INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 162

shown in Table 3.5. The reflectivity is between 99.4% and 99.9% at a fixed
glancing angle of 0.07 ° . According to Equation 2.20, the spectral background
could be higher by a factor of 3-6 for the less reflecting quartz glass in
comparison to the other materials. However, this effect is compensated by a
better flatness and purity of commercial quartz-glass carriers. In addition,
quartz glass is chemically resistant against most acids besides hydrofluoric acid.
The other materials still benefit from the absence of a Si peak so that silicon
and related light elements can be determined. The latter is not possible if
quartz-glass carriers are applied. The most resistant material is boron nitride,
which is even suitable for the analysis of strong acids and solvents. Unlike these
carriers, Plexiglas carriers are only applicable to aqueous solutions or suspen-
sions. Plexiglas, however, is an extremely low-priced material so that it is not
necessary to clean and reuse these carriers. Glassy carbon is preferentially used
for electrochemical applications because of its electrical conductivity [38]. In
general, quartz-glass and Plexiglas carriers are the carriers most used for micro-
and trace analyses.
3.5.2FixedAngleAdjustmentforTXRF(“AngleCut”)
The sample carriers are usually inserted in plastic holders and are either put
manually in their final measuring position or first loaded into a sample changer.
From here they can be brought into the measuring position automatically one
after another. The carriers are slightly pressed—preferably upward—either
against two parallel cutting edges or four ball-points with a clearance of about
20 mm. These reference lines or points are fixed and define the plane of
reflection. They also determine the angle of incidence for the primary beam.
This beam should pass between the cutting edges or ball points and should be
reflected under an angle of about 70% of the critical angle of total reflection.
For quartz glass and Plexiglas carriers this angle amounts to about 0.07 ° . The
fixed position of the carriers is the only position necessary for measurements,
that is, for recording the spectra.
A sample changer is recommended for the investigation of a large number or
series of samples. Changers with a capacity of up to 35 carriers are commer-
cially available. Sample changing can be carried out under computerized
control. The total device is usually incorporated in a plastic chamber for
dust protection. This chamber can be flushed with a gas, such as helium or
nitrogen, and can be evacuated if necessary.
3.5.3Stepwise-AngleVariationforGI-XRF(“AngleScan”)
For surface- and thin-layer analyses, the sample has to be present as a flat disk
capable of total reflection of the primary beam. Wafers are especially suited for
this kind of investigation, for example, Si wafers or GaAs wafers. For that
purpose, sample holders are constructed that grip wafers with a diameter
between 100 and 200 mm. The wafers may be stacked in a magazine and may be