Have you plotted d34S vs 1/%S to see if there is evidence of a consistent composition of the contaminant as a mixing line or cone? If a sulphate or sulphide salt is making it through the extraction procedure then there should be a charge-balancing cation present. We have used EDXRF to investigate the cations of salts that occasionally make it through the extraction.
Dr. Andrew Millard
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Senior Lecturer in Archaeology, Durham University, UK
> -----Original Message-----
> From: Reba Macdonald [mailto:[log in to unmask]]
> Sent: 13 March 2015 22:49
> Subject: high %S amounts in collagen
> Hello everyone,
> A student in our lab has had some unusual results when analyzing sulfur
> of bone collagen. These samples (some humans, but mostly animals) are
> from an 18th century shipwreck in the Gulf of Mexico. The elemental
> percent of S ranges from 0.22 to 1.18 %, with the majority of samples
> falling between 0.4 to 0.7 %. According to Nehlich and Richards (2009),
> well-preserved mammalian collagen should yield 0.21 to 0.35 %S.
> The student had good collagen yields (10-15 %), and CN analyses of the
> same collagen had C:N ratios in the appropriate range. We therefore
> don't think that preservation of collagen is the issue.
> The student followed standard lab procedures for collagen extraction,
> including ultrafiltering of the collagen (30 kilodalton ultrafilter).
> All analyses were performed on an Elementar vario MICRO cube coupled to
> an Isoprime 100, and IAEA S-1, S-3 and NBS-127 were used to calibrate
> the elemental percents. After the first round of S analyses yielded
> high %S amounts, collagen was re-extracted for a small number of samples
> and a NaOH step was added. These too all had high %S amounts.
> Our initial thought was that these samples were contaminated with
> seawater-sulfate. Although this would be surprising given the
> dissolution and rinsing steps involved in the collagen extraction, it
> is an obvious possibility. However, one strike against this theory is
> that none of the samples had particularly high d34S values (seawater-
> sulfate is ~20 permil), and the d34S values of the samples showed great
> variation. Another possibility may be sulfides that precipitated in/on
> the bones before while they laid on the seafloor, but we don't see any
> darkening of the bones or the collagen, and again, the d34S values are
> not uniform. Also, we hope that any contaminants are removed during our
> extraction procedures.
> Our lab has analyzed a number of samples obtained from other shipwrecks
> and we've never seen elevated S percents. Does anybody out there have
> any ideas??
> Dr. Rebecca Macdonald
> Postdoctoral Researcher
> Dept of Anthropology