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)
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