X-Ray fluorescence

When a sample is placed in a beam of X-rays, some of the X-rays are absorbed and the absorbing atoms are excited, or raised to a higher energy state. X-rays with energies characteristic of the particular element are emitted when these atoms decay back to their normal energy states. The intensity of these emitted, or fluorescence, X-rays indicates the abundances of each element in the sample.

Bohr Model

In the atomic model developed in 1913 by Danish physicist Niels Bohr, an atom consists of a positively charged nucleus surrounded by a number of negatively charged electrons that orbit around the nucleus in circles of fixed radii. Only orbits of specific radii are permitted in the Bohr model. The energy required to remove an electron completely from the atom is larger for orbits of smaller radii. Thus, the electrons in orbits of smaller radii are said to have less energy, or to be in a lower energy level. Only two electrons are permitted in each orbit, or energy level. Thus, elements heavier than helium, with an atomic number of two, must have electrons in several different energy levels. The specific energies of these levels depend on the atomic number of the atom involved.

When an atom is placed in a beam of X-rays, a collision between an X-ray from the beam and an electron in a lower electron energy level can result in the ejection of that electron from the atom. This action leaves a vacancy in a low energy level, which is filled by an electron from a higher energy level. The energy given up by the electron when it moves from the higher energy to the lower energy level goes into the emission of an X-ray with a characteristic energy equal to the energy difference between the two electron energy levels. This process, however, leaves a vacancy at the higher energy level, which is filled by an electron from an even higher energy level. Again, an X-ray with an energy characteristic of the energy difference between the two levels is emitted. This process continues until the atom that was disturbed or excited by the X-ray from the incident beam has returned to its ground state, or lowest energy state. The series of X-rays emitted as the atom returns to the ground state have energies characteristic of the atomic number of the atom that was excited. These emitted X-rays, called fluorescence X-rays, form the basis for the X-ray fluorescence method of chemical analysis.

The energy level structure of an atom depends on the charge of its atomic nucleus. Thus, each chemical element has a unique pattern of X-ray fluorescence emission. Chemical bonding into molecules generally disturbs the outer electron shell energy levels, because these electrons participate in the bonding. Because the fluorescence X-rays are associated with the loss of an electron from an inner electron shell, the energies of the emitted X-rays are virtually independent of the molecule in which the element is present in the sample. Thus, the X-ray fluorescence technique of chemical abundance determination is applicable to samples in their natural state and, generally, does not require significant sample preparation.

Development of X-ray Spectroscopy

The first demonstration of X-ray spectroscopy dates to around 1910, when British physicist C. G. Barkla obtained positive evidence of X-ray emission at characteristic energies by each element. By 1912, H. G. J. Moseley had established the relationship between the energies of the fluorescence X-rays and the atomic number of the atom responsible for the emission, laying the foundation for the identification of elements by X-ray emission analysis. This principle was used to validate the existence of the element hafnium, element 72, from its X-ray fluorescence.

Although the potential of this new technique was understood, practical difficulties limited its applicability. The early experiments used an incoming beam of electrons (not X-rays) to eject an inner electron from the sample. This process required the sample to be electrically conductive, and the electron beam caused considerable sample heating. In the mid-1920's, it was recognized that the use of an incoming beam of X-rays would eliminate many of the problems associated with the electron beam, but X-ray sources of high intensity and very sensitive detectors were required. By the mid-1950's, commercial instruments for X-ray fluorescence became available.

Benefits of the Technique

A qualitative measure of the chemical composition of a sample can be obtained by measuring the energies of the fluorescence X-rays that are emitted. These energies can be compared to tables indicating the energies expected for each element. The presence of an element in the sample at a detectable concentration is indicated if X-rays of the energies corresponding to that element are seen in the emitted spectrum. Quantitative chemical analysis can be carried out on the sample by measuring the intensity, or number of X-rays detected, at the wavelengths characteristic of the elements(s) of interest. This intensity is proportional to the number of atoms of that element or elements present in the sample.

The nondestructive nature of X-ray fluorescence analysis makes it the preferred technique for small or rare samples that must be preserved for other types of experimental measurements. The X-ray fluorescence technique is easily adaptable to almost complete automation; thus it is advantageous when a large number of samples needs to be analyzed, such as in mining operations to monitor ore quality.

Complications

In practice, corrections must be made for the efficiency of the X-ray fluorescence process, which depends on the element being detected and the energy distribution of the incoming beam of X-rays. If the sample is thick enough that fluorescence X-rays have a substantial probability of interaction with the sample before escaping and being detected, then corrections for this absorption process must also be made.

These complications delayed the routine application of X-ray fluorescence to the analysis of geological specimens until the early 1960's. By then, appropriate correction techniques had been developed. The most successful of these correction techniques is the preparation of a synthetic control sample of very similar composition to the rock to be analyzed and the comparison of the fluorescence intensities observed from the control to those from the rock under identical analysis conditions. Using this technique, by the mid-1960's most of the major rock-forming elements, particularly aluminum, phosphorus, potassium, calcium, titanium, manganese, and iron, as well as some minor and trace elements, could be measured as accurately using X-ray fluorescence as by the traditional wet chemical or optical spectrograph techniques. For most commonly analyzed rock specimens, the corrections are well determined, and X-ray fluorescence analyses can now be performed down to a sensitivity of about 10 parts per million with better than percent level precision.

