Stable Isotope Geochemistry


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"Löfgren, Lars" <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Tue, 27 Jan 1998 09:38:41 +0100
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Dear listmember,

I want to determine delta values of rat breath, on rats treated with
13C-labeled palmitic acid. We think the easier the better and simply
tried the following procedure. Comments and suggestions for improved
method appreciated as well as aspects on breath analysis on GCCIRMS
without chromatographic separation of the present gases (se below).
Suggestions on suppliers of labeled bicarbonate standards (delta from
-30 to about 100) appreciated as well.

1. Breath was continuously absorbed in a 500 ml 1M NaOH solution for up
to 120 minutes. All CO2 should be absorbed as bicarbonate.

2. At the same intervals as blood samples were taken, up to 1 ml
aliquots of the NaOH solution was collected in exutainer tubes suitable
for automated analysis with the Gilson XL autosampler and the GC IRMS
(VG Optima ITO).

3. 100 ul of phosphoric acid was added and the test tube sealed quickly
and shaken for a few seconds .

4. After 15 minutes  the tubes were run as ordinary breath tests tubes
like the Urea Breath Tests that we perform daily.

Everything seems to works very well for (unlabelled) standards of
bicarbonate, but I experienced strange results (unexpected and
non-reproducible delta values) for the real samples, and do not know why
that should be the case.

I have checked that all CO2 is released from the basic solution when
I have checked that there is no loss of CO2 from the test tube during
the open procedure of adding acid
I have checked that proper amounts of total CO2 is deliberated,
typically 0.1-5% total CO2 in the tubes, comparable to typical

I tried to inject air and breath directly onto my GC combustion IRMS,
set up for the fatty acid methyl esters, and it seemed to work. What I
do not know is if I would expect different results when injecting a gas
mixture of CO2, N2, H2O and trace VOC compared to analysis on a designed
system which includes chromatographic separation of at least N2, CO2 and

Suggestions or comments appreciated, and literature references if

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