Chemistry Reference
In-Depth Information
Total reflection combined with X-ray diffraction (TR-XRD) and with X-ray
photoelectron spectrometry (TR-XPS) seems to be less frequently applied.
However, the combination of TR with X-ray absorption fine structure
(TR-XAFS) increased in use over the last decade. Special attention was turned
to a particular case of TR-XAFS, which is related to the near-edge of X-ray
absorption and which is called TR-XANES or TR-NEXAFS. All of these
combined methods have special capabilities; the improved sensitivity by
excitation in TR geometry can be used for micro- and trace analysis, for
thin-layer and for local analysis. Moreover, the combinations can be capable for
species analysis, for the determination of crystal structures or periodic struc-
tures of stratified layers, and for detection of molecular structures of chemical
compounds or even of electronic structures of atoms.
Local analysis has to be distinguished from microanalysis as it aims at the
determination of elements at defined positions of microstructured samples.
On the other side, species analysis is a completion of element analysis. It is the
differentiation of similar chemical compounds of the same elements but with
different valences. Species analysis is important if it is necessary to determine
the different element species in addition to the elemental composition of a
sample. This happens especially for biological and environmental samples
when problems of physical tolerability, bioavailability, and environmental
mobility have to be solved. Some methods of spectrochemical analyses,
such as XRD and ICP-MS, are suitable for such a species analysis because
different species can be distinguished simply by different lines. Other methods,
for example, AAS and ICP-OES, can be combined with a chromatographic
separation of species. TXRF itself is not capable of such a differentiation
because the spectral peaks of different species do not differ enough apart from
some examples in the low-energy region of X-rays. However, some of the
mentioned combinations in total reflection or grazing incidence geometry are
capable of species analysis even at trace levels.
Several studies on the combination with X-ray diffraction, X-ray absorption,
and X-ray photoelectron emission have already been published in the
1990s [110-119]. All these methods benefit from a reduced background at
grazing incidence and lead to an improved species or structure analysis. A
condensed consideration of all the combinations will be given in the following
sections.
7.3.1CombinationwithX-rayReflectometry
An ideal supplement to GI-XRF is the method of X-ray reflectome-
try [9,31,120-122]. Both methods allow the absolute determination of density,
thickness, and even roughness of layered materials. However, the element
composition of the layers can only be determined by GI-XRF, whereas the
layer thickness can be measured more precisely by XRR. If the results of the
individual fits are not in accord but ambiguous at first, both methods can work
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