Separations by density

These concepts are used to analyze separations in the botde centrifuge, the imperforate bowl centrifuge, and the disk centrifuge. Separation by density difference in other types of centrifuges can be analyzed by analogy. 

Equipment. Centrifugation equipment that separates by density difference is available in a variety of sizes and types and can be categorized by capacity range and the theoretical settling velocities of the particles normally handled. Centrifuges that separate by filtration produce drained soflds and can be categorized by final moisture, drainage time, G, and physical characteristics of the system, such as particle size and Hquid viscosity. 

Most organelle membranes, such as the tonoplast (6) and the Golgi apparatus (7), can be separated by density gradient ultracentrifugation of plant cell homogenates. However, other effective methods for the isolation of the plasma membrane (8,9) have been described. Moreover, another method that uses an aqueous two-phase system for the isolation of ER is also described (10). Those interested in these details for these methods should consult the original articles. 

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

Different grades of both of the ABS and HIPS plastics have specific gravities in the range of 1.055-1.125 g cm-1. As a result such mixtures can not be effectively separated by density gradient procedures. 

Table I. Elemental Analysis and Maceral Composition of Coal PSOC-2 and PSOC-858 Separated by Density Gradient Centrifugation Techniques...

Measured Ni levels on catalyst are expected to supply an independent age marker, as it has been shown that Ni tends not to migrate following deposition on the catalyst (2). In fact, metals (Ni and V) deposition from the feed onto the catalyst is expected to assist the separation by density/age/activity as older catalyst particles exhibit higher metals levels which contribute to an increase in density. Except for the oldest fractions, which contain the highest metals levels, the older portion of catalyst tends to make less coke as the increase in activity due to increasing metals content is overwhelmed by the loss of activity due to crystalline zeolite destruction (1). 

A successful separation by density into fractions of increasing age has been obtained for the USY equilibrium catalyst of this study. The nickel level on each catalyst fraction, which is expected to furnish a stable age marker, increases with increasing density/age. The observed decrease in the V/Ni ratio with increasing age is indicative of interparticle vanadium migration. The trend of increasing Ni level with increasing density has been used to correlate increases in density with "Ni days" in the unit. 

Catsimpoolas N, Griffith AL, Skrabut EM, et al. 1976. Differential Cr uptake of human peripheral lymphocytes separated by density gradient electrophoresis. Cell Immunol 25 317-321. 

Information on the rate of synthesis or turnover of receptors can be obtained from studies with dense amino acids, e.g., in studies with cultured cells. The isolated steroid receptor complexes are separated by density gradient centrifugation and the presence of a faster sedimenting form of the receptor, containing the dense ami-noacids is monitored [25],... 

Phase separation by density Hx (heat out) required H2O wash... 

M. K. Brakke, Arch. Biochem. Biophys., 45 275 (1953). Zonal Separations by Density-Gradient Centrifugation. 

The chemical extraction procedure used to preferentially disassociate carbonate and silicate minerals is described elsewhere in this Symposium Series (12), and, thus, will be described only briefly here. The separation scheme is outlined in Figure 1. Initially, a bitumen-free oil shale is isolated by exhaustive Soxhlet extraction with a methanol/benzene mixture. A portion of the bitumen-free shale oil is then treated with HC1 to produce a carbonate-, bitumen-free oil shale. Following Soxhlet extraction with benzene/methanol to remove carbonate-associated bitunens, a portion of the carbonate-, bitumen-free oil shale is then extracted with HF/HC1 to produce a silicate-, carbonate-, bitumen-free shale. This shale is also Soxhlet extracted to remove silicate-associated bitumens. Finally, a portion of the silicate-, carbonate-, bitumen-free shale is separated by density into three fractions by sink/float techniques using both 15 wt% ZnCl2 in distilled water and pure distilled water as immersion bath media. 

The biosynthesis of Hb Aj. in humans was followed after an injection of [ Fejtransferrin into a human volunteer (B41). The specific activity of Hb Aj increased slowly and at day 60 started to exceed that in Hb Aq. It was concluded that Hb Aj, is formed continuously at a slow rate over the 120-day life span of normal human red cells. Young red cells, separated by density gradient, have lower concentrations of Hb Aj than do old cells (F7). 

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