A plate and method for positioning functional elements in a system for working with fluid-containing samples includes a horizontal work field having a lengthwise dimension and a perpendicularly extending transverse dimension, as well as a robot arm with a functional element, aligned essentially perpendicularly to the work field in a Z direction. The robot arm can move the functional element in at least a partial region of the work field in the X and/or Y direction. The plate includes two light barriers which intersect inside the partial region, each having a transmitter and receiver whose scanning or detection beams each extend in a direction deviating from the X direction and/or from the Y direction.