Techniques for Transmission Absorption Measurements

These are the oldest and most basic sampling techniques for IR spectroscopy and apply to both FTIR and dispersive IR systems. Transmission analysis can handle a wide range of sample types and can provide both qualitative and quantitative measurements. Transmission analysis provides maximum sensitivity and high sample throughput at relatively low cost. There is in some cases substantial sample preparation required. 

The sample or the material used to contain the sample must be transparent to IR radiation to obtain an absorption or transmission spectrum. This limits the selection of container 

A new approach to collecting transmission spectra of solids is the use of a diamond anvil cell. Diamond is transparent through most of the mid-IR region, with the exception of a broad absorption around 2000 cm . A solid sample is pressed between two small parallel diamond anvils or windows to create a thin film of sample. A beam condenser is required because of the small cell size. Very high pressures can be used to compress solid samples because diamonds are very hard materials. As a result, the diamond anvil cell permits transmission IR spectra to be collected of thin films of very hard materials. Hard materials cannot be compressed between salt windows because the salt crystals are brittle and crack easily. 

In general, spectra from sohd samples are used for qualitative identification of the sample, not for quantitative analysis. The spectrum of a solid sample is generally collected when the sample is not soluble in a suitable IR-transparent solvent. There are 

For quantitative analysis it is necessary to measure the path length in order to use calibration curves obtained with the same cell but at different times. If the cell becomes badly etched, the interference pattern becomes noisy and the cell windows have to be removed and repolished. 

The sample and the material used to contain the sample must be transparent to IR radiation to obtain an absorption or transmission spectrum. This limits the selection of container materials to certain salts, such as NaCl or KBr, and some simple polymers. A final choice of the material used depends on the wavelength range to be examined. A list of commonly used materials is given in Table 4.3. If the sample itself is opaque to IR radiation, it may be possible to dissolve it or dilute it with an IR-transparent material to obtain a transmission spectrum. Other approaches are to obtain IR reflectance spectra or emission spectra from opaque materials. 

---