Abstract: The invention relates to a detector arrangement in an instrument for analysis of discrete liquid sample volumes in the microliter and/or nanoliter range contained in microchannel structures that are present in a disc shaped microfluidic device. The suggested detector arrangement comprises a detector head, supported to be driven and controlled for linear displacement and positioning in a first radial plane through a central axis of an associated rotary drive. A rotary member is carried for rotation about said central axis in a second radial plane in parallelism with said first radial plane. A vacuum source is connected with the rotary member and in air flow communication with a planar top surface of the rotary member for a wide area distribution of sub-pressure over said top surface, the rotary member carrying the disc shaped substrate in adhering contact with the top surface for spinning the substrate in equidistant and equiangular relation to the detector head.