In May 2026, the US Environmental Protection Agency (EPA) proposed a significant realignment of its National Primary Drinking Water Regulations for per- and polyfluoroalkyl substances (PFAS). The agency proposed retaining the 4.0 ppt maximum contaminant levels (MCLs) for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) while rescinding and reconsidering regulatory determinations for perfluorohexanesulfonic acid (PFHxS), perfluorononanoic acid (PFNA), hexafluoropropylene oxide dimer acid (HFPO-DA, GenX), and the associated Hazard Index framework.
For analytical laboratories, this creates an asymmetric testing environment. Drinking water compliance programs must continue delivering ultra-trace quantitation for PFOA and PFOS, while environmental, industrial, and food-testing laboratories increasingly require broader screening strategies capable of detecting emerging fluorinated compounds before they enter regulated pathways. This article examines the chromatographic, mass spectrometric, and workflow implications of this shift, with emphasis on EPA Methods 537.1, 1633A, and 1621.
Maintaining Sensitivity at the 4.0 ppt Threshold
The proposed retention of the 4.0 ppt MCLs for PFOA and PFOS preserves one of the most demanding analytical challenges in environmental testing. At these concentrations, contamination from laboratory infrastructure often rivals contamination in the sample itself.
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Fluoropolymer materials remain a major source of background PFAS. Components containing polytetrafluoroethylene (PTFE), including solvent lines, seals, and degasser membranes, can introduce measurable contamination. Laboratories therefore continue replacing fluoropolymer components with polyether ether ketone (PEEK) and other PFAS-free alternatives while implementing rigorous blank control procedures.
The use of PFAS delay columns remains standard practice. Positioned upstream of the injector, these columns separate instrument-derived PFAS contamination from analyte peaks generated by the sample. Laboratories operating near regulatory limits often rely on this approach to maintain acceptable performance of laboratory reagent blanks.
Sample handling requirements remain equally important. EPA Method 537.1 minimizes contact with glass surfaces because target analytes may adsorb to active silanol sites. Polypropylene and polyethylene containers remain the preferred materials for storing standards, extracts, and samples. Method 1633A allows specific high-purity glass consumables to be prepared under controlled procedures, but both methods require PFAS-certified HDPE sample containers.
Isomer Resolution and Quantitation
Manufacturing pathways influence the chromatographic behavior of PFAS. Electrochemical fluorination generates mixtures of linear and branched isomers, whereas telomerization primarily produces linear structures.
These isomers often exhibit different ionization efficiencies and fragmentation behavior. Consequently, chromatographic methods must provide sufficient resolution to ensure reliable quantitation. Under EPA Method 537.1, laboratories report the combined response of linear and branched isomers.
For PFOS, the 499→80 transition remains the preferred quantitation pathway because the sulfonate fragment is consistently produced across branched and linear forms. Alternative transitions may underrepresent certain isomer populations.
PFHxS, PFNA, and GenX: Analytical Priorities Beyond Compliance
Although EPA has proposed revisiting drinking water regulations for PFHxS, PFNA, and GenX, these compounds remain analytically significant. Method 1633A continues to target them across wastewater, groundwater, biosolids, tissue, and surface water matrices.
Method 1633A relies heavily on isotope dilution. Isotopically labeled analogs added before extraction compensate for losses arising from adsorption, extraction inefficiencies, and sample preparation. This approach improves accuracy across complex environmental matrices where recovery effects can vary substantially.
Chromatographically, these compounds present distinct challenges. Shorter-chain PFAS exhibit weaker retention on traditional C18 phases and often elute near the solvent front. Laboratories frequently employ highly aqueous starting conditions and polar-embedded stationary phases to improve retention and peak shape.
GenX introduces an additional complication. During electrospray ionization, the compound readily undergoes decarboxylation. As a result, methods target the stable fragment ion at m/z 285 rather than the intact deprotonated molecule. Careful optimization of source conditions remains essential for accurate quantitation.
