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

Abstract

Here we report the development and optimization of a mass spectrometry imaging (MSI) platform that combines an atmospheric-pressure matrix-assisted laser desorption/ionization platform with plasma postionization (AP-MALDI-PPI) and trapped ion mobility spectrometry (TIMS). We discuss optimal parameters for operating the source, characterize the behavior 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 postionization (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 postionization with ion mobility is presented, and we show the value of ion mobility for the resolution 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.

Solvent-Assisted Laser Desorption Flexible Microtube Plasma Mass Spectrometry for Direct Analysis of Dried Samples on Paper

“…When the IR laser was used as desorption and ionization means barely ionized the compounds, and when ionization occurred, the signal was 3 or 4 orders of magnitude lower than FμTP (Figure S2). The laser is known to have poor ionization efficiency, 42 so the observed ions could be the product of assisted ionization by a heated mass spectrometer inlet. 43,44 To ensure the sample and substrate stability while avoiding excessive heating, we investigated alternative approaches.…”

Lewis Acidic Metal–Organic Framework Assisted Ambient Liquid Extraction Mass Spectrometry Imaging for Enhancing the Coverage of Poorly Ionizable Lipids in Brain Tissue

“…However, in those approaches, multistep sample preparation processes including imprinting, solvent spraying, and tissue rinsing were inevitable, which would result in low recoveries due to transferring processes and incomplete tissue rinsing. Besides the above sample preparation methods, new ionization methods were also developed to enhance the MSI of neutral lipids, such as surface assisted LDI MSI with silicon nanopost arrays, , silver nanoparticles and graphitic carbon nanomaterials, salt doping in the MALDI matrix, and new post ionization (PI) strategies (MALDI-2). − However, in most of the above methods, only the detection of one or two specific neutral lipid classes was enhanced without removing the high-abundance phospholipids, so the peak interference from high-abundance phospholipids to low-abundance lipids could not be reduced. Moreover, the above LDI-based methods are not ambient MSI methods, which have no potential to be applied to living samples or in vivo analysis.…”

A Machine Learning-Driven Comparison of Ion Images Obtained by MALDI and MALDI-2 Mass Spectrometry Imaging

“…Recently, post-ionization techniques have been developed to enhance the ionization of analytes not initially ionized by MALDI to overcome these limitations, boosting sensitivities by up to 2 orders of magnitude. These techniques include low-temperature plasmas, − pulsed RF-Kr lamps, and laser-induced post-ionization (MALDI-2) . MALDI-2 is currently the most widely used post-ionization method and involves directing a second ultraviolet (UV) laser pulse (typically 260–280 nm) through the matrix/analyte plume produced by the MALDI laser pulse.…”