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X-ray Fluorescence
X-ray
fluorescence spectroscopy (XRF) is universally recognized as a very accurate
method of measuring the atomic composition of a material by irradiating
a sample with high energy photons such as x-rays or gamma rays and observing
the resulting x-ray fluorescence emitted by the sample.
In general, x-ray fluorescence spectrometers consist of a source of excitation
radiation (an x-ray tube or a radioisotope), a radiation detector to detect
the stimulated radiation from the sample, and a display of the spectral
output. Computers often are used to perform analysis of the data.
In X-ray fluorescence spectroscopy, the process begins by exposing the
sample in question to a source of x-rays or gamma rays. As these high
energy photons strike the sample, they tend to knock electrons out of
their orbits around the nuclei of the atoms that make up the sample. When
this occurs, an electron from an outer orbit, or “shell”, of
the atom will fall into the shell of the missing electron. Since outer
shell electrons are more energetic than inner shell electrons, the relocated
electron has an excess of energy that is expended as an x-ray fluorescence
photon. This fluorescence is unique to the composition of the sample.
The detector collects this spectrum and converts them to electrical impulses
that are proportional to the energies of the various x-rays in the sample’s
spectrum. Since each element has a different and identifiable x-ray signature,
we can look at specific parts of the emitted spectrum, and by counting
the pulses in that sector, determine the presence and concentration of
the element(s) in question within the sample.
inherently uniform in analyte concentration, or the sample can be prepared such that its volume falls within the linear area of the spectrometer’s field of view. (Note: XRF Corporation’s patented scanning spectroscopy technique overcomes the limitations of previous systems and provides the ability to accurately measure non-uniform samples without significant sample preparation.).
Sources of radiation used in XRF instruments are of one of two types. One method uses sealed sources of radioactive material to provide the excitation of the sample. The advantage of using these materials is that an isotope can be selected that provides a mono-energetic beam of radiation that is optimized for the specific application. The chief drawback of the method is the need for periodic source replacements to compensate for source decay. Typically source replacements are needed once yearly and can cost from $1,000 to $5,000.
X-ray tubes similar to those used at the dentist’s office provide a method of producing x-rays electrically. X-ray tubes emit a broad spectrum of radiation. To achieve a mono-energetic beam of radiation, a target is selected which will fluoresce x-rays of the desired energy when irradiated by x-rays from the tube. This approach eliminates source replacement but the size and power requirements of these systems make their use in portable systems impractical.
Several isotopes can be used for portable xrf applications including 109Cd, 57Co, 55Fe and 153Gd. The isotope or combination of isotopes used for any application is selected based on the ability of the selected isotope(s) to induce fluorescence of the element being measured.
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