This Atomic Perspectives column explains how high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS-GFMAS) quantifies extractable organic fluorine (EOF) as a PFAS sum parameter, complementing LC-MS/MS target analysis, with applications to German river sediments, bean plants, and fluorinated polymer sealing materials.
PFAS (per- and polyfluoroalkyl substances) are a group of several million individual compounds. Many PFAS are extremely persistent, bioaccumulative, and toxic. The analysis of PFAS is challenging because of their various chemical and physical properties as well as the high number of compounds. Hence, complementary analytical strategies are needed that combine target analysis with sum-parameter approaches in which the total amount of a class of compounds is measured rather than identifying and quantifying each individual chemical species. This month’s column will briefly introduce state-of-the-art complementary analytical methods such as high resolution-continuum source-graphite furnace molecular absorption spectrometry-based methods (HR-CS-GFMAS) and how they are applied to different fields of application encountered in the European Union.
PFAS have been an ongoing topic in the scientific and nonscientific communities alike—and for good reason. Extremely persistent, both accumulative and environmentally mobile, toxic to marine and terrestrial organisms, and with more than 7 million possible individual compounds, PFAS give a lot to talk about.1,2 PFAS are defined by a fully fluorinated methylene (–CF2–) or methyl (–CF3) moiety.3 Owing to their exceptional properties regarding lipo- and hydrophilicity, thermal resistance and chemical inertness, they have found their way as an ingredient into a variety of applications, from nonstick cookware and firefighting foams to medical implants all the way to the aerospace industry.4 For these reasons, they have been excessively used since their discovery in the 1940s, and we are now facing the consequences. PFAS have spread across the globe with detections in soil, air, rivers, and biota—becoming a threat to the environment and humans alike.1 Actions have been taken to reduce their use and thus further pollution by listing the so-called legacy PFAS in the Stockholm Convention: perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and perfluorohexanesulfonic acid (PFHxS).5
To limit human exposure even further, drinking water and food legislation has been implemented, defining safety thresholds for the sum of 20 PFAS in drinking water and only 4 PFAS in foodstuffs within the European Union.6,7 However, the regulation of individual substances is like cutting off the Hydra’s heads: new ones emerge to take their place. In response to industrial demand, short-chain alternatives have been introduced, circumventing any imposed regulation, and these have now spread equally throughout the environment.8,9 Given the sheer number of emerging PFAS, individual risk assessments are nearly impossible, though the likelihood is high that most are at the very least not beneficial for one’s health.
The European Chemicals Agency (ECHA) reacted accordingly, going all the way by proposing a class regulation of more than 10,000 PFAS.10 However, analytical methods need to keep pace, both with the detection levels needed to measure threshold limits and with a better understanding of PFAS analysis to enable effective enforcement of the imposed regulation. While target analysis by LC-MS/MS has very low detection limits, the number of substances quantifiable still lags behind owing to the limited availability of labeled internal standards, reaching 57 and 54 PFAS in foodstuffs and groundwater, respectively.11,12 This is an almost negligible percentage compared with the more than 10,000 potentially restricted substances. To capture a snapshot of the remaining PFAS in samples, sum-parameter analyses have been shown to be a promising approach. Because these approaches focus on the presence of fluorine within the samples instead of individual species, the limitation lies in adequate sample preparation before analysis, which removes inorganic fluorine so that the extractable organic fluorine (EOF) or adsorbable organic fluorine (AOF) can subsequently be measured. Two methods with the necessary sensitivities for trace analysis have been established so far: combustion ion chromatography (CIC) and high resolution-continuum source-graphite furnace molecular absorption spectrometry (HR-CS-GFMAS), with the latter gaining more attention recently for being more precise, having shorter sample run times, and for the substantially lower methodological limits of detection compared with CIC, thus offering clear analytical advantages for environmental monitoring, where concentrations are generally low.13
Sum-parameter analysis has proven itself as a powerful complementary tool, able to reveal more accurately the extent of contamination, while the more sophisticated target analysis is most useful for individual risk assessments regarding compounds of high concern such as PFOS and PFOA. Here, we present state-of-the-art sum-parameter analysis for PFAS with a focus on HR-CS-GFMAS applications for the determination of EOF and fluorine in a variety of different matrices.
