All,

A few of us are still having trouble with what is thought to be CO2 tailing
resulting in severely reduced 29/28, caused by CO at the SOURCE.  My carbon
data are not affected.  Our 29/28 has ranged between 0.03 and 0.5 for all
REF/sample peak pairs, except the first sample, which has a normal ratio
(0.7).  REF pulses of N2 sent through the Conflo have good precision and a
normal ratio if they do not immediately follow another sample through the
EA.

We've changed just about everything you can change including the GC column
itself and still get depleted 29/28 when running for N & C simultaneously.
The type of samples (flour & plant material) and C/N (~7) are the same as
we've used for years.  The EA C/N looks fine using its TCD and Eager
software alone.

Did anyone ever conclusively identify what is causing the problem?
Basically, the IRMS seems OK alone.  The EA seems OK as far as doing its
combustion job.  At some unknown point, (when the EA's get a little older?)
something changes and no one knows what it is.  We had a taste of it
initially back in March with 29/28 of 0.5 for one batch of samples, then it
vanished.  Now we can't figure out what the actual problem is enough to
eliminate it and get things back to the way they were before.  This mystery
has us at a standstill.  Any ideas would be appreciated.

Bob

-------------------------------------------------
Robert D. Stickrod II
Idaho Stable Isotopes Laboratory
Department of Forest Resources
College of Natural Resources, 214
P.O. Box 441133, University of Idaho
Moscow, ID 83844-1133, U.S.A.
[log in to unmask]
lab: (208) 885-6512
fax: (208) 885-6226
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