When a gaseous sample is analyzed by its infrared absorption, the technique to be used is considerably different from the technique to be used when the sample is a liquid or a solid. The techniques differ because the spectra differ.

Gaseous molecules, especially those of low molecular weight, are likely to have fine structure in their absorption bands; while condensed phase materials have only broad, structureless bands. The fine structure is very valuable in the analysis of the gases, allowing simplifications in the quantitative analysis method, but the fine structure also restricts the range of absorbance over which gaseous measurements may be made. READ THE TOPIC ABSORBANCE LAW, FAILURE.

Not only does the measurement technique change when going from condensed phases to gases, but the hardware requirements for the measurements also change. Gas studies require higher spectral resolution than the study of condensed phases. If the sample is a solid or a liquid, a spectral resolution of several wavenumbers may be used without loss of spectral information. If the sample is a gas, however, that low degree of spectral resolution will hide the fine structure in the spectrum and thus cause a loss of the ability to distinguish between molecules as well as a loss of the ability to measure correctly the components of mixtures.

The differences between the spectra of gases and condensed phases also affect the detector requirements. A person working with liquids or solids will probably be working in the range of absorbance from about 1 absorbance unit down to the noise level. He will be working at low resolution, so his noise level may well be low, even with a room temperature detector. He will probably be content to use a room temperature DTGS detector. Things are different for a person working with gases. He should not measure in any spectral regions of high absorbance, and furthermore he will be using high resolution, which produces a noise level that is higher than that obtained when working at low resolution. It is therefore necessary that the person seeking a full degree of success in gas measurement should use a nitrogen-cooled MCT detector. SEE THE TOPICS ABSORPTION LAW, FAILURE, HARDWARE REQUIREMENTS FOR QASOFT, AND MCT DETECTOR

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