Isopycnic centrifugation is a commonly used technique in density gradient centrifugation.
Density gradient centrifugation is a powerful method for separating particles based on their density. This is achieved by creating a gradient of increasing density within a centrifuge tube. When a sample containing particles of varying densities is centrifuged through this gradient, the particles will migrate until they reach a point where their density matches that of the surrounding medium. Several techniques leverage this principle. Let's explore some key methods:
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Isopycnic (Equilibrium) Centrifugation: This technique, as highlighted in the reference, separates particles solely based on their density. The gradient medium's density must exceed the density of all particles in the sample. As the particles are centrifuged, they migrate through the gradient until they reach their isopycnic point – the position where their density matches the density of the surrounding medium. Critically, with sufficient centrifugation time, particles will band at their isopycnic point regardless of their initial starting position. This method is also sometimes referred to as equilibrium sedimentation because the particles reach an equilibrium position within the gradient.
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Rate-Zonal Centrifugation: This technique separates particles based on their size and shape in addition to their density. The sample is layered on top of a preformed density gradient, and centrifugation is performed for a specific time. Particles sediment at different rates through the gradient depending on their size, shape, and density. Larger and more dense particles will sediment faster than smaller and less dense particles. The centrifugation time is crucial as particles should not reach their isopycnic points; the separation is based on the rate of sedimentation.
In summary, while both isopycnic and rate-zonal centrifugation are used in density gradient centrifugation, isopycnic centrifugation is particularly noteworthy for its reliance on particle density as the sole separation factor, using a gradient density that surpasses that of all sample components.
