Dear John,

We find the seed oxidation to be largely based on trial (and some error).
You could probably go with a seed oxidation after every 3rd or 4th
injection with a longer backlash time and purge time, AND/OR cut down to
30 sec oxidation than check. Leave your oxygen as it is, you want enough
to oxidize your reactor.

Another tip, reactors can be all different, so check when you change out
yours. 

If you run N isotopes, seed oxidation helps. For carbon, not as vital with
every sample.

Marilyn



Dr. Marilyn L. Fogel,
Emerita Distinguished Professor of the Graduate Division
Equity Advisor for College of Natural & Agricultural Sciences
Dept. of Earth and Planetary Sciences
University of California Riverside
900 University Ave.
Riverside, CA 92521
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Phone: 209-205-6743
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On 10/14/20, 12:25 PM, "Stable Isotope Geochemistry on behalf of John
Howa" <[log in to unmask] on behalf of [log in to unmask]> wrote:

>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 | www.smrtl.org