Hi Andrew
Much more easy is to control higher temperature e.g. 50C or 70C. In 
fact, exact temperature does not matter as long as you are running well 
calibrated CaCO3 standards with your batch of CaCO3 samples and you will 
use them for normalisation of raw values to VPDB. It is common approach 
used  in conflow systems. You have to worry just about keeping 
temperature stable, in order to have standards and samples digested at 
the same temperature. See e.g. Paul  and Skrzypek 2007, (Assessment of 
Carbonate-Phosphoric Acid Analytical Technique Performed using GasBench 
II in Continuous Flow Isotope Ratio Mass Spectrometry, International 
Journal of Mass Spectrometry 262: 180-186) and included references.

Temperature really matters when you are preparing carbonate samples and 
then you are normalising obtained values to VPDB using non-carbonate 
standards (e.g. CO2 gas in dual inlet system). In this scenario, you 
will need to know exact temperature to use appropriate correction factor 
for oxygen e.g. at 25C or 50C (see Sharma et al. 2002).

best regards

Greg


-------------------------------------------------------------

Dr Grzegorz Skrzypek, Assistant Professor

West Australian Biogeochemistry Centre
John de Laeter Centre of Mass Spectrometry
School of Plant Biology, The University of Western Australia
MO90, 35 Stirling Highway, Crawley WA 6009, Australia

Phone +61 8 6488 4584, Fax +61 8 64887925
email: [log in to unmask] http://www.bukibuki.eu



A. Schauer wrote:
> Hi all,
>
> 1) How do you obtain a controlled 25*C for phosphoric acid digestion 
> of carbonate in your lab? My lab varies around 22*C. It seems as 
> though most water baths with heating option only control from 5*C 
> above ambient to 99*C. Do you use a chiller model then or are you 
> lucky enough to have a lab that is 20*C? Please either outline your 
> method if home-built or provide a part number and manufacturer for 
> your bath.
>
> 2) How do I know that my 25*C will be the same as your 25*C?
>
> Thank you.
>
> andy
>
> Andrew Schauer
> Earth and Space Sciences
> University of Washington
> Seattle, WA 98195
>
> 206.543.6327
> [log in to unmask]
> http://depts.washington.edu/isolab/
>