Evaluation of 6 MALDI-Matrices for 10 μm lipid imaging and on-tissue MSn with AP-MALDI-Orbitrap

Mass spectrometry imaging (MSI) is a technique uniquely suited to localize and identify lipids in a tissue sample. Using an AP-MALDI UHR source coupled to an Orbitrap Elite, numerous lipid locations and structures can be determined in high mass resolution spectra and at cellular spatial resolution, but careful sample preparation is necessary. We tested 11 protocols on serial brain sections for the commonly used MALDI matrices, CHCA, Norharmane, DHB, DHAP, THAP, and DAN, in combination with tissue washing and matrix additives, to determine the lipid coverage, signal intensity, and spatial resolution achievable with AP-MALDI. In positive ion mode, the most lipids could be detected with CHCA and THAP, while THAP and DAN without additional treatment offered the best signal intensities. In negative ion mode, DAN showed the best lipid coverage and DHAP performed superior for Gangliosides. DHB produced intense cholesterol signals in the white matter. 155 lipids were assigned in positive (THAP), 137 in negative ion mode (DAN) and 76 lipids were identified using on tissue tandem-MS. The spatial resolution achievable with DAN was 10 μm, confirmed with on tissue line-scans. This enabled the association of lipid species to single neurons in AP-MALDI images. The results show that the performance of AP-MALDI is comparable to vacuum MALDI techniques for lipid imaging.