Sample rotations

Figure C2.3.12. Two-dimensional neutron scattering by EOggPO gEOgg (Pluronic F88) micellar solution under shear witli (a) tlie sample shear axis parallel to tlie beam, and (b) tlie sample rotated 35° around tlie vertical axis. Reflections for several of tlie Miller indices expected for a bee lattice are annotated. Reproduced by pennission from figure 4 of [84]-...

If the Polaroid filters are held fixed and the sample rotated between them, the Maltese cross remains fixed because of the symmetry of the spherulite. 

Figure 10 SIMS depth profiles with end without sample rotation during bombardment by 3-keV O2 at 40° from normal incidence. ...

The samples rotate around the target at lOOrpm by means of an external motor and pulley assembly, and traverse the neutron beam from 1 to 11cm from the target... 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

It is important to stress that ATR absorbance is strongly affected by the sample/crystal contact. Quantitative results are thus difficult to obtain even if the contact is maintained during the sample rotation that is required to record all four polarized spectra. A reference band that does not show significant dichroism is thus most often used to normalize the polarized absorbances in order to obtain quantitative data. For instance, the 1,410 cm-1 band of PET has often been chosen for that purpose, not only for ATR studies but also for specular reflectance (see below) and even transmission studies when the sample thickness is not uniform. It was shown that an appropriate normalization is possible even if no such reference band is available, by using a combination of two bands with orthogonal dichroism [34]. When performing ATR experiments, one should also make certain that the applied pressure does not create artifacts by affecting the structure of the sample. 

Dvinskikh SV, Castro V, Sandstrom D (2004) Heating caused by radiofrequency irradiation and sample rotation in C-13 magic angle spinning NMR studies of lipid membranes. Magn Reson Chem 42 875-881... 

In 2006 Wimperis et al. proposed a method called satellite transitions acquired in real time by MAS (STARTMAS) [142, 202], which allows for the real-time acquisition of high-resolution NMR spectra of spin-3/2 nuclei under MAS. This method combines a train of pulses, similar to CPMG [109, 110], with sample rotation at the magic angle to refocus the quadrupolar broadening in a series of echoes, while allowing the isotropic quadrupolar shift and chemical shift to evolve. 

Molecules in the solid state are in fixed orientations with respect to the magnetic field. This produces chemical shift anisotropic powder patterns for each carbon atom since all orientations are possible (Fig. 2). It was shown as early as 1958 that rapid sample rotation of solids narrowed dipolar-broadened signals [18]. Several years later, it was recognized that spinning could remove broadening caused by CSA yet retain the isotropic chemical shift [19]. 

The dependence of the PL spectra on temperature is again quite complicated. The change from a Gaussian-shaped PL spectrum at RT to a multi-peak spectrum at 5 K for low excitation energies was initially interpreted in terms of an interference effect in the thin porous layer [Ho3]. Later work, however, showed that the peak positions are independent of PS layer thickness, excitation wavelength and sample rotation [Ke3]. The spacing of the peaks is about 61 meV. The origin of this fine structure is still controversial [Ro4],... 

Concerning the membrane itself, phospholipid hydration was characterized in terms of the types of water, bound and bulk, and their exchange rates, as well as direct observation of the intermolecular contacts between the phosphate headgroup and bound water via HRMAS HOESY and between lipids in mixed membranes via HRMAS NOESY.112 Significantly, Zhou and co-workers found little dehydration of the membranes even when rotation rates as high as 9 kHz were used, providing some comfort that the centripetal forces of sample rotation are not changing the structure of the membrane. 

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