HR-CS-GFMAS is a close relative of atomic absorption spectroscopy. However, instead of measuring the absorption of single atoms in the gaseous phase, HR-CS-GFMAS utilizes the formation and absorption of diatomic molecules (molecular absorption spectrometry, MAS). MAS enables the determination of fluorine, whose atomic absorption line lies within the vacuum UV and is thus unsuitable for routine analyses without great effort. In a method initially developed and optimized for inorganic fluorine standards in aqueous solution, Gleisner et al. established gallium as the most responsive molecule-forming agent, with the strongest absorption line at 211.248 nm.14 During measurement, samples are placed alongside gallium (Ga) and matrix modifiers into the graphite furnace. The solvent is evaporated, pyrolysis ensues to remove any interfering matrix components, and finally the sample is atomized, forming the very short-lived gallium fluoride (GaF) at about 1550 °C. The characteristic absorption line is recorded, and the concentration subsequently determined against an external calibration. By optimizing the injection sequence, modifiers and temperatures, Metzger et al. advanced the method for the measurement of organofluorine compounds in organic solvents.15
However, during optimization it was observed that different organofluorines exhibit different absorptions at the same fluorine concentration—meaning that different PFAS produce different responses during measurement. Given the complexity of sample composition, this issue can lead to inaccurate HR-CS-GFMAS measurements by over- or underestimating the EOF content depending on the calibration standard used. Simon et al. tackled this issue and postulated that both the boiling point and the functional group of a given PFAS species affect its stability during measurement, likely through interactions with the graphite furnace and modifiers.16 Further improvements to the previous method were therefore carried out, involving four major steps:
While the pretreatment with Ga positively affected only the sensitivity of the measurement, major improvements regarding response factors were achieved through modifications of the Mg amount. Contrary to what might be expected, a reduction in sample volume increased the precision of measurement. The overall relative standard deviation of response factors across 12 PFAS decreased from 55% (initial method) to 27% (optimized method).
Sensitivities remained high and recoveries for most PFAS were between 70% and 120%. Moreover, investigation of matrix effects showed generally minor interferences, except for chloride, which significantly suppressed signals of carboxylic acids, highlighting the need for adequate removal during sample preparation.
In conclusion, the optimized method provides a more accurate PFAS sum-parameter analysis, especially for ionic PFAS, and reduces species-specific discrimination. While volatile PFAS remain challenging, the method is robust for environmental matrices such as water and solids when combined with appropriate sample preparation.
Let’s take a closer look at some selected applications.
As highlighted in the introduction section, PFAS comprise a huge number of different compounds—hence, complementary analytical approaches are needed to close mass balances and to obtain a holistic picture of the PFAS pollution situation in the environment. To investigate whether the PFAS pollution situation is decreasing or increasing over the years, suspended particulate matter time series samples (2005–2020) from the German Environmental Specimen Bank, covering all large river systems in Germany, were analyzed.17 The samples were analyzed based on the expertise of the collaboration partners via complementary analytical methods covering target analysis (LC-MS/MS), non-target screening (LC/HR-MS), direct total oxidizable precursor assay (dTOPA) in combination with LC-MS/MS, and EOF by means of HR-CS-GFMAS. For EOF analysis, a sample extraction protocol from Simon et al. was applied that does not require solid-phase extraction (SPE) cleanup, as SPE can discriminate against PFAS and thus also lead to underestimation of the EOF sum parameter.18
Figure 1 highlights one major outcome of the study: the identified EOF (iEOF, target analysis) fraction decreased over the years, which is most probably related to a shift in the PFAS in use from known to unknown PFAS. Target analysis alone would indicate that the PFAS pollution situation decreases. The unexplainable EOF (sum parameter, non-target) steadily increased during the same time period, which further underscores the fact that the fraction of unknown PFAS in use increased over this time.
