Surface analytical study of the cause of mass gain of the kilogram prototype

The International Prototype of the Kilogram, which is the only artificial material defining one of the fundamental constants, has been reported to gain mass as time elapses after it is cleaned. The surface of a Pt-Ir alloy that has been used for the newer Kilogram prototypes was analysed by XPS and thermal desorption spectroscopy (TDS) to investigate the cause of mass gain, its time dependence and the effect of the cleaning method. The main cause of the mass gain was found to be adsorption of carbon (in the form of hydrocarbons) and oxygen (as water) from the atmosphere. The amounts of the two contaminant elements were almost saturated 6 months after sample cleaning. Cleaning methods do not make a large difference to adsorption behaviour. Although surface iridium is slightly oxidized, this effect is negligibly small.