Dear Lars,

In a quick look at your method used I see some matters of importance:
1- you should not use 0.1M phosphoric acid meaning you add water to your
reaction, and thus you bias the oxygen isotopic composition, but
phosphoric acid buffered with P2O5 resulting in a socalled '103 percent'
phosphric acid - you need to prepare this yourself (see for instance
Bowen, 1966, Paleotemperature analysis, Elsevier) or buy it at high
price commercially.
2- did you control the temperature of the reaction? The isotopic
fractionation of the reaction is temperature dependable and to know the
isotopic composition of the carbonate the temperature must be known
precisely.
3- degassing the samples on a vacuum line before you react them with the
concentrated phosphoric acid is needed.

I hope this helps,
Pier de Groot.
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Dr. P.A. de Groot
University of the Witwatersrand
Economic Geology Research Unit
Department of Geology
Private Bag 3
2050 Johannesburg
South Africa
Tel. +27 11 7162564
Fax. +27 11 3391697
E-mail <[log in to unmask]>

Visit the combined HOME-PAGE of EGRU-Geology on the Internet:
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