A RESONANCE IONIZATION MASS-SPECTROMETER FOR XENON

A time-of-flight resonance ionization mass spectrometer is described which has been used to measure isotopic ratios of the nine naturally occurring xenon isotopes, with a precision of less than 0.5% for the major isotopes, in samples of 10(-12) cm3 stp of xenon. The precision is essentially limited by counting statistics. A linear mass discrimination up to 0.4% per u, favouring the light isotopes, is exhibited and may be a velocity effect at the detector. No resonance effects non-linear in mass are seen. The sensitivity of the instrument, which has a detection limit of 3 x 10(4) atoms for a particular isotope of xenon, agrees well with theoretical predictions and is comparable to that of the best electron bombardment instruments. Use of lasers with a higher duty cycle and/or a cold finger to concentrate the sample in the ionizing volume could increase the sensitivity by up to two or three orders of magnitude. Interferences due to the non-resonant ionization of hydrocarbons can be measured by tuning off resonance but are negligible in normal operation.