ART CONSERVATION WITH DESI MS IMAGING: DIRECT MAPPING OF COMPOUND LOCALIZATION ON WOOD SAMPLES

Importance of the Topic

Understanding the molecular composition of historic wooden artifacts is critical for conservation and authentication. Ambient ionization mass spectrometry imaging, specifically desorption electrospray ionization (DESI) MSI, provides a noninvasive approach to map endogenous compounds directly on wood surfaces without sample preparation. This capability supports the preservation of cultural heritage objects by revealing chemical patterns linked to wood anatomy, treatment, and degradation processes.

Objectives and Overview of the Study

The primary goal was to demonstrate direct molecular mapping of metabolites in 19th-century French barberry and Cuban mahogany wood samples using DESI MSI. Key aims included:

• Visualizing spatial localization of organic acids, alkaloids, and lipids within wood anatomical features.
• Evaluating the compatibility of DESI with ion mobility separation to resolve isobaric species.
• Assessing the minimal impact of DESI sampling on precious artifacts for repeated analyses.

Methodology and Instrumentation

DESI MSI experiments were conducted under both positive and negative ion modes using a SYNAPT™ XS ion mobility Q-ToF mass spectrometer equipped with a Waters DESI XS source. Key parameters included:

• Solvent: 98:2 methanol:water with 0.01% formic acid at 2 µL/min flow rate.
• Nebulizing gas: nitrogen at 0.9 bar; sprayer voltage: 0.7 kV; cone voltage: 40 V; source temperature: 120 °C.
• Mass range: m/z 50–1200; pixel size: 50 µm; acquisition speed: 0.3–0.5 s per scan.
• Ion mobility separation via traveling wave IMS (TWIMS) to distinguish ions by shape and size prior to time-of-flight analysis.

Used Instrumentation

• Waters SYNAPT XS ion mobility Q-ToF mass spectrometer.
• Waters DESI XS high-performance sprayer source.
• MassLynx™ 4.2 and High Definition Imaging (HDI™) 1.6 software for data acquisition and visualization.
• Lock mass calibration with leucine-enkephalin; ESI/DESI calibration using sodium formate standards.

Main Results and Discussion

French barberry wood samples showed clear localization of berberine and phosphatidic acids along tree‐ring structures, confirmed by overlaying DESI images with photographic cross sections. Negative mode imaging detected hydroxybenzoic and hydroxycinnamic acids, while ion mobility resolved two isobaric free fatty acids differing by 2 mDa. In Cuban mahogany, DESI MSI distinguished metabolites on shellac-coated versus uncoated surfaces, revealing compound distributions associated with coating layers. The minimal ablation of the wood surface allowed multiple imaging passes and preserved anatomical details for correlation with chemical maps.

Benefits and Practical Applications

DESI MSI with ion mobility offers:

• Noninvasive, label-free mapping of endogenous wood metabolites.
• High spatial resolution (50 µm) for correlating chemistry with microanatomy.
• Capability to distinguish isobaric compounds without ultra–high-resolution mass analyzers.
• Preservation of valuable heritage samples through minimal damage sampling.

Future Trends and Potential Applications

Advancements may include integration with other imaging modalities (e.g., microscopy, Raman), quantitative DESI approaches, and expansion to a broader range of cultural heritage materials. Enhanced ion mobility techniques and novel solvent systems could further improve sensitivity and specificity for complex wood metabolites and conservation treatments.

Conclusion

DESI MS Imaging combined with ion mobility separation successfully provided spatially resolved chemical maps of endogenous compounds in historic wood samples without any preparation. The approach preserves artifact integrity and reveals valuable insights into wood chemistry, supporting conservation strategies and scientific studies of cultural heritage objects.

References

• Wishart DS, Guo AC, Oler E, et al. HMDB 5.0: the Human Metabolome Database for 2022. Nucleic Acids Res. 2022 Jan 7;50(D1):D622–31. doi:10.1093/nar/gkab1062.
• Yang X, et al. Comparative metabolomics analysis reveals the color variation between heartwood and sapwood of Chinese fir (Cunninghamia lanceolata). Ind. Crop Prod. 2021 May 26;169:113656. doi:10.1016/j.indcrop.2021.113656.
• Abreu IN, et al. A metabolite roadmap of the wood-forming tissue in Populus tremula. New Phytol. 2020 June 26;228:1559–1572. doi:10.1111/nph.16799.
• Waters Corporation. Publication #720004176EN. April 2012.