Dear Paul,

for me, it looks like isotopic exchange between CO2 and water adsorbed
on the surface of your bellow and/or tubing. What else (than
mass-dependent fractionation) could change the delta 46 AND the delta 45
signal in a way that only delta 18O shows a shift? Have you checked the
mass 18 peak?


Thorsten.


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Thorsten Agemar
Institut f. Umweltphysik
Universitaet Heidelberg
INF 229
D-69120 Heidelberg
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Phone: (+49) (0)6221 546528
Fax:   (+49) (0)6221 546405