Environmental Engineering Reference
In-Depth Information
Semiconductor
detector
Aperture
X-Ray
X-Ray tube
(or a radioisotope source)
λ
Characteristic
of element
Readout
Sample
Figure 9.8
Schematic diagram of an energy-dispersive XRF elemental analyzer
has been treated with lithium and generates an electrical pulse that is proportional to
the energy of the photon. The concentration of the element is determined by
counting the number of pulses.
The energy dispersive XRF has no moving parts and is smaller and cheaper than
the wavelength dispersive XRF. As the semiconductor detector is much closer to the
sample, this results in strong signal and high sensitivity. However, the resolution of
energy dispersive XRF is lower than wavelength dispersive XRF. For the details of
various XRF techniques, readers should consult the topic by Jenkins (1988).
Under the U.S. EPA's SW-846 method (EPA 6200), inorganic analytes of
interest are identified and quantitated using a field portable energy dispersive X-ray
fluorescence spectrometer. Radiation from one or more radioisotope sources (Fe-55,
Cd-109, Am-241, Cm-244) or an electrically excited X-ray tube (Cu, Mo, Ag in
anode) is used to generate characteristic X-ray emissions from elements in a sample.
Up to three sources may be used to irradiate a sample. Each source emits a specific
set of primary X rays that excite a corresponding range of elements in a sample.
When more than one source can excite the element of interest, the source is selected
according to its excitation efficiency for the element of interest.
XRF is unique among all atomic spectroscopic techniques in that it provides a
nondestructive way to analyze samples. This method is well adapted to qualitative
analysis; however, for quantitative analysis, serious problems can be encountered
and reference standards have to be analyzed in matrices that are almost identical in
composition to that of the sample. XRF has been the method of choice for field
applications and industrial process control for the measurement of elemental
composition of materials. Detection limits are poor, in the range of 0.01-10%. It
does not work for light elements such as C, B, because they do not have L or M
shells. XRF as a tool for field screening of heavily contaminated media has become
increasingly important, particularly when hand-held XRF instruments have been
commercially available. Reported uses include brownfield site investigation, testing
metallic soil contamination, contaminated land remediation, industrial hygiene
testing, analyzing bulk soil and powders, elemental dust-wipe analysis, and air
quality-air filter analysis. The detection limits with hand-held XRF are in the ppm
(mg/kg) range.
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