Tuning the instrument for negative CI 3-31
During electron ionization (EI), negatively charged ions form in addition to
positively charged ions. Normally, the negative ions remain undetected
because the ion source and focusing potentials allow only the extraction of
positive ions.
There are limitations to the analytical usefulness of negative ions formed by
electron ionization. For instance, many organic compounds do not form
molecular or quasi-molecular anions under EI conditions but instead provide
spectra dominated by structurally insignificant low mass ions (such as CN-,
F-, and Cl-). Furthermore, the sensitivity for producing negative ions under EI
conditions is several orders of magnitude lower than it is for positive ion
production.
In contrast to the conditions of conventional electron ionization, low energy
electrons are readily captured by many organic compounds without inducing
excessive fragmentation. Consequently, the negative CI ion source has been
optimized for generating a large population of electrons with near-thermal
(low) energy. Introduction of a reagent gas acts as a moderator for the initially
energetic electrons. Anionic reagent gas ions can also be generated for ion
formation by ion-molecule reactions.
Tuning for negative CI mode operation
To tune for operation in negative CI mode:
1. Install the CI inner source.
See also: Changing the EI/CI inner source assembly on page 2-2.
2. In the Tune window, click Ion Mode > CI-.
To configure for tuning in negative CI- mode:
1. In the Tune window, click the Engineer tab.
Note: Record the Detector voltage and the TOF Flight Tube voltage.
2. Set the TOF Flight Tube value to 0.
3. In the CI Tune tab, set the Detector Voltage to 0.
Caution: Take care when changing to negative ion mode after
using the GCT in positive ion mode for an extended period of
time. Perform the following precautionary procedure before
operating the instrument in CI- mode.
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