Laboratory centrifuges are used for separating particles from solutions according to their size, shape, density, viscosity of the medium, and rotor speed. The theoretical basis of this technique is the effect of gravity on particles (including macromolecules) in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity. Centrifugal force (measured as xg, gravity) is used to increase this settling rate depending on the rotation to induce centrifugal force, which accelerates the separation of the liquid from the solids. Centrifugal force causes the solid phase to move through the liquid phase in a straight line and away from the center of rotation. Solid-phase characteristics such as particle density, size, shape, and consistency and the rotation speed for the laboratory centrifuges’ chamber (a basket or bowl, depending on the centrifuge type) of a given diameter also influence how fast the solid phase moves away from the center of rotation. The higher the rotation speed, the higher the G force exerted on the solid phase and the faster the solids accumulate.

Types of Laboratory Centrifuges

The two most common types of laboratory centrifuges are:

• analytical
• preparative

The distinction between the two is based on the purpose of centrifugation. 

Preparative laboratory centrifuges are used to isolate specific particles. This classification is divided into two types: differential and density. Differential centrifuges are used to separate particles from a liquid medium or to separate particles of different masses into separate fractions of the supernatant. Density centrifuges work by spinning two fluids of different densities within a rotating container or rotor with the heavier fluid forced to the wall at the inside of the rotor while the lighter fluid is forced toward the center of the rotor.

Analytical laboratory centrifuges measure the physical properties of particles, such as sedimentation coefficient or molecular weight. Optimal methods are used in analytical ultra centrifugation. Molecules are observed by optical system during centrifugation, to allow observation of macromolecules in solution as they move in gravitational field. The samples are centrifuged in cells (tubes with quartz windows) having windows that lie parallel to the plane of rotation of the rotor head. As the rotor turns, the images of the cell (proteins) are projected by an optical system on to film or a computer. The concentration of the solution at various points in the cell is determined by absorption of a light of the appropriate wavelength (Beer's law is followed). This can be accomplished either by measuring the degree of blackening of a photographic film or by the pen deflection.

Related Information 

CR4 Community—Screen Bowl vs. Pusher Centrifuges

Engineering360—What's the Most Popular Chemical Lab Equipment Being Used By Engineers?

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