Abstract: A mass spectrometer, including a getter-ion pump, in which the sample to be studied is admitted to the spectrometer in pulses at spaced intervals, the rate of gas admission being so chosen, with respect to the pulse duration and interpulse interval, that the amount of gas admitted during each pulse does not exceed the capacity of the ion-getter pump to remove in the interval before the next pulse. The average gas density accordingly does not vary unduly, while the ion concentration during the pulses is appreciably greater than the average ion concentration, and the interior of the spectrometer is in substantially free communication with the gas to be analyzed during the pulses.