EFFICIENCY AND EVALUATION - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 385

spectrometry (ICP-OES) and can also be applied for TXRF. For analysis, a
small specimen of about 10 μ lor10 μ g of a sample has to be deposited on a flat
glass carrier and dried by evaporation. Sample preparation and presentation
necessitate an extremely clean working space, preferably in a clean bench of
class 100. 2 The glass carriers chosen for deposition have first to be checked
for their cleanliness. The expensive quartz-glass carriers ($40 each) can be
cleaned and reused; the cheap Plexiglas carriers (5 cents each) are usually
applied only once.
The next step for TXRF analysis is the recording of an energy-dispersive
spectrum, which is rather straightforward. A total spectrum or a smaller part of
it can be recorded in some seconds, but usually a counting time of 1 or 2 min is
chosen, and for extreme traces 20 min is preset. The total spectrum is recorded
simultaneously, so that even an element that may be in the sample or specimen
unexpectedly will be detected and no element will be overlooked. The
processing of the spectra is likewise simple and rapid, and is usually done
via a software program. Quantification is performed by internal standardiza-
tion, either by addition of the analyte element in a few concentration steps or
by addition of one other element (i.e., previously not present in the sample) in
a single concentration level. The latter method, which is normally applied
for multielement determinations, saves some more time and effort (see

Section 4.4.2.2).
For micro- and trace analyses, only one angle position is used. For surface
and (even more) for thin-layer analyses (GI-XRF), the sample must be tilted in
several steps for an angle scan. The time needed for angle-dependent intensity
profiles can extend to about 1 h or more. The evaluation requires the adapta-
tion of a model with several parameters by an iterative process until calcula-
tions and measurements are in an acceptable correspondence. A correction
of absorption-enhancement effects by a fundamental parameter method is
necessary. Appropriate software programs for the complex calculations are
available.

6.1.2DetectionPowerforElements

Total reflection XRF is especially valuable because of its high detection power.
All elements with atomic numbers Z 11 (sodium) can be detected without a

vacuum. If windowless detectors are used and a vacuum is applied, even the
lighter elements down to Z = 6 (carbon) may be detectable and the other
elements with atomic numbers Z

20 (calcium) can be determined at a higher
sensitivity. Elements can be detected simultaneously if their spectral peaks do
not strongly overlap, but strong interference is rather an exception. Generally,
some 10-15 (but anyhow less than 20) different elements can be determined in

2 Class 100 for a cleanroom means: < 100 particles/ft 3

of < 3500/m 3

with diameters 0.5 μ m; no

particles 5 μ m (after US Federal Standard 209 D).