Microcrystalline powdered samples

In this technique,27 a laser beam is directed onto a solid microcrystalline ( powder ) sample that is sometimes immersed in an index matching liquid. The emitted second harmonic light is collected, filtered, detected, and compared with a standard (usually a urea powder for the organometallic complexes measured to date). This technique is crude The magnitude of the response depends on particle size, and care must be taken in preparation of samples (e.g., sieving samples to ensure a narrow particle size range). 

Note that mosaic artifacts can also occur physically in real spectra when a real powder sample of a model compound exhibits microcrystallinity and thus contains too few different molecular orientations. This phenomenon is rare in X-band EPR and is usually easily solved by grinding the sample in a mortar it is, however, not at all uncommon even for extensively ground samples in high-frequency EPR with single-mode resonators where the sample size is orders of magnitude less than that of an X-band sample. 

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

The MSC has been shown to work well in several empirical studies [9, 10], which showed an improvement in the performance of multivariate calibrations and a reduction in the number of factors in PCA. For example, NIR reflectance spectra of 20 powder samples of microcrystalline cellulose are shown in Figure 4.8a. Due to differences in particles size from sample to sample, there are significantly different baseline offsets. The same spectra are shown in Figure 4.8b after multiplicative scatter correction. The different baseline offsets observed in Figure 4.8a are so large that they mask important differences in the water content of these samples. These differences are revealed in the water absorption band at 1940 nm after the baseline offsets have been removed by MSC. 

FIGURE 4.8 Illustration of multiplicative scatter correction (MSC). (a) NIR reflectance spectra of 20 powdered samples of microcrystalline cellulose, (b) Same NIR reflectance spectra after multiplicative scatter correction, revealing differences in moisture content. 

Figure 3 shows the true stress strain curves for the consolidated microcrystalline powder. Both tensile and compression results show that sample 2B gives the best UTS and ductility. In tension, the 2B sample shows nearly ideal elasto-plastic response without necking until about 17 percent elongation. The... 

The samples were well-pounded microcrystalline powders uniformly spread between two close-fitting non absorbing ribbon tapes of controlled thickness. 

Within the references column, the condition of the solid-state sample will be denoted accordingly S = single crystal, P = polycrystalline, M = microcrystalline powder if the condition of the sample is unknown, it will not be specified. 

Unless denoted otherwise, measurements were carried out at room temperature. Within the references column, the condition of the solid state sample is denoted accordingly S = single crystal, P = polycrystalline, M = microcrystalline powder if the condition of the sample is unknown, it will not be specified. Under the column entitled "doping," the symbols and will denote that Br and r NMR measurements were carried out, respectively. If the nuclide studied is unknown, it will not be specified. 

Under MAS conditions, it has been observed that as long as the magic angle is set correctly, the NMR linewidth of the halogen decreases relative to the static case in microcrystalline powders without exception.The reduction in linewidth does not appear to depend strongly on the relative magnetogyric ratios, but rather on the combined atomic masses of the atoms in the sample. For example, the C1 linewidth of LiCl is reduced by a factor of about 18, while the I linewidth of Csl is reduced by 1.5. The average reduction due to MAS is ca. 5-10. 

When a single pure crystal cannot be obtained. X-ray powder diffraction can be used instead. It can still yield important information about the crystalline structure, such as crystal size, purity and texture, but the data set may not be as complete as X-ray crystallography. The sample under investigation is usually ground down to a fine microcrystalline powder first. Sometimes the sample must be rotated to obtain the optimal diffraction pattern. 

Materials. The adenine was obtained from Calbiochem as A grade microcrystalline powder and was used without further purification. This is a synthetic product, free from contamination with related natural products. It showed only one spot when checked by thin layer chromatography. Most of the powder passed through a 90/x sieve. A sample was also recrystallized from triply-distilled water. 

The success of DNP-NMR for mesoporous materials could be attributed to the large surface area of these samples that favors interaction between the radicals and the surface sites, and thus their detection. But what about powdered samples with much less specific surface area (<100 m g ) A study on microcrystalline organic solids has been published [27]. The surface of silica nanoparticles prepared by the Stober process and functionalized with aminopropylsilane groups was also studied by the authors of this chapter. The idea was to see whether DNP MAS-NMR could help in the detection of the T units (R—SiOs) grafted on the surface of silica composed of Q units. This is a really important issue for the characterization of functionalized particles. 

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