If you are interested indeed only in the organic fraction, I think you can
make your life even easyear. Just go directly to step 3 of Bernhards
suggestion and reduce (volatilize) with CrCl2/HCl to H2S all the sulfides in
your sample (AVS+Pyrite). The filtrate will contain actually the whole
sulfate fraction (except BaSO4, if present), so that in the residue you will
have practically only the "organic sulfur" which you can process to SO2 .

I hope that helps, too

Zsolt  

Dr. Zsolt Berner
Institut für Petrographie und Geochemie
Universitaet Karlsruhe
Kaiserstraße 12
Tel. +49 (0)721 608 4275
Fax: +49 (0)721 697328