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THERMIONIC SURFACE
IONIZATION - Samples
form gas
phase negative
ions by
extraction of
electrons from a hot,
catalytically active
solid surface.
Key parameters
are the
surface composition,
surface temperature,
gas composition
around the
surface, and
polarization of
the surface
relative to
a surrounding
ion collector.
Multiple detection
modes are
obtained through
systematic changes
in these
four parameters.
Some modes
combine reactive
gas phase
chemistry to
decompose incoming
samples, and
then ionize
the decomposition
products by interaction with the surface. In other modes,
intact sample molecules are ionized by direct impact with the surface with
no intervening reactive gas phase chemistry. Some modes are non-destructive
so that sample aromas can be sensed at the detector exit, and series
combinations with other detectors are possible. 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. Polarizer and collector electrodes located more
remotely downstream of the flame selectively measure only long-lived ion
species.
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
constituents.
CATALYTIC COMBUSTION IONIZATION - A hot
catalytic ceramic surface operated in a detector environment containing
Oxygen momentarily ignites a burst of ionization when an individual
combustible compound containing a high concentration of Methylene groups
elutes through the detector. The detection method provides selectivity of
Alkanes vs. Alkenes, as well as saturated vs. unsaturated FAMEs, and the
onset of combustion ionization is associated with the thermionic emission
character of the ceramic surface.
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