Laboratory centrifuge, its types and functions.

Laboratory centrifuge, its types and functions

The centrifuge is used to rapidly sediment particles such as cells, which may be suspended in a fluid. The principle is that the centrifuge exerts a centrifugal force (CF), which is greater than that of gravity, and causes particles in a fluid to sediment. The greater the CF, the faster and more effective is the sedimentation. 

Laboratory centrifuge, its types and functions.

This centrifugal force, which is the outward pull due to rotation, is relative to the speed of centrifuging in revolutions per minute (rpm). The actual sedimentation achieved, however, depends on the radius of the centrifuge. The radius of the centrifuge is the distance between the centre of the centrifuge shaft and the tip of the centrifuge tube. 

The relative centrfugal force (RCF) is calculated from the knowledge of the rpm and the radius (r) as shown in this formula:

RCF (g) = 1.12 x 10⁵ x r (in cm) x (rpm)²

Figure 3.5 shows a nomogram for the calculation of RCF. If the radius of the centrifuge and the rpm are known, RCF can be deduced by joining the two points as shown.

Normogram  calculation of RCF

Types of Centrifuges

Centrifuges have basically two types of rotors, namely, the fixed-angle or the swing-out 

Fixed angle rotor

In the fixed angle rotor head centrifuge, the cups or tubes are held in a fixed angle. This position makes the process of centrifugation more rapid than the swing-out type. Also, there is a less chance of sediment being disturbed when the centrifuge stops. In this type, the particles in the fluid are deposited on the sides of the tube (Fig. 3.6 b).

fixed angle rotor centrifuge

sendimantation  in fixed angel rotor

Swing-out rotor

In the swing-out rotor head, the cups or tubes occupy a vertical position when the centrifuge is at rest, but swing-out and become horizontal during revolution. The particles are sedimented at the bottom of the tube (Fig. 3.7(b)). In some models of centrifuges, interchangable rotor heads are available.

Swing-out rotor centrifuge

sedimation in swing-out rotor

Types of Models of Centrifuge

1. Hand centrifuge 

this is fixed to the bench and the handle is rotated manually. It gives low speeds only. It is now to be found in rural areas of the developing countries where electric power supply is erratic.

2. Battery operated bench centrifuge 

This type of centrifuge is ideal for an area lacking regular electricity. It requires a lot of power so the battery will need regular charging and re-charging.

3. Electric bench centrifuge 

This is the most popular model used in the medical laboratory and they are available in different sizes and models with varying speed ranges.

4.Microhaematocrit centrifuge 

This is used to spin capillary tubes which are used to find the volume of packed red blood cells or for concentration of some blood parasites. It can be either electric or battery-operated. The capillary tubes are filled with blood and are spun in the centrifuge. The percentage of RBC filled 3, 4 or 5 minutes at 10,000 to 15,000 rpm with an RCF of about 14,000 g.

5. Ultracentrifuge 

 This type of centrifuge permits refrigeration at higher speed while protecting the specimen from the heat that is generated by the rotors of the centrifuge. The essential temperature of this centrifuge varies from - 15°C to -25°C during

centrifugation.

6. Cytocentrifuge  

Cytocentrifuge or cytospin is a special centrifuge which is used for transferring cells in suspension to a circumscribed area on a glass slide. This instrument concentrates the cell suspension and minimises cell damage. 

The holder or cup for the cell suspension is a plastic block with an opening on the posterior surface. This plastic block, together with a slide and a filter paper, is spring-loaded onto the centrifuge as shown in Fig. 3.7(c). 

The centrifugal force causes the cells to be deposited in a circular area on the slide. The filter paper absorbs the suspension fluid.column is read off using the haematocrit reader. Usually, the spinning time is set for

Use and Care of Centrifuge

1. Balance the centrifuge by ensuring that the buckets or tubes opposite to each other are of the same weight. 

2. Check that the bucket is properlycushioned.

3. When filling the tubes, make sure to leave the fluid level at least 2 cms below the rim

of the centrifuge tube.

4. Cap the tubes or bottles when using a potentially infectious material.

5. When using a swing-out rotor head, check that the tubes are of proper length so that they will not break while centrifuging dueto swinging out of buckets. 

6. Do not try to stop the centrifuge by handwhile still rotating.

7. Do not open a centrifuge until the rotor stops completely and the buckets are at rest. A locking device is available on some models which prevents the centrifuge from being opened while in operation.

8. Use a non-corrosive disinfectant to clean the centrifuge from time to time.

9. Follow maintenance procedures as given in the manufacturer's manual.