Gas Chromatography Detectors - Ionization Detectors > Page 53


Ionization Detectors

The first ionization detector to be developed was the cross-section detector of Boer (9) which, although competitive at the time, has limited sensitivity (e.g. 10-6 g/ml, about that of the katharometer or the flame thermocouple detector). However, in the 1950s and early 1960s Lovelock introduced the argon ionization detector (20-22), an ionization detector that functioned on an entirely different principle. The argon type detectors utilize noble gases to produce metastable argon atoms which have sufficient energy to ionize most organic compounds.


Noble gases, have their outer octet of electrons complete and, thus, collisions between argon atoms and electrons are perfectly elastic. Consequently, if a high potential is set up between two electrodes in argon, and ionization is initiated (for example by a suitable radioactive source) electrons will be accelerated towards the anode and will not be impeded by energy absorbed from collisions with argon atoms. If the potential of the anode is high enough, the electrons will develop sufficient kinetic energy that on collision with an argon atom, energy can be absorbed, and a metastable atom can be produced.


A metastable atom carries no charge but adsorbs its energy from collisionwithahighenergy electron by the displacement of an electron into an outer orbit. As a result, the metastable atom, on collision with a molecule of an organic compound, can release energy of about 11.6 electron volts as the electron returns to its original orbit. Energy of 11.6 electron volts is sufficient to ionize most organic molecules. Hence, in summary, collision between a metastable argon atom and an organic molecule will result in the outer electron of the metastable atom collapsing back to its original orbit, followed by the expulsion of an electron from the organic molecule. The electrons produced by this process are collected at the anode, generating a large increase in anode current.