The Expanding PFAS Toolkit: Six Methods Driving Environmental Monitoring

1 The Expanding PFAS Toolkit: Six Methods Driving Environmental Monitoring Alexander Beadle Poly- and perfluoroalkyl substances (PFAS) are synthetic compounds that are extremely resistant to heat, water, oil, and other common degradation mechanisms. As a result, these chemicals readily accumulate in the environment where they can remain for hundreds of years, earning them the moniker "forever chemicals." In the past, the extreme stability of these forever chemicals made these compounds attractive for use in firefighting foams, non-stick cookware, and water-repellent fabrics. However, in recent decades, scientific investigations have uncovered associations between exposure to specific PFAS and adverse health effects, including altered immune function, liver and kidney disease, reproductive health concerns, and cancer. In light of these findings, many regulators are responding to these potential harms with more stringent rules on PFAS production and use, as well as launching PFAS monitoring and remediation programs to track and act on the spread of these chemicals in the environment. With PFAS contamination already being widespread in waterways, soils, and the air, appropriately monitoring PFAS contamination requires a toolbox-like approach, with many different analytical techniques supporting monitoring efforts. This listicle will explore the techniques that are central to PFAS regulatory testing, as well as the additional analytical approaches that can provide complementary information for PFAS analysis and quantitation. 1. Liquid chromatography–tandem mass spectrometry: The gold standard for PFAS monitoring Liquid chromatography–tandem mass spectrometry (LC–MS/MS) is the most established technique used for PFAS monitoring, from a regulatory perspective. In the United States, the US Environmental Protection Agency (EPA) issues a list of standard analytical methods for PFAS analysis that have been validated and can be used to support regulatory activities. Analysis via LC–MS/MS forms the backbone of EPA Methods 537 and 533, targeting PFAS analysis in drinking water (in line with the Safe Drinking Water Act), and in Methods 8327 and 1633 for measuring PFAS in non-potable water and solid waste (in line with the Clean Water Act and Resource Conservation and Recovery Act). Listicle THE EXPANDING PFAS TOOLKIT: SIX METHODS DRIVING ENVIRONMENTAL MONITORING 2 To deal with this wide array of different sample matrices, LC–MS/MS is frequently combined with pre-treatment or other sample preparation techniques. This often includes the use of solid-phase extraction (SPE) or ion-exchange SPE to enhance the method's selectivity, sensitivity, and clean-up of target analytes from background. SPE is of particular interest in PFAS environmental monitoring as, under the right conditions, SPE materials can enable "passive sampling" approaches in which PFAS load is measured over days or weeks. This type of approach offers better sensitivity for low-level PFAS and can provide a valuable time-weighted average of PFAS concentrations in the environment that is less affected by periodic fluctuation. Depending on the application, it may also be useful to employ multiple reaction monitoring—also known as selected reaction monitoring—to selectively detect and quantify specific PFAS mixtures. This can help with the monitoring of trace PFAS in complex matrices, which would otherwise be difficult to analyze. Using high-performance liquid chromatography (HPLC) or ultra-high-performance liquid chromatography (UHPLC) in conjunction with tandem mass spectrometry can also be used where exceptionally low limits of detection and high sensitivity are required, such as in contamination control. 2. Gas chromatography–mass spectrometry: Monitoring volatile PFAS While being less effective for non-volatile and high-molecular weight compounds such as long-chain PFAS, gas chromatography–mass spectrometry (GC–MS) can be a highly effective way to monitor volatile PFAS in the air. For example, while the majority of the US EPA's standard analytical methods make use of LC–MS/MS for the analysis of non-volatile and semi-volatile PFAS, Other Test Method (OTM)-50 relies on GC–MS to measure 30 known volatile PFAS in air emissions from stationary sources. This may include emissions from chemical manufacturers, commercial applications, and thermal treatment incineration processes, which can be trapped in air canisters and tested using the OTM-50 method. While not routinely used for regulatory efforts, levels of volatile PFAS present in water or other aqueous samples can also be monitored through the use of headspace SPE GC–MS. This can be used in complement to LC–MS/MS methods for water monitoring, as the GC–MS method requires significantly less sample preparation and thus introduces minimal sources for contamination and analytical error. 3. High-resolution mass spectrometry: Expanding beyond target lists The majority of PFAS monitoring efforts that come from a regulatory lens are "targeted" methods, meaning that they are concerned with the presence of specific, known compounds. In contrast, "non-targeted" (or "untargeted") methods are used for detecting and measuring the semi-quantitative abundance of all detectable molecules in a substance, including unknown compounds. Non-targeted analysis (NTA) plays a key role in PFAS discovery and can be used to highlight emerging PFAS of concern in environmental and biological samples. Due to its extremely high mass accuracy—typically within 1–5 ppm—high-resolution mass spectrometry (HRMS) is able to determine exact molecular formulas with a high degree of confidence, allowing for the identification of unknown compounds. As of a 2019 review, NTA approaches based on HRMS have been used to discover more than 750 PFAS belonging to more than 130 chemical classes in a broad range of different sample matrices, including biofluids, environmental samples, and commercial products. Listicle THE EXPANDING PFAS TOOLKIT: SIX METHODS DRIVING ENVIRONMENTAL MONITORING 3 While NTA methods are not typically used for routine environmental monitoring, the identification of emerging PFAS or PFAS breakdown products is a crucial step in improving monitoring efforts; as new compounds of concern are identified through semi-quantitative methods such as HRMS, these can then be added to the target lists used in targeted PFAS monitoring if analytical standards already exist for these compounds, or standards can begin to be developed. In PFAS analysis, HRMS/MS is also sometimes used to improve molecular structure predictions when characterizing unknown analytes. A subset of NTA, suspect screening compares molecular features against large databases to identify potential matches and can also improve confidence in structural proposals where this is needed. 