Gas Chromatography Detectors - Ionization Detectors > The Electron Capture Detector > Page 69
The photons and metastable helium atoms enter the reaction zone where they meet the eluent from the capillary column. The solute molecules are ionized and the electrons produced are collected at the lower electrode and measured by an appropriate high impedance amplifier. The distance between the collecting electrodes is about 1.5 mm. It is reported that the helium must be 99.9995 pure. The base current ranges from 1 x 10-9 to 5 x 10-9 amp, the noise level is about 1.2 x 10-13 amp and the ionization efficiency is about 0.07%. It is claimed to be about 10 times more sensitive than the flame ionization detector and to have a linear dynamic range of 105. An example of the use of a pulsed helium discharge detector for monitoring the separation of some aromatics on a capillary column is shown in figure 37. The pulsed helium discharge detector appears to be an attractive alternative to the flame ionization detector and would eliminate the need for three different gas supplies. It does, however, require equipment to provide specially purified helium, which diminishes the advantage of using a single gas.
The Electron Capture Detector
Lovelock's work on ionization detectors culminated in the invention of the electron capture detector (25). However, the electron capture detector operates on an entirely different principle from that of the argon detector. A low energy b-ray source is used in the sensor to produce electrons and ions. The first source to be used was tritium absorbed into a silver foil but, due to its relative instability at high temperatures, this was quickly replaced by the far more thermally stable 63Ni source.