Ambient Ionization and FAIMS Mass Spectrometry for Enhanced Imaging of Multiply Charged Molecular Ions in Biological Tissues.

Ambient ionization mass spectrometry imaging (MSI) has been increasingly used to investigate the molecular distribution of biological tissue samples. Here, we report the integration and optimization of desorption electrospray ionization (DESI) and liquid-microjunction surface sampling probe (LMJ-SSP) with a chip-based high-field asymmetric waveform ion mobility spectrometry (FAIMS) device to image metabolites, lipids, and proteins in biological tissue samples. Optimized FAIMS parameters for specific molecular classes enabled semitargeted detection of multiply charged molecular species at enhanced signal-to-noise ratios (S/N), improved visualization of spatial distributions, and, most importantly, allowed detection of species which were unseen by ambient ionization MSI alone. Under static DESI-FAIMS conditions selected for transmission of doubly charged cardiolipins (CL), for example, detection of 71 different CL species was achieved in rat brain, 23 of which were not observed by DESI alone. Diagnostic CL were imaged in a human thyroid tumor sample with reduced interference of isobaric species. LMJ-SSP-FAIMS enabled detection of 84 multiply charged protein ions in rat brain tissue, 66 of which were exclusive to this approach. Spatial visualization of proteins in substructures of rat brain, and in human ovarian cancerous, necrotic, and normal tissues was achieved. Our results indicate that integration of FAIMS with ambient ionization MS allows improved detection and imaging of selected molecular species. We show that this methodology is valuable in biomedical applications of MSI for detection of multiply charged lipids and proteins from biological tissues.