Dear Tim,

we use a sucrose lab standard for the calibration of C and O analyses. For 
d13C analysis we pipette an aqueous solution into tin capsules and dry them 
at 100°C for 30 min in a drying cabinet. When we do so with d18O analysis 
(using silver capsules) we obtain a bad standard deviation and a d18O value 
that is 24.2 instead of 27.5 for samples weighed out as a solid. The solid 
samples do not give a different result when they are dried in the same way, 
thus confirming that the sucrose is anhydrous.

For another sucrose standard the difference is 32.5 (solution) to 36.7 
(solid). Assuming a d18O value lower than zero for the water, it is 
understandable that the difference increases with increasing delta of the 
standard. The water used for the solutions was very clean (Ultrapur, Merck) 
with neutral pH.

These results show that at 100°C there is a clear exchange of O from 
sucrose with water, maybe catalyzed by silver. So, even at a lower 
temperature some caution might be advisable.

Regards,
Joachim




At 12:08 20.07.2005 +0100, you wrote:
>Can sucrose undergo O-isotope exchange with water? In preliminary tests we 
>are finding slight differences in d18O between IAEA CH-6 weighed out into 
>capsules as a solid powder, and the same IAEA CH-6 dissolved in water and 
>pipetted into capsules. All cases used well-homogenised powder and 
>thorough freeze-drying.
>
>Tim H.E. Heaton
>
>NERC Isotope Geosciences Laboratory
>British Geological Survey
>Keyworth, Nottingham NG12 5GG, England
>(www.bgs.ac.uk/nigl/index.htm)
>
>Tel. +44(0)115 936 3401
>Email: [log in to unmask]
>
>
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Dr. Joachim Molkentin
Federal Research Centre for Nutrition and Food (location Kiel)
Institute of Dairy Chemistry and Technology
Hermann-Weigmann-Str. 1
D-24103 Kiel
Germany

http://www.bfel.de

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