Neil:

I use a Micromass Optima, but I hope the following information will help:

The pressure in your mass spectrometer's source or analyzer should be on
the order of 1E-8 to 1E-7 mBar for dual-inlet analyses, and 1E-6 to 4E-6
mBar for continuous-flow analyses, as read by the ion gauge.  If you are
talking about the Pirani gauges that read the pressure in the analyzer and
the inlet, then they should indicate a minimum pressure reading of 1E-3
mBar, but they have been known to drift occasionally, especially when it
gets too warm "under the bench" due to inadequate air circulation.  I have
found that an increase to 4E-3 mBar does not always indicate a leak.  The
end of the Pirani gauges used in the Optima have an end cap that can be
opened.  Inside you will find a couple of adjustment screws, colored blue
in the Optima Pirani gauges.  If you rotate one or both very slightly, you
should be able to return the background reading to 1E-3 mBar.  Remember, a
leak in the vacuum system will show up in the analyzer's ion gauge.  If the
ion gauge pressure has not risen significantly during the same time period,
I'd try adjusting the Pirani gauge first.

Steve


To users of Micromass Prism III SIRMS and all interested parties:

Over the last few months I have noticed the following changes in the analyser
pressure of my Prism III.  From an acceptable pressure of 1E-3mbar the pressure
has gradually increased, first to alternate visually between 1 and 4E-3 and
more recently to permanently sit at 4E-3.  While not disastrous (I can still
run samples, albeit with reduced success) I would be greatful if the list could
point me in the right direction to resolve my problem.  What I have already
done is check my rotary backing pumps and they appear to be functioning
adequately.
 I have also baked the system.  I suspect a small leak somewhere in the vaccuum
tubing in the machine but I would welcome all other suggestions before the
machine
"goes under the knife".

Thanking you in advance

Dr. Neil Ogle
Environmental Engineering Research Centre
The Queen's University of Belfast
Department of Civil Engineering
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