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Centrifuges - Laboratory

Centrifuges - Laboratory


Laboratory centrifuges are very common in laboratories world-wide and have been in use for over 150 years in one form or another. One of the first recorded was in  1864 when Antonin  Prandtl invented the first dairy centrifuge in order to separate cream from milk. Driven by motors (themselves having advanced considerably in technology) a lab centrifuge spins liquid samples at high speeds and separates their components.

Lab centrifuges vary considerably in size, capacity and speed and are used for thousands of tasks where separation of compression/sedimentation is necessary. The centripetal (as per old centrifugal) force created by spinning samples in a rotational space separates substances of greater and lesser density.

Typically a sample will contain on or more types of matter in suspension and to quickly isolate this material, a centrifuge is needed. The sample is placed in a tube (centrifuge tube) often shaped to provide ease of sediment extraction and spun at specified speeds (generally known from experience). A pellet of material is formed at the bottom and the fluid (supernatant) can be removed by a pipette of poured off.

Although most lab and scientists will have a good idea of speeds and timing to separate their samples the actual rate of centrifugation is specified by the acceleration applied to the sample, measured in revolutions per minute (RPM) or relative centrifugal force (RCF).

For the Technical
The acceleration can be calculated as the product of the radius and the square of the angular velocity. Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.
g = RCF = 0.00001118 × r × N2
where:
g = Relative centrifuge force
r = rotational radius (centimeter, cm)
N = rotating speed (revolutions per minute, r/min)
The speed at which sample material settles is a function of their size and shape, centrifugal acceleration and the volume fraction of solids present. This is further complicated by the density difference between the material(s) in suspension and the liquid viscosity.

Laboratory Centrifuge Rotors:

The tubes which are used in a centrifuge to hold the fluid requiring separation are placed within a holding mechanism in the centrifuge.  As one can imagine, a high velocity piece of equipment is potentially dangerous, even more so if the samples are infectious as well! Hence personal safety and sample security are paramount.

A centrifuge will have rotors (the spinning part) contained with a sealed housing with lid. These rotors typically contain two, four, six, eight, or twelve receptacles within which centrifuge tubes may be placed. These rotors are generally supplied as "fixed angle" or "swing out" Fixed-angle rotors are generally made out of light cast materials such as aluminum and used for higher speeds. There are machined holes at specific angles of tilt. Samples are placed within these. A swing-out rotor has a fixed shape to contain "buckets". These buckets can contain different tubes in varying shapes. These buckets "swing" out on to horizontal position when rotational movement has started and swing back to vertical as the centrifuge slows.

Safety as previously mentioned is key. The centrifuge lid will automatically be closed by an actuator and the rotor housing will nearly always have a separate screw down lid within the centrifuge housing. Unless the lid is closed and locked the centrifuge will not spin.  The rotor chamber and the lid must be of high quality and robust enough to survive a rotor failure at full speed. This failure or a seriously shaking centrifuge is often a problem caused by failure to balance the samples and placing samples or blanks of equal weight/mass opposite each other. Some centrifuges may stop working  when shaking or instability is detected.

To reduce the risk of rotor failures, centrifuge manufactures specify operating and maintenance procedures to ensure that rotors are regularly inspected and removed from service or re-rated when they are past their expected lifetime.

Laboratory centrifuge types:

These can be split into several categories:

Microcentrifuges: These devices have become more popular as sample size has shrunk and lab space has become more of a premium. They are rationally used for small  tubes from 0.2 ml to 2.0 ml(micro tubes) and for micro well-plates. They are compact in design with a small bench footprint and can produce forces up to 30.000 g)

Clinical centrifuges are as the name suggests used for clinical applications such as blood collection tubes and due to the nature of the easy sedimentation are generally low-speed.

Multipurpose bench top centrifuges are the most common and cover a whole range of applications, from micro-centrifuge type work, to clinical work, but also a vast array of research, veterinary and life science applications.

"Featured Centrifuges":  are those that have other physical properties other than spin, slow, accelerate, automatic lock down and timing. These may be ultracentrifuges, which have very high centripetal forces or refrigerated device or cooled equipment function.

Decontamination

Centrifuges are typically made of metallic elements to maintain structural strength. Many samples contain hazardous materials and potentially infective materials.  These may become air borne in a centrifuge environment, especially is the tubes have external contamination or are breached due to high speed centrifugation. It is  always wise to be cautious and to also clean thoroughly the rotor, rotor buckets and internal rotor housing with suitable disinfectants and detergents.

There are different suppliers of laboratory centrifuges such as Eppendorf, Thermo-Hera