Fluorescence is initiated by the bombarding a
material with high energy x-rays or gamma rays and exciting the
electrons of the material. XRF is primarily used for chemical and
elemental analysis and is efficient in identifying metals like
term X-ray comes the energy used
to bombard and excite the material and the term fluorescence refers to
the emission of lower energy radiation after higher energy radiation
has been absorbed.
The X-rays have enough energy to force the electron to surpass its ionization energy and actually eject the electron from the atoms in a 'photoelectric effect', the resulting instability forces other electrons to fall into the gaps and thus release energy. The energy is released as a photon with a specific energy relating to the difference in energy of the orbitals. This emitted radiation is characteristic of the specific atoms present in the material. The wavelength of the emitted radiation can be calculated using Plank's Law. The inner K and L shells are most typically involved, on the x-ray spectrum the peaks are labeled as K,L,M,N to show the shell where the electron was originally located.
Images from http://www.amptek.com/xrf.html.
One type of XRF spectrometer is the wavelength dispersive spectrometers that use a diffraction crystal to focus the specific wavelengths on to the detector. The wavelength range is adjusted by changing the angle of the x-rays hitting the surface of the crystal. The x-rays are then passed from the crystal on to the solid surface detector. The angle of the x-ray, the analytical crystal and detector must all be contained in precise measurements of each other, this is referred to as the Rowland Circle. The crystal is tangent to the circle and the slit for the x-rays and the detector are points on the circle. The circle is pictured to the left from http://serc.carleton.edu/research_education/geochemsheets/wds.html.
One other type
of XRF Spectrometer is the Energy Dispersive spectrometer which focuses
all of the emitted x-rays directly onto an energy analyzing
detector. These are not as sensitive as its wavelength dispersive
partner and also have a lower resolution.
Generally the XRF method
is non destructive to the subject being analyzed. The most
accurate readings are achieved when the material is ground down and
formed into a uniform sample, please refer to the Sample Preparation
page for the methods that the toys are prepared for analysis.
This extensive testing protocol is used by industry when toys are
applying for safety ratings so that their products can be marketed in
the United States and abroad. However, the XRF method is very
versatile since it operates with xray that can penetrate most any
substance. The XRF
method can be none destructive to the material and therefore is great
use in the field. It is also very efficient and very cost
effective. Agencies use portable XRF scanners that use
gamma rays as the energy source (Pictured to the right) to voluntarily
test toys at community events and health fairs. These XRF
scanners can also be rented rather inexpensively to test not only
children's toys but surface paints, soil, minerals and any other
household application. Accuracy has improved with these devices,
a major concern was the control of how deep the x-rays penetrated the
sample. This is an issue when testing a material for surface
paint since the substrate which may be lead free would influence and
lower the percentage results of the surface contaminants.
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