Mass Spectrometry Imaging of Lipids using MALDI Coupled with Plasma-Based Post-Ionisation on a Trapped Ion Mobility Mass Spectrometer

Here we report the development and optimization of a mass spectrometry imaging (MSI) platform that combines atmospheric-pressure matrix-assisted laser desorption/ionization platform with plasma post-ionization (AP-MALDI-PPI) and trapped ion mobility spectrometry (TIMS). We discuss optimal parameters for operating the source, characterize the behaviour of a variety of lipid classes in positive- and negative-ion modes and explore the capabilities for lipid imaging using murine brain tissue. The instrument generates high signal-to-noise for numerous lipid species, with mass spectra sharing many similarities to those obtained using laser post-ionization (MALDI-2). The system is especially well suited for detecting lipids such as phosphatidylethanolamine (PE) as well as numerous sphingolipid classes and glycerolipids. For the first time, the coupling of plasma-based post-ionization with ion mobility is presented and we show the value of ion mobility for the resolution and and identification of species within rich spectra that contain numerous isobaric/isomeric signals that are not resolved in the m/z dimension alone, including isomeric PE and demethylated phosphatidylcholine lipids produced by in-source fragmentation. The reported instrument provides a powerful and user-friendly approach for MSI of lipids.