Miguel and others
Probably relevant to Arndt's caution on exchangeable hydrogen, you may want
to look at the work of:
Cormie, A.B., Schwarcz, H.P. and Gray, J. (1994). Determination of the
hydrogen isotopic composition of bone collagen and correction for hydrogen
exchange. Geochimica et Cosmochimica Acta, 58, 365-375.
Tim H.E. Heaton
NERC Isotope Geosciences Laboratory
British Geological Survey
Keyworth, Nottingham NG12 5GG, England
(www.bgs.ac.uk/nigl/index.htm)
Tel. +44(0)115 936 3401
Fax +44(0)115 936 3302
E-mail: [log in to unmask]
-----Original Message-----
From: Arndt Schimmelmann [mailto:[log in to unmask]]
Sent: Thursday, February 01, 2001 3:11 PM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] D/H determination in organic matter,
atmospheric moisture interference
Dear Miguel and others,
You intend to determine D/H in wheat using a TC/EA analyser coupled to a
Delta
Plus XL in an on-line configuration. That is a very fast and economical
method
compared to traditional off-line combustion, with the additional advantage
that
it requires a relatively small sample size. I only want to warn you about
one
complication: It may be difficult for pyrolysis-on-line D/H methods to
compensate for the interference of exchangeable hydrogen in their organic
samples. I estimate the amount of exchangeable hydrogen in wheat to be
around
20% of total hydrogen, because the starch and protein are rich in functional
groups containing isotopically exchangeable hydrogen. That means that ca.
20%
of your analyte is tracking the current D/H composition of ambient
atmospheric
moisture and adding noise to your desired analytical signal from the 80% of
non-exchangeable organic hydrogen. (Note that the D/H of organic
exchangeable
hydrogen is *not* the same like the D/H of the water vapor, but that there
is
an isotope effect causing an offset between organic and inorganic H under
equilibrium conditions). The values you get out of your mass-spec are D/H
ratios for total hydrogen. The D/H of atmospheric moisture can vary
significantly over time, even in one location. Thus you will see different
D/H
ratios of the total hydrogen depending on the season and meteorological
conditions. I don't know about the exact specifications of your inlet
system,
but can you at least 'pre-equilibrate' all your samples in a chamber where
the
moisture is in equilibrium with a laboratory water standard? Let's assume
that
you find a way that such pre-equilibrated samples do not change their D/H
ratios before they will be pyrolyzed in your system. At least this way your
results would be internally reproducible in your lab, regardless of
atmospheric
variance in D/H.
I would very much like to hear ideas from other users of pyrolysis on-line
D/H
determination on this issue.
With my best wishes, Arndt
--
Arndt Schimmelmann, Ph.D.
Senior Scientist
Indiana University
Department of Geological Sciences
Biogeochemical Laboratories
1001 East 10th Street
Bloomington, IN 47405-1405
Ph (812) 855-7645
home (812) 339-3708
FAX (812) 855-7961
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http://php.indiana.edu/~aschimme/
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