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Laboratory Laboratory CentrifugesCentrifuges

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DefinitionDefinition A A laboratory centrifugelaboratory centrifuge

is laboratory equipment, is laboratory equipment, driven by a motor, driven by a motor, spins liquid samples at high speed. spins liquid samples at high speed. There are various types of centrifuges, depending on the There are various types of centrifuges, depending on the

size and the sample capacity. size and the sample capacity. They vary widely in speed and capacityThey vary widely in speed and capacity work by the sedimentation principle, where the work by the sedimentation principle, where the

centripetal acceleration is used to separate substances centripetal acceleration is used to separate substances of greater and lesser density.of greater and lesser density.

comprise a rotor containing two, four, six, or many more comprise a rotor containing two, four, six, or many more numbered wells within which the samples containing numbered wells within which the samples containing centrifuge tips may be placed.centrifuge tips may be placed.

Centrifuges are used inCentrifuges are used in chemistry, biology, and biochemistry for isolating and chemistry, biology, and biochemistry for isolating and

separating suspensions.separating suspensions.

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OperationOperation Increasing the effective gravitational force will Increasing the effective gravitational force will

more rapidly and completely cause the more rapidly and completely cause the precipitate to gather on the precipitate to gather on the bottombottom of the tube of the tube as a "as a "pelletpellet". ".

The The remainingremaining solution is called the solution is called the ""supernatesupernate" or "supernatant"." or "supernatant". The supernatant liquid is then either The supernatant liquid is then either

decanted from the tube without disturbing the pellet, or decanted from the tube without disturbing the pellet, or withdrawn with a Pasteur pipette. withdrawn with a Pasteur pipette.

The The rate of centrifugationrate of centrifugation is specified by the is specified by the acceleration applied to the sample, typically acceleration applied to the sample, typically measured in revolutions per minute (measured in revolutions per minute (RPMRPM) or ) or relative centrifugal force (relative centrifugal force (RCFRCF). ).

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OperationOperation The The particles' settling velocityparticles' settling velocity in in

centrifugation is a function of their centrifugation is a function of their size and shape, size and shape, centrifugal acceleration, centrifugal acceleration, the volume fraction of solids present, the volume fraction of solids present, the density difference between the the density difference between the

particle and the liquid, and the particle and the liquid, and the viscosity.viscosity.

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Rotor objectivesRotor objectivesGenerally spoken, there Generally spoken, there

are two main types of are two main types of rotors:rotors:

Fixed-angle rotorFixed-angle rotor The rotor (mainly made The rotor (mainly made

of aluminium) is very of aluminium) is very compact.compact.

There are boreholes with There are boreholes with a specific angle (like a specific angle (like 45°) within the rotor. 45°) within the rotor.

These boreholes are These boreholes are used for the sample used for the sample tubes.tubes.

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Swing-out rotorSwing-out rotor (= (= horizontal rotor)horizontal rotor) The rotor looks like a cross The rotor looks like a cross

with bucket. with bucket. Within these buckets, Within these buckets,

different tubes can be different tubes can be centrifuged. centrifuged.

For a safe centrifugation, a For a safe centrifugation, a specific adadpter for every specific adadpter for every tube shape is mandatory.tube shape is mandatory.

Rotor objectivesRotor objectives

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DesignDesignRotor objectivesRotor objectives

The rotor is closed by a rotor lid. The rotor is closed by a rotor lid. The rotor is located in a rotor The rotor is located in a rotor

chamber which is covered by a chamber which is covered by a metal centrifuge lid. metal centrifuge lid.

The open lid prevents the motor The open lid prevents the motor from turning the rotor when the from turning the rotor when the rotor chamber is open. rotor chamber is open.

During the run, the lid is locked. During the run, the lid is locked. The lid protects the user from being The lid protects the user from being

injured by touching a rapidly injured by touching a rapidly spinning rotor. spinning rotor.

The rotor chamber and the lid of The rotor chamber and the lid of high quality centrifuges are robust high quality centrifuges are robust enough to survive a rotor failure at enough to survive a rotor failure at full speed. full speed.

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DesignDesignRotor objectivesRotor objectives

After a rotor crash, a centrifuge should not be After a rotor crash, a centrifuge should not be reused as the enormous forces during a crash reused as the enormous forces during a crash may have damaged essential parts of the may have damaged essential parts of the device.device.

