ID: 160
DATING WITH ATOM TRAP TRACE ANALYSIS OF ARGON-39:
METHODS OF SAMPLE PREPARATION
A. KERSTING, T. REICHEL, W. AESCHBACH-HERTIG
Institute of Environmental Physics, Im Neuenheimer Feld 229, Heidelberg, Germany
E-mail address: Ar39 [email protected]
S. EBSER, F. RITTERBUSCH, M.K. OBERTHALER
Kirchhoff-Institute for Physics, Im Neuenheimer Feld 227, Heidelberg, Germany
Abstract: We have developed an Atom Trap Trace Analysis setup of 39Ar as well as an
extraction and separation system for water and ice samples. For the first explicit demonstration
of dating with 39Ar-ATTA, we took large groundwater samples, degassed them in the field and
separated the argon with a gas chromatographic technique. The argon purification of ice
samples is realized with a setup based on a titanium getter. With argon purities of about 98%
and recoveries above 90%, the setup fulfils the sample requirements of the ATTA apparatus for
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Ar dating.
1. INTRODUCTION
The radioactive isotope 39Ar with a half-life of 269 years is the ideal and only tracer for dating
water and ice in the time range of 50-1000 years. However, its routine application is hindered by
its extremely low atmospheric abundance of 39Ar /Ar = 8.23 x 10-16 [1]. Besides Low-Level
Counting, the laser based atom counting method Atom Trap Trace Analysis (ATTA) is the only
feasible technique for the detection of 39Ar with a maintainable effort. For the application of this
technique to environmental samples, various sample preparation setups are required.
2. METHODS
Preparation of groundwater samples:
Groundwater is degassed in the field by an extraction setup based on membrane contactors. For
this purpose, up to 2500 L of water are pumped through a two-stage filtering system and
degassed by a 6 x 28 Extra-Flow membrane contactor [2]. The gas phase is then compressed to
a maximum of 7 bar onto a 9 L sampling cylinder. Although the degassing efficiency slightly
varies between different elements, isotope fractionation of argon is negligible.
For the gas chromatographic separation of argon we use a similar setup as developed at the
University of Bern [3]. The extracted gas is loaded on 9 columns filled with zeolite, which is
cooled down to -130°C. By using helium as a carrier gas, the different gas components are
flushed from the columns. The process is supervised by a quadrupole mass spectrometer which
analyzes the gas at the outlet of the columns. Argon leaves the columns first as it interacts
weaker with zeolite than the other gas components. It is then captured on an activated charcoal
trap and finally transferred to a container for the transport to the ATTA setup.
We demonstrated that the gas chromatographic system is also capable of separating the krypton
fraction of a gas sample. Further developments for routine krypton preparation are currently
underway.
Preparation of ice samples:
Roughly 10% of the volume of an ice block is trapped air. By melting the block under vacuum
conditions, the trapped air is released and then guttered to remove the reactive gases [4]. The
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remaining gas fraction consists to more than 98% of argon, which is transferred to a container
for 39Ar analysis with ATTA.
3. RESULTS
The extraction unit for groundwater requires about 75 minutes for one sample of more than 60 L
of gas with a degassing efficiency of about 90% for argon. With the argon separation system, 5
to 65 L gas samples can be processed, which takes about 7 hours. We obtain argon purities
above 98% while recovering more than 96% of the argon from the gas sample. First tests of
krypton purification led to a purity and recovery of about 60%.
The argon extraction from ice blocks of between 0.5 kg and 10 kg takes about 3 hours and
results in an argon purity of above 98% and a recovery of at least 90%.
4. CONCLUSIONS
Two different systems for the preparation of argon were developed, one for large groundwater
samples and the other for ice samples. Both systems fulfil the requirements for the 39Ar analysis
via the ATTA apparatus and process the samples within a satisfying period of time. Current
developments focus on the preparation of small ocean and lake samples, as well as on an
improved krypton separation routine.
REFERENCES
[1]
[2]
[3]
[4]
LOOSLI, H., A dating method with 39Ar, Earth Planet. Sci. Lett., 63(1) (1983) 51-62.
doi: 10.1016/0012-821X(83)90021-3.
REICHEL, T., Groundwater degassing and separation of argon from air for 39Ar dating
with ATTA, PhD thesis, Univ. of Heidelberg (2013).
RIEDMANN, R. A., Separation of argon from atmospheric air and measurement of 37Ar
for CTBT purposes, PhD thesis, Univ. of Bern (2011).
SCHWEFEL, R., Methoden zur Probenaufbereitung von Eis- und Grundwasserproben
zur 39Ar-Datierung mittels "atom trap trace analysis". Diploma Thesis, Heidelberg
University (2012).
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