Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13a2078o&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13A-2078
Other
1028 Composition Of Meteorites (3662, 6240), 1094 Instruments And Techniques
Scientific paper
In geo- and cosmochemistry noble gases are analyzed for their abundance and isotopic composition by mass spectrometry in the static mode (no pumping). This results in that they are among the elements with the highest detection efficiency. This is especially so for xenon, where during the course of a normal measurement almost all atoms will have been ionized. Not all of them will have been detected, however, even for 100 percent transmission of the mass spectrometer, if individual isotopes are measured by stepped change of the magnetic field setting. On the other hand, except for special cases, noble gas isotopes other than radiogenic 4He and 40Ar usually have abundances too low for useful detection by multicollection using Faraday cups. Multiple ion counting, i.e. parallel detection of all isotopes of a given noble gas element by ion counting, is the obvious choice to improve this situation and gain (in case of Xe with its 9 isotopes) up to an order of magnitude in sensitivity. We have exchanged the standard detection system of our Nu instruments Noblesse noble gas mass spectrometer with a multi-ion counting unit from Nu instruments, the second they have assembled (the other is at Washington University, St. Louis). The detector contains seven channeltrons and (at the high mass side) a Faraday collector. Xenon can be measured by multiple ion counting in two magnetic field steps, one for the seven even-numbered isotopes (masses 124-136) and a second for the high abundant odd- numbered isotopes 129,131Xe, for which we use a shorter counting time. Static measurement of xenon easily leads to memory effects from previously analyzed samples, depending on the history. Hence, often the real improvement is in the gain in speed by which useful data can be obtained, rather than the actual yield, since extrapolating to "time zero" (gas-inlet into the mass spectrometer) can be performed more reliably. "Real" samples will therefore often be measured for a shorter time than required to detect the maximum number of ions. I will report on first experiences with standard samples. Actual samples planned to be analyzed include presolar grains from meteorites, micrometeorites, and interplanetary dust particles.
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