XPS, introduced by the Nobel Prize winner Siegbahn in 1949, is the main technique used for qualitative and quantitative elemental analysis of surfaces. It provides significant information on the chemical bonding of atoms. The absorption of high-energy electromagnetic radiation (X-ray or UV) by surfaces leads to the emission of photoelectrons; those generated in the outermost layers emerge from the surface into the vacuum and can be detected. The measure of the kinetic energy of the emitted photoelectrons allows the determination of the binding energies of electrons and the intensity function (number of photoelectrons vs. kinetic energy), and quantitative results are obtained from the knowledge of the number of atoms involved in the emission process.

Ashkenazy et al., 1997, using X-Ray photoelectron spectroscopy (XPS) pointed out the involvement of nitrogen in lead sorption and the lead-oxygen interaction at the carboxyl group on the basis of the decrease in nitrogen concentration and of the shift of oxygen peak. The same technique confirmed that chromium was sorbed onto grape stalks in both its trivalent and hexavalent forms, and allowed the ascertainment of the oxidation state of chromium bound on pine needles. Furthermore it was used to explain the increase of cadmium and lead sorption onto baker’s yeast after modification of sorbent surface by cross linking cysteine.