Unmodulated infrared radiation

Figure 23.3. Layout of the optics for GC/FT-IR designed to minimize the amount of unmodulated infrared radiation emitted from the glass ends of the light-pipe that reaches the MCT detector. (Reproduced from [10], by permission of the American Chemical Society copyright 1987.)...

Finally, the FT-IR system operates by coding the infrared source with an amplitude modulation which is unique to each infrared frequency. The detector is sensitive to the modulated radiation so that unmodulated stray radiation is eliminated from the experiment, permitting the use of the FT-IR as an in-situ detector in many experiments. For example, an FT-IR has been used to monitor the evolution of coal pyrolysis products within a drop tube furnace (24) and within an entrained flow reactor (25). The latter has been operated up to 1200 C. 

Another application of mid-infrared DR spectroscopy is the measurement of reacting samples in a controlled atmosphere. A number of cells has been designed to be compatible with commercially available DR accessories. A home-built accessory [14] is shown in Figure 16.5 and a typical commercially available accessory is shown in Figure 16.6. The efficiency of these accessories is quite low, so that they are generally used in conjunction with an MCT detector. When the cells are used at high temperature, a considerable amount of unmodulated thermal radiation is... 

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