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The acceleration of the centrifuge is usually expressed as a multiple of the gravitational acceleration (g = 9.80 m/s2) and is called the relative centrifugal force (RCF or "g value"). The RCF depends on the rotor speed (n, r/min in revolutions per minute) and the radius of rotation (r, in mm), which can be expressed as follows:
After this formula is deformed, the speed (r/min) can be calculated when the values ​​of r and RCF are known:
However, it should be noted that the RCF value is not equal everywhere in the centrifuge tube: the value zui large (rmax) near the outer side of the rotor is small (rmin) near the central axis. In applications, the RCF value customarily referred to is the uniform radius of rotation (rav). In addition, it should be noted that the value of RCF is a function of the square of the rotational speed value, so a 41% increase in speed will double the RCF.
Second, the method of centrifugal separation:
1. Differential settlement (precipitation) method:
After centrifuging a mixed suspension at a certain RCF for a certain period of time, the mixture will be divided into two parts, a precipitate and a supernatant. From a suspension, the addition of RCF and the continuous separation of precipitates at a fixed centrifugation time are widely used to separate organelles from cell homogenates. (a) in the centrifuge tube before centrifugation is a suspension containing three large, medium and small particles; (b) after low-speed centrifugation, the precipitate is mainly composed of large particles of zui; (c) further high-speed centrifugation of the supernatant, A second precipitate consisting mainly of medium sized particles is obtained; (d) after the Zui, the remaining small particles are precipitated by centrifugation.
Differential centrifugation is mainly used to separate organelles and viruses. The advantage is that the operation is simple, the supernatant can be separated from the sediment by the dumping method after centrifugation, and the angular rotor with larger capacity can be used. Disadvantages are: 1 separation effect is poor, can not get pure particles at once; 2 wall effect is serious, especially when the particles are large or high concentration, precipitation will occur on the side of the centrifuge tube; 3 particles are squeezed, centrifugal force is too large, Excessive centrifugation time will cause the particles to deform, aggregate and inactivate. 
The zone centrifugation method is a separation method in which a sample is subjected to centrifugal sedimentation or sedimentation balance in a certain inert gradient medium, and the particles are distributed to certain specific positions in the gradient under a certain centrifugal force to form different zones. The advantages of the method are as follows: (1) the separation effect is good, and the relatively pure particles can be obtained at one time; 2 the adaptation range is wide, and the particles having the difference of the precipitation coefficient can be separated, and the particles having a certain buoyancy density can be separated; 3 the particles do not accumulate deformation, It maintains particle activity and prevents mixing of the formed zones due to convection. Disadvantages:
1 Centrifugation time is longer; 2 needs to prepare gradient; 3 is strictly controlled and should not be grasped.
The following technique uses a density gradient in which the solution in the centrifuge tube gradually increases in density from the top of the tube to the bottom of the tube: 
Density gradient.
(a) Continuous density gradient. (b) A discontinuous density gradient formed by flattening the gradients of decreasing concentrations. (c) Single gradient density barrier designed for selective sedimentation of a type of particle 3. Differential zone centrifugation:
The sample is placed on a gentle pre-made density gradient medium and centrifuged. The larger particles settle the smaller particles faster through the gradient medium to form several distinct zones (stripe). This method has a time limit and must be stopped before any zone reaches the bottom of the tube. The differential zone belt centrifugation method is only used to separate particles with a certain difference in sedimentation coefficient, regardless of density. Particles of the same size and density (such as lysosomes, mitochondria and peroxisomes) cannot be isolated by this method.
During centrifugation, due to the centrifugal force, the particles leave the original sample layer and settle along the bottom of the tube at different sedimentation rates. After centrifugation for a certain period of time, the settled particles gradually separated, forming a series of discontinuous zones with clear interfaces. The larger the sedimentation coefficient, the faster the sedimentation down and the lower the zone presented. Particles with a smaller sedimentation coefficient appear in sequence in the upper part. From the point of view of the sedimentation of the particles, the centrifugation must be completed before the sedimentary particles (large particles) reach the bottom of the tube or just after reaching the bottom of the tube, so that the particles are in an incomplete sedimentation state and appear in some specific zones. Inside.
During centrifugation, the position and shape (or bandwidth) of the zone change over time. Therefore, the width of the zone depends not only on the number of sample components, the slope of the gradient, the diffusion and uniformity of the particles, but also the centrifugation. Time related. The longer the time, the wider the zone. Appropriate increase of centrifugal force can shorten the centrifugation time, reduce the band widening caused by diffusion, and increase the stability of the zone interface.
4. Equal density centrifugation:
When there are buoyant density differences between different particles, under the centrifugal force field, the particles may settle down or float upwards, and move along the gradient to a position where their density is exactly equal (ie, equal density points), forming a zone, called Density centrifugation. The effective separation of iso-density centrifugation depends on the difference in buoyancy density of the particles. The greater the difference in density, the better the separation effect, independent of the size and shape of the particles. But the latter two determine the rate, time, and width of the zone to reach equilibrium. The buoyancy density of the particles is not constant, but also related to the original density, the degree of hydration and the permeability of the gradient solutes or the combination of the solute and the particles. For example, some particles are easily hydrated to reduce the density.
This technique separates materials based on differences in buoyancy density. The resolution is affected by particle properties (density, homogeneity, content), gradient properties (shape, viscosity, slope) rotor type, centrifugation rate and time. The particle zone width is proportional to the gradient slope, centrifugal force, and particle relative molecular mass. Several substances can be formed into a density gradient by centrifugation (such as sucrose, glucose, phenanthrene, and Nickden). The sample is mixed with a suitable medium and centrifuged - the various particles form a zone of precipitation at the media band of their equal density. This method requires that the medium gradient should have a certain steepness, and there must be sufficient centrifugation time to form the redistribution of the gradient particles, and further centrifugation will have no effect.
A thin Papillon dropper or syringe with a slender needle can be used to collect the strips within a certain density gradient. Another method can pierce the test tube and collect the contents in sections into several tubes.
Introduction to centrifuge method for laboratory sedimentation centrifuge
First, the calculation of centrifugal force: 2. Density gradient centrifugation: