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| Titel |
Preparation of water and ice samples for 39Ar dating by atom trap trace analysis (ATTA) |
| VerfasserIn |
R. Schwefel, T. Reichel, W. Aeschbach-Hertig, D. Wagenbach |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068124
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| Zusammenfassung |
| Atom trap trace analysis (ATTA) is a new and promising method to measure very rare noble
gas radioisotopes in the environment. The applicability of this method for the dating of very
old groundwater with 81Kr has already been demonstrated [1]. Recent developments now
show its feasibility also for the analysis of 39Ar [2,3], which is an ideal dating tracer for
the age range between 50 and 1000 years. This range is of interest in the fields of
hydro(geo)logy, oceanography, and glaciology. We present preparation (gas extraction and Ar
separation) methods for groundwater and ice samples for later analysis by the ATTA
technique.
For groundwater, the sample size is less of a limitation than for applications in
oceanography or glaciology. Large samples are furthermore needed to enable a
comparison with the classical method of 39Ar detection by low-level counting.
Therefore, a system was built that enables gas extraction from several thousand liters of
water using membrane contactors. This system provides degassing efficiencies
greater than 80 % and has successfully been tested in the field. Gas samples are
further processed to separate a pure Ar fraction by a gas-chromatographic method
based on Li-LSX zeolite as selective adsorber material at very low temperatures.
The gas separation achieved by this system is controlled by a quadrupole mass
spectrometer. It has successfully been tested and used on real samples. The separation
efficiency was found to be strongly temperature dependent in the range of -118 to -130 Ë
C.
Since ATTA should enable the analysis of 39Ar on samples of less than 1 ccSTP of Ar
(corresponding to about 100 ml of air, 2.5 l of water or 1 kg of ice), a method to separate Ar
from small amounts of gas was developed. Titanium sponge was found to absorb 60 ccSTP of
reactive gases per g of the getter material with reasonably high absorption rates at high
operating temperatures (~ 800 Ë C). Good separation (higher than 92 % Ar content in
residual gas) was achieved by this gettering process. The other main remaining
component is H2, which can be further reduced by operating the Ti getter at lower
temperature. Furthermore, a system was designed to degas ice samples, followed by Ar
separation by gettering. Ice from an alpine glacier was successfully processed on this
system.
References:
[1] Sturchio, N. C., et al. (2004), One million year old groundwater in the Sahara revealed by
krypton-81 and chlorine-36, Geophys. Res. Lett., 31, doi:10.1029/2003GL019234.
[2] Welte, J., et al. (2010), Towards the realization of atom trap trace analysis for 39Ar, N. J.
Phys., 12, doi:10.1088/1367-2630/1012/1086/065031.
[3] Jiang, W. et al. (2011), 39Ar detection at the 10-16 isotopic abundance level with atom
trap trace analysis, Phys. Rev. Lett. 106, DOI: 10.1103/PhysRevLett.106.103001 |
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