Bottle Centrifuges

The most common laboratory centrifuge is the bottle centrifuge. Bottle centrifuges consist of a motor-driven vertical spindle to which a horizontal rotor, machined with an even number of sample positions (2-36), is attached. The harness and rotors are covered with a safety shield, which may also serve to reduce air friction and facilitate temperature control. Such units are normally equipped with a timer, tachometer, and manual or automatic braking. Samples may be mixed with the medium prior to loading, or layered on top of a homogeneous medium or density gradient. Bottle centrifuges are usually bench-top units that may operate at speeds up to 30 000 rpm and gmax of 65 000, but are also available as larger, free-standing units that generate centrifugal forces in excess of 100 000g. Sample capacities range from capillary tube to 1 L bottles (4 L total capacity).

Bottle-centrifuge rotors classified as swinging-bucket, fixed-angle, and vertical (Figure 7). In the swinging-bucfeet type, the bottles are in a vertical position at rest but swing outward to a horizontal orientation as the rotor speed increases. In this orientation, the centrifugal force is applied along the length of the tube, making them suitable for rate separations. They may also be used for batch separation of immiscible liquids with some rotors specifically designed to hold separatory funnels to facilitate post-run recovery. However, their high fe-factors make them generally unsuitable for differential pelleting, though some rotors constructed to hold short, large-diameter bottles, are designed for such purposes. Fixed-angle rotors are loaded and operated in a similar manner except that, as the name implies, the tube remains at a fixed angle both at rest and during the run. The fixed angle is typically 20-45° from the vertical, though near-vertical rotors are less than 10° from the vertical. The fixed-angle design provides a shorter pathlength (Figure 7) with a corresponding reduction in run time (lower fe-factor). Particles that reach the outer wall of the tube during the run aggregate and quickly slide down the tube wall to form a pellet in the bottom. This makes the fixed-angle rotor useful for both pelleting (Figure 3) or isopycnic banding (Figure 4). Vertical rotors can be considered as an extension of fixed-angle rotors in which the angle of repose is 0° from the vertical. In this design, the maximum pathlength is equal to tube diameter, thereby providing the lowest fe-factors for a given tube size. Vertical tube rotors are commonly used for isopycnic banding where short run times are important, as compared to near-vertical rotors, which provide short pathlengths yet permit pellet accumulation.

The tubes loaded into both vertical and fixed-angle rotors must be sealed during the run to prevent the contents from escaping as the medium moves up the outer wall at speed. O-ring sealing systems or heat sealing are commonly used. If the volume is kept sufficiently low, this step may not be necessary except to prevent the escape of hazardous aerosols, in which case a plastic screw or push-on cap may suffice.

For pelleting runs, sample recovery entails decanting the supernatant from the top and scrapping or washing the pellet into a recovery vessel or filter. For density-gradient runs, the sample may be unloaded from either the top or bottom of the tube with a pump, a Pasteur pipette, syringe, displacement liquid, etc., or by using soft plastic tubes that may be pierced to facilitate recovery of a targeted central band.

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