3 Summary of advantages, disadvantages, and applica­tion for the techniques of PFAS/fluorine determination

3.1 Suitable methods for PFAS analysis in different matrices

3.2 Challenges in PFAS analyses

1. Sampling: PFAS have been detected in various environment matrices and organisms due to their widespread usage in industrial and consumer products and their unique physicochemical properties. For all different matrices detailed sample collection and preservation methods are needed. Often the focus in the development of analytical techniques lies on the sample preparation and technique itself. Sampling and preservation are often considered as trivial and not considered nor optimised, although recently insights have shown this to be critical aspects of PFAS analysis. Some PFAS are volatile, or not stable for a long period of storage and specific preservation measures are needed. Stability testing and sampling validation should be a part of the method development and validation and should ultimately lead to standardisation of sampling protocols for PFAS analysis.
2. Sample preparation: Different kinds of samples requires different sample preparation techniques. The pre-treatment of sludge, soil, sediment is often more complicated than for example water samples. Generally, samples require preparation steps of drying, sieving, homogenization, extraction, clean-up and concentration before analysis.
   1. The choice of the extraction solvent is critical in the extraction step. For the anionic PFAS, alkalic methanol is often used as extraction solvent. The alkalic solvent is not ideal for the extraction of cationic, zwitterionic and neutral PFAS. The latest needs an extraction with strong acid solvents. To measure a wide class of PFAS compounds, sequential extraction methods are thus necessary and further developments/insights are needed in this field.
   2. In order to reach the required LOQs sample extraction efficiency and concentration are needed, e.g. for water samples. Depending on the choice of sorbent (e.g. SPE anion exchange vs EnviCarb cartridges) different PFAS classes can be retained (or not) and a first discrimination already took place before the actual measurement.
   Sample preparation deserves sufficient attention during the method development process. The extraction and concentration procedure will always be discriminatory and make already a first selection in the different PFAS classes even before measurement!

3. Analytical Method Development: Developing sensitive and selective analytical methods for PFAS detection and quantification is challenging due to the diverse chemical structures and properties of PFAS compounds. Different PFAS may require different analytical approaches e.g., PFAS polymers vs PFCAs. The efforts related to the development of suitable analytical methods have been increased over the last years among all concerned actors (e.g., authorities, academia & industry). It is essential that activities in this field continue, that resources are secured, and that capacity building is considered appropriately. Joint activities by authorities, industry and also standardisation bodies are considered beneficial to increase efficiency. A key focus should be given to develop standard methods, which can be applied commercially.
4. Analyte Complexity: PFAS can exist in various forms, including different chain lengths, isomers, and structural variations. Analysing this complexity accurately can be difficult, especially when trying to distinguish between isomers or measure individual compounds within a mixture. For example, reliable and reproducible analysis of ultra short-chain PFAS still presents a challenge. One of the main challenges at hand is the enormous number of individual PFAS substances that can be encountered. It seems currently impossible to ensure compliance by targeted analysis only. Therefore, it is essential to develop a guidance as regards the testing strategy, defining clearly what proofs need to be provided by duty holders.
5. Low Detection Limits: Individual PFAS compounds are often found in trace amounts in environmental samples and also in consumer products, requiring analytical methods with very low detection limits to accurately measure their mass fractions and concentrations. Not all analytical techniques are suitable to meet the required limits of quantification. However, this is necessary for compliance testing. Efforts are needed to improve the current limits of quantification as the currently proposed limit values are challenging. For efficiency reasons it seems useful to concentrate on selected methods that have the potential to be standardised. In order to ensure legal certainty, authorities should define method detection limits (MDLs) and reporting limits (RLs) based on method performance data and the specific requirements of PFAS.
6. Sample Matrix Effects: Complex sample matrices can interfere with the analytical process and require sample preparation techniques to reduce matrix effects. In particular for polymeric PFAS sample preparation can be difficult. Standard protocols for sample preparation would be an added value for future methods. Although contradictory with the above statement, techniques that require only minimal or no sample preparation should be investigated for its potential for standard use.

7. Reference Materials: Limited availability of both analytical and matrix reference materials for PFAS compounds hamper progress of the current analytical state-of-the-art. Several stakeholders mentioned that synthesis of isotope labelled internal reference standards for PFNS, PFUnDS, PFTrDS etc. are urgently needed. Further, the lack of availability of a reference standard for C₆O₄ was highlighted as a problem. Fortunately, a diverse set of analytical reference standards is already commercially available and the number of available PFAS standards is continuously increasing. Setting priorities and establishing incentives (e.g., research funds) for reference standard providers might speed up the processes. Availability if certified matrix reference materials is very limited, which complicates method trueness validation. Significant change in this respect is not imminent as development of such materials is extremely expensive and time consuming.
8. Non-Targeted Screening: Non-targeted screening methods are still evolving and there is a lack of standardized approaches for comprehensive PFAS analysis. Interpreting non-targeted screening data is complex, manual verification is often needed and thus the process is still quite subjective. Overcoming the challenges associated with non-targeted screening methods for PFAS analysis requires a combination of methodological advancements, standardization efforts, and the development of tools to facilitate data interpretation. Continuous refinement of non-targeted screening methods is crucial. This involves improving the sensitivity, selectivity, and reproducibility of analytical techniques. Collaboration between researchers can lead to the development of more robust and standardized protocols and supports the creation of a comprehensive PFAS database, facilitating the identification of unknown compounds. Furthermore, the development of computational tools for data analysis should be encouraged.
9. Data Analysis: PFAS data analysis can be labour-intensive and the interpretation of results can vary among analysts, especially in non-targeted screening. Automated data analysis tools are still being developed. Concerted efforts are required to advance the development and implementation of such automated data analysis tools. Investing in research and technology to enhance the capabilities of machine learning algorithms, artificial intelligence, and data processing software is essential. In addition, training programs to build up capacities throughout Europe should be encouraged.
10. Analytical Instrumentation: State-of-the-art analytical instrumentation is required for PFAS analysis, and maintaining and operating this equipment can be expensive and technically demanding. Further, availability of high-resolution equipment that is needed for non-target analysis, is less available in routine laboratories due to the complexity to operate and due to lack of legal compliance of non-target data.

11. Background contamination: PFAS are known for their environmental persistence, which can lead to contamination of laboratory equipment and glassware, potentially resulting in false positives if not managed properly. In case a technical guideline is prepared supporting the proposed PFAs restriction the known aspect of background contamination could further be highlighted.