A highly sensitive fluid composition analyzer where a fluid may be placed in contact with a very small area on a material sensitized to change color in the presence of a specific type of compound, to be impinged with light. The light reflected, transmitted and/or scattered by the material may serve as input for the analyzer electronics. The fluid may be pre-concentrated prior to being brought in contact with the material. The area on the material may be a spot having an outside dimension of less than one millimeter.