Pier,
I want  to add certain things to your reply:
1. I think Lars does not want isotope data, in that case temperature is not
an issue.  Besides, even if he wants only the O-18 isotope needs carefully
controlled reaction temperature.  100% carbon from CO3-- ends up in the CO2,
so there is no fractionation there, and no temperature control is
necessary;  however 2/3 of oxygen from CO3--  ends up in CO2, so there is
fractionation, and it needs temperature control .
2. I think flushing the vial with He is enough to remove atmospheric
contamination, it does not require "degassing" under vacuum.
3. You are right about the acid if he wants isotopes, but if not 0.1 M could
be fine  and much better to work with than with the 100 % very viscous one.

Regards: Kinga Revesz

Pier de Groot wrote:

> 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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> University of the Witwatersrand
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--
Kinga Revesz
Chemist
U.S. Geological Survey
12201 Sunrise Valley Drive
Reston, VA 20192
MS 431
(703) 648-5865 Office
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