Abstract
A multiplying digital-to-analog converter (DAC) has been incorporated into the measuring circuit of a conventional double collector mass spectrometer. Inverse feedback of a portion of the signal from the mass-2 peak (H2+) is controlled by the microcomputer so as to exactly cancel the ion current signal of the mass-3 peak (HD+ + H3+). The control to obtain the null point was so successful that we can regard the digital number being sent to the DAC as the proportional value to the ion current ratio. The software simulation of gas exchange in the ionization chamber from the sample gas to the reference gas and vice versa brought about the reduction of the measurement time.
