G-6 Theory and Principles of Operation
See also: Figure titled “GC-FI column position:” on page 9-20.
The position of the emitter in relation to the center line of the extraction rods
is also critical. Small changes to the position can make a large changes in the
beam steering voltage being required. Normally the beam steering is set close
to zero, and the emitter probe micrometer is used to optimize the signal. Small
adjustments to the beam steering can then be made using the beam steering
lens.
When the extraction voltage is higher, the field strength is higher, which
results in improved sensitivity. Adjusting the extraction voltage causes a
change in the source focusing, and the source lenses will need to be
reoptimized.
See also: Tuning in FI mode on page 9-20.
Field desorption
Field desorption (FD) is a technique used primarily for the analysis of
non-polar or thermally labile compounds not amenable to other ionization
modes. The ionization process is soft producing mainly molecular ions, or
fragment ions.
During FD ionization the sample is loaded directly onto the emitter (a 10-µm
tungsten wire covered with carbon microneedles). An electric current is
passed through the emitter wire to raise the temperature. The sample is
ionized on the surface of the emitter. Alternatively, if the sample volatilizes
before ionization, field ionization occurs.
See also: Tuning in FD mode on page 9-27.
General considerations for tuning
In GC, EI and positive CI (CI+) modes of operation intense low mass
background ions continuously appear. In EI mode, these ions are
predominantly mass spectral peaks from the following samples:
• Helium (which is used as the GC carrier gas).
•Background nitrogen.
• Water (from residual air).
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