Abstract: A method and apparatus for determining the concentration of molecules or atoms by measuring the ratio of two fluorescence signals. A sample having fluorescent molecules is exposed to radiation or excitation energy from a first source, which can be a broadband light source. The fluorescence of the sample is detected at two different wavelengths. The concentration of specific molecules or atoms within the sample is determined using the ratio of the two fluorescence signals. Fluorescent molecules can be bound to a human serum or plasma sample to allow determination of the concentration of unbound free fatty acids in the sample.