This sounds similar to the problems I was having recently. I finally turned to the Isogeochem community for help and, after cleaning the drop tube (between autosampler and combustion tube), as suggested by Jason Curtis, my problems disappeared. I have also had lingering 15N enrichment issues due to dirty tubing between the combustion and reduction columns as well.
Stable Isotope Lab
Agriculture and Agrifood Canada
Lethbridge Research and Development Centre
5403 1st Av. S.
I have been having a similar issue with my NA 2500 connected to a Delta Plus.
I was running 15N enriched samples at about 100 permil with no trouble, then a bunch came through that were in the 1000s permil.
During that run, towards the end of it, a tiny N peak appeared in the blanks about 100mV. I though, oh its just an enrichment issue. So I have changed columns, ash trap and chemicals, cleaned the AS, changed He tank, and baked the GC at 100C for 24 hours. I have run a bunch of samples through and just let He flow for days on end.
I still have this 100mV N peak thats ~ 2-4 permil. Can this be enrichment issue if its only 2 permil?!?! I am as confident as I can be there is no leak, my water and Ar backgrounds are low.
I really hate to take everything apart to bake it again if its not an enrichment issue.
On Friday, June 22, 2018, 12:20:58 AM EDT, David Baker <[log in to unmask]> wrote:
In addition to the good advice already given, I would add that getting some samples/standards running through the system seems to help lower the 15N contamination much more efficiently than blowing He through the system and waiting.
Also, it’s easy to get distracted chasing the ghosts of enrichment past and forget about a thorough check for leaks. We recently went through a similar experience and while we were obsessed about 15N demonic intrusions we forgot some fundamentals. Leak fixed and system was stable again.
In my experience, enrichments of a few hundred ‰ don’t have much carry-over. From 1,000 - 3,000‰ you need to plan for ash tube cleaning at least, beyond 5,000‰ you will likely need to replace all reagents/tubes to remove memory. I’m curious if others have seen similar patterns?
Full confession, I’ve analyzed samples nearing high 90’s atom % 15N via EA and the IRMS didn’t explode, though I have been told by engineers this can wear on the mass 30 cup and electronics amplifying that signal.
Hope you will let us know how it turns out.