A METHOD FOR DIGITAL RECORDING FOR MASS SPECTRA

Abstract

A method has been devised for the digital recording of mass spectra and for the completely automatic reduction of the data by means of an IBM 7040 digital computer. Spectral analysis of the base-line noise served as the basis of a decision to sample at half-second intervals. For a scanning rate of 20 seconds per unit mass separation, which is used for our lead trimethyl measurements, it is found possible and advantageous to filter the data digitally, using a filter with a half-power point at 0.15 cycles per second which is phase distortionless. The resulting record has a very much higher signal-to-noise ratio than is available directly from the mass spectrometer.After filtering the data, the computer proceeds to note the locations and amplitudes of maxima and minima in the spectra. It identifies the peaks and selects those that are required for computation. Tails due to pressure scattering are computed and corrections for such tails are applied to all peak heights. Decisions are made for each peak to determine whether maximum peak height or the height at the center of the area should be used, since it is advantageous to use different criteria for different peaks in the spectrum. From these data the spectrum is deconvolved and the isotope ratios for the lead samples are computed.While the procedures adopted are of general interest and are not restricted to any particular measuring system, details are given of the system applied in our laboratory and of the tape format used.