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