Dr. Sarkar,

We have experience trying to analyze soils low in N which will be a similar 
problem.

With 0.1% C in your samples you should have about 1 ugC for every mg of 
sample.  In our Europa 2020 system we have analyzed 20ug C in pollen grains 
in one capsule with no problem.  Depending on your system, it would appear 
you therefore need to try burning 20 mg of sample.  With soils we have got 
good results up to 40 mg of soil in 9x5mm tin capsule.  Therefore, just 
increasing the amount of material may work.  To test that your burn is good 
enough I would start with a similar material with more carbon that you can 
get good results with at say 5mg, then increase the amount of this sample 
burned and see if the results are still consistent with bigger 
samples.  For example, if you have a material with 1% C, then 2 mg should 
be enough for 20ugC in the capsule.  If results are consistent for this 
weight, try 5, 10, and 20 mg of this material.  If the apparent delta 13C 
and %C are the same for the various sizes, then it is reasonable to assume 
that you can get good results for 20 mg of the 0.1% material.

You should also make sure that your system had no dilutor on while the very 
small CO2 peaks are coming out, and have the maximum sensitivity tuning for 
C for these small samples.

Soils have to be ground very finely to work well, I assume the same will be 
true for you rocky material.

Hope this helps,

Paul Brooks.


At 05:35 AM 9/12/2005, you wrote:
>Hi all
>We are in the process of setting up a CFIRMS system with Flash EA and gas 
>bench. We are trying to analyse old sedimentary rocks having ~0.1% TOC for 
>their del 13C. Even after taking several miligram of samples the milivolt 
>signal is very low. Certainly increasing sample size can give better 
>signal but I would like to know if it has effect on the reactor life time 
>and how much can i increase the sample size. or is it that for ultra low 
>carbon concenmtration I have to do offline dual inlet measurements? 
>Suggestions from people doing the similar kind of experiment will be 
>appreciated. also if you can send some PDF papers on the related 
>experimental techniques. Thanking in advance.
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>Anindya Sarkar
>Associate Professor
>Department of Geology and Geophysics
>Indian Institute of Technology
>Kharagpur 721302
>West Bengal, INDIA
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