The PFAS puzzle: from chemistry and consequences to comprehensive analysis

What are PFAS compounds?

Per- and Polyfluoro Alkyl Substances or PFAS have raised concerns from governments, individuals, and experts for decades, despite efforts from some financial interests to suppress information and thereby delay restrictions on this class of compounds.¹ The potential harms of PFAS are wide ranging, and as the label encompasses diverse chemistries, effects observed in one compound will not apply to all others. However, the relative ease of producing a slightly different version of some prohibited PFAS has necessitated both restrictions on individual compounds, and whole group restrictions.

Compounds falling under the PFAS umbrella have been reported to be associated with increased risk of some cancers, immune risks, developmental delays, premature births, decreased vaccine responses, increased cholesterol, and more.² Beyond this, environmental risks include bioaccumulation and biomagnification (also relevant to human health), and similar threats as the ones that face humans, however, much is not yet known, e.g. largely unknown toxicity, especially in terrestrial organisms and aquatic invertebrates and concerning chronic exposure. To date, there is also a lack of generational studies and a proper understanding of the environmental fate of PFAS.³

PFAS have a remarkable ability to repel both water and oils.⁴ Products incorporating PFAS include non-stick frying pans, water repellent clothing, firefighting foam, cellulose based food packaging, medical devices, mining consumables and more. These examples represent diverse areas of application with associated diverse potential routes of both human and environmental exposure. As such, samples with potential relevance for PFAS testing are predictably diverse.⁴

Regulations related to PFAS compounds

Several Eu countries put forward a proposal to restrict PFAS in the EU in 2023.⁵ Consultation and commentary from industry and other stakeholders, and consideration of these concerns in the final construction by ECHA scientific committees have affected and will affect the final formulation of the regulations,⁶ but currently up to 10 000 PFAS are expected to be banned altogether.⁷ The European commission is expected to vote on the legislation during 2025,⁷ with industries granted up to 18 months for achieve compliance with the legislation if passed, and extensions of up to 12 years being possible in fields where viable alternatives have not yet been demonstrated.⁶ Thus, if new and stronger PFAS restrictions are passed, we should expect to see most industries moving away from this class of chemicals, at least in the EU, by 2027. All, in some sense European, industries should continue to follow over the coming years and other jurisdictions could reasonably be expected to follow in the footsteps of the EU to facilitate trade with the large single market, as has been seen previously with other classes of chemicals.⁸ Taken together, it can be inferred that the total need for PFAS testing likely will increase, and that the weight of testing likely will redistribute somewhat from environmental testing over to product testing for regulatory compliance.

Analytical techniques for PFAS testing

Analytical techniques for PFAS testing must cover the array of samples outlined previously. Food and cosmetics, packaging, environmental and tissue samples all have their own specific best testing practices.⁴ Broadly, methods can be classified as targeted, non-targeted tests or centred on fluorine content (Eg. Total Organic Fluorine, TOF). Accredited targeted methods mostly use Liquid Chromatography-Mass Spectrometry (LC-MS), preceded by Solid Phase Extraction (SPE). They offer quantification of specific known PFAS using specific standards.⁴ Non-targeted methods call for the same instruments, but also involve comparing unknown spectra to known references for identification.⁴

Additionally, XPS has successfully been applied in quantifying fluorine in the 0.01 μm surface layer of materials, with the capacity to identify organic fluorine in PFAS through the bonding information.⁹ ToF-SIMS has recently been demonstrated as a method for PFAS detection in water samples, discriminating between common PFAS compounds through Principal Component Analysis (PCA).¹⁰ High resolution Energy Dispersive x-ray Spectroscopy (EDS) detectors coupled with Scanning Electron Microscopy (SEM) imaging, while incapable of directly assessing complex molecular structure, can still reliably map fluorine content across regions in diverse samples. In isolation, this is obviously not sufficient for PFAS characterisation, but with careful consideration, it can yield strong candidate regions for further study using other spatially resolved techniques that instead yield bonding information, e.g. Fourier Transform Infrared (FTIR) microscopy. Beyond FTIR, molecular information relevant to PFAS identification can also be sought with Nuclear Magnetic Resonance (NMR), Ultraviolet (UV) , raman, X-Ray Fluorescence (XRF) spectroscopy, other types of Infrared (IR) spectroscopy, and more.^4,11

Analysis of PFAS with FTIR

In FTIR, the sample is exposed to IR-radiation and an absorption spectrum is gathered based on at which wavenumbers and how strongly the sample absorbs IR-radiation. This absorption is related to vibrations in species, functional groups, and even the molecular backbones of polymers present in the sample. The process of obtaining an informative spectrum is also remarkably quick due to the use of the Fourier transform in signal processing, allowing for the simultaneous collection of data across a wavelength range. Complex polymers can often be identified by comparison using spectral libraries, but if this is unsuccessful, structural information can also be pieced together directly from the spectrum.¹²

Case study

In this case, we were presented with a sample of beads of an unknown recycled plastic, roughly spherical and a few mm in diameter. Differences in surface and bulk composition were identified with FTIR, as were the main constituents of each region (polyethylene and polyester respectively). As the bulk and surface composition of the beads differed, SEM-EDS images were taken to differentiate these areas.

In these images, fluorine-rich regions were identified. In Figure 1, these can be seen in green and are seemingly located in craters in the sample surface. As can be seen in Figure 1, these craters are approximately 10-20 μm in diameter, and therefore, measurable using an FTIR microscope. Figure 2 shows the spectrum obtained from a measurement of one of these craters, alongside a reference spectrum of a common PFAS compound.

Figure 1. In a sample of recycled plastic, we identify high-fluorine regions in shallow pits with a diameter of 10-20 µm using SEM-EDS

Figure 2. In black, an FTIR spectrum gathered at point 1 (marked with a green ring in Figure 1.) using µ-FTIR. In red, a reference spectrum for poly(vinylidene fluoride-co-hexafluoropropylene). Aside from interference from polyester in the sample, the spectra are similar enough to conclude a similar, if not identical, structure.

Summary and conclusions

Here we have discussed the nature of PFAS, their areas of application, properties, as well as associated risks. An overview of relevant legislation was presented, alongside the likely long-term direction of PFAS production and the impact on testing. Analytical techniques were outlined, both through the lens of traditional testing methods and some practical work in PFAS testing. In our case study, SEM-EDS imaging provided candidate locations for PFAS identification. Subsequent FITR measurements yielded high-quality spectra despite the challenges of a small target area. The similarity in the sample and library reference spectra (aside from interference from polyester in the sample spectrum) helps us not only confirm the presence of PFAS but also identify the present PFAS compound as very likely being poly(vinylidene fluoride-co-hexafluoropropylene).

We hope this information will be useful to those who, for different reasons, want to familiarize themselves with PFAS testing. Accredited PFAS analysis methods are briefly discussed here, but we also wish to outline alternative routes to PFAS insights. Public health questions and environmental concerns pose a suite of challenges that, in turn, call for lower detection limits, wider registries of compounds to track, quicker and cheaper testing, testing of an ever-expanding list of product types and matrices, as well as developing methods compatible with continuous testing. In this rapidly changing field, we find it is crucial to approach the issue with a practical scientific mind to try and then use the techniques that work for a specific sample, while continuing to learn more from our colleagues.

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