure 5). For instance, as the

343

spiked concentration of PFOA increased from 12 to 415 µg/L, the recovery of PFOA from

344

PP, PC and G1 tubes improved from 53.7% to 85.5%, 66.7 to 95.1% and 75.3 to 106.3%,

345

respectively. Essentially, the higher the concentration of PFOA in the test solutions, the

346

lower the proportional loss of PFOA onto the container walls, irrespective of the tube-type.

347

This suggests that all plastic and glass tubes tested herein contained a limited number of

348

binding sites on their surface, and can only interact with a finite amount of PFOA. Therefore,

349

as the concentration of PFOA increases, the binding sites on the container walls become

350

increasingly saturated. Similar concentration-dependent results for sorption losses have

351

been reported in the case of other organic chemicals such as pesticides (Sharom and

352

Solomon, 1981) and polycyclic aromatic hydrocarbons (Chlebowski et al., 2016), as well as

353

inorganic substances such as silver nanoparticles (Malysheva et al., 2016).

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ACCEPTED MANUSCRIPT Poor recovery, particularly at low concentrations, can present serious implications. One

355

current topic of interest in PFAS-research is the determination of the toxicity profiles of these

356

chemicals. When conducting ecotoxicological testing, if significant sorption losses occur, the

357

test organisms will be exposed to reduced concentrations of the analytes, resulting in

358

inaccurate toxicity thresholds (Sekine et al., 2015). Similarly, when testing drinking water

359

quality, and comparing to guideline values to determine safety, erroneous risk assessments

360

may be made due to such losses. The lowest PFOA concentration tested in this study (12

361

µg/L) was around 170 times greater than the current USEPA drinking water health advisory

362

limit for PFOA (0.07 µg/L) (USEPA, 2016). Consequently, greater levels of losses may be

363

expected to occur in trace monitoring and health-related studies, causing substantial

364

inaccuracies.

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4. Conclusions and Implications

To our knowledge, this is the first study to systematically quantify and report on sorptive

367

losses of dissolved PFOA observed on tubes made from different materials (PP, PC, PS and

368

glass), despite the serious implications of this issue. References to such losses have been

369

made in the published literature, but remain largely unsubstantiated. Moreover, contrary to

370

what is implicit in standard protocols (e.g. USEPA and ISO methods) and the published

371

literature, our data emphasise that greater sorption losses of PFOA occurred on PP

372

containers than on glass containers. Irrespective of solution chemistry (pH and ionic

373

strength) or concentration of PFOA tested, sorption of PFOA onto the tube-walls increased

374

in the order glass < PC < PS < PP. Proportional losses decreased when PFOA

375

concentrations of test solutions increased. Due to this concentration-effect, losses can be

376

especially exaggerated when dealing with PFAS solutions of low concentrations, for

377

instance, when reporting on the quality of drinking water samples. Filtration of aqueous

378

PFOA solutions also introduced a major source of error, leading to an underestimation of

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ACCEPTED MANUSCRIPT dissolved concentrations. Our recommendation is therefore to avoid filtration of PFAS

380

solutions where possible, unless it is inevitable.

381

It is possible that even when using tubes or filters made of the same materials, sorption

382

losses may vary widely under different conditions, or with different batches and suppliers –

383

hence it is difficult to ascertain what the ‘best’ material for PFOA or other PFASs may be.

384

However, if adsorptive losses are monitored using appropriate controls and measured with

385

suitable precision, it would be possible to minimise errors associated with such losses.

386

Although it is not possible to directly extrapolate from the current dataset for PFOA to apply

387

to other types of PFASs, it is reasonable to suggest that the specific trends may differ

388

depending on the type of PFAS, due to differences in their chain lengths and functional

389

properties. For instance, compared to PFOA, greater losses may occur in the case of

390

perfluorooctanesulfonic acid (PFOS), which binds more strongly than PFOA to most

391

surfaces. Factors such as the co-occurrence of other surfactants and chemicals, as well as

392

the matrix being tested (e.g. freshwater, sea water, soil, sediments, biological fluids and

393

tissues) can also impact sorption dynamics. Consequently, our study highlights the need to

394

account for losses associated with common laboratory ware for each analyte separately as

395

part of sampling and experimental protocols, and carefully consider the choice of suitable

396

materials for use in PFAS-work.

397

5. Acknowledgements

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Financial support from Australian Research Council Discovery Grant DP150101760 and

399

Ziltek Pty. Ltd is gratefully acknowledged. The authors also thank Gavin Stevenson from

400

National Measurement Institute (NMI, Australia) for technical insights on the analytical

401

procedures used by NMI, including laboratory ware and filters.

402

6. Declarations of Interests: None.

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ACCEPTED MANUSCRIPT Figure 1. Percentage recovery of 14C-PFOA filtrate solution (13.58 ± 0.33 µg/L) from different syringe filter membranes. Error bars represent the standard deviation (n = 4). Different letters denote statistically significant differences (p < 0.05) between groups.

