Sue,
I can't think of anything offhand given this level of information other than
data analysis. It is possible for the chromatography to be "pretty good" but
the integration parameters for the early compounds may not be usable for the
later compounds, and this effect will be on the level of per mil in my
experience. As long as it is consistent between samples and standards there
is no analytical problem per se, but to track the effect in more detail you
will have to manually integrate the isotope peaks.
John

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Susan Ziegler
Sent: Friday, April 28, 2006 11:29 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] GC-C-IRMS issues

Hello All:

I am seeking any possible hints to solve a problem that has developed  
in our GC-IRMS system.

At the moment the suite of standards we run for d13C of FAMEs in  
order to check for consistency no longer exhibits a constant  
correction factor across all the compounds in each standard  
chromatogram which range from 14:0 through 23:0 in size. In fact we  
are seeing greater depletion in 13C for the longer (or later RT)  
FAMEs by as much as a few per mil relative to the first compounds  
(14:0).  I have been trying to isolate the variables and this is what  
I've found so far:

1. Flow is constant throughout the entire GC run (2 mL/min).
2. CO2 injected throughout the run remains constant in it isotopic  
composition so does not exhibit the depletion see in compounds that  
must be combusted.
3. I figured it must be the combustion column so it has been replaced  
and we are again seeing the same problem.
4. Peak shape looks pretty good with widths remaining within 15-20s  
and fairly sharp ratio traces.  The ratio traces do become a little  
broader with the last few more deplete compounds but not particularly  
worse than runs where we've gotten good consistent values.

We are using an Agilent (HP) 6890 GC with a split/splitless injector  
in splitless mode.  I have changed the septa (done frequently  
anyway), liner, and the gold seal and washer.  The GC column is very  
new but also has been cut (`0.5m) to see if reactive sites may have  
been a problem.  We are using the metal Valco cross piece and have  
been now for years without a problem, could this have developed  
active sites that may yield such symptoms?

Thanks for any information anyone might have regarding this  
problem...I've been at this for some time now and am in danger of  
pulling out all my hair!

Best regards,
Sue