Learn the definition of a centrifuge machine, two basic rotors, which material to select for sample containers, and how to use a centrifuge without damaging it or yourself!
Definition of a Centrifuge:
A laboratory instrument that separates particles in a solution by spinning it around at high speed. Distinct layers of particles separate based on their different densities, heavier materials will be pushed to the outside of the container. In this way, sedimentation occurs rapidly when compared to only using the force of gravity.
Centrifugation refers to the process in which a mixture becomes separated through spinning it fast.
What is the Relative Centrifugation Force (RCF)?
The Relative Centrifugation Force (RCF) refers to the force that is applied to the mixture and causes the separation of the molecules. The numeric value indicates the multiple of how many times the centrifugal force exceeds the acceleration due to the force of gravity.
RCF = (n/1000)2 x r x 11.18
where n = speed (revolutions per minute) and r = radius in cm
(For more detail on centrifugation, visit My Biomedical Blog for well-laid out explanations.)
Examples of uses of a Centrifuge:
- Large industrial centrifuges are used to separate mixtures, eg cream from milk
- Laboratory centrifuges are most often used for research purposes, eg to separate certain parts of cells or DNA
- Molecular biology labs, eg blood testing
- Isotopic separations in gases
2 Common Types of Rotors:
- Angle Rotors (fixed angle)
Most frequently used for high speed applications.
The oblique angle of the tubes reduces the distance of the path of the particles and requires a greater speed in the process of sedimentation.
Often used for pelleting applications.
- Swing out rotors (variable angle)
The variable angle rotors are used to centrifuge higher amounts of sample at low to medium speeds.
During the centrifugation cycle the tubes are in a horizontal position, the sediment is deposited in the center of the tube and the limits of the phases are formed horizontally.
Which Sample containers should I choose?
Sample containers come in a wide variety of different materials and sizes that must be matched with the rotor type you have selected:
- Metal buckets that the tubes fit in for safety
- Reductors for tubes – autoclavable 121°C
- Falcon tubes – hydrophobic, biologically inert surface for DNA and molecular biology applications
- Glass tubes – suitable for lower speeds, higher level of chemical resistance and ideal for organic solvents
- Plastic tubes – can usually handle higher speeds, but not compatible with all chemicals, such as alcohol and acetone
- Bio tubes with Bio caps – for biosafety; aerosol tight for hazardous samples
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