Hey Alison,
For question # 2: do you rinse samples after acidification?
If not,
one possible reason for the weight increase after 
acidification might be formation of CaCl2. 
Several years ago, running similar experiments 
(but extracting samples on a vacuum  line 
manually) we hypothesized that the following 
reactions might take place in carbonate/Corg 
mixture (we have not pursued the issue to fully confirm the hypothesis):

1) CaCO3 + 2HCl = CaCl2 + CO2 +H2O
2) CaCl2+CuO+2Ag= CaO+2AgCl+Cu
3) CaO+CO2=CaCO3
reaction 1 occurring during acidification and 
reaction 2 and 3 during combustion at 850 C. 
Reaction (3) lead to trapping some of CO2 from 
organic matter in newly formed carbonate.

For question # 1: this problem is known to some 
of us as "the size effect". Part of it is related 
to blanks associated with combustion in 
continuous flow mode (see reference below).

Fry, B., Garritt, R., Tholke, K., Neill, C., Michener, B., Mersch,
F.J., Brand, W., 1996. Cryoflow: Cryofocusing nanomole
amounts of CO2, N2 and SO2 from an elemental analyzer for
stable isotopic analysis. Rapid Commun. Mass Spectr. 10, 953–958

There are probably other issues contributing to 
the "size effect" in addition to the one 
resulting from blank (even when linearity 
obtained with the reference gas pulses is 
perfect). However, I have never found a 
satisfactory explanations for those issues.

Masha

At 01:24 PM 6/29/2006, you wrote:
>Dear List,
>
>I am a graduate student at Syracuse University, working on some sediment
>cores from Lake Tanganyika.  I have a few 
>questions on issues I've encountered.
>
>First of all, we're in the midst of calibrating our MAT 252.  I've been
>doing continuous flow measurements on a number of inhouse standards, and
>have found that the nitrogen isotope values I get show a definite mass
>dependance.  Is this something others have seen as well?
>
>Secondly, I've been testing different acidification techniques, comparing
>vapor acidification and acid wash acidification.  I would like to be able to
>measure my silver capsules before and after they are exposed to the HCl, so
>that I can figure out how much carbonate was removed.  However, I found that
>the capsules actually gain weight after being exposed to the acid.
>Extensive drying didn't account for it, so I don't think it's a moisture
>issues.  Is this a silver chloride crust forming?  Has anyone found a way to
>account for it?
>
>Thanks for your help,
>Allison

__________________________________
Maria Prokopenko

Hess Postdoctoral Fellow
Department of Geosciences
Princeton University
Guyot Hall, Washington Rd
Princeton, NJ, 08544