Digital Recording of Mass Spectra in Geologic Studies. II

Abstract

The digital system described herein can record data from several mass spectrometers on a single magnetic tape. Programs have been written for isotopic abundance calculations of lead, thorium, uranium, strontium, and argon analyses. The programming techniques successfully recognize and reject spurious data. Both chart and digitally recorded data are presented for seven analyses of the U.S. National Bureau of Standards common lead standard SRM 981. Standard deviations of the ratios of the digital data range from 0.018% for the 207Pb/206Pb to 0.038% for the 208Pb/204Pb. The equivalent hand-processed values are 0.044 and 0.066%, respectively. The measured ratios are offset from their absolute values almost entirely by a fractionation component of about 0.16% per mass unit. Other bias introduced by the system is shown to be very small and probably not greater than 0.01% for the digital data and 0.05% for chart-read data. Calibration of the vibrating reed electrometer amplifier with current flowing through the feedback resistor has considerably improved the analytical precision and reduced the system bias.