1、 Introduction
Centrifuge is a mechanical device that uses centrifugal force to separate various components in a mixture of liquid and solid particles or liquid and liquid. Centrifuges are mainly used to separate solid particles from liquids in suspensions, or to separate two immiscible liquids with different densities in emulsion (such as separating cream from milk); It can also be used to remove liquids from wet solids, such as using a washing machine to dry wet clothes; The special overspeed tube separator can also separate gas mixtures of different densities; Taking advantage of the different settling speeds of solid particles with different densities or particle sizes in liquids, some settling centrifuges can also classify solid particles according to their density or particle size.The structure mainly consists of several parts, including motor drive system, refrigeration system, mechanical system, rotating head, and system control.
2、 Classification of Centrifuges
There are many styles and models of centrifuges, including domestic and imported ones, which can be divided into analytical centrifuges and preparation centrifuges according to their usage;It can be divided into:
Ordinary centrifuge (low speed):<8000r/min;
High speed centrifuge: 8000~30000r/rain;
Overspeed centrifuge: 30000~80000r/min;
Ultra high speed centrifuge:>80000r/min;
Classification by structure:
Generally, high-speed and low-speed centrifuges are classified based on their structure and function, with a wide variety and no unified naming standard for each manufacturer. It can be divided into desktop centrifuges, multi tube micro desktop centrifuges, cell smear centrifuges, blood washing desktop centrifuges, high-speed frozen centrifuges, large capacity low-speed frozen centrifuges, low-speed frozen centrifuges, desktop high-speed frozen centrifuges, desktop low-speed automatic balance centrifuges, etc. In addition, there are also triple (five gang) high-speed freezing centrifuges abroad, specifically designed for continuous centrifugation.
According to volume and capacity, it can generally be divided into floor type centrifuges, desktop centrifuges, and mini (handheld) centrifuges. The floor type has a large processing capacity and a large volume, while the mini centrifuge has a small processing capacity and a small volume.
Centrifuges with different capacities, speeds, and temperature controls can be selected based on the experimental purpose and needs.
3、 Turn the head
The rotor is the main accessory of a centrifuge. Simply put, a centrifuge can be divided into two parts: the main unit and a detachable rotor (rotating head). A multifunctional main unit can usually be equipped with multiple rotors, and different models of rotors can meet different needs to achieve its "multifunctional" function. This can effectively reduce procurement costs. However, when choosing a centrifuge, it is necessary to pay attention to selecting one that is convenient for rotor disassembly and assembly. Generally, the rotor of a centrifuge is directly matched with the main shaft, Manufacturers often make the fit between the host and rotor quite high in order to reduce noise. However, this major drawback is that the rotor can easily bite with the main shaft, making disassembly inconvenient. In addition, this fit between the rotor and the motor also has strong directionality. When the rotor direction deviates from the main shaft of the electric machine, it will cause significant noise and affect the service life of the entire machine. In severe cases, it will cause the main shaft to break and the rotor to explode. The most commonly used rotors include fixed angle rotors and horizontal rotors. The more complete the rotor, the more functional it is. Therefore, the selection of the rotor is usually based on the capacity of commonly used samples and the centrifugal conditions. Usually, there is one horizontal and one angular rotating head, one large capacity (relatively low speed) rotor, and one small capacity high speed rotor. Other rotors are selected according to different needs in the work.
(1) Angle type rotating head: An angle type rotating head refers to a rotating head with a certain inclination angle between the centrifugal tube cavity and the rotating shaft. It is made of a complete piece of metal, with 4-12 mechanical holes for installing centrifugal tubes, namely the centrifugal tube chamber. The angle between the central axis of the holes and the rotation axis is between 20-40 degrees, and the larger the angle, the stronger the settlement and the better the separation effect. The advantage of this type of rotating head is that it has a large capacity, a low center of gravity, balanced operation, and a long lifespan. When particles settle, they first collide with the centrifugal pipe in the direction of centrifugal force, and then slide towards the bottom of the pipe along the pipe wall. Therefore, particle deposition occurs on one side of the pipe, which is called the "wall effect". The wall effect easily causes the settling particles to be disturbed by the convection caused by sudden speed change, affecting the separation effect.
The centrifugal tube in the angle rotor can maintain a fixed angle of centrifugation and can use higher rotational speeds. After centrifugation, the sediment is located on the inclined surface at the bottom of the tube. Large capacity rotors can usually be equipped with different "adapters" to "fix" small capacity sample tubes. For example, a 6 x 85ml rotor can accommodate 85ml centrifuge tubes per well, as well as 50ml or 15-20ml centrifuge tubes, achieving multi-purpose use of one substance; However, due to the weight burden, the higher rotational speed of large capacity rotors is relatively lower than that of small ones, so it is necessary to consider whether the higher centrifugal force can meet the experimental requirements. A rotating head with a higher rotational speed is usually a rotor that is suitable for a 2ml centrifuge tube. Sometimes, the price of one rotor can almost buy an additional small centrifuge, and a small desktop high-speed centrifuge can be purchased separately, which can also reduce the loss of expensive equipment. Some small desktop high-speed centrifuges also offer fixed angle rotors of different sizes (paired with 0.5ml and 2ml/1.5ml centrifuge tubes, respectively). In fact, unless a 0.5ml centrifuge tube is frequently used and the sample volume for each centrifugation is relatively large, a 2ml/1.5ml rotor is generally selected, and an adapter (with adapters suitable for 0.2ml, 0.4ml, and 0.5ml tubes) can meet the needs. If PCR tubes are commonly used, it is more convenient to consider equipping an 8-tube rotor.
