The invention relates to the preparation of samples on mass spectrometric sample supports with dispensing of liquids, and particularly to devices and methods to clean the dispenser. During dispensing of hundreds of samples, solved material may crystallize over time as deposit at the capillary tip of the dispenser, which impedes the vertical detachment of the drop in the medium and long run. Therefore, frequently cleaning the capillary tip is essential for a robust operation of the preparation device. The invention proposes to automatically clean the dispenser tip by creating a drop of washing fluid, such as pure solvent, fully enclosing the dispenser tip. The washing fluid is fed through a channel outside the central dispenser capillary to a location slightly above the capillary tip. The size of the hanging drop of washing fluid is photometrically regulated and monitored to prevent it from falling off prematurely.