Moving Beyond Target Lists
The rapid evolution of PFAS chemistry continues to challenge targeted analytical methods. Even extensive target lists capture only a fraction of the fluorinated compounds entering commercial use.
EPA Method 1621 addresses this limitation by using adsorbable organic fluorine (AOF) analysis via combustion ion chromatography. Rather than identifying individual compounds, the method measures the total mass of adsorbable organofluorine present in a sample.
In Method 1621, aqueous samples pass through granular activated carbon, which retains organofluorine species. A nitrate wash removes inorganic fluoride before combustion converts retained compounds to hydrogen fluoride. The resulting fluoride is quantified by ion chromatography.
This aggregate approach provides a valuable screening tool for identifying contamination that targeted LC-MS/MS methods may miss. However, analysts should recognize its limitations. Very short-chain compounds such as trifluoroacetic acid may exhibit poor retention on activated carbon, while highly hydrophobic long-chain species can adsorb to system surfaces and produce low-biased recoveries.
The Rise of High-Resolution Screening
Targeted triple-quadrupole instruments remain the benchmark for regulatory quantitation, but they cannot detect compounds absent from predefined acquisition lists.
To address this gap, many laboratories now supplement targeted workflows with high-resolution accurate-mass (HRAM) platforms such as Orbitrap and quadrupole time-of-flight systems.
These instruments support suspect and non-target screening strategies that can reveal previously unrecognized PFAS. Kendrick Mass Defect filtering remains one of the most powerful tools for identifying homologous fluorinated series, while data-independent acquisition enables retrospective analysis of archived datasets when new compounds become regulatory priorities.
This capability reduces the need to re-extract and reanalyze samples whenever target lists expand.
Building Future-Proof PFAS Workflows
The proposed regulatory separation of PFOA and PFOS from other PFAS highlights a broader trend: analytical requirements are becoming more diverse rather than more focused.
Successful laboratories increasingly combine three complementary capabilities:
- Ultra-trace targeted LC-MS/MS for regulatory compliance.
- Multi-matrix PFAS analysis using Method 1633A.
- Broad screening approaches based on AOF and HRAM technologies.
Together, these approaches provide both regulatory certainty and the flexibility needed to respond to emerging contaminants.
Conclusion
The EPA's 2026 PFAS proposals reinforce the need for adaptable analytical strategies. While PFOA and PFOS remain the primary compliance targets in drinking water, the broader PFAS landscape continues to evolve as new compounds enter commercial use and regulatory priorities shift.
Laboratories that integrate targeted quantitation with class-based screening and high-resolution mass spectrometry will be best positioned to navigate this changing environment. The future of PFAS analysis will depend less on static target lists and more on analytical platforms capable of characterizing both known contaminants and the larger organofluorine universe from which future regulatory concerns are likely to emerge.
References
- US Environmental Protection Agency. Method 537.1: Determination of Selected Per- and Polyfluorinated Alkyl Substances in Drinking Water by Solid Phase Extraction and Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS), Version 2.0. EPA/600/R-20/006, 2020.
- US Environmental Protection Agency. Method 1633A: Analysis of Per- and Polyfluoroalkyl Substances (PFAS) in Aqueous, Solid, Biosolids, and Tissue Samples by LC-MS/MS. EPA 820-R-24-007, 2024.
- US Environmental Protection Agency. Method 1621: Determination of Adsorbable Organic Fluorine (AOF) in Aqueous Matrices by Combustion Ion Chromatography. EPA 821-R-24-002, 2024.
- US Environmental Protection Agency. Proposed Revisions to Compliance Deadlines and Exemptions for Certain PFAS Drinking Water Standards. Federal Register, May 2026.
- Schymanski EL, et al. Identifying Small Molecules via High Resolution Mass Spectrometry: Communicating Confidence. Environmental Science & Technology, 2014, 48(4), 2097–2098.
- McDonough CA, Guelfo JL, Higgins CP. Measuring total PFASs in water: The tradeoff between selectivity and inclusivity. Current Opinion in Environmental Science & Health. 2019;7:13–18. doi:10.1016/j.coesh.2018.08.005.
