The Density Gradient

Synonym Zone centrifugation Isopycnic, density equilibration 

Gradient Shallow, stabilizing—maximum gradient density below that of least dense sedimentating species Steep—maximum gradient density greater than that of most dense sedimenting species 

Centrifugation Incomplete sedimentation, short time, lower speed Complete sedimentation to equilibrium position, prolonged time, high speed 

Isopycnic or sedimentation equilibrium centrifugation involves allowing the sedimenting species to move through the gradient until they reach a 

Acid phosphatase Colhepsin Acid DNase. 8-glucuronidase Acid RNose 

---

Fig. 11. (a) Diffeiential centrifugation (pelleting), where time 1 < time 2 < time 3 < time, (b) Rate 2onal separation in a swinging-bucket rotor, where tube 1 represents the density gradient solution, tube 2 the sample plus the gradient, and tube 3 the separation of sample particles under a centrifugal force, where... 

ASTM-D 1505-85 Standard test method Density of plastics by the density-gradient technique ... 

As the electric field always points in the direction of the electrode, the densities of the electrons and negative ions are set equal to zero at the electrode. It is assumed that the ion flux at the electrodes has only a drift component, i.e., the density gradient is set equal to zero. The conditions in the sheath, which depend on pressure, voltage drop, and sheath thickness, are generally such that secondary electrons (created at the electrodes as a result of ion impact) will ionize at most a few molecules, so no ionization avalanches will occur. Therefore, secondary electrons can be neglected. 

A shadowgraph picture measures the derivative of the density gradient... 

These are all dimensionless combinations of the density and the density gradients and Laplacians. This enables one to obtain the correct scaling properties of the exchange functional. We plot the quantities x and y for the krypton atom in Fig. 12. The exchange energy density within the LDA is given by... 

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-... 

The density gradient resulting from wall friction, in addition to its effects on expl per-... 

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... 

If the density gradient depends only on temperature and not on dissolved chemicals, N can be written as ... 

Note Assume that the density gradient in the water column is not influenced by dissolved solids. 

Since it is assumed that the density gradient is not influenced by dissolved chemicals, the stability frequency, N, can be calculated from the temperature profiles alone ... 

C Isopycnic centrifugation the density gradient forms during centrifugation. Illustration courtesy of Beckman Instruments, Inc. 

---