One of the issues with XPS quantification is removal of a suitable background. The background signal itself comes primarily from the inelastic scattering of electrons which fail to escape during photoemission.
As the name indicates, this is a simple straight line drawn between tow points on the low and high binding energy dies of the peak in question. Its use is primarily historical due to computational limitations in the early days of XPS analysis, however where there is no significant change in background, for example polymers and other materials with large band-gaps usually have a small step in the background over the energy range covered by the peaks and using such a background can give acceptable results.
This method, derived by D.A. Shirley, takes an S-shaped background for each peak, with the shape iteratively defined by background intensity at low and high binding energies and the intensity of each peak intensity. This method has the effect of reducing asymmetry in the peaks.
The Tougaard background (also known as the universal cross-section approach) is an approach for integrating the intensity of the background at a given binding energy from the spectral intensities to higher kinetic energies.
The figure below shows the three different types of background described here. The green area underneath each peak shows the inelastic background signal which is removed by the employed background subtraction method. From the signal removed, it is evident the background plays a significant part in successful data analysis and the same background should typically be used throughout your analysis.
- Neal Fairly, XPS lineshapes and Curve Fitting in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy ed. D Briggs & JT Grant, IM Publications, Chichester UK, 2003, p. 398.
- M.P. Seah Quantification of AES and XPS in Practical Surace Analysis by Auger and X-ray Photoelectron Spectroscopy ed. D Briggs & M.P.Seah , John Weley & Sons, Chichester UK, 1983 p. 204.
- High-Resolution X-Ray Photoemission Spectrum of the Valence Bands of Gold – D.A. Shirley – APS, Phys . Rev.
- Quantitative analysis of the inelastic background in surface electron spectroscopy – Tougaard, SIA, Wiley
- Comparison of background removal methods for XPS – SIA, Wiley
- “Peak fitting in XPS” – CasaXPS