A long-standing challenge in mass spectrometry imaging (MSI) is the accurate identification of isobaric and isomeric biomolecules for structure-specific analysis, which is dependent on high mass resolving power and tandem mass spectrometry. Herein, we present a unique pipeline that enables image acquisition of high-resolution FTMS (500,000 resolution) in parallel with up to 28 ITMS2 without increasing the acquisition time of 1024 ms per pixel. Our developed method exploits the high ion flux from PA nano-DESI MSI and capitalizes on the mass spectrometer’s (OrbitrapTM IQ-XTM TribridTM, ThermoScienfic) ability to perform parallel acquisition. Overall the method provides unique insights for annotation of isomeric and isobaric species in thin tissue sections.
This in-depth chemical characterization approach was applied to annotate previously non-detected oxysterols in white matter multiple sclerosis lesions from human brain tissue. We report the unambiguous identification of 10 oxysterols in the tissue by both their accurate mass from the FTMS scan and their diagnostic product ions from the ITMS2 scans. We map the spatial distribution of cholesterol and its oxidized metabolites in multiple sclerosis lesions and correlate their abundances with lesion characteristics in multiple sclerosis as determined immunohistochemically. Lesions were found to display up to 8-fold decreased cholesterol levels and up to 6-fold increase of oxidized cholesterol metabolites depending on their state. This provides new insights into the altered cholesterol metabolism in multiple sclerosis lesions.
