Per- and polyfluoroalkyl substances (PFAS) have been keeping commercial products slick since 1951. They comprise a class of more than 4,700 synthetic chemicals used in water- and stain-proof clothing and upholstery, non-stick surfaces, food packaging, and firefighting foams. PFAS are highly structurally stable and water-soluble, allowing them to persist intact indefinitely and travel long distances in ground and surface waters. These “forever chemicals” reside in the bloodstreams of the overwhelming majority of animal and human test subjects. The most common PFAS are associated with elevated cancer rates, impacts on fetal and immune development, and endocrine disruption. Accordingly, the maximum limit recommended by the Environmental Protection Agency is 70 parts per trillion, like a grain of salt in an above-ground swimming pool.
In other words, PFAS are here to stay, at least until we develop a scalable technology to remove them from earth’s waterways. In the interim, how do we test for them, quantify and distinguish them, especially from matrices containing so many unknowns, including precursors that oxidize in nature to generate new PFAS themselves? The gold standard for ultrasensitive environmental sample analysis is solid phase extraction coupled to liquid chromatography and tandem mass spectrometry (LC-MS/MS). Collection and processing can incorporate nitrogen gas to concentrate samples, and aid in aerosol generation for electrospray ionization, augmenting detection.
Sample collection itself, however, can impact the reliability of downstream LC-MS/MS analysis. Deceptively mundane guidelines for sample collection account for an invisible contaminating varnish of PFAS on testers, equipment, and surfaces encountered in transit from field sites to laboratories that can inadvertently enter the analytical stream. Therefore, samplers must restrict the range of habiliment and consumables, eschewing water- and stain-resistant clothing, fabric softener, myriad personal care products, Teflon bottles and tubing, permanent markers, chemical ice, and the urge to rest a sample container on the passenger seat of the van.
The EPA methods for sampling and detection of known PFAS from water sources and ambient air belie their ubiquity, with recent and ongoing modifications yet to be optimized. Altogether, these methods address only about 100 known molecules with appropriate analytical standards. Sample collection and detection methods consequently must account for all other unknowns, which can sometimes be quantified in non-targeted analyses by comparison to standards with an adequate degree of structural similarity.
Because of the need to rapidly address this discrepancy, innovators have begun to develop ultrasensitive, miniaturized methods of on-site detection and removal. One promising technology employs labs-on-chips incorporating porous and tunable metal-organic frameworks, consisting of metallic ion clusters conjugated to organic ligands. These materials can perform dually as quantitative current impedance sensors, bypassing the need for complicated and expensive LC-MS/MS detection, and as molecular sponges that far outperform standard water filtration methods of extracting PFAS.
Scaling of this technology for detection and removal has the potential to be integral in sampling, quantification, and mitigation of PFAS. However, validating and generating microfluidic or nanoscale technologies at a sufficient breadth to be truly effective samplers and sensors will require mindsets and resources analogous to the leap from the mainframe to the laptop. Until then, PFAS testers and investigators must maintain neutrality and adaptability in sample collection to account for evolving methods, ubiquitous contaminants, and refractory unknowns.