An assembly for performing controlled heat exchanging reactions is provided. The assembly has a chemical reaction chamber adapted to receive a sample and allow the sample to chemically react; a thermal sleeve having heating elements for making efficient thermal contact with a reaction chamber; an instrument with a housing comprising an electrical connection, a cooling source, a processing area adapted to receive a thermal sleeve, and an optics assembly in optical communication with a chemical reaction chamber; and a circuit for monitoring and controlling the optics assembly and for collecting the output signal of the optics assembly. Preferably there are a plurality of housings and associated modules, each of which may be independently controlled for heat exchanging operations. The reaction chamber comprises two opposing major walls and a plurality of contiguous minor walls, wherein the ratio of thermal conductance of the major walls to that of the minor walls is at least 2:1.