ELECTRON MICROPROBE ANALYSIS, Second Edition,
S.J.B. Reed, Cambridge University Press, 1997, pp: 321, ISBN 0-521-59944-X; Price: $39.95 (pbk)

Electron Microprobe (EM) is a non-destructive analytical tool applied for determining the chemical composition of tiny amounts of solid materials. It can quantitatively analyze elements from fluorine (atomic number Z=9) to uranium (Z=92) at routine levels of concentration as low as 100 µmol/mol. The versatility of EM analysis lets it be attractive for several investigative applications, mainly ranging in materials science and engineering, including archaeometry and geology. Its impact on the technology of innovative materials is important as it directly influences the development of new composites and the possibility to control the relative production route.

This volume is the second edition of a well-appreciated textbook that professor S.J.B. Reed first prepared in 1975 (of the same author see also Electron Microprobe Analysis and Scanning Electron Microscopy in Geology, 1996, Cambridge University). Its aim is mainly educative, describing the characteristic features of the instrumental apparatus. EM can also be usefully applied for spot chemical analysis of solid-state materials at the micrometer scale. An analytical volume as small as a few cubic micrometers, depending on mass concentration, is possible, thus allowing the detection of small compositional variations within an individual crystal, not otherwise observable by bulk analysis.

The book opens with a historical introduction of the development of EM, starting from the work described in the 1951 Ph.D. thesis of Raymond Castaing, which laid the foundations of the theory and application of quantitative analysis by this technique. The volume then, encompassing the physical principles of EM, analyzes the effects of the interactions of the electron beam with matter and the subsequent X-ray emissions. Distinguished from other electron impact techniques (Scanning EM, Scanning Tunneling EM, Auger analysis, X-ray fluorescence), chemical analysis through EM exploits, and even the discrete energies (and wavelengths) of the consequently emitted X-rays photons, typical of the element from which they are produced. Their corpuscular characteristics, producing the phenomena of ionization, scattering and fluorescence, found the rationale for quantitative evaluation.

In its first part, the book deals with the two fundamental components that comprise the modern electron microprobe, the electron-optical system, (i.e. an electron gun and a series of magnetic lenses and apertures to focus the beam onto a sample), and the X-ray spectrometer. Microprobes also usually have imaging systems and their associated detectors for secondary back-scattered electrons. A comprehensive analysis is offered of the exploitable analytical tools, touching all of the quantitative and qualitative characteristics. The two existing types of spectrometers used to measure the generated X-ray spectrum are described: the wavelength-dispersive spectrometer, mainly used for highly quantitative analyses (WDS), and the energy-dispersive spectrometer (EDS).

The X-ray detector used in a WDS is a proportional counter. The book explains that it corresponds to a gas-filled tube with a coaxial wire held at a positive potential and a thin window to permit the passage of X-rays. These ionize the gas, with free electrons attracted to the wire (anode) and positive ones attracted to the body of the counter (cathode). As a result of the potential in the counter, the free electrons are accelerated sufficiently to cause further ionization. This amplifying effect results in an electric charge on the wire, which is proportional to the energy of the X-ray photons.

The EDS utilizes a solid-state detector. The text encompasses the operative principles of proportional electronic counting systems, until the description of Lithium-drifted Silicon detectors (chapter 9). We can read that X-ray photon absorption leads to the ejection of a photoelectron, which condenses the formation of multiple electron-hole pairs, then taken up by an applied bias to form a charge pulse. This last is converted to a voltage pulse, which is amplified, shaped and finally analyzed by the associated electronics. Advantageous for many applications, such as compositional mapping, EDS has poorer spectral resolution and is, therefore, of limited value for quantitative analysis, especially for trace elements.

Examination also indicates rationales and strategies toward peak selection, background corrections, interference sources and reduction, counting, and limits of detection. Finally there is a comparison of energy versus wavelength dispersive analysis.

In the final part, the book discusses X-ray generation and interaction with matter, or stopping power, electron back-scattering technique and modeling, and it provides a means to interpret the different images for areas of different composition in back-scattered images. Mathematical models for absorption corrections are offered, assuming that the attenuation coefficient is known and taking into account also surface effects or layered samples. Factors related to sample composition or matrix effects can affect the X-ray spectrum produced in an electron microprobe analysis and have to be corrected for if an accurate analysis is to be performed. These matrix corrections are usually called “ZAF corrections,” with reference to the three components of matrix effects: atomic number (Z), absorption (A), and fluorescence (F).

Detailed examinations follow. Two factors affecting X-ray emission dependent on atomic number have to be noted: stopping power, or the ability of a material to reduce the energy of an incident ion by inelastic scattering, and back-scattering, i.e. the ejection back out of some of the electrons penetrating the target. Correction of absorption effects can be done by way of an integration process, relating X-ray intensity per unit mass depth to that produced in an isolated thin layer, qualitatively similar for all materials. Small corrections for fluorescence complete the offered prospect toward a correct and complete handling of the technique: at the surface of the sample ionizing X-rays can be generated by previous ionization of other elements with higher electron energy scattered from below, thus enhancing X-ray fluorescence.

Light element analysis (Z < 10), namely Be, B, C, N, O, F is eventually treated, indicating how in this case a wider d-spacing of the monochromator crystal is needed, and soap films, such as lead stearate, or evaporated multi-layers are used. Appendices briefly treat the origin of characteristic x-rays, their energy and wavelength with useful tables and schemes.

This volume, a tutorial on the fundamentals of Microprobe analysis, is directed toward graduate students and advanced undergraduates and will be of use, in particular, to those readers exactly specializing in this application. The buildup is easy enough, starting at a basic level, along its eighteen agile chapters, but it is necessarily sprinkled with mathematics and assumes some background knowledge of physics, especially of optics and atomic theory, for one to benefit from it fully.

Dr. Enzo Ferrara
Materials Department
Istituto Elettrotecnico Nazionale Galileo Ferraris