What makes an ATR Accessory useful in spectrometry?
The heart of an ATR accessory is a trapezoidal or parallelogram crystal mounted in the infrared beam. There are several different crystal materials used for ATR crystals. Their common characteristic is a relatively high refractive index. The infrared radiation enters the crystal through a set of mirrors, and is internally reflected (typically several times) until it exits at the other end. These internal reflections create an evanescent wave, which extends beyond the surface of the crystal into a sample placed on its surface (the evanescent wave decays rapidly with distance from the surface, therefore, the sample must be in intimate contact with the crystal surface). In a typical IR experiment, part of the evanescent radiation is absorbed by the sample producing an absorption spectrum. The resulting absorbance intensity is proportional to the number of reflections of the infrared beam in the crystal and the depth of penetration of the evanescent wave into the sample. These two factors
