X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

20.
3
ϕcrit
= ρ . (III.1)
E
For example, for incident Mo Kα (17.5 keV) radiation and quartz glass as reflector, the
critical angle calculates as 1.7 mrad (= 0.1°).
The preferred types of samples are either aqueous or acidic solutions (Figure III.3).
With special sample preparation techniques, the pg/g concentration level can be reached.
There are no corrections for absorption or secondary excitation necessary due to the sample
formation in a very thin layer. In any case the addition of an internal standard of known
concentration is essential for the quantification (typical elements, preferably not present in the
sample are Co, Ga, Ge, Y …). Rewardingly, the calibration curves are linear over several
orders of magnitude and therefore the calculations for converting the measured intensities to
concentrations are simple and can be based on experimentally or theoretically determined
relative sensitivity curves Srel(Z) as a function of the atomic number Z for all elements in
respect to the internal standard element. The concentration wi of an element i can be
calculated by:
n 1
w = w . (III.2)
i
i
n st Srel
st
Note that nst and wst are the intensity and the concentration of the internal standard element.
Figure III.3: Spectrum of a 3 µL mineral water sample, spiked with 1 ng/µL Ga as
internal standard element. Excitation in TXRF geometry with a multilayer
monochromator by a Mo X-ray tube (50 kV, 10 mA, 1000 s measuring time).
The angular dependence of intensities in the regime of total reflection can be used to
investigate surface impurities, thin near-surface layers, and even molecules absorbed on flat
surfaces. From these angle-dependent intensity profiles the composition, thickness and
density of layers can be obtained. It is the low penetration depth of the primary beam at total
reflection that enables also the non-destructive in-depth examination of concentration profiles
in the range of 1 – 500 nm.