Tools for Analysis

The first requirement for X-ray fluorescence analysis is a beam of X-rays to shine on the sample, usually provided by an X-ray tube or a radioactive source. A typical X-ray tube consists of a high-energy electron beam striking a heavy element target. This target then emits fluorescence X-rays, which escape through a window in the X-ray tube and strike the sample. A smaller and more portable X-ray fluorescence apparatus frequently employs a radioactive source that emits X-rays in its decay sequence. Some commonly used sources are iron-55, cobalt-57, cadmium-109, and curium-242. Intense X-ray beams have become available at particle accelerators, such as the National Synchrotron Light Source at Brookhaven National Laboratory, permitting X-ray fluorescence analysis of smaller samples and at lower elemental concentrations than with conventional laboratory instruments.

Two types of detectors are commonly employed to determine the number and energy of the fluorescence X-rays. The first is a wavelength dispersive spectrometer, which uses a single crystal to diffract X-rays of a particular energy into an electronic counter. The wavelength dispersive detector provides very high energy resolution, allowing nearby peaks from two different elements to be separated. The disadvantage of the wavelength dispersive spectrometer is that only a few X-ray energies can be measured at one time, depending on the number of counters that are placed around the diffracting crystal.

In the early 1970's, the energy dispersive X-ray detector was developed. It consists of a silicon semiconductor doped with lithium that produces an electronic pulse proportional to the energy of the X-ray absorbed by the semiconductor. Thus, this device is responsive to all energies simultaneously, allowing the entire elemental composition of the sample to be determined at one time. The energy dispersive detector has two disadvantages. First, it must be operated at very low temperature, requiring liquid nitrogen to cool it; second, its energy resolution is inferior to the wavelength dispersive detector, causing the energy peaks to broaden and frequently overlap. Mathematical modeling of the peaks' shapes is then required to recover information on the number of X-rays in each of the two overlapping energy ranges.

Applications in Science and Industry

In mining, automated X-ray fluorescence systems are used for the continuous analysis of the zinc abundance in flowing slurries of zinc concentrates. Throughout the mining industry, the X-ray fluorescence technique is employed to analyze ores, tailings, concentrates, and drilled cores. In geology, the X-ray fluorescence method of chemical analysis has been applied to all rock types. Because of its sensitivity to elements present in low abundances and its ease of application to a large number of samples, the X-ray fluorescence technique has been employed in a wide variety of geological investigations.

Recently, X-ray fluorescence has been applied to the analysis of particles collected from the air in order to determine the concentrations of toxic elements. In the analysis of airborne particles, the samples are usually collected by passing measured volumes of air through filter paper and then performing an analysis on the bulk material trapped on the filter paper. With the development of more sensitive X-ray fluorescence apparatus, it is sometimes possible to perform elemental analyses on individual dust particles. In addition, the elemental makeup of the particles is frequently useful in determining the source of the air pollution. In some cases, determination of the chemical composition of the particles has allowed identification of the source of the pollution. In agriculture and food science, the X-ray fluorescence method is used in determining the trace element content of plants and foods. This application has been used to monitor the concentrations of insecticides on leaves and fruits.

X-ray fluorescence has also been applied to problems in medicine. The sulfur content of each of the different proteins in human blood, determined by X-ray fluorescence, has proved useful in medical diagnosis. X-ray fluorescence has also been used to determine the strontium content of blood serum and bone tissue. This use was particularly important during the era of aboveground nuclear testing, when radioactive strontium absorption, particularly by children, was a major problem.

A simple X-ray fluorescence spectrometer flew on each of the two Viking spacecraft that landed on the planet Mars in 1976. These spacecraft provided the first determinations of the major element abundances in the soil of Mars. These measurements confirmed a basaltic composition for the soil, indicating that Mars had experienced planetary differentiation—that is, separation into a metallic core and a stony mantle.

Additional Applications

The development of the X-ray fluorescence technique for medical, chemical, geological, and industrial uses has led to a variety of additional applications. For example, X-ray fluorescence is used to observe automotive and aircraft engine wear by determining the concentrations of the metallic iron, curium, and zinc particles suspended in lubricating oils. By identifying the specific element, it is often possible to identify the actual engine component that is wearing.

The nondestructive nature of X-ray fluorescence has made it an ideal technique for the analysis and authentication of art objects and ancient coins. The elemental compositions of inks, paints, and alloys in the object are compared with the compositions in use at the alleged time of production of the object in question. The widespread availability of X-ray fluorescence apparatuses has given rise to a variety of applications for this technique that were not anticipated at the time of its initial development.

