A comparison between HRAM Orbitrap technology and MS/MS for the analysis of polyfluoroalkyl substances by EPA Method 537

Importance of the Topic

Polyfluoroalkyl substances (PFASs) exhibit extreme chemical stability, leading to their persistence in water supplies and biological systems. Their unique resistance to heat, oil, grease and stains has resulted in widespread industrial and consumer use. However, strong carbon–fluorine bonds hinder degradation, raising concerns about bioaccumulation, endocrine disruption, immune suppression and developmental toxicity. Regulatory bodies such as the US EPA have responded by introducing monitoring programs like UCMR 3 and Method 537 to quantify key PFASs in drinking water.

Study Objectives and Overview

This study evaluates a high-resolution, accurate-mass (HRAM) Orbitrap LC-MS approach versus conventional triple quadrupole tandem MS (LC–MS/MS) for EPA Method 537 target analytes. The goals are to demonstrate that HRAM full-scan and parallel reaction monitoring (PRM) on a Q Exactive instrument deliver sensitivity, accuracy and reporting limits comparable or superior to selected reaction monitoring (SRM) on a triple quadrupole, while adding the capability to detect unknown PFASs in the same extracts.

Methodology and Instrumentation

Sample Preparation

• 250 mL drinking water preserved with Trizma® buffer, spiked with isotopic surrogates.
• Solid-phase extraction on Dionex SolEx HRPHS cartridges, eluted with methanol, dried under nitrogen and reconstituted in 96:4 methanol/water.

Instrumentation

• UHPLC: Thermo Scientific UltiMate 3000 RS system with Hypersil GOLD aQ column (2.1×150 mm, 3 µm) at 30 °C; mobile phases: 20 mM ammonium acetate (water) and methanol; 19 min gradient.
• Mass Spectrometry: Thermo Q Exactive in negative polarity. Full scan at 70 000 FWHM (m/z 100–1100), 1×10⁶ AGC target; PRM at 35 000 FWHM, isolation 1 Da, 2×10⁵ AGC.
• Comparison with a triple quadrupole MS operating in SRM mode under similar LC conditions.

Main Results and Discussion

Chromatographic separation achieved good peak symmetry and retention trends: longer-chain and sulfonate PFASs eluted later. Quantitative performance for six EPA 537 analytes (PFBA to PFTrDA) was evaluated by low-level calibration (0.5–80 ppt). HRAM full scan and PRM provided comparable sensitivity (signal-to-noise) and linearity to SRM. Calculated lowest concentration minimum reporting limits (LCMRLs) met or exceeded regulatory criteria. PFOS quantification benefits from full-scan analysis, which averages branched‐isomer responses and reduces bias inherent in single SRM transitions. Non-target screening in full-scan mode enabled retrospective detection of additional PFASs (e.g., PFDS), confirmed by accurate mass, isotope pattern and spectral library matching. Data-dependent MS² acquisitions combined with Compound Discoverer workflows and Mass Frontier custom explanations facilitated rapid suspect identification.

Benefits and Practical Applications

• Compliance monitoring: HRAM Orbitrap meets EPA Method 537 requirements under the method flexibility rule.
• Improved PFOS quantitation through unbiased full-scan averages of branched isomers.
• Combined targeted quantitation and non-targeted screening in a single analysis enhances laboratory throughput and retrospective data mining.
• Software tools enable efficient data processing, candidate filtering and structure elucidation without exhaustive manual review.

Future Trends and Potential Applications

• Expansion of spectral libraries and suspect lists to cover emerging PFASs and transformation products.
• Integration of orthogonal techniques (19F NMR, MSⁿ) for structural confirmation of unidentified compounds.
• Automated, real-time monitoring workflows for continuous water quality assessment.
• Application of HRAM screening to broader environmental and biological matrices for comprehensive PFAS exposure profiling.

Conclusion

High-resolution Orbitrap LC-MS in full-scan and PRM modes provides quantitative performance equivalent or superior to traditional triple quadrupole SRM for EPA Method 537 analytes, while offering enhanced specificity, unbiased isomer quantitation and the ability to detect unknown PFASs. This approach supports regulatory compliance and advanced environmental monitoring in a single workflow.

References

1. EPA. Per- and Polyfluoroalkyl Substances (PFASs) in Your Environment. EPA website.
2. EPA. Drinking Water Health Advisories for PFOA and PFOS.
3. National Institute of Environmental Health Sciences. Perfluorinated Chemicals (PFCs).
4. EPA Method 537, Determination of Selected Perfluorinated Alkyl Acids in Drinking Water by SPE and LC-MS/MS (2009).
5. EPA. Unregulated Contaminant Monitoring Rule 3 (UCMR3).
6. Winslow SD et al. Statistical Procedures for Determination and Verification of Minimum Reporting Levels for Drinking Water Methods. Environ. Sci. Technol. 2006, 40, 281–288.