Bill Showers wrote:

> Isotope Geochemists,
>
> I am wondering what other labs are using for pyrolysis O-18 standards.  We
> have had pretty good luck with NBS127 (BaSO4), NBS 120C (Florida Phosophate
> rock prepared as AgPO4) and NIST 8542 ANU Sucrose. This gives a range of
> +9.3 to +36.4 O-18.  We tried to add a cellulose standard which should be
> around +20, and used an Aldrich cellulose microcrystalline powder
> (43,523-6).  This cellulose has about 0.5 per mil O-18 variability.  Can
> anyone suggest a cellulose standard that might have less O-18 variability?
>
> Thanks,
>
> Bill
> William J. Showers
> Dept of Marine, Earth & Atm Sciences
> North Carolina State University
> Raleigh NC 27695
>
> (919) 515 - 7143 Office
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>
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>
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Bill
 I am wondering if you are dessicating the cellulose sufficiently before
combustion.  Cellulose is hygroscopic to some extent and this may be causing
your variability.
I use Sigma-Cellulose and it seems to work just fine.  Regards. Leo.