  For sputter depth profiling often sample erosion by Ar+ ions is used. Only a
high purity of the sputter gas and a low contamination level of the ion gun
avoids misleading depth profile measurements results. Here a new measurement
procedure is presented, which monitors these parameters. A Si sample is
sputtered inside the instrument and then the surface concentration of the
elements Ar, C, N and O is measured. Results of such measurements of an XPS
microprobe PHI Quantum 2000, which were recorded over a period of 10 years, are
presented.
