Infrared Absorption

In the infrared absorption method, an infrared light beam is projected through the gas sample, and the intensity of the transmitted light is measured. CO₂ absorbs light with a characteristic peak at a wavelength close to 4300 nm. Several other molecules, such as anesthetic gases (especially nitrous oxide), water vapor, CO, and O₂ also absorb light in this area of the spectrum, interfering with CO₂ measurement, especially if the incident light composition includes wavelengths other than those in a very narrow region around the CO₂ absorption peak. In practical use, the fixed geometry of the gas-sampling cell, a narrow-band infrared light source, and compensating electronic circuits can often automatically correct for interference by other gases. Most expired gas analyzers use infrared absorption.