TRENDS AND FUTURE PROSPECTS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 453

detected under grazing exit angles, that is, at (90 ° ,0 ° ) geometry. In Figure 7.8c,
the primary beam is directed toward the carrier at grazing incidence and the
fluorescence beam is detected at the grazing exit. Both angles are about zero at
this (0 ° ,0 ° ) arrangement, while the angle between both beams is 90 ° . The three
modifications may be called GI-XRF (grazing incidence X-ray fluorescence),
GE-XRF (grazing exit or grazing emission XRF), and GIE-XRF (grazing
incidence and grazing exit XRF). Any further distinction between a fixed and a
variable grazing angle seems to be unnecessary.
All three techniques can involve the effect of total reflection if the relevant
angle is below the corresponding critical angle. This is shown for GE experi-
ments by Figure 7.9. The atoms excited to fluorescence by the primary beam
may be placed above a substrate or within a thin layer on top of a substrate. In
any case, different coherent fluorescence beams may be parallel after reflection
at the substrate and so interfere with each other. The interference pattern of a
standing wave will be highly distinctive if total reflection occurs at or below the
critical angle.
The analytical procedure for all three techniques is the same. Micro- or trace
analyses are performed after a small sample amount is put onto a suitable glass
carrier at a fixed angle. Surface and thin-layer analyses are carried out by tilting
the flat sample and by recording angle-dependent intensity profiles. The
following differences should be considered:
• Under the GE condition the critical angle is determined by the fluores-
cence radiation, whereas under the GI condition it is determined by the
Figure7.9. Beam paths for X-ray fluorescence at grazing exit. The atom excited by the primary
beam is placed (a) above the substrate or (b) within a layer on top of a substrate. Different beams
emitted from the atom may be reflected at the substrate and may interfere with each other. Figure
from Ref. [22], reproduced with permission. Copyright1996, John Wiley and Sons.