The use of X-ray photoelectron spectroscopy to estimate the stability of primary mass standards

Because it is still defined in terms of an artefact, the limit on the accuracy with which the SI unit of mass can be realised is largely limited by the surface stability of the kilogram standard. Not only is this true on the case of the International Prototype used to define the unit but also for national standard copies and indeed for mass standards in general. This paper describes the use of X-ray photoelectron spectroscopy (XPS) to study the accretion of contamination of the surfaces of standard masses. Angle Resolved X-ray photoelectron spectroscopy (AR XPS) was used to characterise the surfaces of platinum-iridium and stainless steel mass standards stored either in air or in vacuum, or which have been transferred between the two media. Various analysis methods for XPS data have been compared both in terms of providing an over-layer thickness on the mass standard and in terms of analysing the composition of this layer of contamination. (C) 2006 Elsevier Ltd. All rights reserved.