F An inexpensive, rugged, and portable flame ionization detector has been designed for the continuous analysis of trace amounts of hydrocarbons. The detector is insensitive to inorganic gases but responds to hydrocarbons in proportion to the carbon atom content.A detectable limit of 1 p.p.b. of hexane is obtainable. As an automotive exhaust analyzer, the unit can first measure total unburned hydrocarbons, and then by introducing an 8-inch silica gel column, the unit can determine methane, ethane, ethylene, acetylene, and propylene. HE QUANTITATIVE estimation of T trace amounts of hydrocarbons is of great importance in air pollution, oil exploration, oxygen manufacture, leak detection, and exhaust gas analysis. McWilliams and Dewar (7,8) have recently reported a flame ionization instrument capable of detecting less than 1 p.p.h. of hydrocarbon. Similar detectors with lower sensitivity have been reported by Harley (4) and Thompson (9). The p r a y argon ionization detector has sufficient sensitivity hut does not respond to C, or C, hydrocarbons. The flame ionization principle was, therefore, investigated for continuous trace hydrocarbon analysis. The detector described is simpler than any reported in the literature, yet has a high sensitivity. It is also more versatile than the Perkin-Elmer continuous hydrocarbon detector and meets the requirements for either a continuous analyzer or a gas chromatography detector.The flame ionization detector makes use of the electron concentrations formed when a hydrocarbon is introduced into a hydrogen flame (1, 9, 10). Figure 1. Front view of detector Inorganic gases such as hydrogen, nitrogen, carbon dioxide, and steam have high ionization potentials (12 t o 16 electron volts) and are not ionized at flame temperatures. These gases do not interfere with the analysis. The mechanism of the ionization of the hydrocarbons in flames is complex and not completely understood. Hydrocarbons are cracked in the preheating zone of the flame t o produce acetylene and large unstable hydrocarbon molecules. These molecules form carbon radicals such as C2 and CH which are in highly activated electronic states. The carbon radicals are believed to react to form carbon nuclei which react with the original hydrocarbon and intermediates t o form carbon particles. The carbon particles, containing ahout 50,000 atoms (S), may then he assumed to behave similarly to solid carbon and to emit electrons due to their low thermionic work function. The thermionic work function for carbon is 4.35 electron volts (2) which is lorn enough for electron emission at flame temperatures. The fact that the conductivity appears t o he a function of the number of carbon atoms and is not a function of their nature supports this contention. APPARATUS Figures 1 and 2 illustrate the electronic and mechanical portions of the unit. One type of burner ( Figure 2) is mounted on a %inch Teflon base through which the electrical and gas connections are made. The entire flame is enclosed in an aluminum cover ii.hich has a fine screen ...
