Improving Detection and Selectivity of PFAS Molecules Using Cyclic Ion Mobility Wideband Enhancement

Significance of the Topic

The growing environmental and human-health concern about per- and polyfluoroalkyl substances (PFAS) requires analytical methods capable of confidently detecting and distinguishing PFAS at sub-ng/L levels in complex biological and environmental matrices. Improved sensitivity and selectivity are essential to reduce false positives from isobaric interferences, to resolve isomeric forms (linear vs branched), and to provide orthogonal descriptors (mass, retention time, ion mobility collision cross section) that increase identification confidence for exposure and regulatory monitoring.

Study Objectives and Overview

This work evaluated the use of a Cyclic Ion Mobility Spectrometry (Cyclic IMS) P20 platform combined with Wideband Enhancement (WBE) to improve detection limits and selectivity for PFAS molecules. The goals were to: (1) exploit PFAS-specific ion mobility drift-time trendlines to synchronise Time-of-Flight (ToF) pusher timing via WBE, (2) increase ToF duty cycle and signal intensity for low-abundance PFAS, (3) enhance separation and detection of branched vs linear isomers, and (4) demonstrate practical limits of detection in LC–Cyclic‑IMS–MS workflows for targeted and non-targeted PFAS analysis.

Methodology

Analytical approach:

• Sample set: dilution series of a Native PFAS mix (PFAC30PAR).
• Chromatography: reversed-phase UPLC using an ACQUITY UPLC BEH C18 column (100 × 2.1 mm, 1.8 µm) at 35 °C, 22 min gradient, flow 0.3 mL/min; mobile phases: A = 95:5 water:methanol with 2 mM ammonium acetate, B = methanol with 2 mM ammonium acetate. A PFAS-free conversion kit and an Atlantis Premier BEH C18 AX isolator column were used for system cleanliness.
• Mass spectrometry: quadrupole–Cyclic‑IMS–ToF instrument with cyclic IMS resolution in the range R ≈ 65–145. Acquisition modes compared included conventional HDMSE, WBE-enabled HDMSMS, LC–Cyclic‑IMS–MSMS and LC–Cyclic‑IMS–DIA‑MS.
• WBE operation: instrument programmed to select PFAS characteristic drift-time trendlines (based on collision cross section–to–mass relationship) and synchronise ToF pusher timing to those windows, thereby increasing effective ToF duty cycle for target PFAS drift times.

Used Instrumentation

The experimental platform was a Waters Cyclic IMS P20 system coupled to a quadrupole and ToF mass analyser. Key components included:

• Cyclic ion mobility cell (Cyclic IMS) enabling multiple passes and high ion mobility resolution.
• Wideband Enhancement (WBE) firmware/method to align drift time windows with ToF pusher timing.
• ACQUITY UPLC system with PFAS-free modifications (conversion kit) and Atlantis Premier BEH C18 AX isolator column for PFAS minimization in flow path.

Main Results and Discussion

Key findings:

• WBE significantly increased analytical sensitivity for PFAS relative to conventional HDMSE acquisition by both improving selectivity (targeting PFAS-specific drift-time trendlines) and raising the ToF duty cycle. Example: signal intensity increases up to ~10× for branched PFOS isomers at 4 ng/L.
• Demonstrated detection of PFOS at 0.4 ng/L with an RMS signal-to-noise ratio of 48 using the combined Cyclic‑IMS + WBE HDMSMS workflow.
• Ion mobility trendlines: PFAS species exhibited distinct drift-time versus m/z trendlines that are shifted to shorter drift times (i.e., appear denser) relative to hydrocarbon-like species of similar m/z due to the fluorinated backbone and resulting collision cross sections. This orthogonal behaviour enables discrimination from isobaric biological interferences.
• Isomer resolution: improved chromatographic peak fidelity and detection of branched PFOS byproducts (e.g., P5MHpS, P6MHpS) was observed when WBE was enabled, aiding isomer-specific monitoring which is important for exposure assessment.

Discussion points:

• The targeted alignment of ion mobility windows with ToF pulsing mitigates the inherent duty-cycle limitation of ToF analysers, concentrating acquisition resources on PFAS-relevant drift times. This produces both higher signal response and lower detection limits for targeted analytes.
• Combining LC retention, accurate mass, ion mobility CCS, and fragmentation (HDMSMS) yields multiple orthogonal identifiers, increasing confidence for both targeted screening and non-targeted detection of emerging PFAS.

Benefits and Practical Applications

Practical advantages of the Cyclic IMS + WBE strategy include:

• Substantial gains in sensitivity for trace PFAS detection in complex matrices, enabling sub-ng/L monitoring relevant to biomonitoring and environmental surveillance.
• Improved selectivity reduces false positives from isobaric endogenous species, streamlining data review and reducing confirmatory analysis load.
• Enhanced isomer discrimination supports exposure assessment where branched vs linear PFAS have differing toxicokinetics and regulatory interest.
• Flexibility to operate in both targeted and non-targeted modes—WBE can be tuned to known PFAS trendlines while Cyclic IMS CCS values support unknown identification workflows.

Future Trends and Opportunities

Potential developments and uses include:

• Expanded CCS libraries and standardized drift-time trendline databases for PFAS to facilitate automated WBE targeting and cross-laboratory comparability.
• Integration of WBE strategies with data-independent acquisition (DIA) and machine-learning algorithms to enhance non-target screening throughput and false-positive filtering.
• Further optimization of cyclic IMS pass numbers and WBE scheduling to balance duty cycle, sensitivity, and chromatographic peak shapes for broad PFAS classes.
• Application to other classes of environmentally persistent halogenated contaminants where ion mobility trendlines differ from endogenous matrix species.

Conclusions

Combining Cyclic Ion Mobility Spectrometry with Wideband Enhancement provides a practical and effective means to increase both selectivity and sensitivity for PFAS analysis. By exploiting PFAS-specific ion mobility behaviour and synchronising ToF acquisition to those windows, the workflow produced up to tenfold signal gains for low‑level branched isomers and achieved PFOS detection at 0.4 ng/L. The approach strengthens identification confidence through orthogonal descriptors (retention time, accurate mass, CCS, fragmentation) and is well-suited for both targeted monitoring and expanded non-target discovery of emerging PFAS in complex matrices.

References

1. Hughes C. Enhanced Glycopeptide Identification Using the SYNAPT XS Q‑ToF with Ion Mobility Enabled Wideband Enhancement. Waters Application Note 720006858; 2020.
2. Enhanced Identification Confidence and Specificity for PFAS Analysis Using Cyclic Ion Mobility Mass Spectrometry Collision Cross Sections. Waters Application Note 720008536; 2024.
3. McCullagh M, Kass I, Lioupi A, Theodoridis G, Plumb R, Dowd S, Adams S. The utility of cyclic ion mobility to improve selectivity and analysis efficiency of environmental PFAS contamination and exposure. Poster presented at ASMS; June 2024.
4. Waters Corporation. PFAS Analysis Kit for ACQUITY UPLC Systems User Guide, Document 720006689.
5. Dodds JN, Hopkins ZR, Knappe DRU, Baker ES. Rapid characterization of per‑ and polyfluoroalkyl substances (PFAS) by ion mobility spectrometry–mass spectrometry (IMS‑MS). Analytical Chemistry. 2020;92(6):4427–4435.