In this issue: Learn about the different window materials available for use in Specac products, including the Omni-Cell, Pearl, High-Temperature High-Pressure (HTHP) and Heatable Liquid Cells.

In FT-IR transmission spectroscopy, the IR beam needs to be transmitted through a liquid or gaseous sample held within a sample cell. The cell has windows that are exposed to the chemical environment within the cell and must therefore be compatible with the sample as well as being transparent to the wavelengths of interest.

If the cell is also to be heated or cooled, pressurized, or placed under vacuum, mechanical compatibility with these conditions is also necessary. As a result, a range of different materials with similar optical properties are needed to suit each application.

There are a few major classes of material which have transmission bands in the infrared: halide salts such as potassium bromide (KBr) and sodium chloride (NaCl) are inexpensive and readily polished into flat and transparent optics; other inorganic compounds such as thallium bromoiodide (KRS-5) and zinc selenide (ZnSe) have been also been widely employed. Semimetals such as silicon (Si) and germanium (Ge), silica glasses, and even diamond are sometimes used as well.

These materials fulfil the most important criteria for infrared applications – namely, a wide and useable transmission band within the wavelengths of interest for IR spectroscopists. But they vary significantly in respect of the other physical properties which come into consideration when working with them.