Improved sample preparation for MALDI–MSI of endogenous compounds in skin tissue sections and mapping of exogenous active compounds subsequent to ex-vivo skin penetration

Localization of endogenous and exogenous compounds directly in tissue sections is a challenging task in skin research. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) is a powerful label-free technique that enables determination of the distribution of a large range of biomolecules directly in tissue sections. Nevertheless, its application in this field is limited in large part by the low adhesion of skin tissue sections to indium–tin oxide-coated (ITO) glass slides. For the first time corona discharge (CD) treatment was used to modify the glass slide surface for improved adhesion. Localization of endogenous cholesterol sulfate was performed directly in human skin tissue sections. A spatial resolution of approximately 30 μm was sufficient for assignment of mass signals to skin structure morphology. Furthermore, imaging of an exogenous model compound, Nile red, was performed directly in skin tissue sections after ex-vivo penetration into porcine skin, enabling determination of the pathway and depth of penetration. Finally, the ion density map of Nile red was compared with its high resolution fluorescence micrograph. This work provides new insights into the application of MALDI–MSI in skin research.