I'm not sure I understand exactly what you are describing. I get the idea
you are running a zero enrichment and you seem to be getting a depletion in
Oxygen but not in carbon. Is that right?  Have you run the zero with the
inlet open all around? Have you run a zero with the bellows closed as well?
Have you run zeros with different inlet valves open and closed as well as
either sample or ref bellows? Are the traces noisy, flat, or slanted or
curved? As a first hand guess it sounds like water. Could be in the cold
finger, the bellows or more likely the caps. All could be isolated by
running zeros with the appropriate valves opened or closed. Of course maybe
there is water in everything.  This is easily taken care of. You can bake
out the inlet, and flame the cold finger and flame the caps. Then rerun the
zero enrichment.

If what you describe is a drift over the course of a zero there are several
possibilities before you jump to other problems.
1) you might want to check that the caps are set-up correctly and to maybe
work at a lower inlet pressure.
2) The amp zero may be incorrect. Rezero it.
3) There may be changing background at the major or the minor mass...but i
think you said you already baked the system...of course maybe it needs more
time and another bake.
4) The voltage supplies are drifting..unlikely but a possibility.
5) The suppressor voltage is faulty...again maybe, maybe not.
6) There is electrical leakage on the collector amp inputs, you might want
to try cleaning the ceramics with alcohol.

Then again you said there was no leak in the inlet, but what about other
leaks- like on the bleed pipe, or in the changeover?

Anyways, best of luck,
Cap

Douglas "Cap" Introne
Stable Isotope Laboratory
Sawyer Environmental Research Center
University of Maine
Orono, ME 04469

Tel. (207) 581-2192
Fax (207) 581-3490
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URL: http://iceage.umeqs.maine.edu/geology/sil/home.htm