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