Assuming no depth profiling experiments are performed, XPS is typically considered to be a non-destructive analysis technique. However there are many cases where the sample analysed can undergo damage. Such changes arise from X-ray irradiation which can induce:
- Desorption of surface species (may lead to surface reduction)
- Reduction of higher oxidation states
- Electric field induced migration of elements within a sample (for example Li+ and Na+ ions have been known to migrate)
Such damage is particularly evident in organic polymers containing unsaturated bonds and halogens (such as PVC and PVA) and catalytic materials with high valence oxidation states. The figure below show the changes in peak shape with prolonged analysis of a modified polyacrylamide. Note how the peak at 286.6 eV changes between consecutive scans (time ca.5 min between acquisitions).
The figure below shows the reduction of the KAuCl4 salt where the two overlaid spectra were recorded ca. 60 seconds apart, clearly the concentration of the lower Au species increases due to reduction of the Au(III) species at ca. 87 eV.
These observations may also be accelerated by dual neutralisation (low energy electron and argon ion) unless gun modifications to the neutralisation parameters are made.
Steps to Improve Analysis
The analyst may consider the following points
- Record reducible elements first and again at end to judge reduction
- Consider multiple measurement areas
- Consider reduced X-ray power (may affect charge neutralisation in some instances)
- Optimise charge neutralisation parameters on a replicate sample
- Degradation of poly(vinyl chloride) and nitrocellulose in XPS (Wiley)
- Degradation of poly(vinyl alcohol) thin films during monochromatized XPS: substrate effects and x‐ray intensity dependence (Wiley)
- Surface Analysis of Polymers by XPS and Static SIMS (David Briggs, Cambridge Press)
- X‐ray induced reduction of rhenium salts and supported oxide catalysts (Wiley)
- Recent advances in dual mode charge compensation for XPS analysis (Wiley)