DESI MS Imaging Using SONAR™: A Novel Method to Image Precursor Ions and Fragment Ions for Molecular Identification in a Single Experiment

Significance of the Topic

Mass spectrometry imaging (MSI) couples spatial distribution and chemical characterization in biological samples.
DESI combined with SONAR acquires both precursor and fragment ion data in a single pass, enhancing identification confidence without additional MS/MS runs.

Goals and Overview of the Study

This study demonstrates integration of SONAR data independent acquisition into a DESI MSI workflow for lipid mapping and molecular identification directly from rat brain sections.
The approach aims to streamline sample analysis and correlate precursor ions with their characteristic fragments within one experiment.

Applied Instrumentation

• Waters 2D DESI stage with solvent sprayer operating at 2 µL/min in 95:5 methanol/water and 5 bar nebulizing gas
• Xevo G2-XS QTof mass spectrometer in SONAR mode (MassLynx SCN 949)
• High Definition Imaging Software v1.4 for data processing
• DriftScope software for multidimensional visualization
• LipidMaps MS tools for lipid identification

Methodology and Instrumentation

Rat brain tissues were cryosectioned and analyzed at –80 °C without pre-treatment.
DESI spray parameters and rapid pixel acquisition (100 × 100 µm) were set to simultaneously collect low energy (LE) precursor spectra and high energy (HE) fragment spectra via a scanning quadrupole window.
This generates a two-dimensional dataset linking precursor m/z and fragment patterns over each imaging pixel.

Main Results and Discussion

LE spectra revealed intense lipid signals between m/z 700–900 in both positive and negative modes, while HE spectra provided MS/MS-like fragmentation from m/z 100–900.
Multidimensional views in DriftScope and MassLynx enabled rotation of the data cube to mine specific quadrupole windows.
A two-step correlation in HDI software first matched fragments to precursors by quadrupole mass setting and then refined associations via spatial Pearson correlation, producing composite fragment ion images.
Positive mode mapping separated protonated, potassiated, and sodiated phosphatidylcholines by characteristic headgroup fragments (m/z 184, 163, 147).
Negative mode exploited deprotonated lipids to generate fatty acid–specific fragments, enabling identification of plasmalogen PEs and PS species through their diagnostic fragment ions.

Benefits and Practical Applications of the Method

• Single-run acquisition of precursor and fragment ion images simplifies workflows and reduces target selection bias
• Enhanced confidence in lipid identification by direct spatial correlation of molecular and fragment signals
• Unbiased data independent acquisition facilitates post-acquisition mining for novel biomarkers
• Compatibility with existing DESI MSI platforms and standard informatics tools

Future Trends and Opportunities

• Integration of SONAR DESI with automated lipidomic databases for high-throughput screening
• Expansion to other biomolecule classes such as metabolites and peptides
• Combining accurate quantitation algorithms with spatially correlated fragmentation for clinical pathology
• Development of advanced informatics pipelines for real-time data interpretation

Conclusion

Implementing SONAR mode in DESI MSI provides a powerful strategy to acquire spatially resolved precursor and fragment ion data concurrently, improving molecular identification and streamlining histochemical analysis in biological tissues.

Reference

1. Eberlin LS et al. Non-destructive, Histologically Compatible Tissue Imaging by Desorption Electrospray Ionization Mass Spectrometry. Chembiochem. 2011;12(14):2129–2132.
2. Gethings LA et al. Lipid Profiling of Complex Biological Mixtures by Liquid Chromatography/Mass Spectrometry Using a Novel Scanning Quadrupole Data-Independent Acquisition Strategy. Rapid Commun Mass Spectrom. 2017;31:1599–1606.