Stability of Reference Masses I: Evidence for Possible Variations in the Mass of Reference Kilograms Arising from Mercury Contamination

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Abstract
Samples of Pt-10% Ir have been studied by X-ray photoelectron spectroscopy (XPS) to assess environmental contamination of reference kilogram masses. Samples were sputtered clean and distributed to four places where reference kilograms are kept at the NPL. All four specimens showed carbon and oxygen contamination which will be reported elsewhere in detail. All four specimens additionally showed mercury contamination, with levels that varied from venue to venue. Of all metals, mercury has the highest vapour pressure, is commonly used in laboratories and reacts with platinum; with hindsight our results are not surprising. The most contaminating venue studied, leading to an effective mass increase of a reference kilogram due to mercury of 0,26 μg/day, was the case enclosing the UK primary balance. The mercury is not removed by washing or by scrubbing with a chamois leather soaked in ethanol and ether. The mercury level is therefore probably stable after a period following manufacture, unless the mass is exposed to a venue with a higher mercury level than previously. Similar studies with stainless steels showed no effect. This problem is, therefore, probably confined to the Pt and Pt-Ir masses only. We recommend that in relation to Pt or Pt-Ir masses (i) all equipment containing mercury be removed from weighing laboratories, (ii) venues be monitored for mercury by XPS, and (iii) further work be done to determine whether best mass stability would be achieved by controlled mercury saturation after the reference mass is made, or by storage in a rigorously contaminant-free environment.