Expanding Spatial\nMetabolomics Coverage with Lithium-Doped\nNanospray Desorption Electrospray Ionization Mass Spectrometry Imaging

Spatial metabolomics has emerged as a powerful tool capable\nof\nrevealing metabolic gradients throughout complex heterogeneous tissues.\nWhile mass spectrometry imaging (MSI) technologies designed to generate\nspatial metabolomic data have improved significantly over time, metabolite\ncoverage is still a significant limitation. It is possible to achieve\ndeeper metabolite coverage by imaging in positive and negative polarities\nor imaging several serial sections with different targeted biomolecular\nclasses. However, this significantly increases the number of tissue\nsamples required for biological studies and reduces the capacity for\nlarger sample cohorts. Herein, we introduce lithium-doped nanospray\ndesorption electrospray ionization (nano-DESI) as a simple and robust\nmethod to increase spatial metabolomics coverage, which is achieved\nthrough enhancements to ionization efficiencies in positive ion mode\nfor metabolites and lipids lacking basic moieties, and improved structurally\ndiagnostic tandem mass spectra for [M + Li]<sup>+</sup> adducts. Specifically,\nsignal intensities were found to be enhanced by 10–1000×\nfor 96 compounds including small molecule metabolites, fatty acids,\nneutral lipids (e.g., diacylglycerols, DAG), and phospholipids when\nlithium was added to the ESI solvent. In addition, proof-of-principle\nresults reveal that lithium-doped nano-DESI MSI was able to comprehensively\nvisualize metabolites and lipids in the prostaglandin (PG) biosynthetic\npathway with PG isomeric resolution in an ovarian tumor section. These\ndata show colocalization of fatty acid (FA) 20:4 containing DAGs,\nFA 20:4 monoacylglycerols (MAGs), and FA 20:4 with PGE<sub>2</sub> and disparate localizations of PGD<sub>2</sub>. Overall, this study\ndescribes a simple and powerful approach to more comprehensively probe\nthe spatial metabolome with MSI.