The invention relates to a method for operating an internal combustion engine. The flat base plate includes a pair of parallel spaced apart upstanding side walls that are individually extend across the flat base plate. In a section perpendicular to an axis of rotation of the roller, a plane of contact between the roller and the condensing member includes at least a part of the portion made of the material that blocks the laser light. The gel cassettes can be used for electrophoretic separation of proteins and protein complexes on native gels, such as native gels that include linear polyacrylamide in the stacker.