Abstract: A substrate for an apparatus for quantitative detection of a component present in a gas or liquid, in which substrate elastic vibrations can be generated, said substrate being provided with a surface layer adapted to bond said component, so that the vibration behavior of said layer is changed. At least the surface portion of said substrate consists of a substance with active sites (A) for covalent bonding situated at its free surface, to which sites (A) molecules of a substance (X) are covalently bonded which, on the other hand, possess active sites (B) for selectively and reversibly bonding by means of a coordinative bond of the molecules of the component (G) to be detected.