INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 129

recorded (an“angle scan”by grazing-incidence X-ray fluorescence or GI-
XRF). Instead of the aforementioned conditions, the following requirements
have to be met:
• The angle of incidence must be capable of being varied between 0 ° and 2 °
in steps of 0.01 ° . This can be realized by tilting the sample around an axis
located on the surface. Great accuracy of the fine-angle control is neces-
sary for quantification and should be better than 0.005 ° absolutely.
• A strong spectral peak of the primary spectrum should be selected by a
suitable monochromator. Only a monochromatic incident beam produces
the angle-dependent intensity profiles that are distinctly determined by
the layered samples. Additional spectral parts, especially strong peaks,
would blur the distinct correlation.
The basic design of TXRF instruments based on these conditions is demon-
strated in Figure 3.1. The primary beam is generated by a high-power X-ray
tube with a line focus. It may either be a fixed or a rotating anode tube. In order
to increase certain peaks in relation to the spectral continuum, thin metal foils
are easily placed in front of the X-ray tube. They work as filters, as mentioned
in Section 1.4.3. By means of a pair of precisely aligned diaphragms or slits, the
beam will be shaped like a strip of paper.
Since the mid-1980s, synchrotron beamlines have been used for excita-
tion [3-5]. This X-ray source certainly provides an ideal primary beam with
natural vertical collimation, polarization, and high brilliance. Of course, it
needs a large-scale machine, the synchrotron, which is now available worldwide
for research work and even for routine analyses (see Sections 1.3.4 and 3.3).
The polychromatic beam of conventional X-ray tubes is deflected by what is
called the firstreflector , and which alters the primary spectrum. For trace
analyses of granular residues, a simple quartz-glass block is sufficient in that
Figure3.1. Basic design of a TXRF instrument. It differs from Figure 1.1b mainly by the addition of
the first reflector. Figure from Ref. [2], reproduced with permission. Copyright1996, John Wiley
and Sons.