Infra-Red (IR) Analyzer is usually used to monitor selectively components which exhibit absorptive characteristics in the infrared region of the wave length spectrum. When a specific molecule of gas (or liquid) is subjected to electromagnetic radiation, it is caused to rotate or vibrate by absorption of energy from an infrared beam of light. The amount of energy transmitted through a sample cell (that is, not absorbed) is monitored and used to measure specifically the stream concentration. By plotting transmittance against wavelength, a spectrogram (or spectrum) is produced which is unique for that molecule. Take spectra of carbon monoxide, methane, ethylene and ethane as an example: ethane is more pronounced in the lower spectrum where transmittance drops to about 5%. The spectra reveals clearly that the ethane component absorbs IR (infrared) radiation energy in the wavelength between 3 and 4 microns. The other components absorb IR energy at other wavelengths. Only the elemental gases, hydrogen, nitrogen, oxygen, chlorine and the rare gases, do not absorb energy in the infrared region.

An infrared analyzer consists of the light source, the sensitization (or optical) section and the detector.

(Source: W. G. Andrew, "Applied Instrumentation in the Process Industries", Gulf Publishing Company, P210)