Techniques for optical analysis of fluid samples using sensors and water enhancing agents for in-line measurements with a continuous flow of the fluid samples are provided. In one aspect, a device includes: at least one reagent dispenser located at an introduction point along a conduit, the conduit being configured to contain a flow of a fluid sample; at least one first detector located at a first detection point along the conduit downstream from the introduction point; and at least one second detector located at a second detection point along the conduit downstream from the first detection point, wherein the at least one first detector and the at least one second detector are configured to make optical measurements of the fluid sample. A method employing the device is also provided.