3-22 Tuning the Instrument
As beam steering is increased from -3.4 V, the air peak at m/z 28 drops
in intensity. However, the sample peak reaches a maximum at around
-0.5 V, and then drops off again as the voltage is increased to +1.0 V. In
this example, a steering voltage of about -0.5 V is optimum.
5. Optimize the focus 2 lens to maximize the intensity of the peak at
m/z 285.
6. Repeat this for focus 1 and focus 3 lenses.
7. In the EI Tune tab, set the Ion Repeller to optimize the size of peak at
m/z 285.
Tips:
• Tuning the focus 1, focus 2, focus 3, and ion repeller can affect the
beam steering value.
• If the ion repeller voltage reaches 5 V, then the ion repeller might
require cleaning.
See also: EI/CI outer source maintenance on page 10-35.
8. Continue to tune the instrument by repeating steps 2 through 5 until
you maximize the intensity of the signal at m/z 285.
Note: Typically the air signal at m/z 28 should be less than 2000 counts
per second for a system with a GC attached and less than 400 counts per
second for a system with an insertion lock or a blanking flange fitted.
When you maximize the sensitivity, enable low mass cut-off, which cuts
off all masses below a specific mass by preventing them from entering
the analyzer.
See also: Low Mass Cut-Off on page B-16.
It is useful to be able to see the air peak (for example, in the previous
tuning step) and particularly useful to check for air leakage. When you
are satisfied that there are no leaks, you can use the low mass cut off to
remove the air peaks (or CI gas reagent ions at low mass in CI+ mode).
Doing so prolongs the life of the MCP detector as it is not exposed to a
constant high ion current. You can repeat step 3 through 6 using the low
mass cut-off to further improve the sensitivity for an analysis.
To Next Page
Loading...
