On problemsin the definition of the International Temperature Scale arising from the variability of the isotopic composition of some substances used for the fixed-points

The definition of the International Temperature Scale, ITS-90, requires the substances used for the realization of the fixed-points, except He-3, to be of 'natural' isotopic composition. The uncertainty of the best fixed-point realizations for some of these substances is today significantly affected by the natural variability of the isotopic composition of the available samples. The paper discusses the main implications of this problem, which is becoming an obstacle to further improvements for some of the fixed-point realizations. In addition, the two main solutions are compared: (i) to define only a reference composition and (ii) to define a reference composition and allow the use of a correction for the deviations from a reference composition.