This invention provides an article of manufacture and method for controlling the application of a coupling agent, such as a silicone oil or mineral oil, on a surface of a tissue prior to contacting the tissue with an apparatus for non-invasive optical measurement of the concentration of an analyte. The article ensures that a specific quantity of the coupling agent is deposited in a uniform layer over the entire target area of the tissue, thereby enhancing both the optical signal and the repeatability of thermal and optical coupling with the components of the apparatus. The article comprises a backing and a layer of coupling agent on at least one major surface of the backing.