Mass Spectrometry Imaging of Lipid Isobars and Isomers with Unit Mass Resolution

Mass spectrometry imaging (MSI) relies on the analysis of complex mixtures of analytes extracted from each location on the sample without any prior separation. Advancements in high resolution mass spectrometers have facilitated the separation of isobars, making these systems popular platforms in MSI studies. However, even with the highest achievable mass resolving power, these instruments cannot separate isomers and closely-spaced isobaric species.

In contrast, multiple reaction monitoring (MRM) mode on the Ultivo triple quadropole mass spectrometer (QqQ) enables the separation of both isobaric and isomeric species. We have implemented nanospray desorption electrospray ionization (nano-DESI) MSI on a QqQ and demonstrated the spatial mapping of isobaric lipids that require a mass-resolving power of >3,850,000 for separation in conventional MS1 mode.

In addition, using unique MRM transitions for each isomer, we have also successfully separated eicosanoid isomers, specifically hydroxyeicosatetraenoic acids (HETEs), in mouse kidney tissues. These byproducts of arachidonic acid oxidation play important roles in inflammation, stroke, and cancer. Nano-DESI MSI in the MRM mode is also ideally suited for isomer differentiation using online derivatization techniques.

We have demonstrated the unique capabilities of this approach for imaging of isomeric species in biological tissues that cannot be separated using other approaches. Collectively, these studies have established QqQ as a powerful and cost-effective platform for molecular imaging of biological systems with unprecedented selectivity.

Presenter: Sara Amer (University Level Teaching Assistant, Purdue University)