Jude,

Assuming you do not accurately know the concentration and isotopic  
composition of carbonate in your samples (if it varies between  
samples), you may not be able to apply a three component isotope mass  
balance to solve for your two unknowns.

Have you considered simply removing the carbonate prior to (re-) 
analysis by EA-MS?  If there is remaining sample, they can be acid- 
treated in silver capsules.  There is a lot of discussion of this  
topic on ISOGEOCHEM under carbonate removal.  One most useful  
methodological reference is:

Verardo, D.J, P.N. Froelich, and A. McIntyre, 1990, Determination of  
organic carbon and nitrogen in marine sediments using the Carlo Erba  
NA-1500 Analyzer, Deep Sea Research, Vol. 37, No.1, 157-165.

Best,

Jonathan Wynn
Department of Geology
University of South Florida
4202 E Fowler Ave, SCA 528
Tampa, FL 33620
(813) 974-9369
[log in to unmask]
http:shell.cas.usf.edu/~jwynn



On Jul 26, 2006, at 6:57 PM, judemaul wrote:

> Folks,
>
> I am characterizing particulate plant derived organic matter input  
> to the soil.  The approach I have taken is planting C3 plants into  
> a soil with a long C4 history.  The particulate organic matter in  
> the soil at the start of the experiment had a del vPDB signature of  
> -14.000 and the new C3 input has a del vPDB of -28.000.  My problem  
> is that the protocol for isolating the organic matter requires that  
> I co-collect the organic matter with the sand fraction of the soil,  
> which contains about 18% carbonate with a del vPDB of 0. I'm  
> wondering if anybody can help me with a three source mixing model  
> that can be run using only one isotope?  Using a separate analysis  
> of the soil I can calculate the amount of carbonate in each sample  
> but I need to determine the proportion of each plant source (C3 or  
> C4) in the remaining organic carbon.
> Ideally I would have removed the carbonate prior to isotopic  
> analysis but at the time of sample prep I didn't realize how much  
> of an issue the carbonate would be.
>
> Thanks
> jude
>
> -- 
> ####################
> Jude Maul
> Department of Horticulture
> Cornell University
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> (607)255-0883 (office)
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