The invention relates to the cleaning of contaminated accelerating or guiding electrodes of ion sources used for ion generation by desorption. A cleaning plate is used that has an outer contour similar to that of a standard sample support plate, and may be equipped with cleaning scrubbers that can be moved out when necessary to contact the electrodes. The scrubbers may include soft covers, and can carry out the cleaning by dry rubbing or with the help of high-boiling solvents for the matrix substances. The moving out of the cleaning scrubbers can be controlled by external light pulses from a laser or video camera spot light. Alternatively, the cleaning plate may be equipped with spray nozzles connected to a reservoir of cleaning fluid which is sprayed onto the electrodes, and the evacuation of the ventilated ion source chamber may be used to initiate the spraying.