Topics - Helium

Helium Helium is an inert, monatomic gas, having an atomic weight of 4. In gas chromatography, helium is commonly used as the carrier gas as it is very inert and solutes diffuse rapidly in it. The high solute diffusivity reduces the resistance to mass transfer in the mobile phase and the column efficiency is consequently higher. Helium must be used as the mobile phase, when the helium detector is employed. As an inert gas, collisions between helium atoms and electrons are perfectly elastic. Thus, if electrons are generated in helium and are then accelerated under a suitable potential, despite collisions, the velocity (and, thus, the energy) of the electrons continually increase. However, when the energy of an electron reaches 19.8 electron volts or more, on collision with a helium atom, energy is adsorbed, one of its electrons changes orbit and a metastable helium atom is formed. These metastable helium atoms, can have energies of 19.8 or 20.6 electron volts and, thus, will ionize the molecules of virtually all substances producing ion and electron pairs. These pairs can be collected by suitably placed electrodes under an appropriate voltage gradient and used as a sensing process for GC detection. The helium detector, however, is difficult to operate as the helium must be extremely pure, any trace impurities quench the production of metastable helium atoms and so the detector does not function. The helium detector, functioning in this manner has been shown to have a high ionization efficiency and is probably the most sensitive detector available. Nevertheless, the main use of helium in gas chromatography is as a carrier gas.