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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.
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