Ion Mobility Separation Coupled With Desorption Electrospray Ionization Mass Spectrometry for High Specificity MS Imaging

Significance of Topic

Ambient ionization mass spectrometry imaging has emerged as a critical tool for spatially resolved molecular analysis in biological tissues. Combining desorption electrospray ionization (DESI) with ion mobility separation (IMS) enhances specificity by resolving isobaric and conformationally similar lipids, enabling accurate mapping of endogenous compounds in complex matrices such as brain tissue.

Objectives and Study Overview

The primary goal was to demonstrate the advantages of integrating high-efficiency gas-phase ion mobility separation into a DESI imaging workflow. A proof-of-principle experiment was conducted on mouse brain sections to illustrate improved lipid discrimination and structural identification directly from tissue surfaces.

Methodology and Instrumentation Used

Tissue Preparation and DESI Imaging

• Snap-frozen mouse brain was cryo-sectioned at 20 μm and thaw-mounted onto glass slides.
• No additional matrix or chemical treatment was applied before analysis.
• DESI spray conditions: 3 μL/min flow rate, 90:10 methanol:water, 120 psi N₂, ±5 kV ionization voltage.
• Imaging resolution: 100 μm spatial resolution with 1 s acquisition per pixel and 100 μm line spacing.

Ion Mobility Separation and Data Processing

• Instrument: Prosolia 2D DESI source coupled to a Waters SYNAPT G2-Si HDMS.
• IMS cell: high-efficiency T-Wave filled with nitrogen at 3 mbar to separate ions by size, shape, and charge.
• Data analysis: Waters High Definition Imaging software with Apex 3D peak picking; DriftScope for two-dimensional m/z versus drift time visualization; DESI MS/MS via CID for structural elucidation.

Main Results and Discussion

IMS-enhanced DESI imaging enabled clear separation of lipid species that overlap in m/z dimension alone. A 2D m/z–drift time plot revealed distinct trendlines corresponding to:

• Triply charged gangliosides (e.g., green trendline in drift time plot).
• Doubly charged acidic glycosphingolipids and ganglioside species (red trendline).
• Singly charged lipids including fatty acids, lysolipids, and glycerophospholipids (blue trendline).

A representative ion image of C24:1 sulfatide (m/z 888.624, drift time 7.81 ms) demonstrated precise spatial localization in mouse brain.
Targeted MS/MS of the doubly charged ion at m/z 917.4 produced diagnostic fragments (loss of sialic acid, glycerol-3-phosphate), confirming its identity as GD1 (d18:1/18:0) ganglioside.

Benefits and Practical Applications

Integration of IMS with DESI imaging offers:

• Enhanced specificity in complex lipid mixtures by resolving isobaric and conformational variants.
• Direct tissue analysis requiring minimal preparation, preserving native chemical distributions.
• In-situ structural characterization via tandem MS without sample transfer, streamlining workflow for lipidomics, pathology research, and pharmaceutical studies.

Future Trends and Possibilities

Advances in IMS-coupled imaging are expected to:

• Enable multiplexed analysis of additional biomolecule classes (metabolites, peptides) in ambient environments.
• Improve throughput and resolution with faster drift cells and refined data acquisition algorithms.
• Integrate machine learning for automated pattern recognition and annotation of molecular distributions.
• Combine with complementary imaging modalities (e.g., optical microscopy) for multimodal correlation studies.

Conclusion

This study demonstrates that DESI imaging augmented with ion mobility separation significantly improves the discrimination and identification of lipid species in tissue sections. The combined approach yields rich, high-specificity datasets that advance molecular histology and lipidomic research.

References

No formal literature citations were provided in the original document.