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Chapter 13 Infrared Spectroscopy Tutorial
Choosing Analytical Wavelengths of Gas Compounds
From the "fingerprint" or IR spectrum by which a compound is identified, we must
choose a good analytical wavelength for a measurement. This is a critical step in
performing a gas calibration. By identifying characteristic main and secondary absorption
peaks unique for that particular compound and recording them for future reference, a
library of analytical wavelengths for a large number of gases can be maintained. The
specific analytical wavelength chosen for each organic compound depends upon two
factors: specificity and intensity.
Specificity
The wavelength chosen for a particular compound must be characteristic of that
compound; it should serve to easily identify and distinguish the compound from other
similar compounds. For example, Figure 13-12 shows spectra of two distinctly different
hydrocarbons: isopropanol (2-propanol) and methyl ethyl ketone. Absorption peaks in the
isopropanol spectrum are present at 8.2, 8.9, and 10.6 microns. Also note that methyl
ethyl ketone has peaks close to 8.9 and 10.6 microns but not close to 8.2 microns.
Therefore, the 8.2 micron peak serves best at distinguishing isopropanol from methyl
ethyl ketone.
13-12
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