• 40Ar/39Ar geochronology;
  • mass spectrometry

Accurate high-precision 40Ar/39Ar ages are limited, in part, by the degree of accuracy and precision of the measurement of the 36Ar atmospheric Ar contamination correction and the mass spectrometer mass fractionation bias (mass discrimination) correction. To improve the measurements of the low-level 36Ar signals, we have implemented digital ion-counting and multicollector data acquisition methods. The switch to the digital ion pulse counting method results in a tenfold improvement in the signal-to-noise ratio relative to analog electron multiplier measurements that are in general use in most 40Ar/39Ar laboratories. The use of ion pulse counting significantly improves low-level signal (36Ar) measurements. The improvement in low-level 36Ar measurements, however, comes at the cost of a reduced dynamic range of the electron multiplier detector, thus requiring the use of an alternate detector at times, such as a Faraday cup or analog multiplier for large signals. In turn, this requires accurate intercalibration of the detectors. Here we present a protocol that addresses these issues, one that closely tracks changes in the mass spectrometer mass discrimination and the detector intercalibration (IC) factor(s) during the time frame of an experiment, thereby improving measurement accuracy. A major advantage of our protocol is that the procedure uses the same aliquots of atmospheric Ar to monitor mass discrimination and detector IC factors, saving a significant amount of measurement time. In addition, this IC protocol may address the cause of reported inaccuracies in the measured isotopic ratio data on the “new” generation multicollector mass spectrometers. Further, we present a “time series protocol” that monitors any temporal drift in the mass spectrometer mass fractionation bias that can occur due to laboratory environmental changes.