Development and Validation of an Automated Online SPE Kit Coupled to Xevo TQ-S micro for the Identification and Quantification of 48 PFAS in Environmental Water Matrices

Significance of the topic

The analytical determination of per- and polyfluoroalkyl substances (PFAS) at ultra-trace concentrations in environmental waters is critical for regulatory compliance, public health protection and remediation planning. PFAS are chemically diverse, highly persistent and often present at ng/L or sub-ng/L levels, requiring sensitive, selective and robust workflows that minimize contamination and support high throughput monitoring programs.

Objectives and study overview

This application note describes development, optimisation and validation of an automated online solid-phase extraction (SPE) trap-and-elute (2D-LC) workflow coupled to a Xevo TQ‑S micro triple quadrupole mass spectrometer for the identification and quantification of the 48 PFAS defined in the UK Drinking Water Inspectorate (DWI) guidance. The work aimed to: achieve low ng/L to sub‑ng/L limits of quantification (LLOQs), reduce analyst hands‑on time and contamination risk, ensure robustness across drinking, river and groundwater matrices, and enable extended unattended operation.

Methodology and experimental design

The method uses an online 2D trap-and-elute approach instead of offline cartridge SPE. Key strategy points: the trap column uses a mixed-mode sorbent combining reversed-phase (RP) and weak anion exchange (WAX) chemistries to retain both short- and long-chain anionic PFAS and reduce breakthrough of neutral/zwitterionic species. Loading solvent conditions (aqueous, pH ~3.0) favour retention on the WAX mechanism; a wash step (ammonium acetate in water, pH ≈5.5) removes weakly retained matrix components and stabilises the background. Elution to the analytical column is achieved by a pH/organic gradient (10 mM ammonium acetate aqueous at pH 5.4 to acetonitrile with ammonium hydroxide to generate high pH), enabling controlled disruption of ionic interactions and chromatographic separation without use of At Column Dilution (ACD).

Analytical run time and sample handling: 24 minutes per sample (including column wash and re-equilibration), injection volume 3000 µL, analytical flow 0.35 mL/min, column temperature 40 °C. Method uses 24 13C-labelled internal standards (ILIS) to correct residual matrix effects and improve quantitation.

Used Instrumentation

Instrumentation and consumables explicitly configured for low PFAS background: ultraperformance LC systems with PFAS‑free solvent lines, CTC PAL‑3 RSi autosampler fitted with PFAS‑free parts, PFAS‑free 8 mL polypropylene autosampler vials, and an isolator column to limit system background. Key devices and columns: Xevo TQ‑S micro Mass Spectrometer with UniSpray ionisation operating in MRM (positive/negative switching); Waters Online SPE UPLC‑MS/MS Kit; Atlantis Premier BEH C18 AX analytical columns; custom Oasis WAX direct‑connect trap column coupled to XBridge BEH C8 direct connect; ACQUITY UPLC solvent management (QSM and BSM). Data acquisition and review: waters_connect for Quantitation (including AME, MS Quan, MS ToolKit). Representative MS source and tuning parameters included desolvation temperature 450 °C, desolvation gas 900 L/hr, cone gas 50 L/hr, impactor 1 kV; compound‑specific cone voltages and collision energies in MRM.

Main results and discussion

Linearity and sensitivity: calibration across 0.05–20 ng/L produced R2 > 0.99 for all 48 PFAS. LLOQs (defined as lowest standard meeting S/N >10 for quantifier and qualifier and ion ratio within ±30%) ranged from 0.05 to 0.2 ng/L. Two MRM transitions were used per analyte except N‑MeFOSE and N‑EtFOSE.

Accuracy and precision: Analysis of the drinking water CRM IRMM‑428 (triplicate) returned concentrations in close agreement with certified values; Z‑scores for the six CRM PFAS were within ±2. Repeatability tests in pooled river water (spiked at +5 ng/L, n=8) yielded %RSD <15% for all targets and retention time variability <0.3%, meeting EPA 1633 initial precision expectations.

Matrix performance and carryover: the online SPE workflow delivered strong matrix cleanup and low background when PFAS‑free parts and an isolator column were used. Carryover testing after the highest standard (20 ng/L) showed no PFAS in subsequent blanks exceeding 30% of respective LLOQs, consistent with EPA 1633 blank acceptance criteria. Groundwater samples with elevated PFAS were quantified accurately (including dilution for out‑of‑range values).

Key method development findings: (1) mixed‑mode WAX + RP sorbent was necessary to capture both short and long chain PFAS—HLB and pure C18 performed poorly for the full 48 panel; (2) acetonitrile outperformed methanol as the organic eluent (better Gaussian peak shapes, lower chemical background and reduced system backpressure; improved separation of branched/linear isomers such as PFOS); (3) optimisation of ionic strength (10 mM ammonium acetate) improved peak shape and reduced tailing; (4) method architecture avoided additional hardware complexity (no ACD) by tuning solvent strength, ionic strength and pH to enable focusing and elution compatibility with the second-dimension column.

Benefits and practical applications

• High sensitivity (sub‑ng/L LLOQs) on a mid‑tier triple quadrupole instrument suitable for regulatory monitoring (EU Drinking Water Directive / UK DWI guidance).
• Automated workflow reduces hands‑on time, contamination risk and variability compared with manual offline SPE and supports extended unattended operation (up to ~60 samples per 24 h).
• Improved precision and reproducibility through consistent online pre‑concentration and reduced operator variability.
• Lower solvent consumption and reduced disposable plastic waste versus offline cartridge SPE, reducing operating costs and environmental footprint.
• Integrated software tools (waters_connect suite) facilitate method development, automated acquisition (AME), quantitation (MS Quan) and streamlined data review.

Future trends and opportunities

Potential extensions of this work include expanding coverage to ultra‑short chain PFAS and additional zwitterionic/neutral PFAS classes by tuning trap chemistry and chromatographic conditions, further lowering LLOQs, and integrating higher degrees of automation for field‑to‑lab sample tracking. Coupling online SPE with high‑resolution MS could assist non‑target and suspect screening while retaining the throughput advantages. Method harmonisation against updated regulatory lists (and evolving standard methods such as EPA 1633A) will remain important for inter‑laboratory comparability.

Conclusion

The automated online SPE trap‑and‑elute method coupled to a Xevo TQ‑S micro provides a validated, robust and high‑throughput workflow for the quantitative analysis of 48 regulatory PFAS in drinking, river and groundwater matrices. The approach combines a mixed‑mode trap sorbent, pH/ionic‑strength mediated elution and targeted MRM quantitation to deliver sub‑ng/L sensitivity, good accuracy against a CRM, repeatability (%RSD <15%) and low carryover. The method reduces manual labour, lowers consumable use and supports regulatory monitoring demands while remaining expandable to emerging PFAS challenges.

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