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Semi-quantitative analysis is possible
by measuring the peak areas of specific elemental core lines (I) and by
applying appropriate atomic sensitivity factors (S), also known as relative
sensitivity factors (RSF), using the general equation:
Cx
= (Ix/Sx) / (∑Ii/Si)
where Cx is the atomic fraction of element
x in a sample[1]. The sensitivity factors can be calculated from
theory or derived empirically from the analysis of standard samples. The use of
standard samples is the preferred method (and is the method applied in the
Kratos line of spectrometers). Peak areas are defined by applying an
appropriate background correction across the binding energy range of the peaks
of interest. In general, three types of backgrounds are used: 1) a simple
straight line or linear background, 2) the Shirley background in which the
background intensity at any given binding energy is proportional to the
intensity of the total peak area above the background in the lower binding energy
peak range[2] (i.e. the background goes up in proportion to the total number of secondary photoelectrons
below its binding energy position) and 3) the Tougaard background (or Tougaard
universal cross-section approach) which offers practical background computation
(based on electron energy losses) with more control over the background shape
then the Shirley procedure[3].
The simple linear background suffers from large peak area changes depending on
the position of the chosen end points and is the least accurate. The Tougaard
background is the most accurate but suffers from complications in practical
use, particularly if there are peak overlaps at binding energies above the
integrated peak. The Shirley background is reasonably accurate and its ease of
use has resulted in its widespread adoption.
References:
[1] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.
[2] M.P. Seah, Quantification of AES and XPS, in: D. Briggs, M.P.Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester UK, 1983, p. 204.
[3] Neal Fairley, XPS lineshapes and Curve Fitting, in: D. Briggs, J.T. Grant (Eds.), Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications, Chichester UK, 2003, p. 398.
