Differential centrifugation

 The difference between differential and density gradient centrifugation techniques are that the later uses solutions of two or more different densities (e.g. sucrose, ficoll) or silica gels which the sample passes. This can separate the sample into multiple layers of different dense particles. The degree of separation or amount of layers depends on the solution or gel. The differential centrifugation technique does not let the sample pass through gradients. The different centrifugation speeds often create separation into not more than two fractions. The supernatant can be separated further in additional centrifugation steps. In each step the centrifugation speed has to be increased until the desired particles are separated. The density gradient centrifugation is usually performed with just one centrifugation speed.Differential centrifugation (also differential velocity centrifugation) is a common procedure in biochemistry and cell biology used to separate organelles and other sub-cellular particles on the basis of sedimentation rate. Although often applied in biological analysis, differential centrifugation is a general technique also suitable for crude purification of non-living suspended particles (e.g. nanoparticles, colloidal particles, viruses). In a typical case where differential centrifugation is used to analyze cell-biological phenomena (e.g. organelle distribution), a tissue sample is first lysed to break the cell membranes and release the organelles and cytosol. The lysate is then subjected to repeated centrifugations, where particles that sediment sufficiently quickly at a given centrifugation force for a given time form a compact 'pellet' at the bottom of the centrifugation tube. After each centrifugation, the supernatant (non-pelleted solution) is removed from the tube and re-centrifuged at an increased centrifugal force and/or time. Differential centrifugation is suitable for crude separations on the basis of sedimintation rate, but more fine grained purifications may be done on the basis of density through equilibrium density-gradient centrifugation. Differential centrifugation (also differential velocity centrifugation) is a common procedure in biochemistry and cell biology used to separate organelles and other sub-cellular particles on the basis of sedimentation rate. Although often applied in biological analysis, differential centrifugation is a general technique also suitable for crude purification of non-living suspended particles (e.g. nanoparticles, colloidal particles, viruses). In a typical case where differential centrifugation is used to analyze cell-biological phenomena (e.g. organelle distribution), a tissue sample is first lysed to break the cell membranes and release the organelles and cytosol. The lysate is then subjected to repeated centrifugations, where particles that sediment sufficiently quickly at a given centrifugation force for a given time form a compact 'pellet' at the bottom of the centrifugation tube. After each centrifugation, the supernatant (non-pelleted solution) is removed from the tube and re-centrifuged at an increased centrifugal force and/or time. Differential centrifugation is suitable for crude separations on the basis of sedimintation rate, but more fine grained purifications may be done on the basis of density through equilibrium density-gradient centrifugation. In a viscous fluid, the rate of sedimentation of a given suspended particle (as long as the particle is more dense than the fluid) is largely a function of the particle size. Larger particles sediment more quickly and at lower centrifugal forces. If a particle is less dense than the fluid (e.g., fats in water), the particle will not sediment, but rather will float, regardless of strength of the g-force experienced by the particle. In contrast, a more specialized equilibrium density-gradient centrifugation produces a separation profile dependent on particle-density alone, and therefore is suitable for more fine-grained separations. High g-force makes sedimentation of small particles much faster than Brownian diffusion, even for very small (nanoscale) particles. When a centrifuge is used, Stokes' law must be modified to account for the variation in g-force with distance from the center of rotation.