According to an aspect of the present invention, a device for fragmenting molecules in a sample by ultrasound is provided. The device comprises a planar-shaped cartridge and a fragmentation instrument that has a vessel-like shape. By means of applying under-pressure in the fragmentation instrument the cartridge is pulled towards the fragmentation instrument and therefore spatially closes a fluid compartment that is comprised by the fragmentation instrument. Afterwards, an ultrasound treatment of the sample, which is comprised in a sample compartment of the cartridge, may be processed. A sealing layer is provided in between the cartridge and the fragmentation instrument in order to hold the fixation of the cartridge tightly and in order to provide for leakage prevention out of the fluid compartment.