The Helium Discharge Detector (HID) is used exclusively for the ultra trace analysis of permanent gases. The detector consists of a small cell containing a beta-emitter radioactive source, and two electrodes, similar to the ECD, except that the electrodes are more closely spaced. Ultra-pure helium is supplied to the cell as carrier gas from the column. The two electrodes are supplied with a high potential difference so that the electrons produced by the emitter are sufficiently accelerated to excite the helium to a higher energy level.

Pulsed discharge helium ionization detector (PDHID)

These excited helium atoms now possess sufficient energy to produce secondary ionization of permanent gases eluting into the cell from the column. The increase in current is subsequently amplified and measured.

Permanent gasses with HID (2 ppm)

The main disadvantage of the HID is the need for ultra-pure helium, and the necessary purification usually has to be carried out in-situ. Also, the detector housing must be purged, preferably using the carrier gas, to avoid the introduction of air.