1) NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride G75 0QF, UK, 2) National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK Geochimica et Cosmochimica Acta (2011), V75(23), pp7494–7501, doi: 10.1016/j.gca.2011.09.042 New high-precision measurements of the isotopic composition of atmospheric argon D.F. Mark (1), F.M. Stuart (1), M. de Podesta (2) Earth’s atmosphere is used as a standard reference gas for mass spectrometric determinations of argon (Ar) isotopes used principally in geochronology. There are three published independent determinations of the Ar isotope composition of modern atmosphere that differ subtly. We have made new high-precision measurements of Ar isotope ratios of five different sources of air using a high-sensitivity multi-collector noble gas mass spectrometer in order to distinguish between them.