Expanding Spatial Metabolomics Coverage with Lithium-Doped Nanospray Desorption Electrospray Ionization Mass Spectrometry Imaging

Spatial metabolomics has emerged as a powerful tool capable of revealing metabolic gradients throughout complex heterogeneous tissues. While mass spectrometry imaging (MSI) technologies designed to generate spatial metabolomic data have improved significantly over time, metabolite coverage is still a significant limitation. It is possible to achieve deeper metabolite coverage by imaging in positive and negative polarities or imaging several serial sections with different targeted biomolecular classes. However, this significantly increases the number of tissue samples required for biological studies and reduces the capacity for larger sample cohorts. Herein, we introduce lithium-doped nanospray desorption electrospray ionization (nano-DESI) as a simple and robust method to increase spatial metabolomics coverage, which is achieved through enhancements to ionization efficiencies in positive ion mode for metabolites and lipids lacking basic moieties, and improved structurally diagnostic tandem mass spectra for [M + Li]⁺ adducts. Specifically, signal intensities were found to be enhanced by 10-1000× for 96 compounds including small molecule metabolites, fatty acids, neutral lipids (e.g., diacylglycerols, DAG), and phospholipids when lithium was added to the ESI solvent. In addition, proof-of-principle results reveal that lithium-doped nano-DESI MSI was able to comprehensively visualize metabolites and lipids in the prostaglandin (PG) biosynthetic pathway with PG isomeric resolution in an ovarian tumor section. These data show colocalization of fatty acid (FA) 20:4 containing DAGs, FA 20:4 monoacylglycerols (MAGs), and FA 20:4 with PGE₂ and disparate localizations of PGD₂. Overall, this study describes a simple and powerful approach to more comprehensively probe the spatial metabolome with MSI.