Abstract <p>A signal processing technique for X-ray photoelectron spectroscopy has been developed based on the interpretation of both the peak area and the energy-loss region adjacent to the photoelectron peaks. The methodology is derived from the method of partial intensities. The results of calculations of partial coefficients obtained in the small-angle approximation are in good agreement with those performed using Monte Carlo simulation. It is noted that calculating partial coefficients in the small-angle approximation allows obtaining analytical expressions that considerably reduce computation time compared with the Monte Carlo simulation method traditionally used for such calculations. This methodology has enabled the analysis of changes in the allotropic structure of materials subjected to helium plasma under conditions simulating the plasma–wall interface in nuclear fusion devices. Changes in the allotrope type of MPG-8 grade graphite and tungsten under plasma exposure have been studied. It has been shown that the surface of the MPG-8 sample acquires a structure resembling pyrolytic graphite as a result of plasma exposure. It has been established that the dielectric permittivity of tungsten does not change when tungsten “fuzz” forms on the surface due to plasma action. Growth of tungsten carbide as a result of plasma exposure has also been observed.</p>