Transmission electron artifacts

Fig. 1-18. Transverse section of compression wood tracheids in tamarack (Larix laricina), showing intercellular spaces (IS), middle lamella (M), the outer (S,), and the inner (S2) layer of the secondary wall, and the lumen (L). The So layer contains narrow, branched helical cavities (HC) as well as two wide drying checks (C), an artifact. Transmission electron micrograph. Courtesy of Dr. T. E. Timell.

Transmission electron microscopy (TEM) can provide valuable information on particle size, shape, and structure, as well as on the presence of different types of colloidal structures within the dispersion. As a complication, however, all electron microscopic techniques applicable for solid lipid nanoparticles require more or less sophisticated specimen preparation procedures that may lead to artifacts. Considerable experience is often necessary to distinguish these artifacts from real structures and to decide whether the structures observed are representative of the sample. Moreover, most TEM techniques can give only a two-dimensional projection of the three-dimensional objects under investigation. Because it may be difficult to conclude the shape of the original object from electron micrographs, additional information derived from complementary characterization methods is often very helpful for the interpretation of electron microscopic data. 

When transmission electron microscopy is used, the specimen has to be extremely thin (on the order of 0.1 to 10 pm) for the highly absorbable electrons to penetrate the solid and form an image. Preparing such a thin solid specimen with minimal artifacts is a very complicated problem that makes sample preparation a crucial step in the use of this technique. Therefore, a substantial part of this chapter (Section 9.3) is devoted to specimen thinning issues in TEM. 

The practical lower limit of emulsion sizing with optical microscopy is on the order of 0.5 (xm. This limit is much lower with electron microscopy, on the order of 0.1 (xm or less with direct observation of frozen samples in a scanning electron microscope, and 0.01 xm or less with replicas and transmission electron microscopy. Sizes smaller than these lower limits can be recognized with each of these techniques, but quantification of the size distribution becomes difficult. Furthermore, at levels of about 0.01 xm, it is extremely difficult to avoid artifacts and subsequent misinterpretations. As mentioned earlier, sample preparation is an extremely important consideration in both optical and electron microscopic techniques. With optical... 

Electron microscopy is in principle ideal for characterization of solid catalysts containing elementary particles of the support of ca 50 nm or larger and particles of the active components of sizes down to 1 nm. The ability to assess the elemental composition on a very small scale by analysis of the emission of X-rays or the electron-loss spectrum has added substantially to the power of the technique. The volume analyzed in transmission electron microscopy is, however, usually very small it is therefore difficult to ensure that the volume studied in the electron microscope is representative for the catalyst. Furthermore the preparation of suitable specimens, that must be thinner than ca 0.1 pm, can also introduce artifacts. It is therefore advisable to combine electron microscopy with results from macroscopic techniques, such as, X-ray line broadening and surface area measurements. If the specimens investigated in the electron microscope are representative for the catalyst, electron microscopy can provide direct information about the build-up of the catalyst even with the fairly complicated catalyst compositions that are sometimes employed to obtain the selectivity required. 

Electron microscopic images of lipoproteins show predominantly spherical shapes. Only nascent HDL appear as stacks of discs by negative-stain transmission electron microscopy. The stacks are artifacts of the method, because in solution nascent HDL... 

This chapter is based both on results reported in the literature and on recent studies of the effect of aging, on the structure of dry archaeological wood. Various insects cause major structural breakdown in dry wood, but enclosure of wood or the presence of toxic extractives in wood appear to have protected many artifacts from attack by insects. Under dry conditions, the effects of age on wood structure appear minimal up to an age of 4400 years. Structural changes are observed only at the ultrastructural level when using transmission electron microscopy. Delaminations in the middle lamella region or in the secondary cell walls are the most commonly reported phenomena. Fissures and loosening of fibers have also been observed. 

Transmission electron microscopy showed Polymer I, as cast from toluene and dried at 121°C for 30 minutes, to have the two phase structure shown in Fig. 4. The diagonal lines extending across the photograph are sectioning artifacts. The polymer shows two continuous phases. 

Polymer II, which uses a different polyester diol from Pol3nner I, but is very similar in other respects has an included second phase which appears in the transmission electron micrograph, Fig. 5, to be about 0.25 y in diameter. The near vertical folds extending across the photograph are sectioning artifacts. 

Fig. 7. Fine structures in the egg and around the ovary revealed by transmission electron microscopy, (a) Adjacent pronuclei in an egg filled with yolk granules. Bar = 10 pm. (b) Adjacent part of the pronuclei in (a). Bar = 5 pm. (c) The viteUine membrane (indicated by seven arrows). The envelope is still not completed, but interrupted. The vitelline membrane separates the egg with yolk granules (below) from the lyrate organ (above). Bar = 5 pm. (d) Ovary with several oocytes. Presumed sperms are visible around the ovary. Bar = 5 pm. (e) Presumed sperms between an oocyte and lyrate organ. Bar = 5 pm. (Q Detailed structure of a presumed sperm of (e). The detailed structure in the sperm cell is different from that shown in Di Palma Alberti (2001). Black bars are not a staining artifact but an unknown structure. Bar = 1 pm. Abbreviations Nl, N2, nuclei O, oocyte S, presmned sperm Yl, tipid-yolk granule Yp, protein-yolk granule (unpubHshed micrographs by Toyoshima Alberti).

The first section is thus divided into two main parts diffraction techniques and their applications, followed by imaging techniques (transmission electron micro-scropy, TEM) and their applications. In both the emphasis is systematically on the possible artifacts and the necessary remedies. ... 

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