Hello everyone.
Let me join in to this conversation, since we published a related paper in
1990 , Method to extract soil water for stable isotope analysis, ( K.
Revesz and P.H. Woods, in Journal of Hydrology, 115 (1990) 397 - 406.), so
we have a lot of experiences with the subject. We observed the same
phenomena, but it was well known, that there is a isotopic shift between
"mobile" and "not mobile" water as they called them in the literature.
There was a paper by Steward, (1972, Clay - water interaction, the behavior
of 3H and 2H in absorbed water, and the isotope effect. J. of Soil Sci.
Soc. Am., 36: 421-426) on this subject and I believe Magaritz from Israel
published papers also.
Our paper describe the azeotropic distillation method that we have been
using in our lab ever since without any problem, analyzing plants, soils,
branches you name it. Developing the technique I remember we tried the
same control experiment that you Renee, namely dry the soil at 200 C for
days and re-wet it with known isotopic composition of water hoping that we
would get back the same values after distillation. It never really worked
for us, especially if the water soil ratio was less than 1:1. Therefore,
we came up with an other experiment to prove the technique. It is in our
paper but the idea was that we mixed and shacked sufficiently amount of
water with soil (dry or wet it does not matter) to be able to decant or
centrifuge one aliquot of water to analyze its isotopic composition. The
isotopic composition of this decanted water should be the same that those
obtained by extracting the water from the soil (after decanting ) by
distillation. If not, the technique could not have been used for that
particular soil (almost never happens). We tested the equilibration
time, soil types, particle sizes, purity of reagents, etc.
This is just an idea how to test a new method.
Happy New Year.
Kinga
Kinga Revesz
Chemist, Stable Isotope Laboratory
U.S.Geological Survey
MS 431
Reston, VA 20192
T:703-648-5865
FAX: 703-648-5274
http://www.isotopes.usgs.gov
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| | "Koehler,Geoff |
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| | <Geoff.Koehler@EC|
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| | Sent by: Stable |
| | Isotope |
| | Geochemistry |
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| | 12/20/2004 10:50 |
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| | Geochemistry |
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| Subject: Re: Cryogenic water extraction: solution |
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Art, Renee, and all,
I think we are talking about oxygen isotopes here, but the same argument
applies. We observed this effect when directly equilibrating soils to
determine the dD and d18O of porewaters. See below for refs.
Interestingly, we noticed a small shift towards more positive d18O values
and no effect on dD values. As Art points out, our best guess for the
reason was exchange or fractionation (or both) between water that is
adsorbed to mineral surfaces and free "bulk" water. Even more
interestingly, we saw this effect with natural core samples when comparing
d18O values obtained by direct equilibration and sampling the pore water
directly from piezometers. The whole thing makes me wonder how well we
really understand these systems - I can think of about a dozen experiments
Id like to do..................
Hope this helps,
Cheers,
Geoff
Geoff Koehler
NHRC Stable Isotope Laboratory
11 Innovation Blvd
Saskatoon, SK.
Koehler, G., Wassenaar, L.I., and M. J. Hendry. 2000. An automated
technique for measuring dD and d18O values of porewater by direct CO2- and
H2- equilibration. Analytical Chemistry 72: 5659-566
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Arndt Schimmelmann
Sent: Friday, December 17, 2004 3:35 PM
To: [log in to unmask]
Subject: Re: Cryogenic water extraction: solution
Hello Renee,
I can offer an explanation that at least partially accounts for your
observed hydrogen isotopic shifts in water. Soil and biomass contain
hydrogen pools that cannot be 'dried away' as water but are
chemically bound, for example as mineral hydrogen in clay minerals
and as organic hydrogen. Some of these H pools are nevertheless
isotopically exchangeable with 'free water'. Also, drying soil or
biomass never completely removes all water hydrogen because some
water is strongly sorbed to surfaces. Sorbed water can also
isotopically exchange its hydrogen with 'free water'.
You added 'free water' with known deltaD to dried soils that
contained bound/sorbed exchangeable hydrogen. After isotopic exchange
(which does not need to reach isotopic equilibrium), the
cryogenically recovered 'free water' has a different deltaD,
depending on (i) the primary isotopic differences between
bound/sorbed and added water pools, (ii) isotope effects between the
participating hydrogen pools, and most importantly (iii) the mass
balance ratio of the hydrogen prools engaging in exchange.
Decreasing the amount of added 'free water' relative to bound/sorbed
water will cause a larger isotopic shift in the 'free water'. That is
what you observed. Your observed depletion in D of the recoverable
'free water' tells us that the initially bound/sorbed water in soil
was relatively D-depleted and/or that there is a significant isotope
effect favoring positioning of deuterium in the bound/sorbed hydrogen
pool.
Hopefully this helps,
Have a Merry Christmas,
Arndt
Renee Brooks wrote:
I have some strange results that I would like to run by you all
for a
possible explanation. We extract water from plant tissues and
soil
using cryogenic distillation for running isotopes on the water.
To QA
the processes, we have taken previously extracted material,
added some
of our working standard water, let that sit for a few days so
that the
water is distributed within the sample, and then extract.
When we do
this, all is fine, the extracted water has the same isotopic
value as
our working standard. However, we have also tried this with
soils that
were air or oven dried, and our results are that the extracted
water is
more depleted than the added working standard. This totally
confuses
me. I would have said that we were extracting additional water
that was
not removed through air or oven drying, but I would expect that
this
water would be more enriched, so our extracted water should be
more
enriched than the working standard. I would expect a more
depleted
result only if we did not extract all the water we added.
However, the
general thought is that cryogenic distillation is more
effective at
removing water than air or oven drying, so I expect that is not
the
problem.
Recently, another lab and ours are comparing cryogenic
distillation with running samples directly on a gas bench using
CO2
equilibrium, and this unexpected result has come up again. We
noticed
that the extracted water became more and more depleted compared
to the
source water with smaller and smaller volumes of water added to
the
soil. We did not see this result with adding water to tree
cores, only
soils. I'm baffled for an explanation, except some aspect of
soil
physics that I don't understand.
Your help in understanding this would be greatly appreciated.
Thanks
Renee
***************
J. Renee Brooks
Western Ecology Division
U.S. EPA/NHEERL
200 SW 35th St.
Corvallis, OR 97333
(541) 754-4684 (Office)
(541) 754-4799 (FAX)
[log in to unmask]
--
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
e-mail: [log in to unmask]
personal home page: http://mypage.iu.edu/~aschimme
home page of the Biogeochemical Laboratories:
http://www.indiana.edu/~geosci/research/biogeochem/
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