Exploring Recently Developed Mass Spectrometry Post-Ionisation Methodologies for Lipid Imaging

<p dir="ltr">Boosted by improvements in technology in the past few years, mass spectrometry imaging (MSI) has become a powerful imaging tool to map analyte distribution of a wide range of species. Most used for lipid imaging, matrix-assisted laser desorption/ionisation (MALDI) is one of the most applied ionisation sources in MSI, however many species remain undetected due to their poor ionisation efficiencies. Therefore, new modalities such as post-ionisation techniques, that promote a second ionisation event of intact species providing not only improvement in increasing molecular coverage but also enhancing sensitivity for many classes of molecules, were introduced in the field. To date, MALDI-2 and plasma post-ionisation are the most widely applied post-ionisation techniques for sensitivity enhancement of species in MSI, consequently enabling high lateral resolution. These new modalities are yet to be fully explored. Therefore, the following chapters cover the evaluation and assessment of the capabilities of both approaches for lipid imaging.</p><p dir="ltr">Chapter 2 explores the interaction of the laser desorption and MALDI-2 process for the selective formation of aromatic antioxidants via REMPI process. This investigation dives into the ionisation process of these selected molecules and provides a methodology for the generation of REMPI-compatible analytes that demonstrates simultaneous improvement in both spatial resolution and sensitivity. With the continued growth of systems implementing MALDI-2 for profiling the distribution of a wide range of species, including for lipidomics, the need for a detailed comparison between ion images generated by both ionisation techniques, MALDI and MALDI-2, drove the investigation performed in Chapter 3. The investigation provided for the first time a complete comparison of molecular features and patterns of ion images with insights into the complementarity of both approaches. By understanding the ion images generated by both methods researchers have the tools available to choose the most effective ionisation technique for their planned analysis. Finally, using a dual-modality approach, Chapter 4 combined plasma post-ionisation with laser-ablation inductively coupled plasma (LA-ICP) to provide a comprehensive analysis of lipids and elements on the same tissue section. This study provided a sample preparation protocol that enables spatial correlations of lipid and elemental distribution on the same tissue section, broadening the repertoire of multimodal MSI techniques.</p>