Windows, Cells, and Materials for Transmission Analysis

The Low cm Cutoff column in Table 4.2 indicates the lowest wavenumber to which a material is infrared transparent enough to allow usable spectra to be measured. For example, KBr begins to absorb strongly near 400 cm , so spectra should not be measured beyond that point when woiking with it. The cost data in Table 4.2 is inexact because the prices of things vary over time. The number of dollar signs is meant to indicate the relative cost of these different infrared fransparent materials. 

Potassium bromide (KBr) is the most commonly used infrared transparent material. It is transparent over a broad spectral range, from 400 cm up through the visible. KBr windows are commonly used in the beamsplitters found in FTIRs, and their low wavenumber cutoff normally determines the low wavenumber cutoff of the FTIR as well. KBr is also relatively cheap and is easy to machine into windows and cells. A major drawback is that it is highly polar, which leads to two problems. First, KBr is hygroscopic, which means it absorbs water from the atmosphere. Over time a thick layer of water can build up on the surface of KBr, masking sample absorbances. Therefore, KBr in all forms—windows, cells, and powder—should 

Infrared Transparent Materials Commonly Used in Transmission Analysis 

The second problem with KBr is that it is water soluble, so any sample that contains liquid water cannot be analyzed using this material. Cesium iodide (Csl) cuts off at 200 cm and has a broader spectral range than KBr. However, it is more hygroscopic and expensive than KBr. It is typically used only when the sample has an important infrared feature between 400 and 200 cm. The final four materials listed in Table 4.2 share the advantage that they are not water soluble, are not hydroscopic, and can be used with samples that contain liquid water. Their major disadvantage is that they are expensive. 

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