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conditions. The filament current is regulated using the total emission current
and not the trap current as in EI operation.
See also:
Tuning the instrument for positive CI on page
Negative chemical ionization
Both positive and negative ions are created in EI and CI sources. Ions of the
required polarity are extracted from the source by applying the appropriate
polarity potential to the source and focusing lenses.
EI operation is not suited to negative ion analysis because as many organic
compounds do not form molecular or quasi-molecular negatively-charged ions
(anions) under conventional EI conditions, but instead provide spectra
dominated by structurally insignificant low mass ions (such as CN
-
Cl
). Furthermore, the sensitivity for producing negative ions under EI
conditions is several orders of magnitude lower than for positive ion
production.
However, low energy electrons are readily captured by many organic
compounds and produce negative ions without inducing excessive
fragmentation. Negative CI operation facilitates this, as the CI ion source has
been optimized for generating a large population of electrons with
near-thermal (low) energy; the CI gas acts as a moderator for the initially
energetic electrons. Negatively-charged CI gas ions can also be generated by
ion-molecule reactions.
See also:
Tuning the instrument for negative CI on page
Field ionization
When using Field Ionization (FI) sample molecules pass in close proximity to
the tips of a mass of carbon needles (dendrite) on an emitter. A pair of
extraction rods are held at a high potential which produces a very high
electrical field around the tip of the carbon needles. These high fields cause
ionization by quantum tunneling of a valence electron
This process is a soft process often producing spectra with little or no
fragmentation.
During field ionization, the tuning of the ion source is strongly dependant on
the position of the emitter in relation to the extraction rods. The closer the
emitter is to the extraction rods, the higher the field strength at the emitter
and, therefore, the higher the sensitivity.
3-25.
-
-
, F
, and
3-30.
Ionization techniques
G-5
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