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 "This e-mail is intended for the recipient only. If you are not the intended recipient you must not use, disclose, distribute, copy, print, or rely upon this e-mail. If an addressing or transmission error has misdirected this e-mail, please notify the author by replying to this e-mail." "Recipients should note that all e-mail traffic on MOD systems is subject to monitoring and auditing."