Centrifugal commonly used terms-centrifugal force and relative centrifugal force

In life science research, centrifugation is an indispensable technical method, and it is the most common experimental item in the laboratory. But do you really understand this technology? Do you know the principle of centrifugation and how to choose the right rotor, centrifuge tube and centrifugation method? In this article, I will introduce these in detail.

Centrifugal principle

To learn a technology, it is necessary to start with an understanding of its principles. We can understand the basic principles of centrifugation through simple experiments.

First, let’s observe the sedimentation of particles under the action of gravity: grab a handful of the mixture of sand and soil and place it in a container filled with water and shake it well, then place the container on the table. Observe that a large number of particles are immediately affected by the gravity of the earth. Settle to the bottom of the container. After a period of time, I saw that the mixture in the container was divided into several layers, and each layer was composed of particles of the same size. The distribution of the particles in the container gradually increased from top to bottom. However, there will still be some small particles moving slowly to the bottom of the container in the water. Because they move very slowly, we may not be able to observe the movement of these particles. There are also some particles floating on the water.

Through the results observed in the experiment, we can conclude that large particles settle to the bottom faster than small particles, while smaller particles settle more slowly. However, some small particles with a heavier density settle faster than larger particles with a lighter density. We can also see that some small particles do not settle in the water and float on the water surface.

Figure 1 Settlement of mixed particles under gravity: The settling speed of particles depends on the size of the particles. Large particles settle to the bottom first, while small particles stay in the upper part of the container.

The above is the main principle of centrifugation. The particles in the liquid move down at a certain rate under the action of gravity. The rate of particle movement is often related to the size and density of the particles, so that the sedimentation of the particles occurs. Of course, the premise is that the density of the particles must be greater than the density of the liquid.

In summary, centrifugal technology is a technology that uses the rotating motion of a centrifuge to generate centrifugal force to separate, analyze, concentrate and purify samples with different sedimentation coefficients.

Centrifugation commonly used terms

Centrifugal force and relative centrifugal force

Although some biological particles can be separated under the action of gravitational field (1×g), such as human blood cells, place the processed blood cells on the table for 1-2 hours. Because different cells have different sizes, white blood cells and red blood cells can be automatically separated. Floor. But if you need to separate smaller biological particles, a force far greater than gravity is necessary. This can be achieved by rotating a centrifuge tube containing suspended particles along an axis. The particles move outward from the axis under the radial centrifugal force, the machine that makes the centrifuge tube rotate is the centrifuge, and the centrifuge tube is loaded and drives the centrifuge tube to rotate along the axis is the rotor. Regardless of whether cells or biological macromolecules are subjected to centrifugal force, the following formula can be used to calculate:

(1)                       F Centrifugal force=m*rw²

F Centrifugal force-the centrifugal force on the particles

m-the quality of the particles

ω-angular velocity

r-the distance from the rotation axis to the particle

But under normal circumstances, the radial force received by the rotating rotor is measured by the relative centrifugation force (RCF: Relative centrifugation force). The so-called relative centrifugal force is the ratio of centrifugal force to gravity, expressed by the following formula:

(2)                       RCF=(RW²)/g

RCF-relative centrifugal force;

ω-angular velocity;

g-acceleration of gravity;

r-the distance from the rotation axis to the particle.

ω is the angular velocity of the rotor, which refers to the number of radians that the rotor turns per second, and its value is:

(3)                    w=2 π*RPM/60=0.10472RPM

RPM (Revolution per minute)-the number of revolutions the rotor has made per minute

Combining formula (2) and formula (3), the relationship between the rotation speed of the same rotor and the relative centrifugal force can be obtained:

(4)                   RCF=1.12r*(RPM/1000)²

r-the distance between the rotation axis and the particle (unit: mm)

It can be seen from the above that the relative centrifugal force of the rotor is proportional to the square of the rotation speed of the rotor, and also proportional to the radius of the rotor. For the same rotor, since the radius is constant, increasing the speed is equivalent to increasing the relative centrifugal force.

In order to facilitate the conversion between rotational speed and relative centrifugal force, Dole and Cotzias used the calculation formula of RCF to produce a nomogram of the relationship between rotational speed “rpm”, relative centrifugal force “RCF” and rotation radius “r”. It is more convenient than the formula calculation method, and it is clear at a glance. When converting, first take the known radius on the r scale and the known number of centrifuge revolutions on the rpm scale, and then draw a straight line between these two points. The intersection with the RCF scale in the figure is the corresponding Relative centrifugal force value.

The centrifugal force nomogram corresponding to the speed and the radius in the above figure, the left column is the radius scale, and the right column is the speed scale. The intersection of the straight line connecting the radius and the speed with the middle RCF gauge can get the relative centrifugal force. Note that if the known rotation value is on the right side of the rpm scale, the value on the right side of the RCF scale should be read, and the rotation value is on the left side of the rpm scale, then the value on the left side of the RCF scale should be read.

In the settlement of the gravity field, the gravity field is usually regarded as a constant value, but the centrifugal force field in the centrifuge tube is not a constant value. Due to the shape and design of the rotor, the distance from the top of the tube to the bottom of the centrifuge tube to the center of rotation is different. In order to calculate the relative centrifugal force value, the average relative centrifugal force can be used to represent the same centrifugal rotor head and bottom. The average value of the centrifugal force. The centrifugal force data in scientific literature usually refers to its average value (RCFav), that is, the centrifugal force at the midpoint of the centrifuge tube.

When labeling the maximum relative centrifugal force (RCF) of centrifuges and rotors, manufacturers generally use the relative centrifugal force at the maximum radius (Rmax) at the highest speed. Please pay attention to distinguish the difference between this parameter and the average centrifugal force (RCFav).

Centrifuges often have a variety of rotors with different shapes, and the size of the centrifugal force field and the centrifugal settlement distance reflected by each of them are different. In actual work, they should be correctly selected and used according to the separation requirements.