Basic Theory of X-ray Fluorescence
X-ray Fluorescence Introduction
Although X-ray fluorescence spectroscopy is no longer regarded
as a new instrumental technique for elemental analysis, ongoing
evolutionary developments continue to redefine the role of this
important analytical tool. From the demonstration of the first
principles in the 1960’s to the development of the first
commercial instruments in the 1970’s, the increasing availability
of affordable computational power has a least been as important
to the desirability and acceptance of the technology as innovative
hardware design. With the widespread availability and use of a
32-bit microprocessor personal computer as the industry standard
platform, X-ray fluorescence spectroscopy has become a useful and
complimentary laboratory tool to other techniques.
X-Ray Fluorescence Theory
An electron can be ejected from its atomic orbital by the absorption
of a light wave (photon) of sufficient energy. The energy of the
photon (hv) must be greater than the energy with which the electron
is bound to the nucleus of the atom. When an inner orbital electron
is ejected from an atom, an electron from a higher energy level
orbital will be transferred to the lower energy level orbital.
During this transition a photon maybe emitted from the atom. This
fluorescent light is called the characteristic X-ray of the element.
The energy of the emitted photon will be equal to the difference
in energies between the two orbitals occupied by the electron making
the transition. Because the energy difference between two specific
orbital shells, in a given element, is always the same (i.e. characteristic
of a particular element), the photon emitted when an electron moves
between these two levels, will always have the same energy. Therefore,
by determining the energy (wavelength) of the X-ray light (photon)
emitted by a particular element, it is possible to determine the
identity of that element.
For a particular energy (wavelength) of fluorescent light emitted
by an element, the number of photons per unit time (generally referred
to as peak intensity or count rate) is related to the amount of
that analyte in the sample. The counting rates for all detectable
elements within a sample are usually calculated by counting, for
a set amount of time, the number of photons that are detected for
the various analytes’ characteristic X-ray energy lines.
It is important to note that these fluorescent lines are actually
observed as peaks with a semi-Gaussian distribution because of
the imperfect resolution of modern detector technology. Therefore,
by determining the energy of the X-ray peaks in a sample’s
spectrum, and by calculating the count rate of the various elemental
peaks, it is possible to qualitatively establish the elemental
composition of the samples and to quantitatively measure the concentration
of these elements.