Use of a double-spike to determine the half-life of 39Ar and the operating characteristics of a mass spectrometer used in geochronological studies

Abstract

A double-spike, containing 36Ar, 38Ar, 39Ar, and 40Ar, was used to investigate the operating characteristics of an 40Ar/39Ar dating system. Analyses conducted whilst the (MS10) mass spectrometer yielded 40Ar/36Ar ratios for atmospheric argon differing by ~4%, indicate the mass discrimination for intermediate isotopes is linear in nature. Runs utilizing a fraction of the spike confirm the validity of a theoretical orifice correction factor. In conjunction with earlier analyses on the double spike, we report a mass spectrometrically determined half-life for 39Ar of 276 ± 3 a, ~3% greater than derived from disintegration rates.