Adriaan van der Veen and Katarina Hafner, “Atomic weights in gas analysis”, Metrologia 51, pp. 80-86 (2014).
The publication of the standard atomic weights of 2009 by IUPAC raised the question to what extent these changes affect the calculated composition of reference and calibration gas mixtures and the associated measurement uncertainty. The smallest uncertainties are attainable in the gravimetric gas mixture preparation, so the assessment was made for this technique. For the first time, the uncertainty contributions to the overall budget due to weighing, molecular weights, and composition of the parent gases are reported separately.
The calculations show that even with the standard atomic weights of 2007, the uncertainty contribution due to the molecular weights can be of the same order as that due to weighing. The standard atomic weights of 2009 and 2011, when used under the same assumptions increase the uncertainties of the amount--of--substance fractions of the abundant components from high-purity parent gases sometimes appreciably.
Based on these calculations and the fact that the atomic weight intervals include sources that are unlikely to be generally relevant for measurements supporting trade, commerce, health and safety, there is a need to make a more in-depth analysis of the atomic weights used in calculations, and to reflect the knowledge about the isotopic composition of relevant materials in the value assignment and uncertainty calculation.