Screening of PFAS compounds in aqueous matrices using adsorbable organic fluorine (AOF) with combustion ion chromatography

Significance of the topic

Per- and polyfluoroalkyl substances (PFAS) are synthetic fluorinated chemicals used for decades in industrial and consumer applications. Their persistence, bioaccumulation potential, and toxicity have prompted stringent regulatory scrutiny. Reliable screening methods for trace levels of PFAS in aqueous matrices are essential for environmental monitoring, regulatory compliance, and protection of human health.

Objectives and study overview

This technical note outlines best practices for implementing U.S. EPA Method 1621, which determines adsorbable organic fluorine (AOF) via pyrohydrolytic combustion ion chromatography (CIC). The goals are to address common contamination sources, describe optimized cleaning workflows, and validate method performance to achieve nanogram-per-milliliter sensitivity for PFAS screening in various water samples.

Methodology and instrumentation used

Sample pretreatment:

• Wastewater or environmental water (100 mL) is spiked with sodium nitrate and passed through a granular activated carbon (GAC) column.
• High-TSS samples require pre-cleaned ceramic wool to capture particulates.
• Columns are rinsed with sodium nitrate solution and DI water to remove inorganic salts.

Combustion ion chromatography:

• Adsorbed PFAS and GAC are pyrolyzed at 1 000 °C under Ar/O₂.
• Halogenated organic fluorine is hydrolyzed and trapped in DI water.
• Fluoride is quantified by suppressed conductivity detection on an ion chromatograph (e.g., Thermo Scientific Dionex HPIC or RFIC with IonPac AS24).

Instrumentation and software:

• Nittoseiko AQF-2100 combustion system with TXA-04 adsorption unit.
• Thermo Scientific Dionex HPIC system and Chromeleon CDS version 7.3.1 or later.
• High-purity reagents: ASTM Type I DI water, Optima-grade methanol, ACS-certified ammonium hydroxide and sodium nitrate.

Main results and discussion

Validation and quality control demonstrate:

• Fluoride retention time reproducibility within ±10·SD and clear separation from water dip and chloride peaks.
• Calibration linearity (1–100 mg/L fluoride) with relative standard errors (RSE) below 10 % using appropriate weighting.
• Method detection limits (MDL) and blank contamination consistently under 1 ng/mL fluoride after rigorous cleaning of GAC columns and combustion boats.
• Ongoing precision recovery (OPR) and initial precision recovery (IPR) standards (15 µg/L PFHxS) within 90–105 % of theoretical values.

Key factors influencing performance include batch-wise cleaning of GAC columns, multi-stage cleaning of combustion boats (soak, methanol scrub, bake at 450–700 °C), and daily conditioning of the adsorption unit and CIC components.

Benefits and practical applications

Implementing these procedures enables:

• Reliable PFAS screening at ppb levels across diverse aqueous matrices (wastewater, surface water, process water).
• High throughput by parallel adsorption channels and automated CIC analysis.
• Compliance with Clean Water Act requirements and EPA reporting thresholds.

Future trends and opportunities

Advancements may include:

• Integration of combustion-CIC with mass spectrometry for PFAS speciation.
• Miniaturized or field-deployable adsorption-combustion systems for on-site screening.
• Automated cleaning cycles and real-time contamination monitoring to further lower detection limits.
• Expansion of AOF screening to novel fluorinated polymers and emerging PFAS classes.

Conclusion

U.S. EPA Method 1621, when supported by stringent cleaning protocols and systematic quality control, offers a robust approach for trace-level PFAS screening in water. Attention to consumable cleanliness, rigorous initial demonstration of capability, and ongoing performance checks are critical to maintain sub-ng/mL detection and ensure data reliability.

References

• U.S. Environmental Protection Agency. Method 1621: Determination of Adsorbable Organic Fluorine (AOF) in Aqueous Matrices by Combustion Ion Chromatography, EPA 821-R-24-002, Office of Water, 2024.
• Christison T., Rumachik N. Application Note AN002748: Screening of PFAS Compounds in Wastewater Using AOF with Combustion IC, Thermo Fisher Scientific, 2024.
• U.S. EPA CompTox Chemicals Dashboard, https://www.epa.gov/comptox-tools/comptox-chemicals-dashboard, accessed June 2024.
• ASTM International, ASTM D1193-99e1: Standard Specification for Reagent Water, 2018.