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.
On 5/20/2016 18:28, Brian N. Popp
[log in to unmask]"
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?
On 5/20/2016 5:35 AM, wbrand wrote:
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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.
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.
Sent from my iPhone
On May 20, 2016, at 7:56 AM, Isabelle
Chery <[log in to unmask]>
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?
Brian N. Popp, Professor
University of Hawaii, SOEST, Department of Geology & Geophysics
1680 East-West Road, Honolulu, Hawaii 96822
Office - (808) 956-6206; Fax - (808) 956-5521
Willi A. Brand, Stable Isotope Laboratory [log in to unmask]
Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
Hans-Knoell-Str. 10, 07745 Jena, Germany Tel: +49-3641-576404
P.O.Box 100164, 07701 Jena, Germany