Thanks to all for the helpful responses.

With regards to CO interference....We still get very similar chromatograms,
with little N tailing and about 40s between the end of the N and the
beginning of the C peak.  Is there a simple way to test for CO coming
through the 28 collector?  Would running carbon alone (i.e., cellulose with
no N) be a way to check?  We've done that at mass 44 up to 8 volts, and
there's still no detectable 28 signal.  Does that eliminate that possibility?