The effect of this varying depth of penetration is to change the relative peak heights compared to a transmission spectrum. The depth of penetration of the evanescent wave is dependent on the wavelength of the light. The ATR technique changes spectra in two important ways relative to the transmission spectrum of the same substance. The ATR spectrum of a substance will be different from the transmission spectrum, in which the light passes directly through the sample. Some of the energy of this wave is absorbed by the sample, yielding a spectrum of the sample. The reflection creates an evanescent wave which penetrates into the sample. The crystal has a higher index of refraction than the sample, so light will reflect off the surface in contact with the sample. In the ATR technique, the crystal is in contact with the sample. When using attenuated total reflectance (ATR) sampling techniques, spectra take on characteristics which are dependent on the angle of incidence of the infrared beam on the ATR crystal, the refractive index of the crystal and the refractive index of the sample.
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