The diagram above shows a typical type of ATR experimental set up. In the example a
parallelogram shaped ATR crystal is covered on two surfaces with a black sample material. The
beam of infrared light traversing through the ATR crystal at a particular angle of incidence is
being internally reflected at the top and bottom surfaces and in this instance there are 6 points of
contact with the sample material (3 at the top and 3 below).
Internal reflection occurs when infrared radiation enters an ATR crystal made of a highly
refractive infrared transmitting material. The crystal is designed to enable internal reflection,
creating an evanescent wave at the crystal surface. This wave extends into a sample that is held
in intimate contact with the crystal material and so absorption spectra can be recorded as a
result. The depth of penetration of the evanescent wave into the sample is a function of the
crystal material and the angle of incidence for the infrared beam traversing through the crystal to
the sample surface interface. Deeper penetration into a sample is achieved with either a smaller
incident angle or a lower refractive index ATR crystal. In general the depth of penetration into a
sample increases with increasing wavelength of light.
The Specac accessories that are used for the analysis of solids and liquids by the ATR technique
are the Golden Gate and Silver Gate single reflection ATR accessories, the Gateway 6 reflection
ATR system and the 25 reflection ATR accessory