Small area X-ray Photoelectron Spectroscopy in the 10’s of micron range has become an important tool for the surface analyst. Two approaches to producing small area XPS information are currently used in commercial XPS instrumentation. One approach employs a micro-focused X-ray probe to limit the sample area illuminated by X-rays during measurement. The second approach, utilised in the AXIS range of spectrometers, is to use a system of electrostatic lenses and apertures to limit the acceptance area of the analyser. In both cases the analysed area on the sample surface is determined by measuring the distance required to receive a 20 to 80% signal increase when scanning the spot across a sharp edge. This is standard practice as described in the literature.
Published work  has highlighted the fact that signal can be obtained over an area many times greater than the quoted spot size when recording XPS data using X-ray microprobe technology. This investigation utilises an approach similar to the aforementioned work to determine quantitatively the total area measured during small area XPS measurements using the virtual probe approach.
 U. Scheithauer, Surf. Int. Anal., 40, 706-709 (2008).