During X-ray photoelectron spectroscopic (XPS) analysis, the sample is exposed to a monochromatic X-ray beam, which causes the emission of photoelectrons from the surface. The photoemission effect is described by simple theoretical relationship as shown in Figure 1 which relates the binding energy of the emitted photoelectron (Eb, characteristic of the element and atomic orbital from which they are emitted) with the kinetic energy of the emitted photoelectron (Ek,  measured during experiment), energy of the primary photon (hν, known and typically constant) and the work function (φ, constant for spectrometer).  

Low-resolution spectrum or survey is obtained by counting the number of electrons detected at each energy value (Ek < hν).  The survey allows identifying elements present at the surface of the sample based on the Eb of the peaks. Relative elemental quantification is possible by calculating the relative concentration of all detected elements (at%), using values of area under the peak (measured) and sensitivity factors (element and atomic orbital specific, provided by the manufacturer). 

The greatest strength of XPS is its ability not only to differentiate elements but also determine their chemical environment. Formation of a chemical bond between two different atoms results in changes the energy of the ground states of the atoms, which in turn changes their respective binding energies. This change in Eb is referred to as chemical shift. To detect such chemical shifts, the analyzer is set to acquire series of spectra with a resolution higher than that used for surveys. These high-resolution spectra are the result of contributions from individual chemical species present in a sample and can be resolved and quantified via curve-fitting.