RONAR-SMITH^® LASER OPTICS & IR IMAGING

ATR Prisms for Infrared Spectroscopy

The criterion for selection of prism material to obtain a good FT-IR ATR spectrum is outlined below. For a good spectrum, it is essential to select conditions to achieve optimum depth of penetration (dp).

· The useful wavenumber (cm-1)/wavelength (l ) range of the material. The wider the useful transmission wavelength range, the better the material for general purpose uses. The depth of penetration increases with decreasing wavenumber/ increasing wavelength.

· Refractive index of material (h 1). This determines the critical angle (q c) of the      material at which total internal reflection phenomena begins to occur. The higher the material refractive index, the lower the critical angle. It is desirable for the angle of  incidence to far exceed the critical angle to avoid spectrum distortion when working with high refractive index (h 2) samples.

· Angle of incidence (q i). Increasing angle of incidence will decrease the depth of    penetration and number of reflections. These will decrease effective pathlength and therefore decrease the absorbance intensity of the spectrum.

· Effective sample contact. It is important to achieve a good sample contact all over the ATR prism sample contact area to obtain a good spectrum. In solid samples it is       important to choose an ATR material that will permit sufficient pressure to be applied without causing damage to the prism or sample in order to obtain a good spectrum. The hardness of the material is important.

· Material reactivity. When dealing with liquid samples, careful attention should be paid to solubility in water, the reactivity of the ATR materials to acids, bases, oxidising and reducing agents and complexes.