A redetermination of the argon content of air for buoyancy corrections in mass standard comparisons

A redetermination of the argon mole fraction in air has been undertaken in two samples of dried natural air using mass spectrometric analysis with reference to a suite of gravimetrically prepared synthetic dry air mixtures. The resulting measurement of the argon mole fraction was 9.332 mmol mol(-1) with a combined standard uncertainty of 3 mumol mol(-1). This is significantly different from the value, 9.17 mmol mol(-1), conventionally employed in the CIPM formula for the determination of the density of moist air during mass standard comparisons. Using the presently reported argon mole fraction value in the CIPM formula rather than the conventional value removes the recently identified discrepancy between the two methods of determining the density of moist air during mass standard comparisons: the CIPM formula method and the air buoyancy artefacts method. Nitrogen, oxygen and carbon dioxide mole fractions in the dry air samples were obtained simultaneously.