Techniques for Reflectance and Emission Measurements

The sample techniques just described are designed for collection of transmission (absorption) spectra. This had been the most common type of IR spectroscopy, but it was limited in its applicahons. There are many types of samples that are not suited to the convenhonal sample cells and techniques just discussed. Thick, opaque solid samples, paints, coahngs, hbers, polymers, aqueous solutions, samples that cannot be 

Typical ATR crystal materials are listed in Table 4.6. Samples must be in actual intimate physical contact with the ATR crystal. The first ATR systems were designed to analyze solids that could be pressed against the surface of the crystal polymers, films, moldable resins, textiles, canvas paintings, and the like. Little or no sample preparation is required. For example, the IR spectrum of a valuable painting could 

Material Spectral range (cm ) Refractive index Penetration depth (p,m) Uses 

Germanium 5,500-675 4 0.66 Good for most samples strongly absorbing samples such as dark polymers 

Diamond 30,000-200 2.4 2.01 Good for most samples, extremely caustic or hard samples 

The sample techniques just described are designed for collection of transmission (absorption) spectra. This had been the most common type of IR spectroscopy, but it was limited in its applications. There are many types of samples that are not suited to the conventional sample cells and techniques just discussed. Thick, opaque solid samples, paints, coatings, fibers, polymers, aqueous solutions, samples that cannot be destroyed such as artwork or forensic evidence samples, and hot gases from smokestacks—these materials posed problems for the analytical chemist who wanted to obtain an IR absorption spectrum. The use of reflectance techniques provides a nondestructive method for obtaining IR spectral information from materials that are opaque, insoluble, or cannot be placed into conventional sample cells. In addition, IR emission from heated samples can be used to characterize certain types of samples and even measure remote sources such as smokestacks. In reflectance and emission, the FTIR spectrometer system is the same as that for transmission. For reflectance, the sampling accessories are different and in some specialized cases contain an integral detector. The heated sample itself provides the light for emission measurements therefore, there is no need for an IR source. There may be a heated sample holder for laboratory emission measurements. 

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