Labeling techniques elastic

Quasi-elastic techniques can also be applied to other types of scattering. For example, as facilitated by the spin echo technique, the method may be may be applied to neutron scattering to look at mobility of smaller regions at larger wavelength light. Isotropic labeling permits studies of motions of selected parts of molecules. 

The macroscopic experimental techniques for determining D, like the Wicke-Kallenbach membrane cell or the packed bed column operate in a transient mode and with a concentration gradient. They yield transport diffusivities. Instead, microscopic experimental techniques like NMR or quasi elastic neutron reflection operated under equilibrium conditions yield self-diffusivities. The latter mechanism leads to spatial uniformization of labeled and unlabeled molecules even without external concentration gradient. The selfdiffusion can also be described by the macroscopic Fickian law or at the microscopic level by a mean square displacement, as applied by Einstein to describe Brownian motion and leading to... 

A number of papers have appeared giving a complete description of this technique which is sometimes known in the literature as elastic recoil detection (ERD). The discussion presented here will therefore be brief. FRES is used to determine the volume fraction versus depth profile of a species labelled by deuteration which was allowed to diffuse into an unlabelled host. The diffusion coefficient is extracted from this profile using a solution to the diffusion equation. 

Elastic recoil detection is one of the few techniques that can quantify hydrogen depth distributions in virtually any material. It is arguable that this technique has had a greater impact on fundamental polymer science than any other ion beam technique, as it can be used in conjunction with deuterium labeling to resolve the depth distribution of nearly identical polymers in blends. 

---