Fluorescence (XRF) is a nondestructive method for the elemental
analysis of solids and liquids. The sample is irradiated by an intense
beam, which causes the emission of fluorescent x-rays. The emitted x-rays
can either be detected using energy dispersive or wavelength dispersive
detector. Either the energies or wavelengths of the emitted x-rays are
to identify the elements present in the sample while the concentrations
(how much) of the elements are determined by the intensity of the x-rays.
XRF is a bulk analysis technique with the depth of sample analyzed varying
from less than 1 mm to 1 cm depending on the energy of the emitted x-ray
and the sample composition. The elements commonly detected range from
sodium to uranium. Lighter elements from boron to fluorine may also
Principles of X-ray Energy Spectroscopy
X-ray Energy Spectroscopy (XES) is a technique for rapid, simultaneous
analysis. When excited by an appropriate source, a sample will emit
x-rays of energies
that are characteristic for the elements composing the sample.
By measuring the
energies of x-rays that are emitted from an excited sample and counting
of x-rays of each energy, XRF allows us to identify which elements are
present in a
sample, and also determine the relative concentration of these elements
within the sample.
The sample can be excited by an x-ray source, by a radioisotope source,
or by an electron
beam. Specifically, in X-ray Florescence Spectroscopy (XRF), generated
x-rays are used
as the primacy source. The primary source "excites" the sample by removing/"knocking
tightly bound electrons from the inner-shell orbital of the excited
atom in the sample.
Relaxation of the excited atom to the ground state is accompanied by
the emission of
Figure 1 illustrates the XRF process.
Figure 1: Schematic of the XRF process. Steps 1 & 2, incident
x-ray knocks out an inner
shell electron, 3, higher shell electron fills the empty vacancy, 4,
excess energy given
up as an x-ray (photon).
Figure 2: Kevex 7000 X-ray system in the Goldwater Laboratories at
Figure 2 shows the X-ray system at ASU that is used for the analysis
of the specimens
in Images of Nature.
The X-rays emitted from the exited sample strike a detector, which is
Solid- State detector in the case of Energy Dispersive Spectroscopy
Florescence (EDS XRF). The detector used in Kevex 7000, Goldwater Laboratories,
silicon drifted with lithium, Si(Li) detector (Figure 3a and 3b).
Figure 3(a):The Si(Li) detector schematic
3(b):The XRF sample chamber with Si(Li) detector
The elements that are present in a sample can be identified by the location
energy peaks along the horizontal axis. Since, in theory, the number
of x-rays produced
is proportional to the number of atoms present in a sample, quantitative
concentrations can be determined from the net intensities of the energy
Most samples can be analyzed "as is" for most qualitative analysis and
Some samples may require sample preparation such as pelletizing or casting
as a fusion disk.
Vacuum compatibility of the sample depends upon the element to be detected.
need not be conductive or insulating. For quantitative analysis the
samples should be
homogeneous with a flat smooth surface. Quantitative and qualitative
can be obtained from a bulk infinite thickness sample. Qualitative elemental
in solids/liquids are of the order of parts per million weight percent
(ppm wt%) range and
qualitative elemental data from boron to uranium can be acquired. The
quality of these results depends on the calibration standards.
1. The Kevex 7000 User's Manual, Kevex Corp. California, 1979.
3. Practical X-Ray Spectrometry by R. Jenkins and J.L. de Vries, published
The Macmillan Press Ltd.
4. Y. Yoneda and T. Horiuchi, Rev. Science Instruments 42:1069 (1971).
5. H. Aiginger and P. Wobrauschek, Nuclear Instrument Methods 114: 157
6. Grieken, Rene E. and Andzrej Markowski. Handbook of X-ray Spectrometry:
Methods and Techniques. Marcel Dekker, Inc., New
York (19993) P. 453.