Bulk samples, electron microscopy

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

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... 

Following early ETEM investigations using environmental cells, environmental scanning electron microscopy (ESEM) has been developed for characterization of surface effects of bulk SEM samples in the presence of gaseous or wet environments (111-114). The method has been applied to the examination of food, wool fibers (111), and polymers (112) and in the conservation of cultural properties (113). Recently, fuel cell catalysts have been characterized using a low-voltage ESEM with a resolution capability of 2 nm (114). 

X-ray diffractograms (Fig. 4) and petrographic analyses (scanning electron microscopy also used for estimating the surface roughness and micro-heterogeneity of samples) indicate the presence of diverse silicates and oxides on the surface and in the bulk of the vitrocrystalline samples. In addition to the nearly ubiquitous quartz, other minerals were found in several samples gehlenite, albite, diopside, portlandite, pyroxenes... 

Electron microscopy has been performed using a sample synthesised at w = 10, [Cd2+]/[S2 ] = 2, and characterized by 430-nm absorption onset, which corresponds to a CdS diameter equal to 25 A. The microanalysis study shows the characteristic lines of sulfide and cadmium ions, indicating that the observed particles are CdS semiconductor crystallites. The electron diffractogram shows concentric circles, which are compared to a simulated diffractogram of bulk CdS. A good agreement between the two spectra is obtained, indicating the particles keep zinc-blend crystalline structure (fee) with a lattice constant equal to 5.83 A. 

The specific surface areas were determined by means of nitrogen adsorption and the metallic surface areas by using adsorption of 3-methylthiophen in liquid phase (ref. 9). The bulk composition of each sample was determined by chemical analysis and expressed by the atomic ratios Al/Ni and M/Ni. The catalysts were observed by transmission electron microscopy (JE0L 200 CX-TEM) and analysed either globally or at point level with a lateral resolution of 1.5 nm by means of a STEM (VG - HB 501) connected to an energy - dispersive X-ray analyser (EDAX). 

The structure (e.g., number, size, distribution) of fat crystals is difficult to analyze by common microscopy techniques (i.e., electron, polarized light), due to their dense and interconnected microstructure. Images of the internal structures of lipid-based foods can only be obtained by special manipulation of the sample. However, formation of thin sections (polarized light microscopy) or fractured planes (electron microscopy) still typically does not provide adequate resolution of the crystalline phase. Confocal laserscanning microscopy (CLSM), which is based on the detection of fluorescence produced by a dye system when a sample is illuminated with a krypton/argon mixed-gas laser, overcomes these problems. Bulk specimens can be used with CLSM to obtain high-resolution images of lipid crystalline structure in intricate detail. 

In addition, most chemical analyses used for the mass closure and tracer approaches are made from bulk samples collected on filters. Consequently, particles with different origin but similar chemical composition can hardly be distinguished. To cope with such overlaps methods based on electron microscopy coupled with X-ray spectroscopy have been developed [10, 11]. 

---