This selective detection method involves a combination of gas phase and  surface ionization processes. A ceramic ion source of moderate Work Function is operated in a gas environment  consisting of a dilute mix of H2 in Air. NP selectivity “turns on” when the ion source is heated sufficiently (600 – 800oC) to ignite and maintain the gases in a chemically reactive boundary layer about the hot ion source. Samples decompose  in the gaseous boundary layer, and electronegative N and P decomposition products are ionized by extracting electrons  from the ion source surface. With this detection, best selectivity versus Hydrocarbons is achieved by limiting ion source  to collector polarization to a low value to allow gas phase positive and negative Hydrocarbon ions to recombine before  reaching the collector. 

This classical mode of GC detection uses a self-sustained H2 – Air flame ignited at a jet orifice  to decompose samples and form positive and negative ions in gas phase reactions. A polarizer voltage and collector  electrode located near the flame jet measure ions formed by combustion of virtually any organic compound, so this is  considered to be a Universal GC Detector. DET has achieved FID detection on some GC models using retrofit NPD  equipment that includes a ceramic tipped jet and a Bare Wire Ignitor/Polarizer probe positioned just above the jet. 

This detection mode features a ceramic ion source having a high Work Function and a collector electrode  positioned several centimeters downstream of a self-sustained flame. The ceramic element functions only as a flame  ignitor and polarizer to collect long lived ion species in the flowing effluent of the flame. Whereas Hydrocarbon ions recombine rapidly with increasing distance from the flame, compounds containing P, Pb, Sn, or Si atoms form ion species  that survive to be collected downstream Use of a H2/CH4/Air flame further improves selectivity versus Hydrocarbons.

Same equipment as Remote FID except uses ion sources of low or moderate  work functions to re-ionize neutral electronegative Halogen or Nitrogen decomposition products in the downstream  effluent of a H2/CH4/Air flame. An added ion suppress electrode at the flame jet can further avoid Remote FID ions. 

A mildly heated (300 – 400oC) low Work Function ion source is operated in  a gas environment containing O2. Sample compounds containing a high concentration of Methylene (CH2) groups  provide the fuel for a momentary burst of flame ionization as each compound impacts the ion source surface. With a low  O2 concentration (Air) in the detector gas, Linear Chain Alkanes and Saturated FAMEs provide the dominant selective  responses. At higher O2 concentration, relative responses to Branched Alkanes, Alkenes, and Mono-Unsaturate FAMEs  are increased. Aromatic and Cyclo-Hydrocarbons have negligible responses.

An uncoated Bare Wire Ignitor/Polarizer Probe and concentric Collector  electrode are operated in a H2 – Air gas environment where high Air flow prevents a self-sustained flame from forming  upstream where H2 and Air first intermix. Instead, flame-like chemistry is ignited and maintained in the immediate  boundary layer of the heated Bare Wire. Like an FID, this mode provides universal detection for all organics, but with  lower signal magnitudes and some enhancement of Aromatic signals versus other Hydrocarbons. 

In a non-GC implementation of thermionic detection, a Thermionic  Ceramic Ionization Transducer is preceded by a heated reactor chamber. The transducer detects selective vapors  thermally evolved from liquid or solid samples placed in the reactor. When the gas flowing through the reactor is Air or  Oxygen, detected vapors include volatilized sample constituents as well as oxidation products of those constituents.