Electron microprobes

The electron microprobe is an analytical tool used to determine the elemental composition of earth materials. The instrument is valuable to geologists because it can analyze very small samples that are approximately 5 microns in diameter.

Development of the Electron Microprobe

Geologists often want to determine the concentration of elements in minerals and glasses. Prior to the invention of the electron microprobe, elemental analysis of geological samples was difficult and time-consuming. Furthermore, the analysis of very small samples, less than 0.1 gram, was virtually impossible. The electron microprobe provides geochemists with a rapid means of determining the elemental composition of geological samples even as small as about 5 microns in diameter. This ability to analyze extremely small samples has provided geochemists with a new understanding of the processes that form the rocks found within the earth.

In 1947, Canadian scientist James Hillier made the first patent application for an apparatus that would use a focused beam of electrons as a source of energy to analyze solid materials. In 1949, French scientists Raimon Castaing and André Guinier presented a paper in Delft, Netherlands, on the application of the electron microscope to the analysis of samples, and they drew heavily on Hillier's earlier work. In 1951, Castaing developed the first usable electron microprobe as part of his dissertation research at the University of Paris. The microprobe introduced a whole new era of analytical research for geochemists. Although crude by today's standards, the instrument that Castaing and Guinier built contained all the fundamental elements of modern, more sophisticated electron microprobes.

Fundamental Process of Analysis

The electron microprobe is a complex machine, but the theory behind the analysis is relatively simple. All materials are composed of atoms. These atoms contain clouds of electrons that surround the nucleus, which is composed of neutrons and protons. These clouds of electrons are called shells and have a variety of geometric shapes. Although the exact positions of the electrons within each shell cannot be predicted, each has associated with it a discrete and known energy. These energy values are different for each element in the periodic table. Shells that are closer to the nucleus are called inner shells, and those farther away are called outer shells.

If an electron can be removed from an inner shell, then an electron from an outer shell will drop into the empty position within the inner shell. As the electron falls from an outer shell into an inner shell, it releases some energy in the form of photons. Each photon will have a characteristic wavelength associated with it, which is a function of this change in energy as the electron moves from one shell to another. Because the change in energies for all the electrons in all the elements is known, one can determine which element was responsible for the production of the photon simply by knowing the wavelength of the photon. The relative concentration of each element can be determined by counting the number of photons of a characteristic wavelength that have been generated. Thus, the type and concentration of each element in a geological sample can be determined.

Electrons are very small but can be removed by other electrons if they are moving fast enough. By accelerating a beam of electrons to a high velocity onto the sample surface, scientists can randomly “knock out” electrons of the shells surrounding the nucleus. That is the fundamental process whereby the electron microprobe performs an elemental analysis of a mineral or gas.

Parts and Process

All electron microprobes contain the following fundamental parts: a filament that acts as a source of electrons, an anode used to accelerate the electrons, a series of electromagnets that focus the beam of electrons, a sample holder, a crystal spectrometer used to determine the wavelength of the emitted photons, a photomultiplier tube used to determine the number of emitted photons, a vacuum chamber that contains all the previous parts, and a computer system for reporting findings.

The filament is a very thin wire that carries an electrical current that causes electrons to be emitted. These electrons are attracted to a positively charged metal plate with a hole in it through which the electrons pass. Because electrons have a negative charge, their path can be bent by electromagnets. Electron microprobes use a series of electromagnets that can adjust the focus of the electron beam from a large surface area (about 5 millimeters) to a very fine point (about 5 microns) on the surface of the sample.

The sample holder on modern instruments usually contains slots for up to ten samples and standards. The standards are used to calibrate the instrument for the material that is to be analyzed and have been subjected to rigorous elemental determinations by independent laboratories. The sample holder can be moved so that many different determinations can be made on each sample. As the sample is bombarded by electrons, photons are emitted in all directions, some of which strike a series of crystal spectrometers. These spectrometers are composed of special crystals whose structures act as diffraction gratings for the photons. Thus, they can be tuned to select specific photons of the characteristic wavelength desired for the element under analysis.

Those photons that pass through the crystal spectrometers are counted by photomultiplier tubes, also called the detectors. Each detector sends a signal to a computer that then converts the number of counts to relative concentrations for the elements being analyzed. The entire system is contained in a vacuum because electrons and photons can be absorbed by the air, which will reduce the lowest concentrations that can be analyzed.

