INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 161

3.5.1SampleCarriers

For trace analyses of granular residues, a carrier is required that serves as a
sample support and as a totally reflecting mirror. Of course, only solid-state
media are suitable. They should be highly reflective because the spectral
background is reduced in proportion to 1 R , in accord with Equation 2.20

where R is the reflectivity. Besides, the carrier has to be optically flat to ensure
its high reflectivity, as predicted by theory. The roughness should be less than
5 nm within an area of about 1 mm 2 , corresponding to about λ /100 where λ is the
mean wavelength of the visible light. The waviness should be less than 0.001 °
within an area of about 1 cm 2 , corresponding to a radius of curvature of about
600 m. Commercially available carriers guarantee these characteristics. They
can be checked by means of a profilometer and a contour meter.
Furthermore, carriers should be free of impurities so that no blank values
appear for elements to be detected. The carrier material itself must not have a
fluorescence peak in the spectral region to be considered. In addition, the
carriers have to be chemically inert when strong inorganic acids and organic
solvents are to be analyzed. The carriers should be easy to clean so that they
can always be reused. Finally, they should be commercially available and
inexpensive.
Several materials have been applied as sample carriers, especially quartz glass
(mostly Suprasil  ), as well as polymethyl methacrylate (generally Perspex  or
Plexiglas) [37], and glassy carbon [38], and boron nitride [39]. One-element
materials like silicon or germanium have also been used [40]. Table 3.5 gives an
overview of different carrier materials [41].
Most carriers are on sale as circular discs with a diameter of 30 mm and a
thickness of 2 or 3 mm, but rectangular carriers are also available. Their critical
angle of total reflection amounts to about 0.1 ° for the Mo-K α radiation as

TABLE3.5.ImportantPropertiesofVariousMaterialsUsedasSampleCarriers

Glassy
Carbon

Boron
Nitride

Quartz
Glass

Carrier Material Plexiglas

Critical angle for
Mo-K α

0.08 ° 0.08 ° 0.10 ° 0.10 °

Reflectivity at 0.07 ° 99.8% 99.8% 99.9% 99.4%

Flatness Good Fair Good Excellent

Purity Good Fair Good Excellent

Fluorescence None None None Silicon

Resistance Insufficient Good Excellent Good

Cleaning Not Necessary Difficult Easy Easy

Price for one a $0.10 $30 $60 $28

a US currency.
Source : From Ref. [2], reproduced with permission. Copyright1996, John Wiley and Sons.