Tim,
as far as I know there can be such exchange but it is indiscernibly slow.
I've never seen it. The complete freeze drying also should not cause
fractionation. The dissolution sampling process is different beast.
Sucrose has as one of its (often useful!) properties that even a seemingly
dilute aqueous solution will have density gradients in cool water. That's not
might, not kinda sorta, but will. Even if there are no recognizable solids on
the bottom of the container, even if you wait a long time, there will be
density gradients and it is difficult for me to imagine there is not some
fractionation between different densities.
About the only feasible solution to this is familiar to sweet iced tea
drinkers: dissolve the sugar in extremely well-stirred hot water, and pipette
the hot water into your capsules.
John

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
Behalf Of Heaton, Timothy HE
Sent: Wednesday, July 20, 2005 6:08 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] 18O/16O of IAEA CH-6 sucrose


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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