  # Useful Tips

## Centrifugation Tips

Security

Always use sealed buckets or rotors when spinning biological samples or dangerous liquids such as acids. A broken tube will result in aerosols – small droplets of the mixture moving through the air. Often it is not known that a tube has broken or leaked until the lid of the centrifuge has been opened. Then it is too late. The act of opening the lid draws the aerosols up into the lab air and the face of the operator.

Calculate relative centrifugal force (RCF)

A rough guide to the ‘g’ force exerted at the popular speed of 3000 rpm is to multiply the centrifugation radius by a factor of 10. So a radius of 160 mm gives approximately 1600 x g.

The error is quite small. The true value is 1609.92 x g. It is calculated from the formula used for all ‘g’ force calculation:

RCF [x g] = 1.118 x Radius [mm] x (Speed [rpm]/1000)²

Calculate speed (rpm) for a relative centrifugal force in a protocol.

If your centrifuge does not have the possibility to control by RCF, then you can calculate the speed to be used that will apply this RCF.

Speed [rpm] = 946 x √(RCF [x g]/radius [mm])

Calculate the radius from a given RCF and rpm

If you know the RCF exerted when a rotor turns at a given speed, you can calculate the radius.

Radius [mm] = RCF [x g] / (1.118 x (Speed [rpm]/1000)²)

Where to measure the radius

Most commonly, the radius quoted is the maximum radius, Rmax, the distance from the centre of the drive axis to the farthest point of the tube from this central point. Most frequently, the radius is measured in mm as this gives a whole number, e.g., 123 mm.

In a swing-out rotor: Measure from the centre of the axis to the inside bottom of the bucket when it is horizontal.

Note: When using adaptors, the above radius value is reduced by the thickness of the bottom of the adaptor.

In an angle rotor, the part of the tube farthest from the centre of the axis is not the bottom of the tube (except for conical tubes) but a point slightly above, to allow for the curvature of the base.

The minimum radius, Rmin, is the distance from the centre of the axis to the surface of the liquid closest to the centre of the axis allowing for this surface to be vertical. Let us consider a rotor with a maximum radius of 160 mm containing a tube where the liquid is filled to a height of 40 mm above the base of the vertical tube.

In a swing-out rotor, simply reduce the value of Rmax by the height of the liquid in the tube, so in our example the Rmin is 160 mm minus 40 mm = 120 mm.

In an angle rotor, you will have to use geometry to calculate the value of the radius. Angle rotors are quoted for the angle from the vertical. If the rotor has a 35° angle (h), then the Rmin is calculated according to:

Rmin = Rmax – (Height of liquid in vertical tube x sin h)

So in our example,
Rmin = 160 – (40 x 0.574) = 160 – 22.96 = 137.04 mm

In fact, this formula is also true for a swing-out rotor, because sin h = sin 90° = 1.

Some protocols require that a defined RCF be applied at a boundary, e.g., between aqueous and a non-aqueous solvents. To calculate this, measure the height of the boundary from the bottom of the tube and subtract this value from the Rmax (in a swing-out rotor).

Extend the life of your centrifuge and accessories

Corrosion is the enemy of the centrifuge and its accessories. Wipe up spillages immediately, dry the surfaces and leave the lid of the centrifuge open.

If there are any surface scratches, clean and thoroughly dry the accessory, then apply a little high vacuum grease to the scratch. Reapply this regularly to maintain protection.

As a general protection, apply a thin film of protective coating on a monthly basis. After washing, rinsing and drying, spray a small quantity of a moisture-resisting fluid such as WD 40 onto the surfaces. This will resist oxidation and corrosion.

Eliminate vibration for better results

Everyone knows that a rotor must be balanced for load. For most people, this means putting tubes or other samples in mirror image positions on opposite sides of the drive axis. However, this is not the whole story.

Imagine that two tubes each are placed in opposite buckets of a swing-out rotor. If two tubes are placed furthest from the centre of the axis, the bucket will spin with the base slightly below the horizontal. If the two tubes in the other bucket are placed closest to the centre of the axis, this bucket will spin with the base slightly above the horizontal. The result will be turbulence as the air alternates in its flow alternately above and below the bucket. The effect will be to create vibration – even if the tubes are perfectly balanced – resulting in a poor separation.

Place tubes so that the load is equally distributed each side of the axis of swing of each bucket. They will then spin horizontally and no vibration will occur.

Centrifuge buckets need to be greased monthly in order to prevent vibration. Use a tissue to remove old grease from under the rim of the bucket and from the pins upon which the bucket swings. Now apply a small quantity of grease to the buckets and rotor pins. Buckets should swing freely when displaced slightly. If they do not, then particles will have become trapped in the grease, so it should be replaced.

Use a refrigerated centrifuge for standard room temperature protocols

There are many protocols at Standard Room Temperature (20 °C). Even if the room is at or below this temperature, it is probable that the sample will be subjected to higher temperatures during centrifugation. Even the most efficient ventilated centrifuges generate heat. And this heat can drive up the temperature… by 5, 10, 15 °C or more, depending upon the design, the length of the run and the environment, including confined space.

Using a refrigerated centrifuge will ensure that the temperature is kept within 1 °C of set temperature.

Some Useful Tips information provided is based on publicly available sources. NÜVE provides it in good faith ‘as is’ in a service to our customers, but cannot take responsibility for its use. We strongly recommend scientists to check this information elsewhere before using it. 