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
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