High-quality trace-level AOF (EPA 1621) and EOF for PFAS investigations

These new services include limits of reporting which are among the lowest available commercially globally, at 0.5 µg F/L for water and 0.2 mg F/kg for soils and solids. This level of sensitivity can provide industry experts with greater confidence when evaluating PFAS trends in water quality monitoring.

 

Targeted PFAS methods quantify a defined list of compounds, typically 40 or fewer by standard US EPA methods, and up to 60 using ALS extended suites. However, thousands of substances meet common PFAS definitions, with reference standards unavailable for most. Targeted PFAS analysis may therefore account for only a small fraction of the organic fluorine present at sites affected by complex sources. AOF and EOF address this gap by measuring broader, operationally defined fractions of organic fluorine, which may include target and non-target PFAS, as well as non-PFAS fluorinated organic compounds.

ALS initial method validation studies demonstrated exceptional precision even at trace levels, with 4% (AOF) and 6% (EOF) relative standard deviation (RSD) at 1 µg F/L – achieving an RSD well below the EPA Method 1621 limit of 20% RSD for mid-level concentrations. Combined with exceptionally low method background and trace-level sensitivity, this precision supports our industry-leading lab practices and provides greater confidence in test results by meeting or exceeding EPA 1621 method criteria.

 

Figure 1. Conceptual representation of fluorine forms

 

Selectivity of AOF and EOF across PFAS classes

AOF and EOF are defined by the sample preparation techniques used to isolate organic fluorine. Depending on the procedure, the recovered fraction may include anionic, cationic, zwitterionic, neutral, and precursor compounds, but no single process can fully recover every PFAS class.

For AOF (EPA 1621), organofluorine compounds are adsorbed on activated carbon. After adsorption and washing prepared activated carbon is combusted, and released fluoride is measured by ion chromatography. The method generally favors medium- and longer-chain compounds, including common anionic PFAS such as perfluoroalkyl carboxylates and sulfonates. It can also capture cationic, zwitterionic, neutral, and precursor compounds, including classes associated with aqueous film-forming foams and industrial formulations. Recovery depends on chain length, functional group, matrix composition, and adsorption and washing conditions. Ultrashort and some short-chain PFAS may be incompletely retained or lost during preparation.

For EOF, organofluorine compounds are extracted by solid-phase extraction (SPE). Recovery of anionic, cationic, zwitterionic, and neutral PFAS depends on the sorbent, solvents, pH, washing, and elution conditions. ALS uses an extraction and cleanup procedure aligned with US EPA Method 1633, which facilitates comparisons between EOF and targeted PFAS or TOP (Total Oxidizable Precursors) Assay data generated using similar extraction chemistry. The sample extract is combusted and the released fluoride is measured by ion chromatography.

For soils and solids, AOF and EOF use the same initial solvent extraction used by US EPA Method 1633. The extract is then diluted with water and processed by activated-carbon adsorption for AOF or solid-phase extraction and cleanup for EOF.

Matrix constituents, including inorganic fluoride, can affect method performance and may require modified preparation or dilution. ALS measures inorganic fluoride and other sample characteristics to evaluate these effects and apply appropriate controls to minimize bias. In some cases, a higher reporting limit may be required to maintain data quality.

AOF and EOF are complementary rather than interchangeable because each targets different PFAS compound types. Method selection should consider the expected PFAS classes, sample matrix, and investigation objective. ALS recommends initial testing of complex PFAS sites for both AOF and EOF to ensure that significant sources and forms of organic fluorine are not missed.

 

Combustion ion chromatography: aggregate organic fluorine measurement

Figure 2. Schematic of combustion ion chromatography. Adapted from von Abercron et al. 2019

Following AOF or EOF preparation, the retained carbon or cleaned extract is analyzed by combustion ion chromatography (CIC). Pyrohydrolytic combustion converts organically bound fluorine to hydrogen fluoride, which is collected in an aqueous solution and measured as fluoride by ion chromatography.

The result represents the aggregate fluorine recovered by the selected preparation technique, and is reported as µg F/L for aqueous samples or mg F/kg for solids. CIC does not identify the individual compounds contributing to the result.

