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Dear Brian,
as many of you have experienced, memory is a bit difficult to predict from first principles. For GC columns, some small amount of many compounds can remain on the column for a longer time, contributing to column bleed. When the same chemical is injected again, a compound-specific washout effect can replace some of the old material and contribute to the new peak. This is a bit like water exchange with the last layer on a metal surface, which cannot be pumped away. However, these are rather small effects and should be curable with some special memory treatment (e.g. additional injection of some neutralizing mixture). The largest effect that bothers most is the apparent tailing right after a highly enriched compound has left the column, prominently visible on the ratio trace. The tailing itself is always present (the memory function is not different for different isotopologues), but the tailing in this case changes the isotopic composition of the baseline. This changing baseline is what causes difficulties for the next couple of eluting compounds and renders quantification a bit more difficult. Once the ratio trace is back to 'normal' the effect should be gone.
Regards    Willi

On 5/20/2016 18:28, Brian N. Popp wrote:
[log in to unmask]" type="cite"> Willi and others

I am curious. How long will it take to get rid of the "blank" created by analysis of a perdeuterated compound before one can return to natural abundance compound specific hydrogen isotopic measurements in this system?

Brian

On 5/20/2016 5:35 AM, wbrand wrote:
[log in to unmask]" type="cite">If your D31 palmitic acid is indeed 100% deuterium labeled you would only see mass 4 (D2+) and some fragment ions at mass 2 (D+). Mass 4 will be a huge peak, as large as the usual mass 2 peak (H2+) for the non-labeled acid. If you have some hydrogen (protium) left in your palmitic acid, most of it will show as mass 3 (DH+). I suspect that there is probably quite a lot of this, which will saturate the mass 3 channel. Moreover, gas chromatography for these H-substituted palmitic acids will be smeared a bit, which is why I would expect a broader GC peak for mass 3. You would probably not see this prominently on the 13C peak.
Regards    Willi




On 5/20/2016 17:16, Marilyn Fogel wrote:
Are all of the hydrogens in the molecule deuterium? If so I don't think this is a chromatography issue.
I am not sure how the collector will handle a completely deuterated compound. Perhaps others will chime in.
Marilyn Fogel

Sent from my iPhone

On May 20, 2016, at 7:56 AM, Isabelle Chery <[log in to unmask]> wrote:

Hello all,

I would like to analyse a uniformly labelled fatty acid with Deuterium (D31 Palmitc Acid) after derivatization to be volatile as methyl ester by Gc pyrolyse IRMS.
I use a Zebron ZB Wax capillary GC column to separate different Fames (C14:0 to C22:0). The injector is at 220°C and the high temperature conversion reactor is at 1420°C.
The D31-Palmitic acid is totally separated from the natural Palmitic acid by GC.
I have beautiful peaks for natural abundance of FAMEs (mass 2 and mass 3) but for the D31-Palmitc acid, the peak shape (only mass 3 is present) is trailing and is not symmetric.
I have changed the insert, the syringe, cut the column, and put a new reactor tube. But the problem is always being.
Does anyone could help me?
Thanks
Isabelle

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Brian N. Popp, Professor
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Willi A. Brand, Stable Isotope Laboratory      [log in to unmask]
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