New instrument for the study of ‘the kg, mise en pratique’: first results on the correlation between the change in mass and surface chemical state

Abstract

In order to study the practical means of a dissemination of the redefined kilogram, the mass laboratory of METAS has built a new and unique instrument, which combines high precision mass measurement and element-specific surface chemical analysis by x-ray photoelectron spectroscopy (XPS) under identical conditions. It is especially designed for the analysis of large samples such as real 1 kg standards and artefacts of up to 100 mm in diameter. In this paper, we first provide a detailed description of the apparatus. In a second step we demonstrate its performance by applying various processes, such as the transfer from ambient to vacuum, cleaning by low-pressure hydrogen plasma, transfer from vacuum to ambient and recontamination in air with time. Real 1 kg PtIr standards and surface artefacts have been monitored at individual steps gravimetrically, and in vacuum additionally by XPS. Values for the gain and loss of mass due to the application of different processes are provided. A model for the short-term recontamination after cleaning is presented, showing the initial growth of contaminants is self-limited.