Seeing the invisible: Preparative strategies to visualise elusive molecules using mass spectrometry imaging

Mass Spectrometry Imaging (MSI) is a powerful tool for the simultaneous visualisation and analysis of multiple molecules in complex biological samples. It provides information on their spatial distribution and is gaining increasing importance as a tool to study health and disease mechanisms. Despite its excellent multi-omics capabilities, some molecules are still not easily accessible via MSI for reasons such as low abundance or poor ionisation efficiency. However, sophisticated (bio)chemical sample treatments such as on-tissue derivatisation, enzymatic digestion, and labelling strategies have been developed to overcome these issues. In this review, we discuss advancements made in the last three decades on On-Tissue Chemical Derivatisation (OTCD), On-Tissue Enzymatic Digestion (OTED), and On-Tissue Mass Tag Labelling (OTMT) strategies, highlighting representative examples and commenting on their respective strengths and weaknesses.