Sampling by internal reflection

Sampling systems based on the multiple internal reflection (MIR) technique, also called attenuated total reflectance (ATR) spectroscopy [916], are very useful for the study of polymer surface degradation [476,1437]. 

In this technique a beam IR radiation is reflected many times, totally within an optical element made from special crystals such as KRS-5 (thallium bromide-iodide TlBr, Til). A portion of energy in the IR beam extends beyond the reflecting surface at each reflection (Fig. 10.73). An absorbing 

The effective path length is fixed in an MIR system as long as the reflecting surface is covered completely to a thickness beyond the beam penetration (a few micrometres). Undissolved particles have little or no effect on MIR measurements. The short effective path, however, limits the sensitivity for measuring minor components (less than 1% concentration). However, the application of the MIR method in FTIR spectrometers allows the detection of even very weak signals, and studies on samples at very low concentrations (0.2-0.5%). 

As the refractive index of the reflectance element approaches the refractive index of the sample, the depth of penetration (dp) will increase. Therefore, by varying the crystal material one can probe to various depths of sample surface. 

The smaller the angle of incidence of the IR beam on the sample surface, the smaller the depth of penetration in the sample. Thus, the 30° internal reflectance element which corresponds to a 60° angle of incidence on the sample will produce a greater penetration into the sample surface than a 60° internal reflectance element of the same material. 

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Many pastes are most easily directly sampled by internal reflection techniques. 

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