Comparison of solid phase microextraction geometries for effective preconcentration of volatile per- and polyfluoroalkyl substances

Importance of the topic

Neutral, volatile per- and polyfluoroalkyl substances (PFAS) such as fluorotelomer alcohols (FTOHs) and sulfonamides are important environmental precursors to persistent ionic PFAS. Their volatility and low environmental concentrations pose analytical challenges for accurate detection and quantification. Robust preconcentration strategies are therefore essential to trace-level monitoring, source apportionment and mass-balance studies of PFAS transformation pathways.

Objectives and study overview

This study systematically compared two solid phase microextraction (SPME) geometries—commercial DVB/Car/PDMS SPME-fibers and larger-volume SPME-arrows—under headspace (HS) and direct immersion (DI) modes. The work evaluated the effect of agitation (cycloid Heatex at 600 rpm vs. orbital shaker at 250 rpm), extraction time and mode on extraction efficiency, linear dynamic range, limits of detection/quantification (LOD/LOQ) and potential competitive adsorption among six neutral volatile/semi-volatile PFAS (4:2, 6:2, 8:2 FTOHs; Me2FOSA, EtFOSA, MeFOSE). The goal was to define optimized SPME workflows for GC–MS analysis of volatile PFAS and to highlight mechanistic factors influencing preconcentration performance.

Methodology

Key experimental features:

• Analytes: 4:2, 6:2, 8:2 FTOHs; N,N-dimethyl- (Me2FOSA), N-ethyl- (EtFOSA) perfluorooctanesulfonamides; N-methylperfluorooctanesulfonamidoethanol (MeFOSE).
• SPME devices: commercial DVB/C-WR/PDMS fiber (10 mm, 80 µm) and SPME-arrow (1.10 mm, 120 µm) conditioned per manufacturer instructions.
• Extraction modes: HS (50 °C) and DI (60 °C) with varied incubation/extraction times (5–90 min).
• Agitation: Heatex cycloid motion at 600 rpm vs orbital shaker (OS) at 250 rpm.
• GC–MS: Agilent 8890/5997C, HP-5MS UI column; EI 70 eV; SIM acquisition. Automated SPME desorption at 270 °C (5 min) in splitless mode.
• Validation: HS-SPME with Heatex optimized (incubation 25 min at 50 °C, extraction 20 min at 600 rpm). Calibration 0.005–25 µg L−1 with isotopically labelled internal standards.

Instrumentation used

Instruments and consumables reported:

• Agilent 8890 GC coupled to 5997C MS (EI, SIM).
• Gerstel MPS autosampler configured for automated SPME and SPME-arrow liner.
• HP-5MS UI capillary column (30 m × 0.25 mm × 0.25 µm).
• Heatex cycloid agitator and standard orbital shaker.
• SEM (Carl Zeiss AURIGA CrossBeam) for coating morphology imaging.

Main results and discussion

Geometry and mode:

• SPME-arrows offered enhanced sensitivity and broader linear dynamic ranges for volatile FTOHs (e.g., 4:2 and 6:2 FTOH). Arrow configurations delivered single broad calibration ranges across 0.005–25 µg L−1 for many analytes and regression coefficients R2 > 0.97–0.99.
• SPME-fibers provided better responses (lower LOQs) for more hydrophobic, semi-volatile analytes such as MeFOSE (fiber LOQ ≈ 0.25 µg L−1; LOD ≈ 0.1 µg L−1), reflecting favorable DI extraction of less volatile species.
• Residual DI–HS plots confirmed a general trend: volatile analytes (higher Henry’s law constants) are preferentially extracted by HS-SPME, whereas hydrophobic/semi-volatile analytes favor DI-SPME, especially at longer extraction times.

Agitation effects:

• The Heatex cycloid agitator (600 rpm) substantially improved extraction kinetics for diffusion-limited compounds by enhancing convective mixing and reducing the boundary-layer mass-transfer resistance. For example, 4:2 FTOH signal with SPME-arrow increased markedly (≈250% at 15 min) with Heatex vs OS at 250 rpm.
• For the least volatile analyte (MeFOSE), Heatex outperformed OS; however, some semi-volatile analytes showed complex responses to agitation type and speed. The Heatex + arrow combination exhibited slightly higher RSDs in some cases, possibly due to septum microleakage at high rpm and larger device diameter.

Competitive adsorption:

• Extraction-time profiles revealed competitive displacement phenomena in multicomponent samples. The most volatile/polar analyte (4:2 FTOH) showed a decreasing signal after ~35 min in mixtures (but not as a single standard), concomitant with increasing signals of more hydrophobic analytes (MeFOSE, EtFOSA), consistent with competitive adsorption and displacement on the mixed DVB/Car/PDMS coating.
• SEM imaging indicated a mixed-phase coating morphology rather than distinct multilayers, which likely facilitates migration of analytes to stronger adsorption sites and enhances displacement effects under prolonged extractions.

Method performance summary:

• SPME-arrow HS with Heatex achieved LODs as low as 0.005 µg L−1 for several FTOHs and broad calibration ranges up to 25 µg L−1.
• SPME-fiber provided lower LOQs for some hydrophobic analytes (e.g., MeFOSE LOQ 0.25 µg L−1) and comparable accuracy (recoveries 94–119%) and precision (RSDs typically < 12%).

Benefits and practical applications

The study demonstrates practical guidance for analytical laboratories targeting neutral volatile PFAS: tailoring SPME geometry and extraction mode to analyte volatility and hydrophobicity yields improved sensitivity and dynamic range. Key benefits include solvent-free integrated sampling/preconcentration, automation potential for high-throughput monitoring, and adaptability for field-deployable sampling when paired with appropriate agitation and SPME geometry choices.

Future trends and potential applications

Opportunities and next steps include:

• Application of the optimized SPME-arrow HS method to complex environmental matrices (air, wastewater, biota) using higher-resolution MS to reduce interferences near the LOQ and to potentially lower LOQs further.
• Systematic evaluation of matrix effects (ionic strength, dissolved organic matter) on HS vs DI extraction equilibria and competitive adsorption dynamics.
• Improved septum and vial sealing strategies for high-rpm cycloid agitation to minimize variability and leakage for larger SPME geometries.
• Exploration of alternative sorbent chemistries and multi-sorbent architectures to mitigate competitive displacement and expand the analyte scope.

Conclusion

This work establishes that SPME geometry, extraction mode, agitation type and extraction time are interdependent parameters that critically determine preconcentration performance for neutral volatile PFAS. SPME-arrows combined with cycloid mixing (Heatex) deliver superior sensitivity and dynamic range for volatile FTOHs, while SPME-fibers can provide lower quantitation limits for hydrophobic semi-volatile PFAS when DI extraction is used. Competitive adsorption can alter responses in multicomponent samples at extended extraction times; thus, extraction time and sorbent selection must be optimized based on the target analyte suite. The presented framework supports method selection for trace-level GC–MS analysis of volatile PFAS and points to concrete improvements and research directions.

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