Hi Stan,

Thank you for your suggestion concerning my background CO2 problem. In this
case though, there is no CO2 to freeze out: I don't use CO2 in the bellows
as reference (bellows are empty and pumped out). CO2 reference comes from
the GC interface, and is shut off at the tank. CO2 signal is far higher
than N2, so not likely to be a leak from air.

You mention positioning of the small magnet: do you mean the two little
permanent magnets mounted on the source? Their position seems fixed, so can
you elaborate on how to adjust them?

Paul

At 09:10 AM 07/04/2005, you wrote:
>Hi Paul,
>
>I conclude from you description that you may have a leak in your change
>over valve.
>I mean that its closing is incomplete. To test this, you should freeze your
>CO2 working standard. The CO2 peaks should disappear thereafter. If so,
>then you have to clean you valve from every dust particle.
>Do not mind about sensitivity. You can recover it, first, by successive
>adjustment of potential of all the electrodes in the ion source,
>second,  by better positioning of small magnet for electron beam focusing,
>and third, by increase of electron beam.
>All the best,
>Stan
>
>
>At 08:35 05-04-07 -0700, you wrote:
>>Hmm. No responses.... Does no one have wisdom they can share on 252 leakage
>>currents?
>>
>>Paul
>>
>>
>>At 02:19 PM 05/04/2005, you wrote:
>>>Finnigan users:
>>>
>>>Our 252 source has been giving me a lot of grief in the last week: I have a
>>>leakage current in addition to some other difficulties.
>>>
>>>Two weeks ago the filament was damaged: still functioning but not centered.
>>>I replaced it, but opted not to do a full cleaning. It was fine for a few
>>>days, but sensitivity started to drop and the leakage current light came on
>>>(not immediately).
>>>
>>>I took the source out a second time, and gave it a full cleaning. No
>>>change: poor sensitivity and still a leakage current.
>>>
>>>I took it out a third time, and swapped out a number of ceramic pieces: all
>>>four ceramic posts, and all of the ceramic spacers (some had
>>>discoloration). I also paid particular attention to the leads to the
>>>feedthrough: none were touching.
>>>
>>>So, what other causes might there be for a leakage current?  Am I missing
>>>anything?
>>>
>>>In addition to the poor sensitivity, I'm also seeing something completely
>>>new to me: a CO2 signal when the change over valve is closed (~10
>>>millivolts on mass 44, 45, and 46).
>>>
>>>- the bellows are empty and pumped out.
>>>- vacuum is good (1x10^-8mbar).
>>>- seems to be slowly decreasing over time (days).
>>>- baking the source has no effect.
>>>- background is zero just off peak.
>>>- mass scan over the entire range shows the background is only at mass 44,
>>>45, and 46. Normal nitrogen traces.
>>>
>>>Any ideas on either leakage current or the background problem? Might they
>>>be related?
>>>
>>>Paul Eby
>>>
>>>
>>>[log in to unmask]
>>>Biogeochemistry Lab (E-Hut, room 102)
>>>School of Earth and Ocean Science
>>>University of Victoria
>>>
>>>PO Box 3055
>>>Victoria, B.C.
>>>Canada
>>>V8W 3P6
>>>
>>>phone: 250-721-6183
>>>fax:   250-472-4620
>>>
>>>
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>>
>>Dr. Stanislaw Halas
>>Mass Spectrometry Laboratory
>>Institute of Physics, UMCS
>>20-031 Lublin, Poland
>>phone: +48 81 537-62-75
>>fax: +48 81 533-36-69
>
>
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