GAS MEASUREMENT REQUIRES A LONG OPTICAL PATH.
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 that is to be measured is frequently only a minor component in air, nitrogen, or other 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. The minor components thus may be measured by infrared absorption, providing that the optical path is long enough.