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Figure 2. Percentage recovery of dissolved PFOA after filtering through three different filters types (PP, RC, GF) when using a range of 14C-PFOA concentrations (0 – 528.90 µg/L). Error

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bars represent standard deviation (n = 3).

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Figure 3. Average concentration of 14C-PFOA remaining in solution after 1, 2 and 7 days when in contact with different centrifuge-tube materials. Error bars represent the standard deviation (n = 3). Concentration of 14C-PFOA used was 21.32 ± 0.07 µg/L. Different letters

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denote significant differences between days on individual centrifuge tube types.

Figure 4. Effect of solution chemistry, pH (a) and ionic strength (b), on the average percentage recovery of PFOA when in contact with different centrifuge tubes. Test

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conditions ranged from pH 3 to pH 9 and ionic strength ranged from 0 to 100 mM CaCl2. Concentrations of 14C-PFOA used were 20.79 ± 0.07 µg/L and 19.18 ± 0.10 µg/L, for pH and

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ionic strength tests, respectively. Error bars represent the standard deviation (n = 3). The lines across the data points represent linear regressions.

Figure 5. Average percentage recovery of 14C-PFOA in different centrifuge-tube types when in contact with increasing concentrations of 14C-PFOA (0 – 415.19 µg/L) in solution. Error bars represent standard deviation (n = 3). The lines across the data points represent moving averages.

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Table 1. Specifications of different filter-types tested for sorption of PFOA. Membrane

Housing

Pore size

Diameter

Source

code

type

material

(µm)

(mm)

(product number)

PP

Polypropylene

Polypropylene

0.45

30

MicroAnalytix (30AP045AN)

GF

Glass fibre

Polyvinyl chloride

20

25

Millipore (SLAP02550)

PVDF

Polyvinylidene fluoride

Polypropylene

0.45

PES

Polyethersulphone

Polypropylene

PES+GFa

PES with GF pre-filter

PTFE

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Filter

Millipore Millex (SLHV033NK)

0.45

35

MicroAnalytix (MS SF35PS045)

Polypropylene

GF 1, PES 0.45

35

MicroAnalytix (MS SF35GPS045)

Polytetrafluoroethylene

Polypropylene

0.45

25

Sartorius (17576-K)

RC

Regenerated cellulose

Polypropylene

0.45

25

Sartorius (17765-K)

CA

Cellulose acetate

Acrylic resin MBSb

0.45

28

Sartorius (16555-K)

CA+GFa

CA with GF pre-filter

Acrylic resin MBS

GF 0.7µm2, CA 0.45

28

Sartorius (17829-K)

NY

Nylon

Polypropylene

0.45

25

ProSciTech (WS1-04525N)

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a

These filter units have two filter membranes in one housing unit, where the GF pre-filter membrane preceded the PES or CA membranes.

b

MBS - methacrylate butadiene styrene

ACCEPTED MANUSCRIPT Table 2. Specifications of different tubes tested for sorption of PFOA. Tube code

Material

Lid material

PP1

Polypropylene

Polyethylene

Capacity

Source

(mL)

(product number)

10

LabServ (LBSCT1202)

Polypropylene

Polyethylene

10

LabServ

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PP2

(LBSSP1201)

PS1

Polystyrene

Polyethylene

10

Rowe Scientific (S10316UU)

PS2

Polystyrene

Polyethylene

10

ThermoFisher

PC

Polycarbonate

Polypropylene

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(LBSCT1002)

10

ThermoFisher

G1

Glass

Screw cap, Teflon liner

Rubber

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G2

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(NAL 3118-0010)

12

10

Kimble Kimax (45066A-16100) BD Vacutainer (366430)

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Recovery of PFOA (%)

100 80

ab

a

a

ab

60 40

c

d e

b

e f

0

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Filter membrane type

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ACCEPTED MANUSCRIPT 100

PFOA recovery (%)

95 90 85 80

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PP RC

75

GF

70 100 200 300 400 500 PFOA concentration (μg/L)

600

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Day 1 Day 2 Day 7

ab a

A

B

b

A

PP1

PP2

60

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80

40 20 0

G2

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PS PC G1 Tube material

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Recovery of PFOA (%)

100

ACCEPTED MANUSCRIPT

100 90 80 PP1 PS2 PC G1 G2

70 60 50 75 150 225 300 375 Solution (PFOA ng/mL)

450

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0

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Recovery of PFOA (%)

110

105

105

100

100

PP1

95

95

PS2

90

90

PC

85

85

80

80

75

75

G1 G2

70 3

4

5 6 7 pH (In solution)

8

-10 10

30 50 70 CaCl2 (mM)

(b)

90 110

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(a)

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70

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PFOA recovery (%)

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Highlights:

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Sorption of PFOA onto lab-ware can cause analytical bias during routine procedures. Filtration of PFOA solutions can lead to underestimation of dissolved concentrations. Sorption losses of PFOA onto polypropylene tubes was greater than on glass tubes. Sorption losses decreased when concentrations of PFOA in test solutions increased.

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• • • •