Abstract: A method and apparatus for determining a characteristic of a substance is disclosed. The apparatus comprises a light source, a prism, a sensor chip located on the prism, focusing optics located between the light source and the prism, a detector, collimating optics located between the prism and the detector, and calculation means for determining the characteristic of the substance. The sensor chip comprises a metallic film and a transparent substance. The metallic film is operatively arranged to reflect light from the light source. The transparent substance comprises a material having an index of refraction matched to an index of refraction of the prism. The sensor chip is operatively arranged to receive a sample of the substance.