Development of online coupling between supercritical fluid chromatography and 18 T FTICR MS for the molecular characterization of wood bio-oil

Significance of the topic

Understanding the molecular composition of lignocellulosic bio-oils is critical for valorization, quality control and downstream upgrading. Bio-oils produced by pyrolysis are extremely complex mixtures containing thousands of isobaric and isomeric species spanning a wide polarity range (from nonpolar aromatics to highly oxygenated sugars and phenolics). Coupling high-resolution separation with ultrahigh-resolution mass spectrometry enables detailed molecular-level characterization that informs process optimization and targeted fractionation strategies.

Objectives and overview of the study

This study develops and optimizes an online coupling between supercritical fluid chromatography (SFC) and an 18-tesla Fourier transform ion cyclotron resonance mass spectrometer (18 T FTICR MS) to characterize wood-derived bio-oil at the molecular formula and isomer level. The goals were to adapt chromatographic and spectrometric parameters to the constraints of both instruments, validate the approach with a panel of 24 representative standards, and apply the method to analyze a pine bio-oil, demonstrating high chemical coverage, mass accuracy and isomer separation capability.

Methodology

The workflow combined reversed/normal-phase-capable SFC for broad polarity coverage with negative-mode ESI FTICR MS detection to leverage ultrahigh mass resolving power and sub-ppm mass accuracy. Key methodological steps included:

• Selection of a 24-compound standards mixture representing typical bio-oil families (fatty acids, polyphenols, saccharides, small aromatics) to optimize chromatographic and MS parameters.
• Systematic optimization of injection solvent and volume, co-solvent composition, make-up solvent, flow rate, accumulation time and free induction decay (FID) settings to balance chromatographic speed and FTICR acquisition requirements.
• Implementation of a fast data acquisition strategy (reduced processing method down to ~2.4 s/scan for targeted runs) while preserving sufficient resolution for formula assignment.

Used instrumentation

The main instruments and hardware choices reported were:

• SFC: Waters Acquity UPC2 system using a BEH stationary phase (100 x 3.0 mm, 1.7 µm).
• Mobile phase conditions: CO2 with co-solvent (MeOH/ACN mixtures). Typical SFC operating conditions included a flow rate of 0.7 mL/min and an automated backpressure regulator (ABPR) set to 150 bar.
• Make-up solvent for ESI coupling: Methanol/water with 0.2% NH4OH delivered at ~0.3 mL/min.
• Injection conditions: 10 µL injections in acetonitrile for standards and bio-oil extracts.
• Mass spectrometry: 18 T FTICR MS (tims-MRMS configuration referenced) operating in negative electrospray ionization (ESI−). Instrumental parameters included accumulation times on the order of 450 ms, FID durations ~1.677 s, mass range m/z 120–1500, and transient lengths corresponding to 16M data points for highest-resolution acquisitions.

Main results and discussion

Key outcomes from method development and application to pine bio-oil:

• Analytical performance: The online SFC–18 T FTICR MS method achieved mass resolution up to ~1,400,000 at m/z 400 and mass accuracy better than 0.1 ppm in optimized runs.
• Molecular coverage: More than 10,000 molecular formulas were initially resolved in ESI(−) experiments, with a reported formula attribution count reaching 12,434 after data processing and assignment routines.
• Isomer separation: The chromatographic dimension effectively separated many isomeric species that would otherwise be indistinguishable by direct infusion FTICR MS, increasing confidence in formula-to-structure relationships for abundant classes such as phenolics, fatty acids and carbohydrates.
• Standards validation: The panel of 24 standards spanning varied polarities (e.g., palmitic acid, vanillic acid, ferulic acid, levoglucosan, glucose, xylose, taxifolin, phloretin, naphthalene, 2,6-di-tert-butyl-4-methylphenol) guided parameter optimization and confirmed that both polar saccharides and less-polar aromatics could be retained and separated by the SFC conditions used.
• Trade-offs: Instrumental compromises were necessary — SFC favors higher flow rates and rapid separations, while FTICR MS requires long transient acquisitions for maximum resolving power. The authors optimized scan speed and flow rate to obtain a pragmatic compromise (scan times in the 1–10 s range for FTICR depending on resolution target) that still allowed chromatographic peak delineation.

Benefits and practical applications

The combined SFC–18 T FTICR MS platform offers several advantages for bio-oil characterization and broader complex-matrix analysis:

• Enhanced molecular specificity: Chromatographic separation reduces isobaric/isomeric interferences, improving formula attribution and enabling semi-quantitative comparisons across fractions.
• Broad polarity coverage: SFC with appropriate co-solvents provides a unified chromatographic approach to analyze nonpolar aromatics up to polar sugars and polyphenols in a single run or using short complementary gradients.
• High-confidence assignments: Ultrahigh resolving power and sub-ppm accuracy from 18 T FTICR MS permit unambiguous elemental formula assignments even in dense spectral regions.
• Applications: The method is suitable for feedstock screening, process monitoring in pyrolysis or upgrading units, development of fractionation strategies, and detailed fundamental studies of thermal depolymerization chemistry.

Future trends and potential applications

Anticipated developments and extensions driven by this work include:

• Faster acquisition strategies and hardware/processing improvements (e.g., optimized transient handling, parallel accumulation, or advanced denoising algorithms) to better match chromatographic peak widths while retaining ultrahigh resolution.
• Expansion to complementary ionization modes (positive ESI, APCI) and derivatization workflows to increase coverage of neutral, basic or thermally labile species.
• Integration with ion mobility spectrometry or tandem MS workflows to add structural constraints for isomer differentiation.
• Automation and standardized data-processing pipelines for high-throughput formula assignment, class-based visualization and reproducible reporting for industrial QA/QC contexts.

Conclusion

The study demonstrates a practical and powerful online coupling between SFC and an 18 T FTICR MS, enabling deep molecular-level characterization of wood-derived bio-oil. By systematically optimizing chromatographic and FTICR parameters and validating the approach with representative standards, the authors achieved ultrahigh mass resolution, excellent mass accuracy and substantial molecular coverage, while also obtaining chromatographic separation of many isomers. The platform provides a valuable analytical route for research and industrial applications focused on bio-oil valorization and complex mixture analysis.

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