Frank As far as I'm aware the IAEA C3 standard is holocellulose rather than alpha-cellulose. Therefore there will be some hemicelluloses present and it will not be truly homogeneous. However, these differences in structure may not be significant. We measured the 13C/12C values from a nitrated sample of it: Mean -24.78 n 14 s.d.(n-1) 0.11 However, 13C/12C values from nitrated samples would appear to be depleted by 0.30 per mille (Palaoklimaforschung 15, 141-155). We decided to use a commercial alpha-cellulose as a standard (Sigma Cellulose ref 92F 0243) and typically have a precision of 0.05 per mille on 13C/12C measurements... Let me know if you want some. Best wishes. Iain ************************************************************************* Iain Robertson + Godwin Institute for Quaternary Research, + University of Cambridge, Free School Lane, + Tel. ++ 44 (0)1223 334881 Cambridge, UNITED KINGDOM CB2 3RS + Fax. ++ 44 (0)1223 334871 ------------------------------------------------------------------------- Godwin Institute homepage http://www.esc.cam.ac.uk/GIQR.html *************************************************************************