In <[log in to unmask]>, on 11/25/02
at 10:37 PM, Andrew Calvert <[log in to unmask]> said:
>The baselines and noise on the electron multiplier on our MAP 216 are
>quite high because several samples with large 37Ar signals were run
>before they had decayed sufficiently. When this has happened in the
>past, I have bitten the bullet and patiently waited for the dark current
>to decay away. As I understand it, the dark current is caused by the
>electron capture decay of 37Ar atoms buried in the multiplier. As
>expected, the dark current decays with a roughly 35 day half-life.
>This time around I am considering a light bake on the collector end of
>the MS. Hopefully that will liberate some of the 37Ar and speed up the
>process. However, following 300øC bakeouts after filament replacements,
>it typically takes a week or more to get backgrounds down to a
>reasonable level. Has anyone tried wrapping only the multiplier end and
>heating it up to 100-150øC for a day or two while pumping on it?
I don't know if it will solve your problem, but I don't think it will do
any harm, so I'd say go for it. I have done this sort of partial bake
several times in the past (sometimes trying to track down excrutiatingly
small leaks), and the effects are small. Most of your blank will be in the
source area, so the only down side would be if you cause a bit of junk to
migrate from the collector end to the source end. That will probably
settle down pretty quickly.
--
Chris M. Hall, Assistant Research Scientist
Dept. of Geological Sciences, University of Michigan
Quando omni flunkus moritati
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