Environmental Engineering Reference
In-Depth Information
13.2.5 X-Ray Photoelectron Spectroscopy (XPS)
A sample, introduced in an ultrahigh vacuum chamber, is bombarded with an X-ray
beam. The kinetic energy (in electron volts, eV) of emitted electrons of all elements
(except H and He) present at the surface (analyzed depth between 1 and 10 nm) is
measured with a precision of about 0.2 eV. Shape and position of peaks depend on
the chemical state of the element (the so-called chemical shift effect). Area of peaks
used in combination with sensitivity factors allow to calculate atom fractions with a
detection limit of a few tenths of atom percent. A detailed analysis of certain well-
resolved peaks allows quantifying functionalities present at the surface. On most
recent systems the minimum spatial resolution is of about 15
m for XPS
analysis and XPS imaging, respectively. In most cases XPS can be considered as a
nondestructive technique.
X-ray photoelectron spectroscopy (XPS) is generally regarded as an important
and key technique for the surface characterization and analysis of biomedical
polymers. This technique, also called ESCA (Electron Spectroscopy for Chemical
Analysis), provides a total elemental analysis, except for hydrogen and helium, of
the top 10-200
μ
m and 5
μ
(depending on the sample and instrumental conditions) of any
solid surface which is vacuum stable or can be made vacuum stable by cooling,
chemical bonding information is also provided. Of all the presently available
instrumental techniques for surface analysis, XPS is generally regarded as being
the most quantitative, the most readily interpretable, and the most informative with
regard to chemical information. For these reasons it has been highly recommended
and used by researchers for the analysis of polymers.
Surface analysis by XPS involves irradiating a solid in vacuum with mono
energetic soft X-rays and analyzing the emitted electrons by energy. The spectrum
is obtained as a plot of the number of detected electrons per energy interval versus
their kinetic energy. Each element has a unique spectrum. The spectrum from a
mixture of elements is approximately the sum of the peaks of the individual
constituents. Because the mean free path of electrons in solids is very small, the
detected electrons originate from only the top few atomic layers, making XPS a
unique surface-sensitive technique for chemical analysis. Quantitative data can be
obtained from peak heights or peak areas, and identification of chemical states often
can be made from exact measurement of peak positions and separations, as well as
from certain spectral features [
7
].
XPS can be used to characterize the surface of all types of materials, quite
exclusively solids (powders or bulk specimen) as it is the case in our group:
biomaterials, catalysts, ceramics, fibers, glass, metals, minerals, polymers. The
laboratories sharing the XPS facilities have an uncommon expertise in using the
method for analysis of microbial cells and adsorbed phases on one hand and of
catalysts and dispersed materials on the other hand.
Å
