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Karyne,

To analyse d13C of organic materials (or any other material), it is
important to have a reference material similar in composition/structure to
the sample material (as having a delta value not far from that expected for
the sample value). Using just CO2 is not sufficient, since it only relates
the CO2 produced by the conversion of your sample to the carbon scale, but
it does not include the conversion process. CO2, as used in He-CF on-line
systems is to monitor the proper working of the MS or any eventual shift
taking place (useful but not sufficient for proper calibration). The
reference material, to tie the sample result to the carbon scale, has to
pass the complete conversion-purification procedure as applied to sample
materials to make certain not to bias your measurements in the end (and
assumed you did not make any other mistakes in that process).
By the way, a solid does not need to be carbonate, Fausto, but also can be
an organic compound.

Comparing organic compounds, where CO2 or CO is produced depending on the
conversion method used ('reduction/pyrolysis'), with a reference material
where CO2 is produced by acid digestion (carbonate reacted with phosphoric
acid) is a risky matter.

I know, the international standard for carbon is the NBS19 (carbonate), also
for organic compounds. This, to my opinion, is a far from satisfactory
situation. The idea is to tie the CO2(organic) to the CO2(carbonate
standard). Are we than talking about the same carbon scale is the big
question.... Or is it more a situation where we have two carbon scales
'tied' to the same standard but not representing the same values? These
scales once were 'defined' separately (organic materials and carbonates) in
relation to the same old PDB standard.
I certainly realize there is no simple answer to this problem.
I just want to bring this forward to the list, and hopefully it generates
some discussion.

Coming back to your question, Karine. A provider for organic reference
materials is IAEA and also Indiana University has a number of materials
available (you may ask Arndt Schimmelmann).

Looking forward for your reactions,
Pier.


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Dr. Pier A. de Groot
Pastoor Moorkensstraat 16
2400 Mol - Achterbos
Belgium
Tel. +32 (0)14 326 205
e-mail: [log in to unmask]

Visit my WEB-site about my ³Handbook of Stable Isotope Analytical
Techniques² at:
http://users.pandora.be/handbook/index.html
last update: 6 October, 2003.
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> Dear Karine,
> do you think is the same to use a gas (CO2) rather than a solid carbonate
> having this range of isotopic composition?.
>
> Ciao
> Fausto
>
>
> On Tue, 25 Nov 2003 10:52:57 +0100, Karine Charlier
> <[log in to unmask]> wrote:
>
>> Dear Isotopists,
>>
>> I'd like to measure delta13C of organic matter with values between -60
>> and -100 per mil.
>> Then I'm looking for a solid standard with delta 13C below -60 per mil.
>> Do you know  where I can find it ?
>>
>> Thanks in advance,
>>
>> Karine
>
>