Elemental Mapping Techniques

Within the geological sciences, the electron microprobe has become one of the most valuable tools to modern geochemists. It is used to find the composition of virtually all naturally occurring minerals and glasses with an accuracy previously unattainable. The electron microprobe can thus be used to understand the variation of the composition of naturally occurring materials over very small distances. The electron microprobe has been used to understand the origin and evolution of rocks, to determine the composition of new minerals, to develop new theories for the formation of ore deposits, and to help study how the earth has been affected by human activities.

Prior to the invention of the electron microprobe, the best that a geochemist could hope for was the bulk composition of single large crystals found within rocks. Furthermore, older analytical techniques were, for the most part, highly destructive to the sample under investigation. Thus, once the analysis was complete, the sample was usually lost and no check on accuracy was possible. With the invention of the electron microprobe, these problems were solved. The sample is prepared by making a highly polished surface. By virtue of the very fine focus available on the electron microprobe, many points on the sample can be analyzed. This process is called elemental mapping of samples.

By using elemental mapping techniques, geochemists can determine the variation of the composition from the core to the rim of minerals. Much information on the origin and evolution of rocks can be deduced using this method of research. The composition of igneous rocks (those rocks that have solidified from a molten magma) often changes during the crystallization history. By determining the chemical variation in minerals that formed during the crystallization of the magma, geologists can understand better how and in which sequence these minerals formed. Quite often, the composition of certain minerals found in rocks is a function of the temperatures and pressures of the environment in which they formed. Thus, these two parameters can be determined using an electron microprobe. That is especially valuable for metamorphic rocks (those rocks that formed from previously existing rocks as a result of a change in temperature and pressure) as well as for igneous rocks. Sedimentary rocks (those rocks that form at or near the earth's surface) are also good candidates for study using the electron microprobe. Many sedimentary rocks have been subjected to a variety of processes during their formation, which include erosion, transportation, and deposition. The very small changes in the composition of the minerals found in sedimentary rocks often reflect processes that formed them.

Analyzing Rare and Valuable Materials

The electron microprobe, used in the years following its creation to investigate samples collected from the earth, was central to the scientific study of the origin and evolution of the rocks that were collected on the moon. Thus, for example, the modification of the moon's surface by meteorite impacts has been understood, in part, by using the electron microprobe on samples collected by the Apollo mission astronauts. Furthermore, geologists have been able to determine the composition of the materials collected on the moon, which, in turn, has helped them in understanding the origin of the earth.

Geologists are continuously finding minerals that have never been described. Although the crystal structure of these new minerals can be determined using an X-ray diffractometer, the composition is best found using the electron microprobe. Most minerals that are discovered today are rare, and it is imperative that they be able to be preserved. For this reason, the electron microprobe is an ideal instrument to use.

Economic ore deposits are occurrences of rocks or minerals that may be extracted from the earth at a profit. Exploration geologists need to understand how known ores formed so that they can find new deposits. One key part of this research is understanding which processes cause a variation in the mineral composition found in such occurrences. Thus, large mining companies often own electron microprobes as analytical tools.

Application to Environmental Geology

The earth has been affected by the industrial activity of humankind. In an effort to repair the damage caused by this activity, geologists study how certain activities have affected earth materials and how to prevent further damage to the earth.

The application of the electron microprobe is also widespread in the area of environmental geology. One example involves the use of clay minerals to act as a barrier to toxic materials by absorbing them onto the clay's surface. By finding the amount of a given toxic material that has been absorbed by a particular clay, geologists can decide whether the clay is a suitable barrier in that case. Environmental scientists have needed the aid of the electron microprobe to provide quality microscopic-scale analyses of geological materials to aid in resolving other issues as well. These include the amount of hazardous material released by mining operations into the environment, and the migration of toxic substances in groundwaters.

Principal Terms

diffraction: the bending of waves around obstacles, a process that allows photons of a specific wavelength to be analyzed

electron: one of the fundamental particles of which all atoms are composed; it has an electrical charge of −1

electron shell: a region around the nucleus of an atom that contains electrons; each electron in each shell will have a specific energy associated with it

photon: a form of energy that has the properties of both particles and waves; electromagnetic (light) radiation

Bibliography

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