What is the current feeling about the preparation of ancient bones for
stable isotopic analysis of 'collagen'?
Our approach has always been toward a very gentle removal of carbonate with
dilute mineral acid to avoid excessive hydrolysis. In our somewhat
operational approach, the 'collagen' would be the organic residue after acid
treatment and rinsing. We are not naive enough to assume that collagen
survives intact and unaltered over the centuries in all samples - but short
of a full blown protein sequencing analysis and/or extensive NMR work, you
have to draw the line somewhere. Occasionally we find samples that require a
more vigorous treatment presumably because of occluded carbonate that gets
missed the first time around.
Recently, however, we found our values for carbon to be significantly
different (several per mille) from a lab that uses an alkali treatment
following the acid. Their reasoning involved removal of humic acid - which
we had never considered to be a problem. 'Humic acid' is, after all, an
operational definition for the organic matter that can be extracted under
certain conditions. And you might reasonably make the argument that the
carbon that is left from an ancient protein might have altered enough to be
extractable under those same conditions.
I guess our question is in several parts: What are others doing to isolate
the 'collagen' from ancient bones? Have we totally missed the boat with
respect to more current approaches? Is there a feeling that a problem
exists with some other carbon fraction 'creeping in' over time? Has anyone
investigated the existence of large isotopic variability over the length of
a long bone - ancient or recent?
Any and all responses and references would be welcome -
Coastal Science Laboratories, Inc.
6000 Mountain Shadows Dr.
Austin, Texas 78735
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