We consider the FTIR measurement of substances in gaseous state gases and vapours. By gases we mean those compounds that at room temperature have a vapour pressure higher than one atmosphere, such as CO2, CO, NO2, NO, methane, dimethyl ether and arsine. By vapours we mean the gaseous phases of compounds that normally are liquids or solids (room temperature vapour pressure less than one atmosphere).
Long path cells are necessary for gas measurements mainly because of the low density of the gaseous samples.
A gas consists of fast-moving molecules that occupy a volume about one thousand times greater that the volume of a comparable mass of condensed phase material. Furthermore, a gas to be measured is frequently only a minor component in air, nitrogen, or another gaseous matrix material. The matrix gas may be totally transparent to infrared radiation, as in the case of dry air, or at least it is likely to be partially transparent.