 

Complementing targeted PFAS with AOF and EOF

AOF and EOF provide the greatest value where complex or poorly characterized mixtures of fluorinated compounds are likely. Used alongside targeted PFAS methods, they can indicate whether a substantial organofluorine fraction remains unexplained and provide more information about its nature. Some recommended applications include:

• AFFF-impacted sites: Airports, military installations, fire-training areas, refineries, tank farms, terminals, and well sites which may have complex PFAS mixtures resulting from historical or current foam use, system testing, and releases. These mixtures can include precursors and proprietary components not covered by routine targeted methods.
• Landfills and wastewater systems: Landfill leachate and wastewater can contain PFAS from diverse sources, as well as precursors and transformation products outside targeted lists. AOF and EOF can support comparison of locations, waste streams, influent, effluent, and changes over time.
• Industrial facilities: Fluoropolymer processing, metal plating, coatings, textiles, electronics, and other operations involving fluorinated materials may release process aids, by-products, or other organofluorine compounds not included in targeted methods.
• Treatment and remediation progress: Testing before and after activated carbon, ion exchange, separation, or destruction processes can indicate whether the broader organofluorine fraction is decreasing, rather than only targeted PFAS. Results can also help identify potential transfer between phases or formation of unmeasured transformation products.
• Property assessment and due diligence: Screening can identify potential organofluorine concerns where site history is incomplete to help determine whether additional characterization is warranted.

Figure 3. Non-target PFAS in AFFF may be measurable by AOF/EOF.

Comparison with targeted PFAS or TOP assay test results can support fluorine mass-balance studies to identify samples with unexplained organic fluorine. Spatial and temporal patterns may then help identify source areas, assess migration, evaluate treatment stages, and prioritize further sampling.

AOF and EOF results are expressed on a common fluorine basis. For comparison to PFAS data from LC mass spectrometry, the conversion factor is the proportion of fluorine in the PFAS molecule by mass. PFOS or PFOA proportions are often used as a proxy for this conversion. By mass, PFOS is 65% fluorine, and PFOA is 69% fluorine.

 

AOF and EOF do not identify the compounds responsible for an unexplained organic fluorine fraction, so the sample results should not automatically be attributed entirely to PFAS. Results can instead demonstrate the need for expanded targeted analysis, precursor testing, high-resolution mass spectrometry, and/or additional site characterization.

 

Table 1: Advantages and Limitations of AOF & EOF

Technique	Relative Advantages	Key Limitations
AOF	Broad molecular-weight and charge-class coverage, including neutral, anionic, cationic, and zwitterionic PFAS.	Ultrashort and some short-chain PFAS may be poorly retained.
EOF	Coverage aligns with EPA Method 1633. Better retention of some short-chain and ultrashort PFAS than AOF.	Coverage depends on extraction conditions and does not quantitatively include polymeric PFAS (e.g. Teflon).

Sample collection and handling

Table 2 summarizes sample collection requirements for AOF and EOF. Samples are collected in HDPE containers with linerless caps. Chemical preservatives are not necessary for these analyses. Samples should be protected from light and refrigerated during transport and storage.

ALS provides AOF and EOF testing for aqueous and solid matrices through our North American Waterloo laboratory, supported by testing for an extensive suite of targeted PFAS both locally and through the USA laboratory network. The AOF Water procedure follows EPA Method 1621 prescriptively, making it appropriate for situations in which the method is required. The ALS Waterloo laboratory holds ISO/IEC 17025 accreditation through CALA for AOF and EOF testing in water and solid matrices. Refer the CALA scope of accreditation for ALS in Waterloo for current accreditation details.

Table 2. AOF and EOF sampling requirements

 

Please contact your ALS Project Manager for further information or to arrange for sampling supplies.

References

1. Von Abercron et al. (2019), Determination of adsorbable organically bound fluorine (AOF) and adsorbable organically bound halogens as sum parameters in aqueous environmental samples using combustion ion chromatography (CIC), Science of the Total Environment, 673, 384–391.
2. US EPA Method 1621, Determination of Adsorbable Organic Fluorine (AOF) in Aqueous Matrices by Combustion Ion Chromatography (CIC), EPA 821-R-24-002 (2024).
3. US EPA 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).