4-Aminoazobenzene: A novel negative ion matrix for enhanced MALDI tissue imaging of metabolites

Endogenous metabolites play key functions in many important physiological and biochemical processes. The comprehensive in situ detection and direct imaging of metabolites in bio-tissues by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is very important for understanding complex and diverse biological processes and has become an essential aspect of spatial omics. In this work, 4-aminoazobenzene (AAB) was successfully screened and optimized as a new negative ion (-)MALDI matrix to enhance the in situ detection and imaging of metabolites in tissues using MALDI-MSI. Obviously, AAB exhibited superior properties in terms of ultraviolet absorption, background ion interference, matrix morphology, and metabolite ionization efficiency. AAB was used for in situ detection and imaging of metabolites in rat brain and germinating Chinese yew seed tissue sections, where 264 and 339 metabolite ion signals were successfully detected and imaged using (-)MALDI-MS, respectively. In addition, high-resolution imaging of mouse eyeball section using MALDI-timsTOF MSI with spatial resolution of up to 10 μm was successfully carried out, showing that AAB is an efficient (-)MALDI matrix for capturing high-resolution images of metabolites in biological tissue sections.