Nondispersive IR systems

Nondispersive IR systems are good for measuring concentrations of specific compounds under industrial and other similar circumstances. As can be readily understood, they are not generally used as research instruments and do not have scanning capabilities. However, they are robust and enduring and can be used for the continuous monitoring of specific compounds. Nondispersive systems are very common in NIR apph-cations, as discussed in Section 4.7. 

In industry, it is often necessary to monitor the quality of a product on a continuous basis to make certain the product meets its specifications. This online, real-time approach to analysis is called process analysis. IR spectroscopy is often the method of choice for process monitoring of organic chemical, polymer, and gas production. It is usually not feasible to use laboratory IR instruments under production conditions because they are too delicate, too big, and too expensive. Nondispersive systems have therefore been developed that are much sturdier and can be left running continuously. Many nondispersive systems have been designed for the NIR region. These will be discussed in Section 4.7. The mid-IR region is used mainly for monitoring gas streams. 

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Figure 4.31 Schematic of a positive-filter nondispersive IR system.

Figure 4.30 (a) Schematic ofatwo-filterwheel, multi wavelength filter photometer. (From Coates, J., Vibrational spectroscopy, in Ewing, G.W., ed.. Analytical Instrumentation Handbook, 2nd edn., Marcel Dekker, Inc., New York, 1997. With permission.) (b) Schematic of a positive-filter nondispersive IR system. 

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