Single-Cell Mass Spectrometry Imaging Using Desorption Electrospray Ionization Coupled To Orbitrap Mass Spectrometers

Desorption electrospray ionization-mass spectrometry imaging (DESI-MSI) enables ambient, matrix-free molecular imaging with minimal sample preparation. This protocol describes a workflow for single-cell MSI of OVCAR-8 cells using DESI coupled to Orbitrap mass spectrometers. The system integrates a DESI sprayer, a modified mass spectrometer interface, a motorized XYZ stage, and an optical breadboard for high-spatial-resolution imaging. Cells are cultured on gridded glass coverslips, washed with ammonium formate solution to reduce salt interference, dried, and analyzed under ambient conditions. Solvent is delivered continuously using a nano-liquid chromatography system, and nebulizing nitrogen gas is regulated to maintain spray stability. The protocol also describes raster imaging, pixel control, and optical-to-mass spectrometry image registration for single-cell localization. High mass resolution and mass accuracy provided by Orbitrap detection enable differentiation of closely related molecular species and support single-cell metabolomic analysis. This protocol provides an adaptable strategy for implementing high-spatial-resolution single-cell MSI using multiple Orbitrap mass spectrometer platforms.