PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 227

The operating voltage of the X-ray tube will usually be set to a fixed value if
a quantitative analysis is also planned. The sensitivity values needed for
quantification are not applicable to any but a fixed operating voltage. As
already mentioned, values of 50 and 25 kV are commonly used. The operating
current of the tube, on the other hand, can be selected in a wide range, normally
between 5 and 60 mA. Its particular setting is irrelevant even for quantification.
However, an upper limit is determined by the dead time of the detection unit.
The current has to be limited to such a value that the dead time losses are kept
below 63%. For trace analyses, this current should be chosen if at all possible. If
the applied current is first set too high, it should be lowered; otherwise, the
sample mass has to be reduced. Finally, the count rate meter should indicate a
value between 300 up to 1 000 000 cps.
4.2.3.1Energy-DispersiveVariant
Normally, the intensity is measured in counts or number of photons depending on
the photon energy, N ( E ). For energy-dispersive spectrometry of X-rays (EDS),
this measurement is carried out simultaneously for all indicated photons and
shown on a display. Only a few parameters have to be set for EDS. The energy
scale can normally be restricted to 10, 20, and 40 keV. The decision depends on
the analytical problem and its approach. Excitation mode (a) requires a 40 keV
recording; mode (b), the usual 20 keV setting; and mode (c), the lower 10 keV
setting. If the multichannel analyzer (MCA) has the usual number of 2000
channels, the channel width will consequently be fixed to 20, 10, and 5 eV,
respectively. A shaping time of 50 μ s is usually selected for a Si(Li) detector; a
shaping time of only 2 μ s is preferable for an SDD. It should be increased when a
better spectral resolution is wanted and should be decreased when a higher count
rate is desirable. Finally, the energy axis has to be calibrated or recalibrated. This
procedure should be be repeated once or twice a day. For that purpose, some
special carriers are prepared and always reused. They are loaded with the residue
of an aqueous solution of certain standard elements. Molybdenum is preferred
for excitation mode (a), iron for mode (b), and titanium for mode (c).
After a warm-up phase of about 1 h, the individual spectra of the different
samples can be recorded. A live-time is preset between 10 and 1000 s; 100 s
setting is commonly selected, and 1000 s setting is only chosen for the detection
of ultratraces. The increasing spectra can be observed on a color display during
measurement. At the end, the spectra are coded and stored for the subsequent
evaluation and analysis. Of course, samples are first examined in one certain
excitation mode. After such examination, the mode may be changed and the
samples may be examined in the next mode.
4.2.3.2Wavelength-DispersiveMode
For wavelength-dispersive spectrometry of X-rays (WDS), a spectral range has
to be chosen first, which should be investigated for a sample or a set of samples.