A DID is an ion detector which uses a high-voltage electric discharge to produce ions.  The  detector  uses  an  electrical  discharge  in  helium  to
generate high energy UV photons and metastable helium which  ionizes  all  compounds  except  helium. The ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.