Biology Reference
In-Depth Information
For those researchers who are engaged in research outside of the United States, analysis of
stable isotopes frommineralized tissues may pose equally challenging obstacles, as sampling
and exporting permission processes are diverse and variable. For thorough treatment of
issues related to international research programs in skeletal biology, interested readers
should consult
Turner and Andruskho (2011)
for practical guidelines related to international
collaboration. In my own experience, a minimum of six months was required to obtain
permits to sample an archaeological collection for a stable isotope project. This adds an
important logistical concern for projects.
Regardless of where samples come from, it should be mentioned that current methodolog-
ical protocols only require minimum amounts of samples. For example, extraction of collagen
only requires ~10 g of bone and extraction of apatite from tooth enamel only requires ~15 mg
of enamel. Moreover, cutting-edge methods involving
laser ablation
have been utilized by
some researchers interested in strontium isotope analysis. Laser ablation has been applied
to Neandertal samples and was described as a relatively nondestructive method by the
authors and ultimately required much less sample than traditional methodologies (
Richards
et al., 2008
). Regardless, researchers should confirm minimum sample standards with their
collaborating laboratories.
Tissue Extraction: Collagen
The issue of sample preparation requires some discussion of the types of tissues that can
be analyzed by stable isotope analysis. The first tissue to be studied was bone collagen, as
researchers had been using this tissue for radiocarbon dating prior to the late 1970s. As other
authors in this volume have highlighted (see Trammell and Kroman [Chapter 13]), bone is
comprised of both organic and inorganic compounds. The organic component, called
Type-1 collagen, comprises approximately 30% of dry bone. In its pure form, Type-1 collagen
is approximately 35% carbon and 11
e
16% nitrogen by weight (
van Klinken, 1999
), making it
the ideal tissue for stable isotope analysis of these two elements.
Researchers interested in collagen isolation should review the various approaches that are
available (
Longin, 1971; Schoeninger and DeNiro, 1984
;
Brown et al., 1988; Tuross et al., 1988;
Ambrose, 1990
). Each of these procedures has been used with success and involves wet
chemistry protocols that demineralize the bone and separate out the collagen component.
Researchers should keep in mind that taphonomic processes (see Marden et al. [Chapter
9], this volume) might result in poorly preserved bone, and therefore poorly preserved
bone collagen. Consequently, one should choose an appropriate extraction protocol tailored
to these circumstances (
Brown et al., 1988
).
As
Katzenberg (2008)
aptly points out, it is of utmost importance for a skeletal biolo-
gist interested in analyzing bone collagen to demonstrate that the final product under
analysis is actually bone collagen and not some other chemically altered, taphonomic
byproduct.
DeNiro (1985)
was the first to suggest that researchers should evaluate the
ratio of carbon-to-nitrogen (C/N) in their collagen samples. In “good” samples of
collagen, the C/N ratio will fall somewhere between the values of 2.9 and 3.6, which
reflects the C/N ratio of 3.2 typically found in modern, taphonomically unaffected
bone samples. In performing literature searches on stable isotope analyses of bone
collagen, researchers should certainly take note of those studies that report C/N ratios
