I understand that the peaks within one run are consistent, but you find offsets of 50 permil between runs. If so, I would check if the reference gas (or whatever you want to call it) is biased (sometimes). Did you try to have several on-offs within one run (where no peaks elute) to check for ref gas stability over time?
Von: Stable Isotope Geochemistry <[log in to unmask]> Im Auftrag von mwolhowe
Gesendet: Donnerstag, 20. September 2018 22:25
An: [log in to unmask]
Betreff: [ISOGEOCHEM] Multi-hour-scale instability/offset in GC-TC-IRMS
Greetings all. I'm spinning our D/H GC-TC-IRMS back up after a month of disuse, and I've been running into a perplexing problem when checking standards.
I'm seeing massive offsets in raw delta values (the ones calculated vs the ref gas injections) from few-runs to few-runs. For example: If I run a sequence of standard injections (range of Schimmelmann alkanes) for 24 hours, about half the runs come out at around where they should be, and half the runs come out up to 50(!) permil too negative. If I plot offsets from known values versus retention time, each compound is split into two clusters of 'high' and 'low' delta values. The two run 'types' seem to alternate randomly every 1 to 5 runs (a run takes about 70 minutes).
Now, I've seen instances where things like this happened with a visible pattern (regular, quasi-sinusoidal offsets in a GC-C-IRMS over the course of the day that appeared to track with the room's air conditioning, for example), but nothing completely random and abrupt like this. Does anyone have any experience with anything like this, or thoughts for what to dig into?
Context: The chromatography is fine; the retention times are stable; peak height and area are stable; H3+ factor is nice and low; source has recently been tuned; air leaks as per argon are negligible; reactor is newly installed and conditioned; GC column just had a meter trimmed off the end of it and the inlet was re-assembled (new liner, new ferrules, etc); K factor for the GC is about where one would expect; ref gas values are stable and reproducible with a run; column bleed/background is where it's always been.
Setup: Thermo Trace GC Ultra -> Thermo GC-TC interface (furnace/reactor) -> Thermo GC-C III (open splits and backflush plumbing) -> Delta V Plus IRMS.
Thoughts anyone? Thanks!
Dr. Matthew Wolhowe
Research Scientist/Engineer III
Lab Manager, Oceanographic and Geochemical Stable Isotope Facility
School of Oceanography
College of the Environment
University of Washington