Biomaterials Tutorial

Electron Spectroscopy for Chemical Analysis (ESCA)

Maxi Boeckl
University of Washington Engineered Biomaterials

Electron Spectroscopy for Chemical Analysis (ESCA)is atechnique also referred to as X-Ray Photoelectron Spectroscopy (XPS).  It is one of the most widely used surface analytical techniques.  It provides valuable information about the chemical surface composition of approximately the top 80 Å of a sample surface. 

The technique is based on the principle of photoemission.  When photons from a high energy X-ray source impinge on a sample surface, the photon’s energy can be directly transferred to a core-level electron in the sample molecules [1].  If the photon energy exceeds the binding energy (BE) of the core-electron, the electron may be ejected as a photoelectron.  Its kinetic energy (KE) is then measured by an energy analyzer.   Through the simple equation, the kinetic energy can be related to the binding energy.

where is the energy of the photon.

Figure 1:  a) Survey spectrum showing the peaks for the individual elemental orbitals.  b) Blown-up region of part a) shows spectrum at high resolution of the sulfur 2p region.  A specialized program was used to fit curves under the peak to identify the individual orbitals.

The binding energy of the photoelectron is characteristic of the element and molecular orbital from which the electron originated, leading to a spectrum of peaks for each orbital of the elements present on the sample surface [2, 3].  The area under each peak is proportional to the number of emitted electrons of that specific binding energy, and thus relates to the number of atoms of that type present.

Bioengineers use ESCA to characterize the chemical composition of biomaterial and model surfaces and to test for contamination of samples.

References:

1. Practical surface analysis, vol. 1. In: Briggs D, Seah MP, editors. Auger and X-ray photoelectron spectroscopy, 2nd edition. New York: John Wiley & Sons, 1995.
2. Ratner BD, Castner DG. In:  Vickerman JC, editor. Surface analysis – The principal techniques, 1st edition. New York, NY: John Wiley & Sons Ltd; 1997. p. 43-98.
3. Methods of surface analysis. Walls JM, editor. Cambridge Cambridge: University Press, 1989.

For more information about ESCA:

http://nb.engr.washington.edu/moreinfo/tutorials/techniques/esca/