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Types of ATR Crystal

Attenuated total reflection spectroscopy (ATR) is a research technique used to study infrared light. Infrared light is passed into a crystal and then into another object. The spectrum created between the crystal and the research object is analyzed and the data is recorded. ATR crystals themselves are installed in different types of research equipment. The types of crystals used for research are defined by four characteristics: their refractive index, chemical and physical properties, spectral range and sensitivity.

Refractive Index
- The refractive index is the factor by which the phase velocity of a beam of light is decreased relative to its velocity in a vacuum. The type of ATR crystal being used for research must have a higher refractive index than the other objects being studied. Most organic material has a refractive index around 1.5. ATR crystals span between 2.5 to 4.0. When the crystal's index is not in correct proportion to the material being studied, it can affect the spectrum being analyzed and result in false data.
Chemical and Physical Properties
- ATR crystals are also typed according to their physical and chemical properties. Pressure and temperature conditions of the experiment dictate the composition of the ATR crystal being used. Researchers also consider whether or not the crystal will have a chemical reaction when it contacts the material being analyzed. Improper crystal selection can result in the formation of caustic acids or noxious gasses. ATR crystals are made of zinc selenide, germanium, zinc sulfide, silicon or diamond. A combination of germanium, arsenic and selenium, called AMTIR, is also available.
Spectral Range
- The spectral range of an ATR crystal determines the wavelengths of light it absorbs when the light passes through it. Every crystal has a cutoff point on the wavelength spectrum, but those points are different for each ATR crystal. The range is determined by the type of material the crystal is made from and the length of the cut. Researchers must choose a crystal that has a compatible spectral range with the material being studied.
Sensitivity
- The sensitivity in an ATR crystal refers to the number of reflections it can produce when a beam of light is directed into it at a particular angle and to the depth the light can penetrate into the crystal. Light introduced at a 45-degree angle typically produces between nine and 20 reflections in a crystal. Researchers using highly sensitive crystals introduce light at higher angles to reduce the number of reflections in the ATR crystal, and low sensitivity crystals have light introduced at lower angles.

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