Waters GCT Premier Operator's Manual page 361

In CI+ mode, reagent gas ions dominate the spectrum. If the entire ion signal
from these intense peaks was allowed to reach the detector, then the detector
would rapidly degrade, causing gain loss.
In the GCT Premier, the transmission of these ions into the pushout region of
the TOF is reduced by restriction slits in the ion source. The primary ion beam
exiting the inner source is partially mass-dispersed by a magnetic field. The
field deflects low mass ions more than high mass ions. Therefore, the beam
does not travel along the centerline of the instrument.
These low mass ions are incident on the restriction slits and so do not enter
the analyzer. This introduces some mass discrimination into the spectra,
which you can use to reduce the intensity of the low mass ions. In practice the
low mass ions are so large that a proportion of the ion beam still enters the
analyzer; the intensity of these ions must be monitored and minimized during
tuning.
You must consider two effects when tuning in these modes.
• If the ion beam is badly focused in the source, then the beam will appear
more homogenous with respect to low-mass and high-mass ions. This is
equivalent to a poor mass resolution in the source region. In this case, it
will be difficult to significantly reduce the intensity of the low mass ions.
• The beam steering allows the primary ion beam to move across the face
of the restriction slit, which adjusts the amount of mass discrimination.
When tuning in these modes, the ion signal from a reference compound
in the middle of the mass range should be monitored (approximately
m/z 300), as well as the ion signal from the most intense low mass
background ion.
In MassLynx, you can open two peak displays to monitor these two separate
mass regions simultaneously. At the point of best focusing, small changes in
lens voltage should have much smaller effect on the higher mass ion and a
much greater effect on the low mass ion.
General considerations for tuning G-7