Cristina,
I'm not sure what analyzing equipment you are using, but in general if you
are doing elemental analysis - mass spectrometry then the Cl is always going
to be worse than the SOx. I have had some success with HCl fumigation, but it
is to me way too much of a hassle to plumb properly and to keep checking
subsamples to see if "all" the carbonate is gone.

I have been running EA-IRMS on solid (powder) samples using tin capsules with
ease for some years now. Unless there are high levels of carbonate I have
found it best to run samples before and after loss on ignition, and back
calculate the organics. You may be surprised that the inorganic carbon
remaining after LOI is higher than you think.
John

John Furey
(601)-634-2778
(601)-634-4002 fax
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US Army ERDC EP-P
3909 Halls Ferry Rd
Vicksburg, MS 39180  

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Cristina Castanha
Sent: Thursday, December 22, 2005 12:50 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] carbonate removal method

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