Dear Cap,

As you rightly suspect, the purification of samples like these is a 
rather messy and frustrating business. Depending on the flow 
parameters I would even suspect some CO2 to be trapped together 
with the water in your proposed -90 C cold trap. CO2 quite happily 
"dissolves" in water, the more so the lower the temperature.

I don't know what your instrument setup is, so my suggestion has to 
be taken with a pich of salt. If you can hook up a GC to your IRMS, 
no problem. If not, a little improvisation might be needed.

I wonder if you could condense all volatiles into a liquid N2 cold 
trap and transfer (after flash heating) the condensables with a He 
stream of 3 ml/min into a PoraPLOT Q GC column. Under isothermal 
conditions, e.g. 50 C, you should be able to separate the CO2 as a 
nice concentrated peak from all the other junk and "heart-cut" this 
peak into your system.

I envisage a setup that purges He through your sample in flask A and 
condenses the purged compounds in a coiled cold trap B which can be 
resistance heated (SGE makes very nice glass lines SS tubing which is 
well suited for such a purpose). A short transfer line connects the 
cold trap with the PoraPLOT Q column which can be placed e.g. in a 
thermostatic water bath if you are short one GC. The connection of 
the column to your IRMS must allow to heart-cut the desired CO2 peak 
whilst flushing all the undesirables to atmosphere.

Hope, you don't find this approach not to far-fetched.


Good luck.

                      Wolfram


*****************************
Dr. W. Meier-Augenstein
Hon. Lecturer in Chemistry

University of Dundee
Dept. of Anatomy & Physiology
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DUNDEE  DD1 4HN
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