Physics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.353g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 353
Physics
1
Resonance Ionization Spectroscopy, Xenon
Scientific paper
Refrigerator Enhanced Laser Analyser for Xenon (RELAX) [1] is an ultrasensitive mass spectrometer designed for the analysis of xenon from meteorites. It combines a selective laser resonance ionization ion source using a Nd:YAG based laser system for the generation of UV light pulses at 10Hz with a low volume (400cc) time-of-flight mass spectrometer. A cryogenic sample concentrator is used to increase sensitivity to the point where one count per second is produced from a sample of 1000 atoms, and this, combined with the continuous measurement of all isotopes and baselines implicit in the time-of-flight technique gives an effective sensitivity 2 orders of magnitude in excess of conventional, single-collector, magnetic sector instruments. In the past year the data acquisition system has been radically altered by the addition of discrimination against electronic noise and the development of a pulse counting system for small quantities of gas (<5,000 atoms of a single isotope). Peak fitting is still used to determine the heights of the peaks in the mass spectrum corresponding to individual isotopes. However, the fitted peak height can now be corrected if the isotopic abundance is sufficiently low for the count rate to be below the saturation level (approximately 5cps). This has resulted in an increase in precision in the measurement of the low abundance light isotopes ^124Xe and ^126Xe. Precisions of close to 10% are achievable in under 10 minutes in calibration aliquots that contain approximately 600 atoms of ^124Xe. Gas can be released from samples using either an argon ion laser microprobe or a tantalum filament microfurnace while being observed through the laser port via a video monitoring system. The blank from the sample extraction chamber in which either the filament furnace or the laser microprobe sample mount can be fitted is not measurably higher than the dynamic blank of the spectrometer (10^-15ccSTP xenon total) however, at high filament temperatures (~1000 degrees C) the filament furnace produces a larger blank (up to 5 x 10^-15ccSTP) over the 5 minute duration of a typical sample extraction. No corresponding increase in blank has been noted when the laser microprobe is in use. High resolution stepped pyrolysis analyses of acid residues from the Murchison meteorite have been performed using the filament furnace in preference to the laser probe because of the greater stability and reproducibility of its temperature control. It is hoped to obtain an optical pyrometer in the near future to allow temperature measurements to be made during sample release. References: [1] Gilmour J. D. et al. (submitted) G Rev. Sci. Inst.
Gilmour James D.
Johnston W. A.
Lyon Ian C.
Turner Gary
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