A reflectance sampling technique. In ATR, infrared radiation impinges on a prism of infrared transparent material of high refractive index. The total internal reflectance based design assures that the light reflects off the surface of the crystal at least once before leaving it. The infrared radiation sets up an evanescent wave which penetrates a small distance above and below the crystal surface. Samples brought into contact with the surface will absorb the evanescent wave giving rise to an infrared spectrum. This sampling technique is useful for liquids, polymer films, and semisolids.