The cover equipped with the angle rotor helps to reduce friction, reduce noise, and prevent accidents. When using the angle rotor, it is necessary to cover it. If the centrifugal chamber is not covered, it will generate significant eddy current resistance and friction heating, which adds additional burden to the motor and refrigerator of the centrifuge and affects the service life of the centrifuge. In addition, there is a type of airtight rotor that can undergo airtight centrifugation to prevent accidental leakage, which is important for improving the experimental environment.
⑵ Horizontal swivel: This type of swivel is composed of 4 or 6 freely movable hanging baskets (centrifugal sleeves). When the rotating head is stationary, the bucket is suspended vertically. When the rotating head speed reaches 200 to 800 revolutions per minute, the bucket swings to the horizontal position. This type of rotating head is most suitable for density gradient zone centrifugation. Its advantage is that the gradient material can be placed in a centrifuge tube that remains vertical. During centrifugation, the separated sample belt is perpendicular to the longitudinal axis of the centrifuge tube, unlike the angle between the interface of the sample sediment in the angular rotating head and the centrifuge tube, Therefore, it is beneficial to separate various product bands from the tube after centrifugation. Its disadvantage is the long settling distance of particles and the long time required for centrifugation.
The horizontal rotor is equipped with 4 or 6 hanging baskets, which are vertically hung on the rotor when stationary. The hanging baskets have different adapters to meet the needs of centrifugal pipes of different capacities. When the rotor speed exceeds 600rpm and the centrifuge tube reaches a horizontal position, the sample settles in the direction of axial movement along the centrifuge tube, and then settles at the bottom of the tube for easy collection. Because it is not as stable as a fixed angle rotor, the horizontal hanging basket has a much lower rotational speed compared to a fixed angle rotor, which is limited to the range of low-speed centrifugation. The hanging basket has different adapters, which can accommodate various types of centrifuge tubes and even meet the needs of plate centrifugation. For example, some brands of large capacity centrifuges can be equipped with multi-purpose adapters, allowing multiple microplates or culture plates, deep well plates, PCR plates, slide boxes, purification plates for large-scale nucleic acid purification kits, and even centrifugation of cell culture bottles to share the same horizontal rotor as large capacity centrifugation bottles, Helps to save money while meeting diverse needs. So choosing a rotor with a wider adaptation range is better.
A horizontal rotor can cause centrifugation to settle at the bottom of the tube rather than on the inclined surface of the tube wall, but the speed of the horizontal rotor is often much slower than that of fixed rotors of the same model - sometimes it cannot meet the needs of high-speed centrifugation. For trace samples that require high speed but also require sedimentation at the bottom of the tube, a drum shaped rotor can be selected. The centrifuge tube is horizontally fixed on a bracket inside this unattractive "circular biscuit box", It can be centrifuged at a relatively high speed (14000rmp) while ensuring that the sediment accumulates at the bottom of the tube.
⑶ Zone with rotating head: The zone with rotating head does not have a centrifugal tube and mainly consists of a rotor barrel and a detachable top cover. The rotor barrel is equipped with a cross shaped partition device, which divides the barrel into four or more fan-shaped small chambers. There are conduits inside the partition, and gradient liquid or sample liquid is pumped in from the inlet pipe in the center of the rotating head. These conduits are distributed around the rotor, and the partition inside the rotating head can maintain the stability of the sample belt and gradient medium. The settling of sample particles in the zone rotating head is different from that of angle and external pendulum rotating heads. Under the action of radial scattered centrifugal force, the settling distance of particles remains unchanged, so the "wall effect" of the zone rotating head is minimal, which can avoid the disorder of zone and settling particles, achieve good separation effect, and have the advantages of high speed, large capacity, easy recovery gradient, and no impact on resolution, Make it possible to use ultracentrifugation for preparation and industrial production. The disadvantage of the zoned rotating head is that the sample and medium directly contact the rotating head, which requires high corrosion resistance and complex operation.
(4) Vertical turning head: The centrifuge tube is placed vertically, and the settling distance of sample particles is the shortest. The centrifugation time is also short, making it suitable for density gradient zone centrifugation. After centrifugation, the liquid surface and sample zone need to undergo a 90 degree turn, so the deceleration should be slow.
(5) Continuous flow rotating head: It can be used for the concentration and separation of a large amount of culture solution or extraction solution. The rotating head is similar to the zone rotating head and consists of a rotor barrel, a rotating head cover with inlet and outlet, and auxiliary devices. During centrifugation, the sample liquid continuously flows into the rotating head from the inlet. Under the action of centrifugal force, suspended particles settle on the wall of the rotor barrel, and the supernatant flows out from the outlet.
More and more new types of rotors are emerging in response to new needs. The material of the rotor has also been continuously innovated from aluminum alloy, which is lightweight, high-strength, and low-cost. Ultra light rotors such as carbon fiber rotors and plastic rotors have emerged, and aluminum alloy molds have been used to press hollow angular rotors to effectively reduce weight, thereby reducing the load on the motor and increasing the sample capacity. Nowadays, most rotors, adapters, and covers can be directly sterilized at high temperatures, greatly improving their corrosion and temperature resistance, which can meet special needs. However, when the rotor is running at high speed, centrifugal stress will be generated internally, which has a certain limit of use. It is necessary to refer to the instructions for proper storage and use. When the centrifugal stress exceeds a certain limit, it will cause premature damage to the rotating head, and even accidents. The manufacturer's instructions should be strictly followed.
