To everybody dealing with cellulose!

I had suggested a little ring test some months ago using a IAEA
standard cellulose. Here are the results.

The background:
Since I am using a combustion technique at a rather low temperature 
for my cellulose samples, I could not use graphite standards like most 
other labs do - the graphite simply won't react. Therefore I started 
to use IAEA cellulose for internal check of my precision.

The IAEA informed me that their C3 standard was likely to be 
inhomogeneous on a mg-scale  for 13 C and was not published for 
such standardization purposes, but to the best of my knowledge 
that had never been properly checked. This was for the reason of 
the proposed test.

I had sent strips of C3 cellulose out to five laboratories that had 
kindly agreed to take part in the test, four of which have provided 
results so far. A summary of the results and methods used is 
given below:

METHODS

lab. 1: +CuO offline, 850 C, 2hrs
lab. 2: +CuO offline, 900 C
lab. 3: +CuO offline, 750 C, 4 hrs
lab. 4: +CuO offline, 600 C, 6 hrs
lab. 5: element. anal.

(guess which one's my lab....)

The results are corrected for differences in other (approved) 
standards where those data were supplied by the participants.

SUMMARY OF RESULTS

lab no.      no. spls.     mean (ppm)      95 % conf. interval (+-)

1               5               -24.53                0.06
2               3               -25.15                0.05
3               2               -24.11                0.38
4               7               -24.45                0.05
5               7               -24.83                0.15

All values taken together produce a mean of -24.64 ppm +- 0.13 ppm.

This is much better than numbers published by Rozanski et al. (1992, 
Radiocarbon 34(3), 506-519). For C3 a value of -24.91 +- 0.49 (!) was 
given there. The precision of intercomparison tests for other 
standards (C1 to C6) was also in that range.

As one of the participants pointed out, he was "astonished at the 
wide error bands of carbon" in that publication. He remarked that 
"there is clearly a need to check the carbon scale as it is already done 
for hydrogen and oxygen." I can only agree.

The results of this little test have been assuring, not only for the 
overall homogeneity of C3 in terms of stable carbon isotopes, but 
also for my use of the low combustion temperature. It is, by the way, 
interesting how many slightly different temperatures are used for 
cellulose combustion. It seems not to make any difference, anyway...


Frank









----------------------------
Frank Pawellek
Stable Isotope Laboratory
Department of Geography
University of Wales, Swansea
Singleton Park
Swansea SA2 8PP
Wales / GB

Tel. +44 1792 295148
Fax. +44 1792 205556
email [log in to unmask]
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