INFICON Explorer Portable Gas Chromatograph - Manual (Page 331)

Appendices
329
Dissipation of the HF energy in the gas inside the lamp causes the gas to glow with a
bright blue-violet light. UV light with a wavelength of 120 nanometers (nm) is also
produced. UV light of this wavelength carries 10.6 electron volts (eV) of energy.
The UV light is emitted from the lamp and is directed at the stream of carrier gas
eluting from the column. When light of this energy hits the eluting molecules, they
may become ionized; that is, electrons may be knocked off the molecules.
Many of the chemicals considered pollutants, including most hydrocarbons, are
ionized. The permanent air gases (argon, carbon dioxide, nitrogen, oxygen, water
vapor etc.) require a relatively high energy for ionization, and are not ionized by the
10.6 eV UV lamp.
Whether or not a certain molecule is ionized depends upon its Ionization Potential
(IP). If the IP of a molecule is less than the energy of the light, 10.6 eV, it will most
likely be ionized. If the IP is greater than 10.6 eV, it is not readily ionized. Most of
the permanent air gases including nitrogen and water vapor have IPs over 12 eV.
This means that the carrier gas and the sample matrix are not ionized.
A continuous electric field is maintained between a repeller electrode and a collector
electrode in the PID cell. Once they have been ionized by the lamp, the ionized
molecules in the PID cell move in the electric field, generating a current which is
proportional to the concentration of the ionized molecules in the detector cell. An
electrometer circuit converts the current to a voltage, which is then further
converted to a digital signal and fed to
Explorer's microprocessor.