Gas Chromatography Detectors - Ionization Detectors > The Helium Detector > Page 64

The thermal argon detector is not commercially available. However, its high sensitivity, freedom from radioactivity and electron producing ancillaries make it a very simple detector to fabricate and operate. The fact that the sensor can be constructed from very inert material and thus accommodate very corrosive gases could also be an advantage for certain applications.


The Helium Detector

The helium detector works on exactly the same principle as the argon detector, but metastable helium atoms are produced by the accelerated electrons instead of metastable argon atoms. Metastable helium atoms, however, have an energy of 19.8 and 20.6 electron volts and thus can ionize, and consequently detect, the permanent gases and, in fact, the molecules of all other volatile substances. As a consequence, contaminants in the helium can be extremely deleterious and the helium must be extremely pure or the production of the metastable helium atom production will be quenched by traces of any other permanent gases that may be present. When first developed a very complicated helium purifying chain was necessary to ensure its optimum operation. However, with high purity helium becoming generally available, the detector can now be used to detect concentrations of organic vapors at 10-13 g/ml or less.

As an alternative to a radioactive source, electrons can be generated by electric discharge or photometrically and these can be accelerated in an inert gas atmosphere under an appropriate electrical potential to produce metastable atoms. This procedure is employed in the helium detector that is manufactured by the GOW-MAC Instrument Company. It is claimed that the detector does not depend solely on metastable helium atoms for ionization and for this reason is called the Helium Discharge Ionization Detector (HDID). A diagram of the GOW-MAC sensor is shown in figure 34.