Streaked spot

Many fonns of disorder in a surface structure can be recognized in the LEED pattern. The main manifestations of disorder are broadening and streaking of diffraction spots and diffuse intensity between spots [1]. 

Diffraction is usefiil whenever there is a distinct phase relationship between scattering units. The greater the order, the better defined are the diffraction features. For example, the reciprocal lattice of a 3D crystal is a set of points, because three Laue conditions have to be exactly satisfied. The diffraction pattern is a set of sharp spots. If disorder is introduced into the structure, the spots broaden and weaken. Two-dimensional structures give diffraction rods, because only two Laue conditions have to be satisfied. The diffraction pattern is again a set of sharp spots, because the Ewald sphere cuts these rods at precise places. Disorder in the plane broadens the rods and, hence, the diffraction spots in x and y. The existence of streaks, broad spots, and additional diffuse intensity in the pattern is a common... 

Fig. 2. Evolution of an ED pattern on tilting the specimen about an axis perpendicular to the lube axis. (a.b,c) The spots A and B as well as C and D approach one another. In (d) the spots A and B coalesce. In (f) the spots C and D form a single symmetrical streak. The positions of the spots oo.l remain unchanged. On moving the spots A and B as well as C and D describe arcs of the same circles centred on the origin [9].

Fig. 3. (a) Diffraction pattern of a well formed rope (superlattice) of armchair-like tubes. Note the presence of superlattice spots in the inset (b). The broadening of the streaks of 1010 type reOexions is consistent with a model in which the SWCNTs have slightly different chiral angles. 

A combination of 2-propyl ether in the first dimension with a chlorinated solvent mixture in the second was able to resolve both 3-chloro-5-sultone and the dimer sultone, which are not separated by the ether alone, plus all of the other sultones discussed. The sensitivity was increased by a factor of 2-4 because even with very large samples the spots are clearly resolved. When developed in one dimension, a very large sample gives a background streak and very faint spots cannot be seen. 

Usually, the samples completely dissolved in a solvent, preferably chloroform, are applied as narrow streaks or as discrete spots about 1.5 to 2.0 cm from the bottom... 

Figure 4. Electron diffraction pattern (bottom left) of a typical disordered wol-lastonite specimen. Evidence of the disorder comes from the streaked (Tc odd) diffraction spots. The fringes shown in the dark-field image (a) are 7 A apart. Dark-field image b, taken from the streaked diffraction spots, shows a one-dimensional image of the disordered wollastonite. Dark-field image c shows a two-dimensional image which shows the haphazard stacking of the triclinic and monoclinic...

All of these methods fail to show the presence of the epimeric form of the tetracyclines and in most instances streaking of the spots is a problem. A basic improvement in the paper chromatography of these antibiotics was achieved by Selzer and Wright (47) and Kelly and Bryske (48) when they reported methods for the pretreatment of the paper with com-plexing agents to bind the metallic ions which may be present. 

Instead of a microscope slide, you usually use a 12 X 12-in. glass plate and coat it with a thick layer of adsorbant (0.5-2.0 mm). Years ago, I used a small paintbrush to put a line (a streak rather than a spot) across the plate near the bottom. Now you can get special plate streakers that give a finer line and less spreading. You put the plate in a large developing chamber and develop and visualize the plate as usual. 

In TLC, the sample is applied as a small spot or streak to the marked origin of stationary phase supported on a glass, plastic, or metal plate. The sample solvent is allowed to evaporate from the plate that is then placed in a closed chamber containing a shallow pool of mobile phase at... 

A typical gel electrophoresis apparatus is shown in Figure 11.23. The thin gel slab referred to above is contained between two glass plates. The slab is held in a vertical position and has notches at the top where the samples to be separated are spotted or streaked. In the configuration shown in the figure, only downward movement takes place, and thus only one type of ion, cation or anion, can be separated, since there is only one direction to go from the notch. 

Samples are applied as discrete spots or as streaks with capillary or micropipettes as volumes of between 1 and 20 /A. Damage to the surface must be avoided in order to achieve a regular pattern of the separated components. Appropriate reference mixtures must be applied to the same plate as the samples to assist in identifying the test components. 

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