Thanks to all for their comments and 
"ciao" to Pier, nice to hear from you again!

My doubts were rosen by the following link:
http://www.bgc.mpg.de/service/iso_gas_lab/activities/index.shtml

in which was stated:
__________
With A. Schimmelmann (Univ. Indiana, Bloomington) we engaged in generating
a new (organic) ä18O reference material for on-line oxygen isotope analyis.
We purchased a larger amount of high purity benzoic acid from Merck and
exchanged the oxygen with 18O enriched water (pH 0, 90°C, 3 weeks reaction
under reflux conditions) for 1/3rd of the material. The exchange reaction
was made in collaboration with the inorganic chemistry institute of the
University of Jena (M. Doerr and W. Weigand). The two materials were then
isotopically measured using a high temperature pyrolysis technique (Jena
Isolab) and a medium temperature off line decarboxylation reaction (Indiana).
Both Experiments resulted in similar values for the two materials, oone
at about +23 per mill and the enriched form at about +71 per mill vs. VPDB.
The material was forwarded to the IAEA in Vienna for further handling. The
work done in Bloomington was sponsored by the IAEA. 
________________

...and since Isolab seems to be the producer I was wondering who was right.

Ciao
Francesca Serra