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

Abstract

The new cleaning techniques of low-pressure hydrogen plasma and UV/ozone were studied on stainless steel used in the manufacture of high-quality weight pieces, rhodium and gold using x-ray photoelectron spectroscopy (XPS) and a quartz crystal microbalance (QCM) for direct mass determination. Both cleaning techniques were directly compared with respect to the achieved cleanliness, the remaining contaminants, the speed of action and the change in mass. Both techniques proved to be very efficient for the removal of carbon compounds. Hydrogen plasma additionally also reduces oxides and oxygen compounds, whereas UV/ozone leads to an oxidation of weight steel and rhodium. Plasma cleaning is two to four times faster than UV/ozone and the remaining amount of contamination is lower. Cyclic cleaning using both techniques by turns clearly demonstrated that both modern methods delivered for each technique different but repeatedly and reversibly the same surface chemical state. The change in mass monitored directly with a QCM is in good agreement with the XPS data. The recontamination of the gold-coated quartz crystal after hydrogen plasma was monitored with the microbalance in situ. The gain in mass due to the accumulation of carbonaceous contamination can be explained by a self-limited growth model proposed recently.