Enhancing Ion Yields in MALDI-2-MSI Using Dopant-Assisted Post Ionization for High-Resolution MS-Imaging of Lipids and Metabolites.

Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) is the most widely used label-free method for mapping the molecular distribution of lipids and metabolites from tissue sections and cell cultures. To mitigate the problem of partially low MALDI ion yields for several of these analyte classes, different post ionization techniques have recently been developed─for example, MALDI-2, where a second UV-laser pulse intersects the initial particle plume to generate additional charges. Here, we show that the MALDI-2 signal intensities of important analyte classes can be further increased upon introducing reactive dopant vapor into the MALDI-2 ion source. Dopant assistance as an optional add-on was realized in prototype form on MALDI-2 ion sources of a Spectroglyph Injector-Orbitrap MSI setup and a timsTOF fleX MALDI-2 instrument. Acetone, anisole, toluene, 2-propanol, and chlorobenzene were tested as dopants. Acetone proved to be the most effective and robust dopant for the given setup and MALDI matrix, and thus was used for further parameter optimization on both instruments. We demonstrate that with dopant-assisted (DA-)MALDI-2, a signal increase of up to 5 orders of magnitude, as compared to regular MALDI, is, for instance, achieved for several classes of sterols, glycerophospholipids, and sphingolipids. Next to the increased intensity of signals for abundant lipids, the total amount of annotated signals (based on MS1) was also majorly increased by the use of dopants. We demonstrate the potential for biomedical imaging applications with the examples of murine spleen and cerebellum sections, analyzed in the positive- and negative-ion mode.