User:Spacepotato/Draft:Equilibrium density-gradient centrifugation

Equilibrium density-gradient centrifugation is a method used to separate a mixture of particles or solutes based on their density. One of its applications is in biology, where it is used to separate subparts of cells. In this technique, a gradient medium is prepared in the centrifuge vessel whose density decreases with height, either continuously or discontinuously. The medium is centrifuged together with the material to be separated until it reaches equilibrium. This forces particles in the mixture to move to the height where the medium matches their density (if the medium has an interface with a discontinuous density step, the particles may end up resting on top of the interface.)

Gradient media
Many gradient media have been used. Some criteria for selecting a medium are as follows:
 * Ability to produce the required range of densities.
 * Viscosity. Higher viscosities will increase the time to come to equilibrium.
 * Osmolality. Osmotic effects may change the density of the components being separated.
 * Ionic strength.

Materials that have been added to water to create gradient media include: Sucrose. At high concentrations, this is viscous and has high osmolality. Glycerol. Sorbitol. Mannitol. Dextran. Cesium sulfate. Potassium bromide. Cesium chloride.
 * 1) Sugars and related compounds:
 * 1) Salts:

Iohexol (sold for this purpose under the names Accudenz, Nycodenz, and Histodenz.) Metrizamide. Iodixanol. Diatrizoate.
 * 1) Colloidal silica.
 * 2) Iodine compounds:

Creating the gradient
The density gradient in the medium may be preformed prior to the centrifugation, or it may be created by the equilibrium between diffusion and sedimentation during the centrifugation.

A preformed gradient may be created by hand by overlaying layers of lighter mixtures of gradient medium on top of heavier layers, or by underlaying heavier layers beneath lighter layers. Alternatively, devices known as gradient mixers or gradient makers can be used to create a preformed gradient. These devices consist of two chambers, containing solutions of differing concentrations, which are gradually mixed to create the gradient. They can be used to create linear, concave, convex and exponential gradients.