Attenuated total reflectance

The penetration depth into the sample is typically between 0.5 and 2 micrometres, with the exact value determined by the wavelength of light, the angle of incidence and the indices of refraction for the ATR crystal and the medium being probed.

The accessibility, rapid sample turnaround and ease of use of ATR with Fourier transform infrared spectroscopy (FTIR) has led to substantial use by the scientific community.

The signal to noise ratio obtained depends on the number of reflections but also on the total length of the optical light path which dampens the intensity.

[citation needed] Typical materials for ATR crystals include germanium, KRS-5 and zinc selenide, while silicon is ideal for use in the Far-IR region of the electromagnetic spectrum.

[4][5] Due to the ATR geometry and the resulting evanescent wave, it is possible with this technique to study transport phenomena and sorption kinetics through thin films.

Water-soluble proteins to be investigated require Polyhistidine-tags, allowing the macromolecule to be anchored to a lipid bilayer, which is attached to a Germanium crystal or other suitable optical media.

Light undergoes multiple internal reflections in the crystal of high refractive index, shown in yellow. The sample is in contact with the crystal.
An ATR accessory for IR spectroscopy.