At the conclusion of a selective binding assay (e.g., immunological or nucleic acid), an enzyme is present in modulated concentration and/or activity to moderate a chemical reaction such as the cleavage of o-nitrophenyl-.beta.-D-galactopyranoside. After a controlled period, the enzymatic product is transferred to the gas phase and concentrated relative to other components in the enzymatic reaction mixture, such as by extraction into ethyl ether, injection into a gas chromatography column and detection by flame ionization or electron capture. Kits for such assays are also disclosed.