At 01:29 PM 11/30/98 +0100, you wrote:
>Dear isotope geochemists,
>
>we want to analyse the d18O of molecular oxygen in soil gases.
>We use a mass spec equipped with a double inlet system.
>The problem we are facing is to purify the oxygen for isotope analysis.
>
>The only procedure for separation of oxygen from other gases (including
nitrogen and noble gases) that I am aware of, involves conversion of oxygen
to CO2.
>If possible, however, I would like to analyse molecular oxygen.
>(We run molecular oxygen extracted from silicates routinely on our mass
spec, and it works fine)
>
>Can anyone comment on one of the following questions or point out
literature that could help ?!
>
>(1) Are there any adsorbants, molecular sieves... that could selectively
trap nitrogen and help to purify oxygen for isotope analyses?
>
>(2) How clean must molecular oxygen be to get reliable results for d18O?
>
>(3) What are the problematic interfearing species in the mass spec that
could influence the d18O analysed?
>
>Any comments are greatly appreciated!
>
>Thanks.
>
>Rainer
>

Hi Rainer,

At our lab, we are measuring different species present in the atmosphere
for oxygen isotopes and therefore need to purified it. For your special
case, it is relatively easy to remove the N2 matrix. You can first remove
all condensable gases with a cold trap at liquid N2. The only remaining
gases should be N2, O2 and Ar. Cryotransfert all these gases in a molecular
sieve (5A) cooled at liquid nitrogen temp. Then you have to slush this
molecular sieve at -123C using  ethanol (in general ethanol freeze at -117
but in our lab for an unknow reason some old ethanol slush can reach -125C.
I guess it has something to do with supercooling effect like water).
Eventually you can use pentane which freeze at -130C (Cool it in a hood).
All O2 should be released in 50 min. Use an other molecular (13X work well)
to trap the O2 during the extraction.  With this inexpensive method Ar came
with O2 and it is impossible to separate these two gases. You will still
have around 5% of N2 but no interference in the mass spec has been seen at
this level . For more information you can read  Cliff and Thiemens, Anal
Chem 1994, 66, 2791 and Thiemens and Meagher, Anal Chem 1984, 56, 201.

The other way will be to use a GC.
         Joel Savarino
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