Gradient media

As a consequence of high osmolarity, macromolecules or organelles are dehydrated resulting in altered sedimentation behavior thus, different buoyant densities for nucleic acids and mitochondria are observed in different density gradient media, as illustrated in Table 5.3. Data of some density gradient media are given in Table 5.4. 

In Figure 6 the result of a rapid density gradient centrifugation is shown using this system. It deals with a graft copolymer latex of butadiene and styrene acrylonitrile. The gradient media are different mixtures of 3-butene-2-ol and ethylene glycol. 

Rubidium chloride is used in preparing rubidium metal and many rubidium salts. Also, it is used in pharmaceuticals as an antidepressant and as a density-gradient medium for centrifugal separation of viruses, DNA, and large particles. Other applications are as an additive to gasoline to improve its octane number and as a catalyst. 

Table 5.3. Buoyant density of mitochondria and DNA from Micrococcus luteus (G+C 71%), dependent on the applied density gradient medium...

Density gradient medium Cone. (% w/v) Molarity Density C (g/ml) Refract. 20... 

Ulmer, A. J. and Flad, H. D. (1979) Discontinuous gradient separation of human mononuclear leucocytes using Percoll as gradient medium. J. Immunol. Methods 30, 1-10. 

In rate-zonal centrifugation (or velocity sedimentation) the sample is loaded as a thin layer on the top of a density gradient medium. During centrifugation, the sample separates into bands, and the particles are separated on the basis of their different sedimentation coefficients (s). For biological particles, the coefficient s is mainly related to the size of the particles. This process is illustrated in Figure 13.7(a). 

Thermal Precipitation. Particles in a thermal gradient medium move in the direction of negative gradient, that is, from hotter to colder regions (7, 8). Based on this principle, the instrument typically consists of two parallel round microscopic plates and a heated wire in between as shown in Figure 36. The sample is drawn between the plates, and... 

Fig. 1. Sucrose density gradient profiles of the 5-30% PEG fraction in the presence of (o) and absence of ( ) 5.0 mM GMP in the gradient medium. Glycine incorporation into purines...

Layer blood carefully over Ficoll-Triosil in centrifuge tubes, using a ratio of two volumes of blood to one volume density gradient medium. 

The creation of any RID profile in the gradient medium corresponds to a distribution of the gamma-irradiation doze on the surface of the sample of polymer and is dependent on three factors [17,76] ... 

The differences in buoyant density as well as molecular size and shape provide a means for separating the nucleic acids by isopycnic ultracentrifugation, a centrifugation technique in which particles sediment through a gradient medium (CsCl or sucrose) until they reach a zone of equal densities and band at that position. 

Further developments are required in diffusion technology to obtain gradient elements with the required performance, and better systematic studies of synthesis of the matrix, formation and fixing of gradient medium RID, and determining the precise interconnection between RID formed and the initial gradient composition, experimental conditions, physicochemical and optical characteristics of gradient elements obtained. 

A simple law, known as Darcy s law (1936), states that the volume flow rate per unit area is proportional to the pressure gradient if applied to the case of viscous flow through a porous medium treated as a bundle of capillaries,... 

Now the force per unit volume exerted on the porous medium by the pressure gradient in the gas is -grad p, where p, as distinct from is the physical pressure of the gaseous mixture. This is the force which must be balanced in our model by the external forces acting on the dust particles, so... 

Though a porous medium may be described adequately under non-reactive conditions by a smooth field type of diffusion model, such as one of the Feng and Stewart models, it does not necessarily follow that this will still be the case when a chemical reaction is catalysed at the solid surface. In these circumstances the smooth field assumption may not lead to appropriate expressions for concentration gradients, particularly in the smaller pores. Though the reason for this is quite simple, it appears to have been largely overlooked,... 

The purpose of all flux models is to express the fluxes in a porous medium in terms of gradients in pressure, composition and temperature. Isothermal flux models are therefore all of the general form... 

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