Removal of mercury contamination on primary mass standards by hydrogen plasma and thermal desorption

Abstract

The removal of a high mercury contamination on a Pt reference mass by thermal desorption was studied directly by x-ray photoemission spectroscopy (XPS). Subsequently the contamination mechanism was investigated. Samples of PtIr and AuPt exposed to vapour of mercury in air were studied using XPS and gravimetric mass determination. We find an extremely rapid mercury contamination which takes place within minutes and reaches an initial equilibrium state after 2 h to 4 h. Roughly 1 to 2 monolayers of mercury adsorbs directly on the metal surface. A natural contamination of carbon and oxygen compounds is at the top. Due to the accumulation of mercury, we find a gain in mass which corresponds to 20 µg to 26 µg for a PtIr standard. XPS data from a historical Pt standard give strong evidence for further average mercury accumulation of (1.3 ± 0.1) µg/year during a period of more than a century. This can be explained by a two-step mechanism presented in this study. The speed of contamination depends on the initial surface conditions. Polishing activates the surface and results in an enhanced accumulation of mercury. Natural contamination by C and O can delay but not prevent contamination. We further demonstrate that the mercury contamination can be removed by both hydrogen plasma and thermal desorption. The removal of mercury by hydrogen plasma can directly be attributed to the synthesis of gaseous mercury dihydrides at low pressures.

Cleaning of mass standards: II. A comparison of new techniques applied to actual and potential new materials for mass standards

“…As very recently reported [29] mechanical polishing can activate the surface and result in an enhanced speed and level of contamination. The different contaminations obtained for an unpolished gold coating and polished samples support this.…”

“…It is well known in surface physics that polishing alone cannot remove all surface contaminants [22,23]. Furthermore, it was shown that mechanical treatment can increase the surface activity [24][25][26][27][28][29].…”

Thermal desorption mass spectrometer for mass metrology

“…Storage conditions (air, vacuum, inert gas) and cleaning methods (alcohol, UV/Ozone, plasma, or thermal desorption) are also factors that affect mass stability. [12][13][14][15][16][17] To understand how surface behaviour affects mass stability after the artefact has been cleaned using different methods and stored under different conditions, it is essential to characterize surface quality by rugosimetric methods (for example, optical scatterometer or X-ray reflectometer, SNOM, or atomic force microscopy 18,19 ) to evaluate mass stability by gravimetric methods by means of mass comparators as well as to identify surface contaminants using spectrometric techniques (for example, XPS (X-Ray photoelectron spectrometry) or ToF-SIMS (Time-of-Flight Secondary ion mass spectrometry)).…”

Towards a new SI: a review of progress made since 2011

“…In particular, the effect of mercury contamination on the mass of platinum-iridium mass standards has received renewed attention, as has the general topic of improved methods of removing carbonaceous surface contamination from artefacts [26][27][28][29][30][31][32][33].…”