1. Principle
A body is rotated in a circular movement at speed. This creates a force that drives the body away from the centre of the circular movement (Fig. 3.35). To calculate the relative centrifugal force (rcf) for an individual centrifuge, measure the radius (r) of the rotor arm (in cm) and the number of revolu- tions per minute (rpm) and use the formula below: rcf = 1.118 x 10-6 x r x (rpm)2
For example, if the radius is 25 cm and the rpm is 1300 rev/min, the rcf is about 50g.
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| Fig. 3.35 Principle of centrifugation |
Components of a centrifuge (Fig. 3.36) A centrifuge consists of:
— a central shaft or spindle (A) that rotates at high speed;
— a head (E), fixed to the shaft, with buckets for holding the centrifuge tubes;
— tubes (T) containing the liquid to be centrifuged.
When the spindle rotates the tubes are subjected to centrifugal force. They swing out to the horizontal and the particles in suspension in the liquids in the tubes are thrown to the bottom of the tubes. The particles are compacted at the bottom of the centrifuge tubes.These particles form the centrifuge deposit which can be separated from the supernatant fluid and examined. The deposit may contain, for example:
— blood cells;
— parasite eggs (in diluted stools);
— cells from the urinary tract (in urine).
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Fig. 3.36 Components of a centrifuge A: central shaft or spindle; E: centrifuge head; T: centrifuge tubes.
2. Types of centrifuge
Hand-operated centrifuge (Fig. 3.37)
This is operated manually by turning a handle. It takes two or four tubes. The hand-operated centrifuge can be used:
— to examine urinary deposits;
— to concentrate certain parasites in stools.
The speed is insufficient for satisfactory separation of erythrocytes from plasma in blood.
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Fig. 3.37 A hand-operated centrifuge
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Important:
● Clamp the centrifuge firmly on a stable support (edge of a table).
● Balance the two diametrically opposite tubes perfectly as described in the in- structions for use, section 3.3.3.
● Keep your distance while operating the centrifuge.
● To stop the centrifuge, do not slow down the turning of the handle. Pull the handle out of the machine with a sharp movement.
● Remove the tubes slowly and carefully (so as not to disturb the deposit).
● Lubricate the spindle of the centrifuge regularly.
Warning: The hand-operated centrifuge can cause serious injury, so follow the instructions above carefully.
Electric centrifuges
Electric centrifuges are more accurate than hand-operated centrifuges and should be used whenever possible. Electric centrifuges are used with two types of head — the “swing-out” head and the “angle” head.
“Swing-out” head (Fig. 3.38)
The head is designed to swing the tubes out to a horizontal position during centri- fuging. This is the type most frequently needed.
“Angle” head (Fig. 3.39)
The “angle” head holds the tubes at an angle of about 45° during centrifuging. It is useful for certain techniques, e.g. agglutination tests in blood-grouping by the test- tube method.
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| Fig. 3.38 A centrifuge with a swing-out head |
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| Fig. 3.39 A centrifuge with an angle head |
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| Fig. 3.40 Types of bucket for a centrifuge |
Buckets (tube holders)
There are several types of bucket for use with electric centrifuges (Fig. 3.40). The choice depends on the model of centrifuge:
— buckets designed to hold one round-bottomed or conical tube only;
— buckets that hold two round-bottomed or conical tubes;
— buckets that hold nine small (precipitin) tubes, etc. Some models are also fitted with:
— a timer that stops the centrifuge automatically when the time is up (e.g. after 5 or 10 minutes);
— a cooling chamber that prevents heating of the specimen during centrifuging;
— a revolution counter, i.e. a dial with a needle that indicates the speed of the machine during centrifuging (this is useful for some methods of concentra- tion of parasites).
Battery-operated centrifuges
Small battery-operated centrifuges are sometimes used to measure the packed cell volume in haematology.
3. Instructions for use
You should always follow the manufacturer’s instructions when using the centrifuge.
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Fig. 3.41 Balancing centrifuge tubes
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Installing the centrifuge
The centrifuge must be placed on rubber pads or a mat on a flat level surface.
Balancing the tubes
If the tubes are numbered, place them as shown in Fig. 3.41:
— tube 1 opposite tube 2;
— tube 3 opposite tube 4.
Balance the tubes that are opposite each other by weighing them in their buckets on the open two-pan balance.
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Fig. 3.42 Balancing centrifuge
tubes by adding
liquid to the lighter tube
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| Fig. 3.43 Balancing centrifuge tubes by adding water to the bucket containing the lighter tube |
To balance: either add more of the liquid to be centrifuged to the lighter tube (Fig. 3.42); or add water to the bucket containing the lighter tube (using a wash bottle; Fig. 3.43).
If only one tube of liquid is to be centrifuged, balance it with an identical tube filled with water.
Preventing breakage of tubes
Always pad the bottom of the buckets with the rubber cushions provided with the machine. This protects the bottom of the centrifuge tubes.
Using a wash bottle, add a little water between each tube and its bucket.
Safety precautions
● Check that the tubes are the correct size for the centrifuge. Tubes that are too long or too small may break.
● Fill the tubes to no more than three-quarters full to prevent spillage in the bowl.
● Always balance the centrifuge buckets before starting the centrifuge. Failure to do this can cause excessive wear or the centrifuge may move.
● Ensure that the lid is closed before starting the centrifuge.
● When starting the centrifuge, gradually increase the speed, turning the knob slowly, until the desired speed is reached.
● Stop the centrifuge gradually (some models have a brake that can be applied). Never try to slow the centrifuge down with your hand.
● Never open the centrifuge until it has come to a complete stop.
● Remove the tubes slowly and carefully.
Cleaning and maintenance
For details of cleaning and maintenance of centrifuges, see section 3.5.3.
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