Detection of PFAS in Aqueous Samples by Matrix Assisted Laser Desorption Ionisation Time-of-Flight (MALDI-TOF) Mass Spectrometry

Significance of the topic

Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative contaminants of increasing regulatory and research concern, particularly in aqueous environments. Rapid, low-cost screening methods that indicate presence of PFAS in water can accelerate monitoring, triage, and follow-up confirmatory analysis. This study demonstrates a proof-of-principle workflow using benchtop linear MALDI-TOF mass spectrometry to detect representative PFAS in aqueous samples with minimal sample preparation.

Study objectives and overview

The primary objective was to develop a simple, rapid MALDI-TOF method to detect representative PFAS (PFOA, PFOS, PFHxA, PFBS) in water. Key aims were to (1) identify suitable MALDI matrices and acquisition parameters, (2) evaluate spot homogeneity and sampling strategies using MALDI imaging, (3) determine approximate limits of detection (LODs) for model PFAS, and (4) apply the workflow to real bottled and pond water samples with orthogonal confirmation by LC-QTOF MS after SPE.

Methodology

The approach minimized sample preparation: aqueous samples or standards were mixed 1:1 with a MALDI matrix solution then 1 µL was spotted onto a MALDI target. Two matrices were optimized: norharmane (1 mg/mL in MeOH:H2O 70:30) and 1,8-bis(tetramethylguanidino)naphthalene (TMGN, 5 mg/mL). MALDI-MSI (imaging) was used during optimization to assess spot homogeneity and guide acquisition strategies. Representative analyte peaks monitored included m/z 413.1 for PFOA and m/z 499.1 for PFOS; 13C-PFOS (m/z 507.1) was used as an internal standard for PFOS.

Used instrumentation

• MALDI-8030 EasyCare benchtop linear MALDI-TOF mass spectrometer (Shimadzu) operated in negative polarity, m/z 100–1000 acquisition range, 5 shots per profile at 200 Hz.
• MALDI imaging and data processing: IonView (Shimadzu) for spot imaging and visualization.
• Solid-phase extraction: Chromabond WAX SPE columns for sample cleanup prior to LC-MS confirmation.
• Confirmatory analysis: LCMS-9050 Q-TOF mass spectrometer (Shimadzu) using data-independent acquisition (DIA) and LabSolutions Insight Explore for MS/MS library matching.
• General consumables: standard PFAS solutions (Sigma), solvents (methanol, acetonitrile), SpeedVac concentrator for extract concentration.

Main results and discussion

• Matrix selection: of several tested matrices (CHCA, DHB, DHAP, DAN, norharmane, TMGN), norharmane and TMGN performed best. TMGN produced more homogeneous spot distribution than norharmane, improving sampling consistency.
• Spot homogeneity and acquisition strategy: MALDI-MSI revealed analyte accumulation toward spot edges (particularly with norharmane). Implementing an annular raster (sampling ring-shaped regions) improved signal-to-noise for norharmane spots, although it requires precise positioning or slide strategies (e.g., FlexiFocus) for routine use. A software data quality filter can also ensure sufficient signal at each acquisition point.
• Limits of detection (approximate): PFOA ~5 ng/mL (5 ppb); PFOS ~250 pg/mL (250 ppt); PFHxA ~200 pg/mL (200 ppt); PFBS ~1 ng/mL (1 ppb). Sensitivity varied by PFAS subclass and matrix.
• Application to real samples: bottled and pond water were screened using the optimized TMGN method. Several samples showed peaks at m/z values corresponding to PFAS standards. For example, both bottled waters contained a PFOA-corresponding peak; one bottled sample also showed PFBS. One pond water sample contained PFHxA and PFOS signals, while another pond sample showed no detectable PFAS by MALDI screening.
• Orthogonal confirmation: SPE enrichment followed by LC-QTOF DIA and library MS/MS matching confirmed the MALDI-flagged PFAS identifications. Library similarity scores (SI) reported were high for PFOS (98–100), good for PFOA (80–91), and variable for PFBS and PFHxA (77–96), supporting the initial MALDI screening results.

Benefits and practical applications of the method

• Speed: sample-to-result screening within minutes after simple mixing and spotting, enabling high-throughput triage of environmental samples.
• Minimal sample preparation: no SPE required for initial screening, reducing cost and time per sample.
• Low resource footprint: entry-level, compact MALDI-8030 instruments are robust and suitable for labs with limited space or resources.
• Flexible application: the workflow is suitable for a variety of aqueous sample types and can be integrated with downstream confirmatory LC-MS workflows when positive indications are found.

Limitations and considerations

• Quantitation challenges: spot inhomogeneity and variable ionization among PFAS classes imply that multiple internal standards and careful calibration would be needed for reliable quantitation.
• Sensitivity differences: LODs are analyte- and matrix-dependent; some PFAS (e.g., PFOS, PFHxA) showed much lower LODs than others (e.g., PFOA, PFBS).
• Need for confirmatory analysis: MALDI-TOF provides presumptive identification based on m/z; orthogonal confirmation (e.g., LC-MS/MS with SPE) is necessary for regulatory or definitive reporting.

Future trends and potential applications

• Screening networks: deployment of compact MALDI platforms for rapid field or regional screening could prioritize samples for resource-intensive confirmatory testing.
• Method refinement for quantitation: development of robust internal standard panels, improved matrix formulations, and automated deposition techniques (to improve spot homogeneity) could extend the method toward semi-quantitative or quantitative workflows.
• Integration with imaging and sampling strategies: automated rastering, optimized target slides (e.g., hydrophobic/hydrophilic patterns), or controlled deposition (robotic spotting) could increase reproducibility and throughput.
• Expanding analyte coverage: optimization for a broader PFAS library and coupling to software libraries and machine learning for pattern recognition could enhance detection of emerging PFAS chemistries.

Conclusions

The study demonstrates that a benchtop linear MALDI-TOF system with minimal sample preparation can serve as a rapid, cost-efficient screening tool to indicate the presence of multiple PFAS in aqueous samples. TMGN was identified as a preferable matrix for spot homogeneity and reproducible detection; imaging-informed acquisition (e.g., annular raster) improved data quality. However, variable sensitivity across PFAS subclasses and spot inhomogeneity limit direct quantitation and require confirmatory LC-QTOF analysis after SPE for definitive identification. The workflow is well suited for rapid triage in environmental monitoring and can be further developed toward higher throughput and semi-quantitative applications.

References

• Application note: Detection of PFAS in Aqueous Samples by MALDI-TOF Mass Spectrometry, Shimadzu MALDI-8030 EasyCare Application News (Apr 2026).
• Instrumentation and software referenced: MALDI-8030 EasyCare; IonView; LCMS-9050 Q-TOF; LabSolutions Insight Explore; Chromabond WAX SPE (product and vendor names as cited in the source).