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.
