Joachim
Thanks for that - it's not just me then. We also found that the dissolved then dried sucrose gave d18O 3 to 4 per mille lower (and much worse precision) than the non-dissolved sucrose. And this was for freeze-drying; so substantial O-isotope reaction along the line of Wolfram's comment occurs at room temperature.
Pity. Pipetting multiple standard samples from solution is so much less tiresome than weighing out solids.
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]
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
Behalf Of Joachim Molkentin
Sent: 28 July 2005 12:53
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] 18O/16O of IAEA CH-6 sucrose
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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