Sample preparation

A cryo-microtome (Leica CM 1860 UV, Leica Biosystems, Wetzlar, GE) was pre-equilibrated to −20 °C more than half an hour before cutting. Samples were sectioned at 10 µm thickness and thaw mounted on pre-cooled Indium Tin Oxide coated (ITO) slides. Slides were dried in desiccator for 15 min prior to matrix application.

Twelve layers of 7 mg/ml norharmane matrix solution (2: 1 chloroform/methanol (v: v)) were applied to the tissue sections using a HTX TM-sprayer (HTX Technologies, Chapel Hill, NC) at a 30 °C nozzle temperature and 0.120 mL/min flow rate. Norharmane and chloroform were obtained from Sigma Aldrich (Zwijndrecht, the Netherlands). Methanol was obtained from Biosolve BV (Valkenswaard, the Netherlands).