Tracer diffusion

Tracer Diffusivity Tracer diffusivity, denoted by D g is related to both mutual and self-diffusivity. It is evaluated in the presence of a second component B, again using a tagged isotope of the first component. In the dilute range, tagging A merely provides a convenient method for indirect composition analysis. As concentration varies, tracer diffusivities approach mutual diffusivities at the dilute limit, and they approach selr-diffusivities at the pure component limit. That is, at the limit of dilute A in B, D g D°g and... 

For the special case of self-diffusion (tracer molecules dynamically... 

When the concentration levels are higher, such as Ni diffusion between two olivine crystals with the same compositions except for Ni content (e.g., one contains 100 ppm and the other contains 2000 ppm), it may be referred to as either tracer diffusion or chemical diffusion. Tracer diffusivity is constant across the whole profile because the only variation along the profile is the concentration of a trace element that is not expected to affect the diffusion coefficient. 

Self-diffusivity tracer diffusivity intrinsic diffusivity Thickness... 

PET offers the possibility to quantitatively measure the myocardial blood flow (MBF). MBF tracers can be divided into two groups. The first group is freely diffusible and represented by [ 0]H20. These tracers do not show any specific absorption and their distribution is completely determined by diffusion. Consequently, the measurement of the MBF is based on the first-pass extraction and clearance data. Because of the low heart-to-blood radioactivity ratio, the freely diffusible tracers provide myocardial images with low signal-to-background ratios. The second class is composed of highly extractable heart tracers. The tracer p NjNHs belongs to this family. These radiolabeled compounds are characterized by a selective extraction and retention in the myocardium. The... 

Sr was more stable in vivo but Ag was more lipophilic. These results suggest that generator-produced isotopes such as Rb-82 (T% = 75 sec) sequestered inside cryptands may be useful freely diffusible tracers for measuring blood flow by positron emission tomography. It would be more convenient to make this measurement with generator-produced isotopes than with water from cyclotron-produced oxygen-15 (Th = 122 sec). 

Since the tracer ions (e.g., Na " ) diffuse among particles (e.g., Na0 that are chemically just like themselves, one often refers to the phenomenon as sef/-diffusion (tracer diffusion is a more explanatory term) and to the diffusion coefficient thus determined as the. s -diffusion coefficient. 

Kety SS (1987) Cerebral circulation and its measurement by inert diffusible tracers. In Encyclopedia of Neuroscience, Volume I (Adelman G, ed), pp 206-208. Boston Birkhuser. 

W. Liu and G. Haller. Strange eigenmodes and decay of variance in the mixing of diffusive tracers. Physica D, 188 1-39, 2004. 

Gifford, F. A. (1980) Smoke as a quantitative atmospheric diffusion tracer, Atmos. Environ. 14, 1119-1121. 

In tracer diffusion, however, where the attention is focused upon individual solute molecules that are labeled in some manner, molecular interactions between solvent and tracer, solvent and unlabeled solute, and tracer and unlabeled solute will all contribute to the observed rate of tracer diffusion. Tracer diffusion can be analyzed by considering a ternary system solvent, solute (A), and labeled solute (tracer. A ). The molar fluxes of the individual solute species can be written directly from Equation 4-13 ... 

Self-diffusion, tracer diffusion and tracer impurity diffusion... 

SELF-DIFFUSION, TRACER DIFFUSION AND TRACER IMPURITY DIFFUSION 205... 

Rose CP, Goresky CA, Bach GG (1977) The capillary and sarcolemmal barriers in the heart - An exploration of labeled water permeability. Circ Res 41 515-533 Tancredi RG, Yipintsoi T, Bassingthwaighte JB (1975) Capillary and cell wall permeability to potassium in isolated dog hearts. Am J Physiol 229 537-544 Yipintsoi T, Dobbs Jr WA, Scanlon PD, Knopp TJ, Bassingthwaighte JB (1973) Regional distribution of diffusible tracers and carbonized microspheres in the left ventricle of isolated dog hearts. Circ Res 33 573-587... 

In equation (5.6) k. is the external (diffusive or electrostatic) force on an ion i. In the conductance case, the external force is the applied electrostatic field. For the self-diffusion of a tracer, the external force is the gradient of the chemical potential created by the gradient of (isotopic) concentration of the diffusing tracer. In the acoustophoresis, it is the sound pressure wich induces a differential displacement of each ion so that an induced electric field appears. 

The use of isotopes or tracers is a common means of studying diffusion. Tracer methods permit measurements of self-diffusion, that is, the diffusion of the crystal components in a crystal. Furthermore, they allow measurements of diffusion in homogenous materials, that is, without imposing chemical gradients (when one disregards the difference in atomic weight between atoms in the crystal and of the tracer). 

A common technique is to deposit a very thin film of radioactive isotopes on a plane surface of a sample, and, after subsequent diffusion anneal, determine the activity of diffusion species as a function of distance from the plane surface. If the thickness of the sample is very much larger than the penetration depth of the tracers, the solid can be considered semi-infinite. Furthermore, if the diffusion is homogenous (e.g. taking place by lattice diffusion), the concentration of the diffusing tracers normal to the plane is through solution of Fick s second law with appropriate boundary conditions given by... 

Optimized mobility fimctions were obtained using the PARROT optimization code to evaluate the composition and temperature dependenee of using the available experimental diffusion tracer, intrinsic, and chemical diffusivity data. The experimental data were weighted giving preference to the tracer diffusivity, Z), data, which are independent of concentration, as seen by equation (6)... 

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