PFAS Testing Methods and Guidance for Sample Filtration

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• Filtration product selection table for PFAS testing
• Datasets for PFAS extractable testing
  
• What are poly- and perfluoroalkyl substances (PFAS)?
• Requirement for filtration in PFAS test methods
• Concerns about PFAS contamination from sample collection and preparation materials
• Sample filtration in PFAS testing
• Recommended syringe filters, membrane filters, and filter holders for membrane filters
• References

Filtration Product Selection Table for PFAS Testing

Datasets for PFAS Extractable Testing

• Detection of PFAS contaminants in water after filtration with Millex^® PES and nylon devices using modified EPA 537.1. 
• Detection of PFAS contaminants in methanol samples after filtration with Millex^® PES and nylon devices using LC-MS/MS according to modified EPA 1633. 
• Detection of PFAS contaminants in water after filtration with hydrophobic 0.2 µm and 0.45 µm polypropylene disc filters and hydrophilic 0.2 µm and 0.45 µm polypropylene disc filters in Swinnex^® devices using modified EPA 537.1. 
• Detection of PFAS contaminants in methanol samples after filtration with three different lots of hydrophilic 0.2 µm polypropylene disc filters in Swinnex^® devices in methanol solvent using modified EPA 1633. 

What are Poly- and Perfluoroalkyl Substances (PFAS)?

PFAS are poly- and perfluoroalkyl substances known as “forever chemicals” and comprise a group of over 4,000 varieties of long- and short-chain perfluorinated compounds.¹ PFAS are used in a variety of industries for their excellent oil-, water-, temperature-, chemical-, and fire-resistant properties and noted for their use in polymerization reactions of fluoropolymers such as Teflon^® by companies such as 3M and Dupont. Products that contain PFAS and related compounds are ubiquitous in industrial and consumer products, including product packaging, cosmetics, non-stick cookware, stain repellents, polishes, paints, coatings, and firefighting foams.

The excellent properties and broad use of PFAS have led to the persistent accumulation of these man-made chemicals in environmental and biological matrices, recently linked to liver damage, cancer, weakened immune system, and high cholesterol in humans.^1-3

Requirement for Filtration in PFAS Methods

In response, agencies in the US and Europe have taken regulatory action. The Stockholm Convention proposed regulations for two of the most common PFAS compounds — perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) — with certain exemptions, effective in 2020. The US Environmental Protection Agency (EPA) published an Action Plan in 2019 followed by recommendations for testing water matrices for PFAS compounds under the Safe Drinking Water Act in early 2020, with a drinking water advisory concentration of 70 parts per trillion (ppt). In October 2021, the EPA published its PFAS Strategic Roadmap, which outlines their extensive approach to addressing PFAS from 2021 through 2024. Most recently, the EPA released drinking water advisories for four PFAS compounds (PFOA, PFOS, Hexafluoropropylene Oxide (HFPO) dimer acid and its ammonium salt, and PFBS and its potassium salt). The European Union (EU) drinking water directive, which includes a limit of 0.5 µg/L for all PFAS, took effect in January 2021. Additionally, the European Chemicals Agency (ECHA) submitted a restriction proposal in January 2022 for PFAS in firefighting foams, with several other proposals expected through 2023. Additional PFAS substances are on the list for evaluation under REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals). In response to rapidly evolving regulatory proposals and actions, academic and industrial testing labs have developed analytical methods for testing and monitoring PFAS in a variety of matrices, such as those listed in Table 2. These regulations are important to understanding the extent of human exposure and environmental contamination to inform future remediation efforts.

Concerns about PFAS Contamination from Sample Collection and Preparation Materials

In recent studies, there has been concern about adding PFAS contamination to samples from a variety of sources including collection bottles, solvents, storage vials, tubing components, and any other plastic that contacts the sample. This includes membrane filters, filter holders, and syringe filter housing used for clearing particulates from sample matrices. Some filters may show trace amounts of contamination that may interfere with LC-MS/MS detection of PFAS and resulting data, especially among increasing sensitivity requirements.⁶ Another concern specifically for consumables is the adsorption of PFAS compounds, such as onto filtration media or SPE sorbent. For filter devices, this is dependent on many attributes - most importantly the filter type, the solvent being filtered, and the type of PFAS molecule.^4-5 For example, in some cases, both contamination and sorption onto filtration media can be reduced by washing with methanol.^6-7

Sample Filtration in PFAS Testing

In all analytical methods, sample preparation should be carefully considered. However, in PFAS workflows, additional factors could complicate PFAS analyses downstream. These include potential PFAS contamination from filters or other consumables that contact the samples and adsorption of PFAS compounds onto consumables, leading to loss of recovery. Thus, we tested PES, nylon, and nylon-HPF syringe filter devices within PFAS detection workflows EPA 537.1 and EPA 1633 to determine levels of contamination resulting from membrane extractables. We also tested polypropylene cut disc membrane filters (0.2 µm and 0.45 µm hydrophilic and hydrophobic polypropylene for EPA 537.1 and 0.2 µm hydrophilic polypropylene for EPA 1633). We found that none of the filters had any detectable levels of PFAS contamination above the reporting limits (RL). Adsorption of internal standards leading to some loss of recovery occurred primarily for nylon and hydrophobic polypropylene membrane materials, which varied according to PFAS type, chain length, and filtrate material (methanol vs. water). Filtration in methanol demonstrated better recoveries of the same standards for nylon. This supports the suggestion that rinsing with methanol can reduce binding of PFAS compounds to filter materials. Hydrophilic polypropylene performed similar in both methanol and water.

Therefore, when filtration of higher particulate samples is needed in a PFAS workflow, PES, nylon, and nylon-HPF Millex^® syringe filters, as well as polypropylene cut disc membrane filters, provide a suitable option. 

Alternative diameter PES Millex^® syringe filters and Millipore^® cut disc membranes are also available.