Absolute isotope amount ratio measurements on gases: Part I: Measurements of isotope amount ratios—Basic theory

A basic measurement model and corresponding measurement function for measuring isotope amount ratios on various gases is described and discussed, as the measurand is different from the one used in differential measurements (δ values).Performed on similar types of ‘hardware’ (a gas isotope ratio mass spectrometer) Isotope Amount Ratio Measurements (‘IARM’) are not intended to replace the traditional (delta) isotope measurements but to complement them: providing “absolute” isotope amount ratio values when there is a need for them. The basic theory is discussed while in a subsequent paper, illustrations by experimental values will be described.The measurement functions used in performing ‘IARM’ describe a transparent and metrologically traceable relationship between an isotope amount ratio of an element (the quantity intended to be measured i.e. the ‘measurand’) to its measured ion current ratio (the quantity subject to measurement), based on the gas flow dynamics in the gas inlet system of the mass spectrometer and due consideration of isotope fractionation effects.