IONIZATION - Samples
electrons from a hot,
products by interaction with the surface ( i.e., NPD,
FTID). In other modes,
intact sample molecules are ionized by direct impact with the surface with
no intervening reactive gas phase chemistry (i.e., TID). Some modes are non-destructive
so that sample aromas can be sensed at the detector exit, and series
combinations with other detectors are possible (i.e., Tandem TID). Several modes use Air as the
main detector gas, so that stand-alone applications involving selective
detection of samples in ambient Air streams is possible.
FLAME IONIZATION - Samples decompose and form ions
in gas phase reactions with radical species such as H, O, and OH that are
present in self-sustained flames. A polarizer voltage and ion collector
located near the flame effectively measures ions formed by combustion of
most organic compounds (i.e., FID). Polarizer and collector electrodes located more
remotely downstream of the flame selectively measure only long-lived ion
species (i.e., Remote FID).
REACTOR THERMIONIC IONIZATION ANALYSIS
(RTIA) - In a non-GC implementation of thermionic detection, a
thermionic 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 and transducer is Air or Oxygen, detected vapors include volatilized
sample constituents as well as products of oxidation of the sample
CATALYTIC COMBUSTION IONIZATION (CCID) - A hot
catalytic ceramic surface operated in a detector environment containing
Oxygen momentarily ignites a burst of flame ionization when an individual
combustible compound containing a high concentration of Methylene groups
elutes through the detector. Aromatics and Cyclo-Hydrocarbons are NOT
ignited in this process. Variations in the Oxygen concentration provides
additional selectivity of Alkanes vs. Alkenes, Linear vs. Branched
compounds, saturated vs. unsaturated FAMEs, as well as mono-unsaturates vs.
onset of combustion ionization is associated with the thermionic emission
character of the ceramic surface.
CERAMIC MATERIAL CONSIDERATIONS -
DET ion sources are comprised of a wire core covered by layers of ceramic
coatings. Additives in ceramic sub-layers are selected to improve the
hardness and semiconductor electrical conductivity of the ceramic. Additives
in the surface coating are selected to impart a desired catalytic ionizing
activity to the ion source surface.