An instrument and a method for the automated thermal treatment of liquid samples are disclosed. An inter-distance between a temperature-controlled receptacle for loading with a plurality of vessels for containing the samples and end portions of optical fibers can be varied, wherein the receptacle is configured to form a thermal communication with the loaded vessels and wherein the optical fibers have first and second end portions. The first end portion and the second end portion of each optical fiber is fixed with respect to each other for transmitting light, wherein the variation of the inter-distance allows the vessels to be loaded to or unloaded from the receptacle and to enable detection of light from the samples contained in the one or more receptacle-loaded vessels.