Stable Isotope Geochemistry


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John Howa <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Wed, 14 Oct 2020 15:25:03 -0400
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Greetings, all.

To demonstrate that I've done my due diligence on the IsoGeoChem mailing list, I'm resurrecting this thread from a few months ago about Bill Holmes' GC IsoLink II tips. I have the tips and have read them thoroughly, but could use some additional guidance from the community using this setup, which seems quite a bit bigger than in my previous big project!

We have been using seed oxidation each injection (we are analyzing carbon isotope ratios of acetylated steroids in the realm of 50-100 ng per injection per compound). The default settings (according to the guide) are 2 min oxidation, 1 min purge, and 2 min backflush after seed oxidation. However, when using this method, there seems to be so much oxygen left in the system that during the run, the 46/44 trace rises pretty terribly. This is (from what I can tell) due to the massive amount of O2 in the source during the run, as when I've seen the 32 trace it maxes out the middle cup. Obviously this isn't ideal for the life of the source/filament as well as getting stable backgrounds, and I know too much oxygen will also burn out the reactor quickly. For reference, when seed oxidation is not used, the 46/44 background doesn't rise above S/N. 

My first thought is that I'm overloading the system during seed oxidation because the O2 regulator on the IsoLink set too high. The only way I know how to check this (without tearing things apart) is to measure the flow rate through the measuring point (stainless steel tube) on the front of the IsoLink. In the guide, it says to "Check O2 flow monthly to be sure O2 flow is correct. It should be 0.5-1.0 mL/minute above BF flow. This
must be measured at MP (measuring point)." However, when I measure the backflush (which is 2.4-2.5 mL/min, set according to recommendations) and then turn on the oxygen, the flow rate only increases a tiny, almost imperceptive amount (0.2-0.3 mL/min) and decreases right afterward to baseline, which suggests I in fact have the flow too low (this is all during the seed oxidation, prior to purge).

Any suggestions? I'd like to keep from burning out my filament and reactors too quickly! We are using a Trace 1300 GC and a Delta V Plus in addition to the GC IsoLink II.

John D. Howa

Analyst III | Sports Medicine Research and Testing Laboratory
10644 South Jordan Gateway | South Jordan, UT, USA 84095 |