A coated waveguide holder-humidifier to supply moisture in the case of moisture-sensitive reactions in order that quantitative results might be obtained with a gradient light analytical detector which will quantitatively measure atmospheric contaminants by comparing changes in light transmission through the coated waveguides before and after exposure. The coated waveguide holder-humidifier comprises a container, a porous material capable of absorbing water and desorbing water vapor mounted within the container, means to hold one or more waveguides within the container, and one or more openings in the container to allow fluid (liquid or gas) sample to contact a waveguide. Also, a method for improving the sensitivity of gaseous reactants on a waveguide where the reaction is moisture sensitive comprising humidifying the environment of said waveguide providing an atmosphere which has a substantially constant humidity normally in the range of 85 to 100% at which said reaction is not appreciably affected by changes in ambient relative humidity.