Hi Mimi,
equilibration depends critically on the water temperature.
Have a check for the waterbath temperature with a
0.1 degree C resolution thermometer. I guess you
will find similar patterns like in the isotopes,
in parallel to the heating wires and possibly
cold water input that keep the water temperature
constant everywhere, but at different
temperatures at different places. THe effects you
find are really large, but in principle the same
I found earlier (for CO2 / O18).
My solution was to have an external
temperature-stabilized waterbath and pump this
water through the equilibration bath at high
speed. At least move the water in your bath
around quickly.
The amount of water is also important. NOte that
smaller amounts of water need longer time to
reach equilibrium, as all the gas has to be
"isotopically washed" in less water (Roether,
1970).
This might help for 1. - 3., #4 sounds like a leakage, or frozen watertrap.
Good luck,
Rolf
>Hello,
>
>I am a new graduate student at Syracuse
>University, working on a multi-tracer approach
>to local groundwater problems. Among these
>tracers are oxygen/hydrogen isotopes, which we
>measure using a water equilibration bath (GFL
>1086; 24 samples (3 rows of 8 bottles); 24 Deg
>C; 8 hrs equilibration) connected to a Finnigan
>MAT252 mass spectrometer. Despite great
>efforts, I keep having the following problems
>(with great consistency….):
>
>1. The first sample bottle of each row (samples
>1; 9; 17) yield consistently bad numbers
>(several per mil off), despite gas pressure
>readings being ok.
>
>2. The spread in d18O values for samples from
>the first row of bottles (samples 1-8) is very
>large, with variations of up to 1.5 permil. The
>variations of samples in rows 2 and 3 (samples
>9-24; except 9 and 17 - see above) are much
>smaller (within 0.3 permil). Changes in
>equilibration times does not influence or
>improve this pattern (tests were done for 4, 5,
>6 and 8 hours).
>
>3. Another pattern we consistently see is a
>slight but steady increase in isotope values for
>each individual row (it goes down again at the
>first sample of the next row), resulting in a
>cyclic pattern.
>
>4. Last but not least, once in a while the 24
>sample bottles seem to have very different
>amounts of gas in them but fortunately, this
>does not happen very often!
>
>Has anybody experienced similar problems? Or
>even better: does anybody know how to
>approach/solve any of these problems?
>
>I truly welcome any insights and suggestions on
>how to improve my oxygen isotope measurements
>with the existing equipment.
>
>Best regards,
>Mimi
>
>
>--
>Mimi (Soumitri Sarkar)
>Graduate Student and Teaching Assistant
>Room #212, Heroy Building
>Department of Earth Sciences
>Syracuse University
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