Figure 1: The proportions of the EOF mass fractions in German rivers for iEOF, NTS, dTOPA and not explainable EOF are shown for time-dependent effects. (iEOF = identified EOF, NTS = non-target screening, dTOPA = direct total oxidizable precursor assay, EOF = extractable organically bound fluorine). Reprinted from reference 17 with permission from Elsevier (License Number: 6318030864523).
Only a complementary analytical approach accurately reflects the actual PFAS situation and allows for a valid assessment of the environmental exposure.
Once PFAS enter the environment, they spread throughout all environmental domains—from the soil into groundwater and surface water, as well as into plants—and ultimately end up back in our bodies through the food chain. In a further study, Gehrenkemper et al. therefore examined the distribution of various PFAS in plants. Here, too, complementary analytical methods were used—target and sum parameters—to close mass balances (see Figure 2).19 French bean plants (Phaseolus vulgaris) were cultivated on soil amended with ten different PFAS compounds that differed in chain length and functional group structure. The tested substances comprised three short-chain (C4–C5) and five long-chain (C7–C10) perfluorinated carboxylic acids, as well as one sulfonic acid and one sulfonamide compound. To evaluate compound-specific uptake patterns, each PFAS was applied individually to the soil. In addition, one mixture containing the investigated PFAS at equal fluorine mass fractions (w(F)) and four PFAS mixtures of unknown composition were examined.
After six weeks of growth, the plants were harvested and divided into four compartments: roots, stems, leaves, and fruits. These plant compartments, together with the soil samples, were analyzed for EOF using HR-CS-GFMAS and for the sum of ten target PFAS using LC-MS/MS. Figure 2 depicts the setup of the study.
Figure 2: PFAS spiking experiments in bean plants (Phaseolus vulgaris). Upon growth either with known PFAS or unknown mixed PFAS spikes, the plants were divided into the roots, stems, leaves, fruits and the soil and extracted and analyzed by means of complementary sum-parameter and target analytical approaches. Figure reproduced from Gehrenkemper et al.19 under the Creative Commons Attribution (CC BY) 4.0 license.
The results showed that the three short-chain PFAS accumulated predominantly in the fruits, whereas the long-chain PFAS were mainly retained in the roots. For the single-compound treatments, both analytical approaches yielded largely consistent results. In contrast, the PFAS mixtures of unknown composition exhibited substantial discrepancies between EOF and target PFAS concentrations, with EOF values being systematically higher.
These results highlight (i) the need for complementary analytical approaches to close mass balances and (ii) the differing properties of various PFAS in terms of bioavailability and distribution within plant compartments. In addition to their impact on food supplies, the results also provide intriguing insights into potential phytoremediation strategies for PFAS-contaminated soils.
In addition to environmental applications, HR-CS-GFMAS is also suitable for material investigations—for example, polymers. As our economy transitions from a linear to a circular economy, environmentally friendly materials and products play a crucial role—analytical methods are needed to develop and test materials and products that are safe and sustainable by design. Because of their favorable chemical and physical properties, PFAS are also found in many materials and products, such as polymers.
Elastomers represent a subgroup of polymers, which are often disposed of in landfills or waste incineration plants as no reliable recycling strategy has been established. These polymers may contain fluorinated compounds as additives, as part of their structure, or as residues from the manufacturing process. To monitor the release of fluorine, a new processing protocol is presented by Kampf et al. in the form of a standard operating procedure (SOP) for time-efficient sample preparation, subsequent extraction and screening of EOF from sealing materials,20 based on the solid–liquid extraction approach by Simon et al.18
Here, extraction with acidified methanol as well as leaching in ultrapure water was investigated for 12 different (non-)fluorinated polymers. These materials were shredded and then sorted into different size fractions, with the 125 µm to 250 µm fraction accounting for the most promising results in terms of extraction efficiency, material losses, and time required for sample preparation. Moreover, consistent and reproducible results for extractable fluorine content were obtained in both methanolic and aqueous extraction (see Figure 3).
