RELAX - AN ULTRASENSITIVE, RESONANCE IONIZATION MASS-SPECTROMETER FORXENON

RELAX is a resonance ionization, time-of-flight mass spectrometer to w hich a cryogenic sample concentrator has been added. This has resulted in an increase in sensitivity by a factor greater than 100. The sampl e concentrator consists of a localized cold spot in the ion source, on to which the sample condenses, and a heating laser to release the cond ensed sample into the ionization region. The lifetime against detectio n of a sample atom is close to 20 min, which corresponds to a count ra te of 1 cps from a sample of 1000 atoms, while the mass resolution is 300 (10% peak height). Sensitivity depends on the return time of sampl e atoms to the cold spot (10 s) and the fraction of these atoms subseq uently ionized (approximately 1%). The minimum sample size which can b e measured is limited only by blank, which is currently 2 X 10(-15) cc STP total xenon and isotopically atmospheric (this can be attributed to the large aliquots of xenon admitted to the instrument during devel opment, and so may be expected to decrease with time). The precision o f abundance measurements has been improved by the incorporation of pul se height discrimination and pulse counting detection for the less abu ndant isotopes. The design, construction, and operation of the spectro meter in its new configuration are described with particular attention to abundance extraction. The effects of the sample concentrator on io nization efficiency and discrimination are discussed in detail, as are interferences from nonresonantly ionized hydrocarbons and the means o f accounting for them.