A vacuum-compatible polymer coating for stabilizing nanoparticles and volatile compounds in vMALDI-MS analysis

Matrix-assisted laser desorption ionization mass spectrometry operated under high vacuum (vMALDI-MS) enables molecular analysis and spatial mapping of metabolites in biological tissues through protonation or deprotonation mechanism. However, its application is constrained by matrix sublimation under high vacuum and low ionization efficiency for redox-active species lacking labile protons. We developed a vacuum-compatible polymer coating for vMALDI-MS adaptable to two complementary ionization strategies, deprotonation using the volatile proton sponge 1, 8-bis(dimethylamino)naphthalene (DMAN) for acidic metabolites, and electron-transfer ionization on (Bi₂O₃)₀·₀₇(CoO)₀·₀₃(ZnO)₀.₉ nanoparticles for redox-active species without labile protons. Polyvinyl alcohol (PVA) is introduced to form a protective thin film that concurrently prevents nanoparticle dispersion during laser ablation, suppresses organic matrix sublimation, and preserves tissue integrity. A home-built ThermoSonic FilmMaster was established for the deposition of both organic matrices and inorganic nanoparticles. We validated this approach using standard compounds and tissue homogenates, in which volatile short-chain fatty acids, indole derivatives, and aldehydes were putatively annotated by accurate mass matching. We further applied it to image brain sections from 4T1 tumor-bearing mice, where less volatile metabolites such as long-chain fatty acids and lipids were spatially resolved. This approach complements ambient ionization techniques and provides a coating approach for stabilizing nanoparticles and volatile compounds in vMALDI-MS analysis.