A reaction cartridge comprises a first liquid reservoir having a first aperture exposed to the atmosphere and a second aperture, a first flow channel connected to the first liquid reservoir, a reaction chamber communicating with the first liquid reservoir by way of the first flow channel, a second flow channel connected to the reaction chamber and a third aperture connected to the second flow channel and exposed to the atmosphere. A probe carrier is arranged in the reaction chamber and externally supplied liquid is stored in the first liquid reservoir. The reaction cartridge is adapted to move liquid between the liquid reservoir and the reaction chamber by increasing or reducing the pressure at the third aperture.