Abstract: A device for detecting substances in a liquid film by measuring chemiluminescence, fluorescence or absorption using techniques such as flow-injection analysis schemes, liquid chromatographies, and gas chromatographies. The device can be used as a micro-volume mixing or reaction substrate for a variety of measurements. The device comprises at least two film supports that are separated by a small gap. One or more applicators are positioned along the gap to supply fluids to the gap. The applicators are positioned near the gap at a spacing that is near the necking length of the fluid supplied by the applicator. A thin, continuous film of the mixture of fluids to be examined is supported in the gap between the film supports. The film extends through the examining chamber and through drop and overflow detection probes. Also disclosed is a mixing or reaction chamber which comprises an enclosable mainframe into which a plurality of applicators are butted near a gap defined between two film supports.