The rotor must be balanced by placing The rotor must be balanced by placing samples or blanks of equal mass opposite each samples or blanks of equal mass opposite each other. Since most of the mass is derived from other. Since most of the mass is derived from the solvent, it is usually sufficient to place the solvent, it is usually sufficient to place blanks or other samples of equal volume. blanks or other samples of equal volume.

As a safety feature, some centrifuges may stop As a safety feature, some centrifuges may stop turning when wobbling is detected (automatic turning when wobbling is detected (automatic imbalance detection, see Safety).imbalance detection, see Safety).

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TheoryTheory During circular motion During circular motion

the acceleration is the product of the the acceleration is the product of the radiusradius and the and the square of the square of the

angular velocityangular velocity and it is and it is traditionally named traditionally named

"relative centrifugal force" "relative centrifugal force"

((RCFRCF). ).

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TheoryTheory The The accelerationacceleration is measured in multiples of is measured in multiples of

""gg“, the standard acceleration due to gravity at “, the standard acceleration due to gravity at the Earth's surface, or × "the Earth's surface, or × "gg", and it is given by", and it is given by

wherewheregg = acceleration = accelerationrr = rotational radius (centimeter, cm) = rotational radius (centimeter, cm)NN = rotating speed (revolutions per minute, = rotating speed (revolutions per minute, r/min)r/min)

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Calc. of RCF: Calc. of RCF: NOMOGRAMNOMOGRAM To calculate the RCF value at any point along the

tube or bottle: measure the radius, in mm, from the center of

the centrifuge spindle to the particular point. draw a line from the radius value on the right

hand column to the appropriate centrifuge speed on the left hand column.

the RCF value is the point where the line crosses the center column.

The nomogram is based on the formula:

R = Radius in mm from centrifuge spindle to point in tubeN = Speed of spindle in RPMRCF = (11.17 x 10-7) RN2

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Types of centrifugeTypes of centrifugeThere are at least five types of There are at least five types of

centrifuge:centrifuge: preparative centrifugepreparative centrifuge analytical centrifugeanalytical centrifuge angle fixed centrifugeangle fixed centrifuge swing head centrifugeswing head centrifuge haematocrit centrifugehaematocrit centrifuge

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Centrifuge tubesCentrifuge tubes Centrifuge tubesCentrifuge tubes or or centrifuge tipscentrifuge tips are tapered are tapered

tubes of various sizes made of tubes of various sizes made of glassglass or or plasticplastic. . They may vary in capacity from tens of mm, to They may vary in capacity from tens of mm, to

much smaller capacities used in micro-centrifuges much smaller capacities used in micro-centrifuges used extensively in molecular biology used extensively in molecular biology laboratories. laboratories.

The most commonly encountered tubes are of The most commonly encountered tubes are of about the size and shape of a normal about the size and shape of a normal test tubetest tube (~ (~ 10 cm long). 10 cm long).

Micro-centrifuges typically accommodate micro-Micro-centrifuges typically accommodate micro-centrifuge tubes with capacities from 250 centrifuge tubes with capacities from 250 μlμl to to 2.0 2.0 mlml These are exclusively made of plastic.These are exclusively made of plastic.

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Centrifuge tubesCentrifuge tubes Glass centrifuge tubesGlass centrifuge tubes can be used with can be used with

most solvents, but tend to be most solvents, but tend to be more more expensiveexpensive. They can be cleaned like other . They can be cleaned like other laboratory glassware, and can be laboratory glassware, and can be sterilizedsterilized by autoclaving. by autoclaving.

Plastic centrifuge tubesPlastic centrifuge tubes, especially micro-, especially micro-centrifuge tubes tend to be centrifuge tubes tend to be less expensiveless expensive. . Water is preferred when plastic centrifuge Water is preferred when plastic centrifuge tubes are used. They are more difficult to tubes are used. They are more difficult to clean thoroughly, and are usually inexpensive clean thoroughly, and are usually inexpensive enough to be considered enough to be considered disposabledisposable

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Centrifuge tubesCentrifuge tubes

Three microcentrifuge tubes: 2 mL, 1.5 mL and 200 μL (for PCR).

Four screw-top micro-centrifuge tubes.

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Safety aspectsSafety aspects Centrifuge servicing includes full strip Centrifuge servicing includes full strip

down, chemical cleaning and mechanical down, chemical cleaning and mechanical breakdown so that each individual breakdown so that each individual component is cleaned and re-calibrated component is cleaned and re-calibrated before re-installation. before re-installation.

