Density gradient method

August to June. Because the density of the ER differs with the season, we collected ER by the linear sucrose density gradient method. 

The density of the liquid mixture can be determined using a pyknometer or it can be derived from a previously determined calibration curve. For the determination of densities less than unity (e.g., polyethylene), ethanol/water mixtures are suitable for densities larger than unity one may use mixtures of water with aqueous salt solutions (40% CaCl2 solution = 1.40 g/ml 72% ZnCl2 solution = 1.95 g/ml). The density gradient method, which is an elegant var-... 

Leukocytes are prepared from EDTA-blood (approximately 3-5 ml) by the density gradient method. EDTA-blood is mixed by inversion and 5 ml is slowly added to 1 ml dextran solution. The blood and dextran solution are carefully mixed so that formation of foam is avoided. The mixture is allowed to stand for 1 h. If further time for sedimentation is required, it has to be noted as it may affect the resulting enzymatic activities. The time needed for proper sedimentation depends on sample quality and should not exceed 3 h. The upper phase including the white cells is transferred to another tube and spun at 600-1000 g for 10 min. The supernatant is... 

Density measurements were made at room temperature, >20°C, using the density gradient method. Duplicate runs were made in all cases. 

Several groups have demonstrated excellent quantitative correlation regarding estrogen receptor analysis between the DCC assay and sucrose density-gradient methods (Al, C2, HI), although Chester et al. (C3) found the DCC assay for estrogen receptor to be more sensitive than the sucrose density-gradient analysis and this difference was more pronounced when molybdate was absent. 

One historically important application of density gradient methods is the classic Meselson and Stahl (1958) experiment in which DNA strands of different densities (incorporating natural N or enriched with N isotopes) were used to demonstrate the semi-conservative replication of the DNA double helix that is central to molecular biology. 

The density of a polymer sample can be determined by the density gradient method, as described by ASTM D1505. In this method, solutions A and B are prepared with densities in the range of interest. Solution A, with the lowest density, and solution B, with the highest density, are combined in a glass tube to form a vertical column of liquid in which the density varies linearly from the bottom to the top. The column is calibrated with glass beads of known density. Plastic samples are dropped in the column and will rest at the level corresponding to their density. The density of the plastic is calculated from the position of the sample compared to that of the calibration beads. 

The British standards BS 2782, Methods 620 A-D [6]. are identical to ISO 1183. ASTM D 792 [7] covers the displacement method and has two procedures, one for the displacement of water and one for that of other liquids. It is not clear why it has been split in this way, and it is notable that there is no mention of using a sinker, nor does it include a pycnometer method. The density gradient method is given in ASTM D 1505 [8] and is very similar to the ISO procedure. There is also an ASTM method for density of polyethylene by means of ultrasound, ASTM D 4883 (9). This works on the principle of measuring sound velocity in the plastic, which correlates to density. The apparatus requires calibrating with reference materials but is claimed to give accuracies of 0.08% or better. The use of the method would mostly be in quality control, and it is questionable whether it should have been standardized. Essentially it describes the use of a commercial in.strumcnt with no apparatus details, not even the frequency used. 

The rate of crystallization of polyethylene terephthalate was measured using the density-gradient method (Keller et al., 1954) by following the changes in density as a function of time for a given crystallization temperature. Data for T = 383 K are tabulated as follows ... 

Density-gradient separations of cytoplasmic fractions from liver have been carried out to study the distribution of activity of several enzymes (Thomson, 1959). Ribosomes and polysomes have been fractionated to elucidate the mechanisms of protein synthesis (Watson, 1964), Various viruses have been studied with the density-gradient technique polio virus (Levintow and Darnell, 1960), Rous sarcoma virus (Crawford, 1960), Shope papilloma virus (Williams et ai, 1960), and adenoviruses (Allison et ai, 1960). Density-gradient fractionation has been particularly useful for separation of DNA molecules from various species of bacteria (Rolfe and Meselson, 1959) and from animal cells (Kit, 1961), Other types of molecules, e.g., antibodies, lipoproteins, and rheumatoid factor have been isolated by density-gradient methods [see Charlwood (1966) for examples]. 

Protein precipitates cannot disturb separation as they may do in the density gradient method. Precipitates remain in the compartment where they first collected. They can be separated from the solution later by centrifugation. 

To the right. Fig. 4.15 illustrates Type 4 instrumentation, a particularly easy method of density determination at a fixed reference temperature, the density gradient method. Figure4.16 depicts the analysis method. The sample, checked for uniformity and freedom from attached air bubbles, is placed in a density gradient column, and its... 

The bulk density p of even small samples may be readily determined using a density-gradient tube or by immersion in salt solutions of known density. The latter method is well adapted for rapid work in routine analyses. The density-gradient method is currently the standard method employed by numerous workers [10]. A density-gradient column contains a liquid whose density increases continuously from the meniscus down to the base [11]. Such liquids can consist, for... 

The question whether also the liquid-like fluid may exist in micropores has been solved theoretically. Using mean field and density gradient methods, Evans and his co-workers "(ref.21" have shown that adsorbed fluid "condensation" should disappear, when the diameter R of the cylindrical micropores approaches to d the molecular diameter. According to the simplest form of their theory the critical temperature T Cml of the fluid confined in cylindrical micropores is determined by the approximate relationship (T -T (m))/T d/R, where T is the bulk critical temperature. From this relation it follows that e.g. benzene (d O.4-0.68 nm, cf. "(ref.3)") adsorbed at room and higher temperatures on zeolites and microporous active carbons behaves as a supercritical fluid. But the co-operative condensation of benzene at room temperatures is not excluded in large supermicropores of radii R>1.2 nm, ... 

Under ideal conditions, the accuracy to which a sample s density can be measured is equivalent to that obtained from the density gradient teehnique. A standard method for measuring polymer density by densimetiy ean be found in ASTM D 792. Densimetry lends itself to automation [74]. As a method of calculating the degree of crystallinity, densimetry suffers from many of the same disadvantages as the density gradient method however, it is much quicker, and, when automated, it is less subjeet to operator error. 

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