Abstract: A device for determining the concentration of a compound in an aqueous phase in a dynamic manner and while flowing, and a device (300) for determining the concentration of a compound in a gaseous phase and soluble in an aqueous phase. The device (200, 300) for determining the concentration of a compound in a gaseous phase includes elements (302) for transferring the compounds present in the gaseous phase to an aqueous phase then determining in a dynamic manner and while flowing, the concentration of the compounds in this aqueous phase by fluorescence spectroscopy. The devices (300) are robust, conveyable and less costly, and have greater temporal as well as spatial sensitivity than the devices of the state of the art.