INFICON Explorer Portable Gas Chromatograph - Manual (Page 333)

Appendices
331
In Explorer, these GC design requirements have been met by fabricating an array
of 21 inert, low-volume gas switching valves, with interconnections, in an
internally-heated monolithic assembly. The other GC components (injection port,
precolumn, analytical columns, and detector) are mounted and thermally linked to
this valve array, to create a compact analytical engine.
Electron Capture Detector
The electron capture detector (ECD) is sensitive to chemical compounds called
electrophilic compounds, which attract free electrons. The ECD is especially
sensitive to hydrocarbons which contain atoms of chlorine or bromine.
In the ECD cell there is an electron source which generates electrons at a constant
rate, and a collector electrode for collecting electrons. When an electrophilic
compound passes through the ECD, it captures some of the generated electrons, and
causes the rate of collection at the collector electrode to fall. This decrease in
electron current is the detector response.
The ECD cell contains a cylinder of nickel foil through which the carrier gas flows.
Coated on the foil’s inner surface is a quantity of the radioactive isotope nickel-63
which emits electrons at the rate of about 500 million per second.
Electrons emitted from the nickel-63 collide with and ionize the carrier gas, creating
more electrons. The resultant ions and electrons travel to the collector electrode
under the influence of a pulsed voltage applied between the nickel-63 electron
source and the collector electrode. This flow of ions and electrons is the cell
background current. The pulse frequency is varied to maintain a constant average
background current. An electrophilic compound in the detector decreases the cell
current as the compound captures electrons and forms ions, which travel more
slowly than electrons. The pulse frequency of the polarizing supply is automatically
raised to maintain a constant current in the cell. This frequency is the detector
output signal and is fed to
Explorer’s microprocessor.
You must use nitrogen carrier gas as the oxygen in air will absorb electrons and
reduce the sensitivity of the ECD.