Near-Infrared Spectra of Gases

In this chapter, we will consider spectral contours of gases in the NIR and compare them with those in the MIR and with those in solutions. It will be shown that the NIR fingerprint region is very similar to that in the MIR. These comparisons will be followed by a quantitative application of NIR for determining the compositions and energy contents of natural gas. Finally, it will be shown that NIR can be used effectively in searching spectral libraries with unknown spectra of mixtures. 

FIGURE 21.1 (a) MIR spectra of typical environmentally important gases, (b) NIR spectra of the same gases. 

Comparison of Pressures Used for Measuring MIR and NIR Spectra of Organic Gases 

One of the few practical applications of NIR to gases has been its proposed use as a monitoring system for natural gas. In the United States, the composition of natural gas and its British Thermal Unit (BTU) content in transmission lines is measured at each state boundary. At present, gas samples are taken offline and analyzed automatically by gas chromatography (GC). In the case of several transmission lines crossing at a specific location, a system is set up to automatically sample each of the lines periodically. Flow meters are then used to measure the volume of gas consumed by each customer. 

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