Thanks Ty and Ethan for the reference.  This should get me on the way.

>DIC 13C extraction methodolgies were studies by
>
>Hassan, A. A., Methodologies for extraction of dissolved inorganic carbon
>for stable carbon isotope studies:  Evaluation and Alternatives,  U.S.
>Geological Survey Water-Resources Investigations 82-6, 51 p., 1982.
>
>>Dear Isogeochemers,
>>I was wondering if I could obtain a survey of the relative merits of sampling
>>ground water for the del-13C of inorganic carbon with either of these two
>>
>>methods:
>>1.  Collecting water poisoned by HgCl2 and then acidified by phosphoric acid
>>in
>>the lab.
>>2.  Collecting water with BaCl2 or SrCl2 allowing BaCO3 or SrCO3 to form.
>>Filtering the precipitate and then acidifying with phosphoric acid in the lab.
>>
>>Are there any publications that compare the accuracies of each approach?
>>Any advice would be greatly appreciated.  Thanks, T.C. Onstott
>>