PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 206

A certain pretreatment is generally required, in contrast to conventional
XRF. Samples have to be prepared as solutions, suspensions, fine powders, or
thin sections. Solids must be ground or dissolved. For a determination of
ultratrace components, the matrix of the sample should first be separated and
removed. For that purpose, all techniques that have already been tested and
combined with other methods of atomic spectroscopy, for example, with
atomic absorption spectrometry (AAS) or ICP-OES, can be used. Certain
precautions have to be taken in dealing with small samples, and working with
a clean-bench (a laminar flow cabinet) is mandatory for critical steps of
sample preparation.
On the other hand, TXRF is a variant of energy-dispersive X-ray spectrom-
etry and shares all the convenient features. The complete spectrum is recorded
simultaneously within seconds; it is displayed on a screen, and the registration
can be observed continually during the measurement. A dedicated computer is
usually incorporated for advanced processing of the spectra. Automatic peak
identification is made possible, enhancing the speed and ease of a qualitative
analysis. A visual comparison of two complete spectra enables a fingerprint
analysis.
Quantitative analysis by TXRF is essentially facilitated by the use of only
small amounts of sample. Troublesome matrix effects do not arise—neither
absorption nor enhancement effects. Quantification can therefore be carried
out after the addition of an element serving as the internal standard. For a
single-element analysis, the analyte can itself be used as the standard. For
multielement analyses, any element not present in the sample can be chosen
as the standard against which all the other elements are to be determined. In
this case, the different sensitivity values for these elements are needed. They
have to be determined prior to analysis, but only once for each new
instrument.
Surface and thin-layer analyses can be carried out by TXRF only in
combination with a stratified etching of flat and even samples. However,
another simple variant for a direct analysis is given by GI-XRF (grazing
incidence XRF). The glancing angle of the primary beam has to be varied in
the region of total reflection and the peak intensity of concerned elements
has to be recorded simultaneously. The angle-dependent intensity profiles
give a first qualitative picture of contaminants, layers, and/or a substrate.
However, quantification by an internal standard is not possible because the
primary beam not only passes through a thin upmost layer but also pene-
trates to a greater depth when the critical angle of total reflection is
exceeded. To get a quantitative description of the layered system, an
algorithm has to be applied that is already known from conventional
XRF and is called fundamental parameter method. It is based on a simple
model in order to calculate fluorescence intensities of the individual elements
while allowing for matrix effects. Only one external standard is needed. The
fundamental data can be obtained from tables or partly be calculated by use
of equations.