Centrifugation

Centrifugal force acts differentially on particles based on their density and mass. Particles sediment at rates proportional to the centrifugal force they experience. Heavier particles sediment first among equal-density components, while denser particles sediment first among equal-mass components. After spinning, the pellet settles at the bottom and the supernatant liquid can be decanted for further processing.

Differential centrifugation separates particles by incrementally increasing speed until target particles isolate, working best when particles have similar densities. Density gradient centrifugation uses a preformed or developing density gradient where each component stops moving when reaching a layer with equal density, resulting in separate bands. This method is more sophisticated and effective for complex mixtures like cell lysates.

Sedimentation rate depends on particle density and mass. Among particles of equal size, denser particles experience greater centrifugal force, causing them to sediment faster. Larger particles tend to be heavier and sediment first, but when size is similar, density becomes the determining factor for sedimentation order.

When a particle's density equals the solution density at a particular layer, the centrifugal force acting on it becomes zero, and sedimentation ceases. The particle remains isolated in that layer, creating distinct bands of separated components. This equilibrium principle allows precise separation of analytes with different densities in a single centrifugation run.

After centrifugation, the supernatant liquid is decanted from the pellet. Depending on the application, either the pellet or supernatant is retained for further purification. Sometimes the supernatant undergoes additional rounds of centrifugation and downstream processing, such as extraction or precipitation and co-precipitation methods.

Complex mixtures such as cell lysates contain numerous components with overlapping densities and sizes, making simple separation difficult. Particles may not form distinct bands due to similar physical properties or interactions between components. Researchers use sophisticated centrifugation techniques like density gradient centrifugation to overcome these challenges and achieve better resolution.

When particles have equal density, larger particles sediment first because they are typically heavier. When particles have equal mass, denser particles sediment first due to higher sedimentation rates. The relative contributions of size and density determine sedimentation order, with centrifugal force acting proportionally on each particle's mass and density.