Using Targeted MRM MS Imaging With DESI to Visually Localize Isobaric And/or Low Abundance Lipids

Significance of the Topic

The localization of isobaric and low-abundance lipids within biological tissues is essential for deciphering their roles in cellular processes and disease mechanisms. Traditional discovery workflows often fail to resolve compounds with nearly identical masses or low ionization efficiency. The integration of DESI-MSI with targeted MRM acquisition addresses these limitations by offering enhanced specificity and sensitivity without extensive sample preparation.

Objectives and Overview of the Study

This study aimed to implement a targeted DESI-MSI workflow using multiple reaction monitoring (MRM) on a Xevo TQ Absolute Triple Quadrupole mass spectrometer to visualize and distinguish isobaric, isomeric, and low-abundance lipid species in rodent brain tissue sections.

Methodology

• Sample Preparation: Snap-frozen rodent brain tissues were cryosectioned at 18 µm thickness and thaw-mounted on glass slides.
• DESI-MSI Acquisition: A DESI XS source with an HPS sprayer delivered 95:5 methanol/water at 2 µL/min and 10 psi nebulizing gas. A heated transfer line (25 °C for positive mode, 450 °C for negative mode) enhanced desorption and ion transfer.
• Mass Spectrometry: The Xevo TQ Absolute operated in positive and negative polarity, unit mass resolution (0.7 Da), and MRM mode. Diagnostic transitions targeted phospholipid headgroup fragments, fatty acid product ions, and eicosanoid isomers (HETEs).
• Data Processing: MassLynx v4.2 acquired data and HDI Software 1.8 visualized spatial ion distributions at a 25 µm pixel size.

Instrumentation

• Xevo TQ Absolute Triple Quadrupole Mass Spectrometer
• DESI XS Ion Source with High-Performance Sprayer
• Heated Transfer Line
• MassLynx v4.2 and HDI 1.8 Software

Main Results and Discussion

The targeted DESI-MSI MRM approach successfully differentiated isobaric phosphatidylcholine species at m/z 782.55 and 806.55 by monitoring distinct headgroup fragments. In negative mode, isobaric and isomeric phosphatidylethanolamine and phosphatidylserine lipids showed unique spatial patterns based on fatty acid fragment transitions. Five hydroxyeicosatetraenoic acid isomers (5-, 8-, 11-, 12-, and 15-HETE) were simultaneously imaged with sufficient signal-to-noise ratios, revealing discrete localizations in brain subregions.

Benefits and Practical Applications

• Targets isobaric and isomeric lipids with diagnostic product ions for unambiguous identification.
• Reveals differential spatial distribution of endogenous molecules, aiding functional interpretation.
• High specificity and sensitivity facilitate visualization of low-abundance lipids directly from tissue.

Future Trends and Applications

Advances may include combining MRM with ion mobility separation for enhanced isomer resolution, higher spatial resolution imaging, and application to diverse tissue types. This workflow has potential in lipid biomarker discovery, pharmacokinetic studies, and real-time clinical diagnostics.

Conclusion

The integration of DESI XS with targeted MRM on a triple quadrupole platform provides a robust, high-resolution imaging solution for localizing challenging lipid species in biological tissues, expanding capabilities in spatial lipidomics.

References

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3. Powell WS, Rokach J. Biosynthesis, Biological Effects and Receptors of Hydroxyeicosatetraenoic Acids (HETEs) and Oxoeicosatetraenoic Acids (Oxo-Etes) Derived From Arachidonic Acid. Biochim Biophys Acta. 2015;1851(4):340–355. doi:10.1016/j.bbalip.2014.10.008.
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