![]() Since the IR beam used in FTIR spectroscopy contains a broad range of wavelengths, the penetration depth is different for each wavelength. ![]() The penetration depth is subjected to changes depending on varying incident wavelengths. Most organic samples have a refractive index of 1.5 therefore this value is widely used for calculation. N 2 … refractive index of sample (rare medium)Īccording to the above equation the penetration depth for an ATR cell made of zinc selenide, a sample with a refractive index of 1.5 and a wavelength of 10 µm is approximately 2.0 µm. N 1 … refractive index of ATR crystal (dense medium) Λ … wavelength of incident light in vacuum The angle of incidence of the IR beam has to match the calculated critical angle for each cell material in order to create total internal reflection. Because of their high refractive indices, ATR crystals are said to be optically dense whereas typical samples have low refractive indices and therefore are optically rare. To obtain total internal reflection, ATR crystal materials must have a higher refractive index than the tested sample material. Table 1 Common materials for ATR crystals critical angle calculated for n2=1.5 table taken from Griffiths P.R., de Haseth J. ![]() Many organic samples have mid-IR refractive indices of around 1.5 therefore, the critical angle for ATR crystal materials can be calculated for a refractive index of 1.5.
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