Analytical electron microscopy techniques

Reiche, I., Yignaud, C., Favre-Quattropani, L., and Menu, M. (2002b). Fluorine analysis in biogenic and geological apatite by analytical transmission electron microscopy and nuclear reaction analysis. Journal of Trace and Microprobe Techniques 20 211-231. 

Aerosol Heterogeneity. The variation of the chemical composition from particle to particle within an aerosol size class has been probed in a number of ways. Single-particle chemical analysis has been achieved by using the laser Raman microprobe (25) and analytical scanning electron microscopy (26). With the electron microscope techniques, the particle can be sized as well as analyzed chemically, so the need for classification prior to sample collection is reduced. Analyzing hundreds to thousands of particles provides the information necessary to track the particles back to their different sources but is extremely time consuming. 

Asbestos can be determined by several analytical techniques, including optical microscopy, electron microscopy, X-ray diffraction (XRD), light scattering, laser microprobe mass analysis, and thermal analysis. It can also be characterized by chemical analysis of metals by atomic absorption, X-ray fluorescence, or neutron activation techniques. Electron microscopy methods are, however, commonly applied for the analysis of asbestos in environmental matrices. 

Coupling FFF with other techniques can enhance measurement capabilities. Here, the possibility of taking fractions after the FFF separation is of great advantage. The use of photon correlation spectroscopy, for example, to determine the size of spheres eluted from sedimentation FFF yields both size and density [75]. Further comparison can be achieved with electron microscopy. In principle, every analytical technique (spectroscopy, microscopy, chemical analysis, etc.) can be performed off-line on fractions from FFF. 

Volume, surface and hydration properties of low-molecular compounds and macromolecules in solution are required in the biosciences for numerous purposes (i) correct application of many physicochemical techniques (e.g., analytical ultracentrifugation, solution scattering and diffraction techniques, electron microscopy, modeling approaches). 

Microscopy is an unusual scientific discipline, involving as it does a wide variety of microscopes and techniques. All have in common the abiUty to image and enlarge tiny objects to macroscopic size for study, comparison, evaluation, and identification. Few industries or research laboratories can afford to ignore microscopy, although each may use only a small fraction of the various types. Microscopy review articles appear every two years m. Jinalytical Chemistty (1,2). Whereas the style of the Enclyclopedia employs lower case abbreviations for analytical techniques and instmments, eg, sem for scanning electron microscope, in this article capital letters will be used, eg, SEM. 

Materials characterization techniques, ie, atomic and molecular identification and analysis, ate discussed ia articles the tides of which, for the most part, are descriptive of the analytical method. For example, both iaftared (it) and near iaftared analysis (nira) are described ia Infrared and raman SPECTROSCOPY. Nucleai magaetic resoaance (nmr) and electron spia resonance (esr) are discussed ia Magnetic spin resonance. Ultraviolet (uv) and visible (vis), absorption and emission, as well as Raman spectroscopy, circular dichroism (cd), etc are discussed ia Spectroscopy (see also Chemiluminescence Electho-analytical techniques It unoassay Mass specthot thy Microscopy Microwave technology Plasma technology and X-ray technology). 

The STEM instrument itself can produce highly focused high-intensity beams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneous crystallographic information can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over 1,000,000. 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

Figure 1. Three levels of analysis for catalyst materials, a) bulk analysis of an entire catalyst pellet, b) surface analysis and depth profiling from the surface inward, c) analytical electron microscopy of individual catalyst particles too small for analysis by other techniques.

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

The recent development and application of methodologies sensitive at the single cell wall level has shown that traditional bulk analytical techniques average out important intrinsic heterogeneity in sampled populations. By exploring the diversity of cell walls using novel cryopreservation techniques for electron microscopy and non-invasive... 

Analytical studies, using scanning electron microscopy and electron microprobe techniques (see Textbox 10), revealed that the mixture used for making this particular type of ceramic material consists mainly of clay, comminuted... 

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