Regular servicing and maintenance Regular servicing and maintenance ensures that your benchtop centrifuge ensures that your benchtop centrifuge works effectively and provides accurate works effectively and provides accurate mixing of samples which can affect results mixing of samples which can affect results in clinical trials.  in clinical trials. 

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Safety aspectsSafety aspects The load in a laboratory centrifuge must be The load in a laboratory centrifuge must be

carefully balancedcarefully balanced. Small differences in mass of . Small differences in mass of the load can result in a large force imbalance the load can result in a large force imbalance when the rotor is at high speed. This force when the rotor is at high speed. This force imbalance strains the spindle and may result in imbalance strains the spindle and may result in damage to centrifuge or personal injury. Some damage to centrifuge or personal injury. Some centrifuges have an automatic rotor imbalance centrifuges have an automatic rotor imbalance detection. The control software immediately detection. The control software immediately discontinue the run when imbalance appears.discontinue the run when imbalance appears.

Before starting a centrifuge, an accurate check of Before starting a centrifuge, an accurate check of the the rotor lockagerotor lockage as well as the lid lockage is as well as the lid lockage is mandatory. mandatory.

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Safety aspectsSafety aspects Centrifuge rotors should Centrifuge rotors should never be never be

touched while movingtouched while moving, because a , because a spinning rotor can cause serious spinning rotor can cause serious injury. injury. Modern centrifuges generally have Modern centrifuges generally have

features that prevent accidental contact features that prevent accidental contact with a moving rotor as the main lid is with a moving rotor as the main lid is locked during the run.locked during the run.

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Always run the centrifuge with full or evenly distributed Always run the centrifuge with full or evenly distributed sample tubes. Failure to do so can distort the centrifuge sample tubes. Failure to do so can distort the centrifuge rotor and or damage the centrifuge. Sample tubes come in rotor and or damage the centrifuge. Sample tubes come in different weights which mean the rotational mass must be different weights which mean the rotational mass must be equal prior to switching the centrifuge on. equal prior to switching the centrifuge on.

Ensure the centrifuge lid is securely shut. Our Ensure the centrifuge lid is securely shut. Our benchtop benchtop centrifugescentrifuges come with a safety mechanism to prevent come with a safety mechanism to prevent operation unless the lid is securely sealed, however it is operation unless the lid is securely sealed, however it is always advised to check prior to switching on. always advised to check prior to switching on.

Only use manufacture specific accessories designed to Only use manufacture specific accessories designed to work with your centrifuge, failure to do so can cause work with your centrifuge, failure to do so can cause damage or injury. damage or injury.

Ensure your centrifuge is kept clean at all times. Dirt, Ensure your centrifuge is kept clean at all times. Dirt, grease and grime can degrade the efficiency and quality of grease and grime can degrade the efficiency and quality of the centrifuge over time. Regular servicing is paramount. the centrifuge over time. Regular servicing is paramount.

Ensure centrifuges are stored in a clean dry area to Ensure centrifuges are stored in a clean dry area to prevent condensation damage to moving parts. prevent condensation damage to moving parts.

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Safety aspectsSafety aspects Because of the kinetic energy stored in the rotor head Because of the kinetic energy stored in the rotor head

during high speed rotation, those who have experienced during high speed rotation, those who have experienced the loss of a rotor inside of an ultracentrifuge compare the the loss of a rotor inside of an ultracentrifuge compare the experience to having a bomb explode nearby.experience to having a bomb explode nearby.

When handling dangerous samples (like biohazard), the lid When handling dangerous samples (like biohazard), the lid of the rotor needs to have a special gasket (aerosol-tight) of the rotor needs to have a special gasket (aerosol-tight) to prevent contamination of the laboratory. to prevent contamination of the laboratory.

The rotor can be loaded with the samples within a hood The rotor can be loaded with the samples within a hood and the rotor lid is fixed on the rotor. Afterwards, the and the rotor lid is fixed on the rotor. Afterwards, the aerosol-tight system of rotor and lid is transferred to the aerosol-tight system of rotor and lid is transferred to the centrifuge. Fixation of the rotor within the centrifuge is centrifuge. Fixation of the rotor within the centrifuge is done without opening the rotor lid again. After the run, done without opening the rotor lid again. After the run, the rotor (incl. lid) is removed from the centrifuge to the the rotor (incl. lid) is removed from the centrifuge to the hood (closed system) for further steps.hood (closed system) for further steps.