1. A sensor device for the detection of magnetic particles in a sample chamber with a contact surface at which magnetic particles can collect, comprising: a) a sensor unit for detecting magnetic particles in a target region on the contact surface and/or in at least one reference region on the contact surface; b) a magnetic field generator for generating a magnetic field that shall guide magnetic particles to the contact surface; c) an evaluation unit for determining from the detection signals of the sensor unit an auxiliary parameter that is related to the magnetic particles and/or their movement but that is independent of binding processes taking place in the target region between magnetic particles and the contact surface.