Dear Gerhard and Clemente.
Two BaSO4 standards have been prepared by Stan Halas and me with del34S
values near zero and -33per mil. We anticipate that they will also be
useful as 18O reference materials with the latter reference having a
negative value. (We have yet to finalize the 18O data) Because S-isotope
analyses by CF-IRMS have become routine with a few microgram-S and  thermal
ionization IRMS in the sub microgram range is improving, we spent much time
homogenizing the sample. It turned out not to be a trivial task as ball
mills etc tended to compact the powder even though we felt that we had
removed water. We are checking homogeneity by CF-IRMS as SO2.
In any case, these materials should be handed over to the IAEA within days.
Roy Krouse.