Utility of Desorption Electrospray Ionization (DESI) for Mass Spectrometry Imaging

Importance of the Topic

Desorption electrospray ionization (DESI) imaging enables direct molecular mapping of surfaces under ambient conditions, overcoming limitations of traditional vacuum‐based methods. Its minimal sample preparation, non‐destructive workflow, and compatibility with downstream histological staining make it a powerful tool for spatially resolved analysis of lipids, metabolites, and other small molecules in diverse sample types.

Objectives and Study Overview

This work describes the application of DESI imaging on time‐of‐flight mass spectrometers (SYNAPT G2‐Si and Xevo G2‐XS) to demonstrate spatial localization of biomolecules in tissue sections, plant materials, pharmaceutical tablets, fingerprints, and microbial colonies. The goal is to establish a streamlined workflow from sample mounting to molecular image generation and co‐registration with optical microscopy.

Methodology

Thin sections (10–15 µm) of fresh‐frozen samples are mounted on glass slides without further pretreatment. An optical scan is acquired and imported into High Definition Imaging (HDI) software to define the region of interest. The DESI sprayer deposits charged microdroplets at predefined X,Y coordinates while a 2D moving stage rasteres the surface line by line. Pixel size is determined by stage speed and acquisition rate (typically ≥50 µm). After data collection, mass spectra are processed and visualized in HDI, and the same section can be stained with hematoxylin & eosin (H&E) for histological correlation.

Used Instrumentation

• Waters SYNAPT G2‐Si HDMS Mass Spectrometer
• Waters Xevo G2‐XS Q-TOF Mass Spectrometer
• DESI Imaging Source with 2D Linear Moving Stage
• High Definition Imaging (HDI) Software
• Cryostat (Leica) for Tissue Sectioning

Main Results and Discussion

Multimodal imaging in both positive and negative ion modes yields rich spectra of phospholipids and metabolites. In mouse brain, distinct lipid distributions (e.g., PG (44:12), ST (d18:1/24:1), PC species) align with anatomical features. Versatility is demonstrated by mapping oleic acid in porcine liver lobules, visualizing latent fingerprints via specific ion signatures, distinguishing two small molecules in bacterial colonies, and differentiating tumor versus normal regions in human liver based on PE(O-34:3) and PG(36:2) localization. These examples highlight DESI’s sensitivity, spatial fidelity, and ability to perform multiple analyses on a single sample.

Benefits and Practical Applications

• Minimal sample preparation and ambient‐pressure operation
• Non‐destructive workflow enabling subsequent histology or alternative imaging
• High sensitivity for small molecules, lipids, and metabolites
• Multimodal capability: positive/negative ion modes, DESI and MALDI on the same section
• Flexibility across sample types: tissues, plants, pharmaceuticals, forensics, microbiology

Future Trends and Opportunities

Advancements may include sub‐50 µm spatial resolution, integration with ion mobility separations, real-time intraoperative applications, and coupling with artificial intelligence for automated feature recognition. Expansion into clinical diagnostics, pharmaceutical quality control, and environmental surface analysis will further establish DESI imaging as a routine tool.

Conclusion

DESI imaging on TOF mass spectrometers provides robust, high‐throughput spatial mapping of small molecules in complex samples. Its minimal preparation, ambient operation, and compatibility with complementary techniques position it as a versatile platform for research and applied analyses.

References

1. Heeren RMA, Skraskova K. DESI imaging of mouse brain tissue sections. Maastricht University, The Netherlands.
2. Malone J, Saalbach G. DESI imaging of human liver samples with histological correlation. Imperial College London; NRES Committee London–South East, Study ID 1/LO/0686.