Small-angle scattering resolution

The small angle scattering resolution is determined by the closest approach to the primary beam (Fig. 27). This is hmited by the aperture slits which produce a diffuse halo. The guard slits are only used to limit the diffuse scattering without cutting into the primary beam. For a given size of the aperture slits — — the size of the... 

High resolution multidimensional NMR experiments can provide the dendrimer chemist with a wealth of additional information extending far beyond the determination of the molecular structure. In the interpretation of (2D)-NOESY (NOESY=nuclear Overhauser enhancement spectroscopy) spectra, a knowledge of the spatial interrelationships between protons in different parts of the dendrimer scaffold can be acquired from proton-proton NOE interactions. At the same time, the prevailing conformation of the dendritic branches in the solvent used can be deduced from this information. Furthermore, studies of dendrimer/sol-vent interactions and the influence of solvent on the spatial structure of the dendrimer are also possible [22]. Thus the information content of such NMR experiments resembles that of small-angle scattering experiments on dissolved dendrimers (see Section 7.6). 

Fig. 27. Geometry of a focussing small angle scattering camera after the focussing elements. The resolution is limited by the intense background which is due to scattering from the aperture sUt. The guard slit only limits the diffuse scattering without cutting into the primary beam...

Fig. 28. Calculation of the vertical resolution -L- of a small angle scattering camera. Lj corresponds to the distance of the guard slit to the detector plane (see Fig. 27). The other parameters are indicated in the text...

A considerably higher resolution is expected for a small angle scattering camera at a dedicated source like the NSLS at Brookhaven or the projected ESRF which is due to a smaller source point, a smaller vertical divergence and an optimum location of the optical elements. 

Figure 2.13 Schematic elastic scattering curve of a spherical colloid (e.g. a microemulsion droplet) in solution. As a rule of thumb q 1 is an approximate measure for the spatial resolution of the used scattering experiment. At low values of q (i.e. in the Cuinier region of the scattering curve) the overall size and shape of the particles as well as correlations between different particles can be monitored (typically by static and dynamic light scattering). At high values q, the internal structure of the particles, i.e. the local structure of the interfacial film is resolved (e.g. by neutron or X-ray small angle scattering and neutron spin-echo spectroscopy (NSE)).

Slits. Slits are used to shape the beam and to reduce parasitic scatter aroimd the primary beam. The latter is of overriding importance in small-angle scattering beamlines since it is the amoimt of parasitic scatter that is the most important factor in determining the possible low angle resolution and therefore the maximum size of the structures that can be observed (see Fig. 3). 

The characteristics of small-angle scattering does not allow atomic resolution. It is desirable, therefore, to substitute the nuclear scattering lengths, b, with a continuous scattering length density function, p, which averages the b-values over an appropriate volume V, which should be small compared to the instrumental resolution. p is thus defined as... 

Plasma lipoproteins are apparently a continuous, pauci-modal spectrum of particles. By careful separation methods, subfractions can be achieved which show symmetrical boundaries in the analytical ultracentrifuge and appear as uniform by electron microscopy. To be safe, however, one has to consider the limits of resolution of these methods even with all possible precautions taken, a size variation of about l5% would appear as a homogeneous sample by these techniques Together with the above mentioned structural dynamics, this notion has to be respected in the interpretation and discussion of small-angle scattering data from lipoproteins. 

Small angle scattering is a technique for measuring spatial correlations at large distance. The resolution is lost at the atomic and even at the monomer unit scale, typically we can not expect to have an accurate structural information for length smaller than 10 A. The experiment can be achieved either using light. X-ray or neutrons. 

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