Manfred,
you can also check 
Werner, Kornexl, Rossmann and Schmidt, Anal Chim Acta 319, (1996), pp. 159-164
and
Koziet, J Mass Spectom 32, (1997), pp. 103ff
for more suitable methods for the on-line determination of d18O in organic
samples. 
I measured some ethanols using the Werner et al method; the samples I
analyzed were distilled  ethanol samples used for SNIF-NMR. They contain
about 5 % of water, and I checked if the water had any influence, but
obviously the conversion temperature of 1080°C was not high enough to also
convert water to CO in our system. (I added up to 50 % of water and there
was no change in d18O).

Hope this helps

Barbara
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Dr. Barbara E. Kornexl
UFZ - Umweltforschungzentrum Leipzig-Halle GmbH
Sektion Analytik
Permoserstr. 15
D-04318 Leipzig

Tel.:   ++49/341/235-   2252
Fax:                    2625	
E-Mail: [log in to unmask]
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