Figure 3: Mass fraction of extractable fluorine upon extraction with either acidified methanol (MeOH, n = 3) or ultrapure water (UPW, n = 2) from different sealing materials. Extracts were analyzed via HR-CS-GFMAS. Figure reproduced from Kampf et al.20 under the Creative Commons Attribution (CC BY) 4.0 license.
Extraction with organic solvent yielded measurable fluorine content in all samples. In water, 9 of 11 samples leached fluorine, as shown in Figure 3. This raises concern regarding the environmental impact of disposed polymers when coming into contact with rainwater and groundwater.
A strength of this method is its adaptability and the easy introduction of further sample preparation steps (for example, SPE); thus, the user is offered a fit-for-purpose method for determining fluorine in polymers, particularly in view of regulatory aspects and safe-and-sustainable-by-design materials.
PFAS are among the emerging contaminants of our time, and regulatory efforts are ongoing to decrease the environmental pollution situation on the one hand and avoid the release of PFAS from materials and products on the other hand. For environmental monitoring as well as the development of safe and sustainable materials, appropriate analytical methods are needed. However, given the enormous number of possible PFAS, only a holistic analytical approach is sufficient for closing mass balances—combining target, non-target, and sum-parameter approaches.
Within our research group, we develop sum-parameter methods based on HR-CS-GFMAS applicable to different environmental as well as material-related matrices as highlighted in this article. Our research approach is based on rapid screening using HR-CS-GFMAS (EOF) followed by target and non-target LC/HR-MS analysis. EOF provides a more realistic assessment of PFAS exposure levels and enables the closing of mass balances, while the subsequent target and non-target analysis makes it possible to identify PFAS and to identify and eliminate their sources. Thus, only complementary elemental as well as molecular instrumental analytical approaches deliver information needed for a holistic evaluation of the PFAS pollution situation.
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Sebastian Kampf majored in biochemistry and is currently a PhD candidate at the Federal Institute for Materials Research and Testing (BAM) in Division 1.1 – Inorganic Trace Analysis (ITALab). His research focuses on method development for determining PFAS in varying matrices with emphasis on industrial materials.
Pia Schünemann is a PhD candidate at the Federal Institute for Materials Research and Testing (BAM) in Berlin, Germany, in Division 1.1 – Inorganic Trace Analysis (ITALab). Her research focuses on the development of new methods for the determination of fluorine in environmental samples related to the seasonal and spatial co-occurrence of PFAS and trace organic contaminants in a river system.
Carolin Dietze completed her bachelor’s degree in geoecology at the Technical University Bergakademie Freiberg, Germany. She is now a master’s student of geoecology and environmental science, focusing on soil and water protection as well as hydropedology, where she is conducting a temporal and spatial analysis of PFAS in soils in Saxony.
Björn Meermann is the Head of Division 1.1 – Inorganic Trace Analysis (ITALab) at the Federal Institute for Materials Research and Testing (BAM) in Berlin. He studied chemistry at the University of Münster and received his doctorate in Analytical Chemistry in 2009, followed by research stays at Ghent University (Belgium) and the Federal Institute of Hydrology (Koblenz, Germany). His team’s research lies at the intersection of materials, environment, and the life sciences, where they investigate the release of elements, elemental species, and (nano)particles from materials into the environment and their potential uptake by organisms and cells, with the aim of assessing their impact on the environment and ultimately supporting new “sustainable” materials in the context of the circular economy by using techniques such as ICP-(ToF)-MS and HR-CS-GFMAS-based methods.
Robert Thomas, CSci, CChem, FRSC is column editor for Atomic Perspectives. He is Principal Scientist for Scientific Solutions; Team Leader, Montgomery County K-12 STEM Volunteer Program; and Assistant Adjunct Professor of Chemistry at the University of North Dakota.
