Contrast variation

The above description implies that contrast variation and matching can be employed to enhance or suppress the contribution of a relaxation signal from selected subunits of a system. Only the motion of those structures that contribute to the SANS intensity are seen in the corresponding NSE experiment. However, for NSE a few restrictions are to be observed  

The majority of the sample should be deuterated to optimize the intensity to background ratio, in particular at Q l-2 nm where for typical soft matter samples the coherent intensity drops below the incoherent contribution 

Prominent examples of successful application of contrast variation are the investigations of the single chain dynamics of polymers in melts. Here a mixture of about 10% h-polymer in a matrix of d-polymer is used. Further details are obtained by investigating d-polymers that contain only a h-labelled section, i.e. at the ends, at branching points or at its centre in a fully deuterated matrix. 

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The second temi of the above equation gives an important adjustment to contrast variation between the... 

Keywords. Dendrimers, SANS, SAXS, Contrast variation... 

Small-Angle Scattering Experiments Contrast Variation. 185... 

We have presented a new method of using SANS to elucidate the radial structure of dissolved dendrimers [23, 24]. It has been demonstrated that SANS in conjunction with contrast variation [32-37] is a valid tool to determine the internal structure of dendrimers. The main result of [23,24] is the clear proof that the density distribution has its maximum at the center of the molecule. Hence this corroborates the general deductions of theory as discussed in the preceding section. 

SANS offers the unique ability to address this problem by change of the contrast between solute and solvent through use of mixtures of deuterated and protonat-ed solvents [32,33]. In the following section the method of contrast variation as applied to the analysis of dissolved dendrimers will be discussed. 

The central idea of contrast variation is shown in Fig. 5. The dissolved object is depicted as an assembly of scattering units with different scattering power. The entire object occupies a volume in the system depicted by the shaded area in Fig. 5, left-hand side. The local scattering length density r) is rendered as a product of the shape function T (f) and the local scattering length density inside the object ... 

More recently, Loppinet and co-workers have used both SAXS and SANS (with contrast variation methods) to characterize the morphology of dilute PFSI solutions having a volume fraction of polymer less... 

Contrast variation measurements objects are essentially composed of steroid of steroids per unit length of objects is... 

The regio- and enantioselectivity of the cationic nickel catalyst seems not to be affected by the nature of the counter ion in the case of the cyclization of 27a (entries 7, 8). In contrast, variation of the counter ion for substrate 27b influenced not only the activity but also the catalyst selectivity. The enantioselectivity increased from 33% to 54% in the opposite order to that observed for the activity (entries 12-14). Intriguingly, the order of activity imparted from the different anions for the cyclization of 27a differs signiflcantly from that observed for 27b. 

In addition to travel delays, a day may be spent in dialyzing the sample against D20 buffers for contrast variation experiments. 

For contrast variation the EL.O content of a solution has to be replaced by D20. Usually a series of samples is prepared with varying EL.O/D.,0 ratios. For each solution in the series, there must be a buffer with exactly the same composition of aqueous phase, induding small solute molecules and ions. The exchange of IL,0 for D20 is usually performed by dialysis, and so the last dialysis must be taken to equilibrium. 

For food systems in solution and showing no preferred orientation, the most useful data analysis methods are radius of gyration Rg, variation of Rg with contrast, and particle mass. Experimental data are frequently obtained using contrast variation with a series of samples with varying H20/D20 ratios in the aqueous phase of the sample. 

The contrast variation method gives the ability to highlight or blank out different phases of a multi-phase system and adds greatly to the power of SANS. 

Controversial discussions have surrounded the three-dimensional structure of flexible dendrimers in solution during the past two decades. Now, on the basis of numerous SANS experiments using the contrast variation technique, the idea that isolated flexible dendrimers in good solvents do not take on the originally predicted dense shell structure but instead assume a dense core structure [47] appears to be gaining general acceptance. This means that in such dendrimers the segment density reaches a maximum at the centre of the molecule and decreases towards the periphery (cf. Fig. 7.6). 

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