Characteristics and applications of RELAX, an ultrasensitive resonance ionization mass spectrometer for xenon

Abstract

RELAX is an ultra sensitive, time of flight, resonance ionization mass spectrometer specifically for measuring xenon isotope ratios. RELAX combines a resonance ionization source, a cryogenic sample concentrator and a time of flight mass spectrometer. It is the most sensitive instrument currently available for determining xenon isotope ratios. We report recent modifications to the spectrometer and discuss the properties and performance of the instrument in its current configuration. We also present the first data from attempted analyses of individual presolar SiC grains from the Murchison meteorite.

The lifetime against ionization of xenon atoms in the spectrometer is about 100 s. Isotope ratios for the major xenon isotopes can be determined more precisely than 1.5 ‰. The detection limit (3σ over the blank) is 950 atoms ¹³²Xe, and is limited only by variations in the blank, which is currently about 1000 atoms of ¹³²Xe. At peak sensitivity, sample sizes are restricted to less than ∼10⁶ atoms. Thus to achieve precise measurements of isotope ratios it is necessary to take an average from multiple analyses. Repeat analyses of air calibration samples are reproducible to 1 ‰ for the major xenon isotopes over several weeks, allowing us to determine precise and accurate isotope ratios of large samples by averaging multiple analyses.