Principal Terms

atomic number: the number of protons, or units of positive charge, in the nucleus of an atom

Bohr model: a model of the atom in which electrons move in circular orbits around a positively charged nucleus, with orbits of only certain discrete energies being permitted

energy level: the energy of an electron in one of the permitted orbits of the Bohr model of the atom

fluorescence: light emitted as the result of the decay of an atom from an excited state back to its ground state

ground state: the configuration of an atom such that all of its electrons are in the lowest energy levels that are permitted

X-ray: light in the wavelength range from 10^!8 meter to about 10⁻¹⁰ meter, spanning the range from the ultraviolet to the gamma rays

Bibliography

Adler, I. X-ray Emission Spectrography in Geology. New York: Elsevier, 1966. This classic 258-page text describes all aspects of the theory of X-ray fluorescence analysis as well as the practical aspects of sample preparation, sensitivity, and methods of interpreting the resulting data. Well illustrated and intended for college-level geology students, the book provides a clear introduction to the method of X-ray fluorescence analysis and a thorough bibliography.

Bush, Laura. “The Dynamic World of X-ray Fluorescence.” Spectroscopy 26 (2011): 40-44. Provides current information on X-ray fluorescence technology and applications. Written in a nontechnical manner accessible to the layperson, but contains enough detail to be relevant to professionals in the field of X-ray fluorescence.

Dzubay, T. G. X-ray Fluorescence Analysis of Environmental Samples. Ann Arbor, Mich.: Ann Arbor Science, 1981. This 310-page book describes the applications of X-ray fluorescence analysis to problems of atmospheric science, particularly the chemical characterization of airborne particulate matter. Each chapter contains a reference list of scientific journal articles describing particular applications of the technique. While intended for professionals, most sections should be understandable by college-level science students.

Goldstein, Joseph, et al. Scanning Electron Microscopy and X-ray Microanalysis. 3rd ed. New York: Springer, 2003. An excellent resource for anyone working in a SEM-EMPA lab.

Jenkins, Ron. X-ray Fluorescence Spectrometry. 2d ed. New York: Wiley, 1999. This college-level textbook clearly describes the entire process of X-ray fluorescence analysis, beginning with a historical account of the development of the technique. The sources of X-rays, the X-ray fluorescence emission process, and the various types of detectors are described in detail.

Klockenkeamper, R. Total-Reflection X-ray Fluorescence Analysis. New York: Wiley, 1997. A clear description of the procedures and protocols associated with X-ray and fluorescence spectroscopy. Appropriate for the college student without much background with the field. Illustrations, index, and bibliographical references.

Liebhafsky, H. A., and H. G. Pfeiffer. “X-ray Techniques.” In Modern Methods of Geochemical Analysis, edited by R. E. Wainerdi and E. A. Uken. New York: Plenum Press, 1971. The process by which X-rays are produced and the interaction of these X-rays with matter are thoroughly discussed. A schematic electron-shell diagram clearly illustrates the various X-ray energies emitted in the X-ray fluorescence process. Although intended for college-level readers, this well-illustrated, 25-page chapter should be suitable for students who have completed a high school chemistry course.

Maxwell, J. A. Rock and Mineral Analysis. New York: John Wiley & Sons, 1968. This comprehensive textbook emphasizes the analysis of rock composition by wet chemical techniques but devotes Chapter 11 to the X-ray fluorescence technique. The emphasis in this 22-page chapter is on sample preparation, precision of the analyses, and experimental complications. Suitable for college-level readers.

Pella, P. A. “X-ray Spectrometry.” In Instrumental Analysis, edited by G. D. Christian and J. E. O'Reilly. 2d ed. Boston: Allyn and Bacon, 1986. This well-illustrated chapter describes the process of X-ray fluorescence and the instrumentation normally employed. Some of the complications in inferring chemical compositions from the X-ray spectra are also described. The textbook, intended for undergraduate science students, includes an extensive bibliography.

Pinta, Maurice. Modern Methods for Trace Element Analysis. Ann Arbor, Mich.: Ann Arbor Science, 1978. Chapter 8 of this well-illustrated book describes all aspects of the X-ray fluorescence method and its application to geological, biological, and industrial samples. An extensive reference directs the reader to original sources for a variety of applications of X-ray fluorescence analysis. This 53-page chapter is suitable for advanced high school students.

Potts, Philip J., and Margaret West, eds. Portable X-ray Fluorescence Spectrometry: Capabilities for In Situ Analysis. Royal Society of Chemistry, 2008. This text provides an overview of the limitations and capabilities of the new instruments available in X-ray fluorescence spectrometry. Written for the undergraduate student.

Robinson, J. W., Eileen M. Skelly Frame, and George M. Frame III. Undergraduate Instrumental Analysis. 6th ed. New York: Marcel Dekker, 2004. This textbook, intended for undergraduate science students, includes a chapter on X-ray spectroscopy, which discusses the applications of the technique to chemical abundance determinations. The medical, industrial, and scientific applications of X-ray fluorescence are described.

Tertian, R., and F. Claisse. Principles of Quantitative X-ray Fluorescence Analysis. New York: Wiley, 1982. This comprehensive 385-page text describes all aspects of X-ray fluorescence analysis. Individual chapters describe the process by which fluorescence X-rays are emitted, the instrumentation employed, sample preparation, and the procedure for interpreting the observed spectra. Each chapter contains a comprehensive reference list. Suitable for college-level science students.