Abstract: An apparatus and method for monitoring the presence in a liquid of a material which is at lest partially immiscible in the liquid are disclosed. The apparatus includes a container and an overflow outlet for allowing a surface layer of the liquid to flow out of the container. A slick of the material forms in the container on a surface of the liquid. The container includes a settling portion with an adjustably angled baffle and a second, fixed baffle located therein, a concentrating portion for concentrating the slick downstream of the settling portion, and an intermediate constricting portion for gradually constricting the surface of the liquid. An ultraviolet light source and visible light detector are provided for detecting the fluorescent radiation produced by irradiating the surface of the liquid and the material.