Dear Lars,
From the comments by Pier de Groot and Andrew Tait, you might want to
consult the following references.

Krishnamurthy, R. V., Atekwana E. A., and Hillol Guha, 1997. A simple,
inexpensive carbonate-phosphoric acid reaction method for the determination
of the ?13C and ?18O of carbonates. Analytical Chemistry, 69, 4256, 4258

Swart, P. K Burns, S. J. and Leder, J.J.  1991. Fractionation of the stable
isotopes of oxygen and carbon in the carbon dioxide during the reaction of
calcite with phosphoric acid as a function of temperature and technique,
Chemical Geology, 86, 89-96

Al-Aasm, I H., Taylor, B. E., and South, B. 1990.  Stable isotopic analysis
of multiple carbonate samples using selective acid extraction.  Chemical
Geology, 80,119-125.



Eliot Atekwana                  Assistant Professor
Department of Geology           (317) 274 7969 (office)
IUPUI                           (317) 274 7484 (dept.)
723 W Michigan Street           (317) 274 7966 (fax)
Indianapolis IN 46202-5132      e-mail:  [log in to unmask]
www.geology.iupui.edu


Dear listmember

I was asked to determine carbonate content in a solid powder and did the
following with my VG Optima GC-IRMS normally in use for simple breath
analysis.
Is this a good method?
Improvements suggested?
* 100mg solid powder put in 1.8 ml gas tight glass vials
* Vials flushed with pure helium in a glove chamber to reduce background
CO2
* 200 ul 0.1M phosphouric acid (>10x molar excess) added with a syringe via
the septa
* mix 5 min wait 30 minutes
* Headspace gc-irms to get total CO2 and isotope ratios as a bonus

Got nice results anyway, but have to be sure I did not do any major
methodology or practical mistakes
Thanks!
By the way - would it be possible to tell what carbonate was in the
material from the delta values?
Got -3.4 for my unknown samples and -8.0 from my Na2CO3 standard. Does it
tell me anything else then that it was not laboratory air CO2 in my
samples?
Best wishes,

Lars