4. Ion mobility spectrometry–mass spectrometry: Improving PFAS separation Chromatography techniques are often coupled with mass spectrometry in order to separate matrix components from the sample analytes and to improve overall sensitivity. However, there are alternatives to chromatographic methods that can also be used for PFAS analysis. In ion mobility spectrometry–mass spectrometry (IMS), the sample is ionized and sent through a drift tube to be separated according to their charge and mass, but also according to how the ions interact with a buffer gas flowing in the opposite direction to the ion stream. Highly charged ions and/or smaller ions will move more quickly through the apparatus, while bulky ions will be slowed through their collision with the gas molecules. This separation method can easily be nested into existing GC and LC–MS workflows. It can also be a useful complement to LC–MS/MS data, as the use of this different separation mechanism provides an additional dimension of information about the analyte molecules; because the time that an analyte spends in IMS is determined largely by effective physical area (also known as the "collision cross section" of an ion), this may help to identify unknowns and distinguish between chemical isomers which share the same molecular weight but have very different molecular structures. IMS–MS has been increasingly applied in PFAS analysis over the past several years. A number of studies have demonstrated the effective use of the technique for the rapid characterization of PFAS in environmental water samples. Another example, this time a study of passive samplers in aquatic environments near a fluorochemical manufacturer in North Carolina, found that IMS–MS was able to identify 36 known PFAS as well as elucidate 8 new PFAS species that had previously gone unreported in environmental media. 5. Total fluorine and organofluorine methods: Measuring the unknown fraction All of the analysis methods above are "direct" or "specific" methods for PFAS measurement—meaning that they are concerned with looking at individual compounds. In contrast, "indirect" or "non-specific" methods focus less on detecting and quantifying individual PFAS and instead consider the total PFAS load in a sample. Listicle THE EXPANDING PFAS TOOLKIT: SIX METHODS DRIVING ENVIRONMENTAL MONITORING 4 Examples of indirect fluorine testing commonly applied in PFAS monitoring include: • Total fluorine: Measures the sum of all forms of fluorine present in a sample. • Total organic fluorine (TOF): Measures only organic (carbon-bound) fluorine, while ignoring natural minerals containing fluorine salts. • Extractable organofluorine (EOF): Similar to TOF, but instead of analyzing a complete raw sample, it is only able to capture the portion of organic fluorine that extracts into a solvent; this is often required for solid samples, tissues, or very complex matrices. • Absorbable organic fluorine: Similar to EOF, but for the screening of organic fluorine compounds dissolved in water. Each of these methods measures a slightly different fraction of the total fluorine load, though the general idea is the same—to use fluorine as a proxy for estimating the total PFAS concentration in a sample. The majority of these analyses are performed using combustion ion chromatography, with particle-induced gamma-ray emission and high-resolution continuum source graphite furnace molecular absorption spectrometry being the two next-most common techniques. In a similar fashion, the total oxidizable precursor (TOP) assay offers another way to capture a broader array of PFAS compounds. Many of the unmeasured PFAS in direct methods are PFAS precursors, which may exist in the environment before being converted into PFAS through oxidation. The TOP assay helps to capture this fraction of compounds in aqueous samples by oxidatively converting PFAS precursor compounds into perfluorocarboxylic acids. By comparing this to the initial sample, it is possible to estimate the concentration of PFAS precursors present in samples such as industrial effluent, landfill leachate, and other environmental sites of interest. 6. Sensor systems: Real-time monitoring for rapid response Sensor-based solutions for PFAS monitoring are an emerging technology, geared towards enabling fast and portable in-field testing. While LC–MS/MS remains the gold standard for PFAS analysis, it requires laboratory infrastructure and operation by highly trained personnel; in contrast, the optical, electrochemical, and biosensing systems currently being developed for PFAS monitoring can deliver a simple result (such as a visible color change) for rapid environmental monitoring. However, there are still many significant barriers facing the commercialization and routine adoption of sensors for PFAS monitoring. The majority of these technologies are highly specific, meaning that they will only respond to one specific PFAS analyte. Additionally, many of the current sensors still have relatively high detection limits that render them unsuitable for measuring the ultra-low PFAS concentrations that are relevant to regulatory efforts. Listicle THE EXPANDING PFAS TOOLKIT: SIX METHODS DRIVING ENVIRONMENTAL MONITORING 5 The outlook on environmental PFAS monitoring PFAS monitoring is an essential aspect of protecting human and animal health, as well as a key part of guiding environmental remediation efforts. LC–MS/MS is a core technique for PFAS monitoring, largely due to its extreme sensitivity. As a result, LC–MS/MS forms the backbone of regulatory testing approaches targeting the analysis of PFAS in environmental samples. Where a more comprehensive view of PFAS load is needed, or where LC–MS/ MS is limited in its applicability, experts can turn to a wider analytical toolbox—including techniques such as GC–MS and HRMS—which may be more suitable for analyzing certain sample types or for studying emerging PFAS of concern. More novel technologies, such as sensor systems, may one day further support this toolbox by allowing for more flexible in-field testing. Unlike traditional laboratory techniques, these systems may be more easily integrated into monitoring programs to enable more frequent PFAS testing and to facilitate the early identification of PFAS hotspots in the environment. Sponsored by About the author: Alexander is a science writer and editor for Technology Networks. He writes news and features for the Applied Sciences and Analysis & Separations communities, leading the site's coverage of topics relating to environmental science, food and beverage analysis, and analytical techniques. Before joining Technology Networks, Alexander worked as a freelance science writer. He holds a masters degree in Materials Chemistry from the University of St Andrews, Scotland. Listicle