Thankyou Thomas, Clemente and Hilary for your responses. I have done a series of rough experiments using different pretreatments on purified bone mineral. This material was already significantly lower in organic content than fresh bone (ca. 1%), nevertheless there were isotopic offsets between treatments. - Combusting at different temperatures resulted in an enrichment in oxygen isotopic composition with increasing temperature due to oxygen exchange or decomposition (from 13.8 permil at 450 deg C to 19.5 at 900) - suggesting that the true value is less than 13.8?. - Treatment with 30% H2O2 only resulted in a value of around 16.7 permil. - Plasma ashing for 10+ hours resulted in a value of 12.8, while plasma ashing for a few hours of both the bone mineral and the treated bone mineral resulted in a value for both of around 16 permil. For fresh bone, pretreatments would need to be even more rigorous. Plasma ashing may be effective, but would appear to take several hours. Perhaps, chemical pretreatment combined with plasma ashing would do the trick. With the O'Neil method it seems to me that isotopic offsets due to organic interference could be a result of reduced yields of silver phosphate, or organic carbon incorporated with the precipitate that results in the production of CO on combustion. Either way, as Hilary says, it is essential to separate any organic material before the precipitation step. If I cannot do this by chemical means, plasma ashing, or some combination there of I would certainly be interested in the details of the cerium method. Incidentally with fresh bone I have made black crystals and even what appears to be metallic silver, with inadequate pretreatment. Any further comments would be appreciated. Cheers Cameron. Cameron McDonald Chemistry Department University of Otago Dunedin, NZ [log in to unmask] Ph: +64 (0)3 479-7915 (wk) 24 Pitt Street Dunedin, NZ Ph: +64 (0)3 474-0603 (hm) "When life hands you lemons just grab a bottle of tequila and some salt"