In principle, we might be able to help.  We have found similar issues 
with soils from Nepal, and have used a thermal analysis system attached 
directly to an IRMS to determine C isotope ratios in CO2 evolved during 
a heating cycle.  This easily determines del13C for the carbonates, 
separately from organic matter, and gives the quantities of organic C 
and carbonate in the sample.

The thermal analysis method has the advantage that no chemical 
preparation is needed; let me know if you want us to run some samples 
on a trial basis!

David Manning


On 22 Dec 2005, at 18:50, Cristina Castanha wrote:

> I am trying to decide on the lesser of evils regarding elimination of
> inorganic C before 13C determination on solid samples by mass 
> spectrometer.
>
> Our samples are Oklahoma soils. They don't fizz when acid is dropped 
> and
> they all have less than 0.2% gravimetric CaCO3 equivalent. But because 
> this
> level of inorganic C could still raise the 13C content by 4 per mil I 
> would
> like to compare the 13C content of treated vs untreated soils.
>
> Browsing the archives I find that HCl or H2SO3 treatments are 
> preferable to
> H3PO4 or the HF treatments. The Harris et al (2001) HCl fumigation is,
> however, purported to consume some organic C and thereby alter the 13C
> content of the remaining organic matter, whereas the Verardo
> et al (1990) H2SO3 method is purported to damage the analyzing 
> equipment.
>
> Is it clear that the Verardo et al method -- in which 0.4 ml of 8% 
> sulfurous
> acid is added to each capsule -- is more aggressive on the mass 
> spectrometer
> than the HCl fumigation with 12M HCl?
>
> Can someone please outline the appropriate adjustments to the Verardo 
> et al
> method (e.g. using use Ag, not Al, capsules?).  And, is there any 
> reason why
> one can't fumigate with H2SO3 instead of dropping it in each capsule 
> several
> times? If fumigation is plausible, what strength is recommended?
>
> Thanks very much
>         -Cristina
>
>
Professor of Soil Science
School of Civil Engineering and Geosciences
University of Newcastle
Newcastle upon Tyne
NE1 7RU

telephone (+44) 0191 222 7893