PFAS Analysis of Leachate: Evaluating Key Sample Preparation and LC–MS/MS Parameters for Improved Performance

Significance of the topic

Landfill leachate is an analytically challenging, highly variable matrix that concentrates per- and polyfluoroalkyl substances (PFAS), many of which are environmental and human-health concerns. Robust, reproducible PFAS quantitation in leachate is essential for source characterization, regulatory compliance, and remediation planning. This study evaluates critical sample-preparation and LC–MS/MS parameters that govern sensitivity and reproducibility for a broad range of PFAS classes in complex leachate matrices, with the aim of improving routine analytical performance beyond the baseline EPA 1633A workflow.

Objectives and study overview

• Assess how SPE sorbent choice and drying conditions influence recoveries, particularly for neutral and hydrophobic PFAS.
• Compare chromatographic column performance under repeated matrix exposure and solvent storage.
• Systematically evaluate LC–MS/MS ion-source parameters (nebulizing/drying gas, interface temperature and voltage) across PFAS classes to identify class-dependent ionization drivers.
• Provide practical recommendations to enhance accuracy, sensitivity, and robustness for landfill leachate PFAS analysis.

Methodology and sample preparation

• Samples: Real landfill leachate samples (southern U.S.) were processed following EPA Method 1633A with modifications explored for drying and sorbent selection.
• SPE: Six commercially available weak anion exchange (WAX) cartridges (labelled A–F) were evaluated for extraction efficiency of the EPA 1633A target list, with special attention to neutral, less ionic PFAS.
• Drying optimization: EPA 1633A drying steps were extended/modified to improve removal of residual water from the sorbent bed to enhance retention and elution of hydrophobic neutral analytes.
• Chromatography: Four C18 columns (including Shimadzu GIST and three alternate C18 phases) were compared using peak area, peak height, asymmetry, and HETP under repeated batch runs and after storage in acetonitrile.
• Mass spectrometry: LC–MS/MS analyses used a Shimadzu LCMS-8060 operated in negative ESI. Source parameters (nebulizing gas, drying gas, interface temperature, and interface voltage) were systematically varied to construct a source-response heat map for EPA 1633A analytes.

Instrumental setup

• LCMS: Shimadzu LCMS-8060 triple quadrupole MS operated in negative electrospray ionization (ESI) mode.
• Columns: Shimadzu GIST C18 and three alternate C18 phases were evaluated.
• SPE sorbents: Six WAX cartridges (A–F); sorbents E and F correspond to MilliporeSigma WAX with 200 mg and 500 mg bed volumes, respectively.

Main results and discussion

• SPE performance and neutral PFAS: Neutral PFAS exhibited poor and variable recoveries on standard WAX sorbents. Two internal standards—D3-NMeFOSA and D5-EtFOSA—showed particularly low average recoveries (~40–59% and ~36–53%, respectively), indicating weak retention of hydrophobic, less ionic analytes on WAX phases. Only sorbents E and F (MilliporeSigma 200 mg and 500 mg) consistently met target recovery ranges for these compounds.
• Drying optimization benefit: Extending and optimizing EPA 1633A drying steps yielded an approximate additional 12% recovery for the evaluated neutral PFAS, consistent with the concept that residual water on the sorbent disrupts hydrophobic interactions and reduces elution efficiency.
• Chromatography and matrix robustness: Two columns produced higher signal and better peak shape while the others provided higher efficiency. The Shimadzu GIST column demonstrated superior robustness, showing ≤15% signal change after repeated matrix exposure and acetonitrile storage, whereas another evaluated C18 lost 20–50% signal—especially for long-chain PFAS and precursor compounds—implying greater susceptibility to matrix accumulation and signal degradation.
• Ion-source class-dependent behavior: Source response was PFAS-class dependent. Short-chain perfluorocarboxylates (PFCAs) were more sensitive to nebulizing and drying gas settings (aerosol-driven ionization), while perfluorosulfonates (PFSAs) benefited more from increased interface temperature (thermal desolvation). Increasing interface voltage consistently reduced analyte signal, consistent with in-source fragmentation. Overall interpretation: tune gas flows to favor PFCA detection and raise source temperature to improve PFSA response, while avoiding excessive interface voltage.
• Native leachate profile: The most abundant native PFAS detected was 5:3 FTCA at ~239 ng/mL, in line with known uses (e.g., carpeting) and typical landfill inputs.

Benefits and practical applications

• Improved recoveries for neutral PFAS: Selecting appropriate WAX sorbents (e.g., MilliporeSigma 200/500 mg bed) and increasing sorbent drying can materially improve recovery of hydrophobic neutral PFAS, reducing false negatives and bias in mass balance or source apportionment studies.
• Class-specific source tuning: Adjusting nebulizing/drying gas and source temperature according to PFAS class increases sensitivity without wholesale reconfiguration. This is particularly useful in mixed-class matrices like leachate where both short-chain PFCAs and long-chain PFSAs coexist.
• Column selection for routine work: Using matrix-robust stationary phases such as GIST reduces long-term signal degradation and maintenance/replacement frequency when analyzing heavily contaminated matrices.
• Method refinement for regulation and monitoring: Integrating these optimizations into EPA 1633A-based workflows can strengthen compliance monitoring and research studies by lowering analytical uncertainty in complex matrices.

Future trends and potential uses

• Alternative sorbent chemistries: Development and testing of mixed-mode or hydrophobic-enriched sorbents specifically targeted at neutral and highly hydrophobic PFAS will improve extraction breadth beyond WAX limitations.
• Automated drying and sorbent conditioning: Instrumented SPE automation that controls drying efficiency could standardize recovery improvements and reduce inter-operator variability.
• Adaptive source tuning: Software-driven, analyte-class-specific source parameter sets could be implemented to maximize sensitivity for multi-class PFAS runs within a single sequence.
• Broader applicability: The optimized practices demonstrated here can be extended to other high-matrix aqueous samples (industrial wastewater, biosolids leachates) and integrated into regulatory method updates.

Conclusion

• PFAS analytical performance in landfill leachate is governed by class-dependent extraction, chromatographic robustness, and ionization mechanisms. Neutral PFAS require attention beyond standard WAX SPE—appropriate sorbent selection and efficient drying improve recoveries significantly.
• Chromatographic phase choice impacts long-term signal stability in matrix-rich samples; the Shimadzu GIST column showed superior resistance to matrix-induced signal loss.
• Ion-source optimization should be performed with PFAS class in mind: aerosol processes favor PFCAs, thermal desolvation favors PFSAs, and excessive interface voltage can suppress signal via in-source fragmentation.
• Implementing these targeted optimizations enhances sensitivity and reproducibility for PFAS quantitation in landfill leachate, supporting more reliable monitoring and research outcomes.

References

1. U.S. Environmental Protection Agency. Method 1633A: Analysis of Per- and Polyfluoroalkyl Substances (PFAS) in Aqueous, Solid, Biosolids, and Tissue Samples by LC-MS/MS; EPA 821-R-24-005; Washington, DC, 2024.
2. Raynie, D. E.; Watson, D. W. Understanding and Improving Solid-Phase Extraction. LCGC North America 32(12), 2014.
3. Omaojo, U.; Quinete, N. PFAS in Municipal Landfill Leachate: Occurrence, Transformation, and Sources. Science of the Total Environment 2024, 908, 168197.