Hi Claire,

If you have run anything with S in it, it will start to coat the mol
sieve in your GC column.  In this instance, baking the column will make
things worse.  If you have mol sieve, it is easy and inexpensive to
repack your stock column. While you are at it, you may want to replace
the short stainless steel transfer line to the GC.

Another thing, I always set my purge pressure a little higher than the
carrier, so that when I open the isolation valve, flow always goes in
the right direction.


Please keep us posted,

Ben

 
Benjamin Harlow
Manager, Stable Isotope Core Laboratory
Washington State University
School of Biological Sciences
G-81 Eastlick
Pullman, WA  99164-4236
 
Office:  509-335-6161
Lab:     509-335-6154
Fax:     509-335-3184
 
Laboratory for Biotechnology and Bioanalysis (LBB2) Stable Isotope Core
http://www.isotopes.wsu.edu
 
 

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Lock Claire
Sent: Tuesday, November 25, 2008 3:39 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] TC/EA retention time problems

 
Hello everyone,

I have a persistent but random problem with my TC/EA, but the problem is
sporadic and I haven't managed to track down the cause.

When measuring CO we observe changing retention times.  The variation
occurs between two things, firstly our "normal" run time, the N2 peak
elutes around 200 seconds and the CO peak elutes around 350 seconds,
secondly the "delayed" run or N2 elutes around 350 seconds and CO around
850 seconds.  Whichever retention time is observed is consistent for the
entire sample run.

We have observed this change with the same reactor tube, same gas
pressures, temperatures etc. The difference has occurred between loading
different samples i.e. the only thing that has been done is closing the
isolation valve re-loading the auto-sampler, purging it and re-opening
the isolation valve.

I have baked the GC, but this has not helped.
I have changed the reactor several times but the problem seems random
I have varied how I purge the samples and open the isolation valve with
purge left on and switched off, this hasn't affected it.
I have tried resetting the TC/EA
I have checked the ends of each line and they all seem fine.  I've
changed several ferrules which hasn't helped.

The argon background is around 5-8 mV. Stability and linearity of the CO
ref gas are good.  Mass 28, 29 and 30 backgrounds are stable.  

The heaters seem to be working ok, so I guess there must be something
wrong with the carrier flow, maybe a blockage which occassionally moves?
But I have been unable to locate it.

I am just about to change the reactor again and examine the one that's
in there to see if anything has become displaced or blocked, but if
anyone has any experience of similar problems, I'd be very grateful for
any insights.

Many thanks in advance for your help,

Claire  
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