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] ********************************************************************* This e-mail message, and any files transmitted with it, are confidential and intended solely for the use of the addressee. However, the information contained in this e-mail may subsequently be subject to public disclosure under the Freedom of Information Act 2000 and, unless the information is legally exempt from disclosure, the confidentiality of this e-mail and your reply cannot be guaranteed. If this message was not addressed to you, you have received it in error and any copying, distribution or other use of any part of it is strictly prohibited. Any views or opinions presented are solely those of the sender and do not necessarily represent those of the British Geological Survey. The security of e-mail communication cannot be guaranteed and the BGS accepts no liability for claims arising as a result of the use of this medium to transmit messages from or to the BGS. http://www.bgs.ac.uk *********************************************************************