A04 Routine Low-Speed Centrifugation
Low-speed centrifuges are used to separate large particles, such as cells, from fluids.
- Equalize the masses of the tubes. For low-speed centrifuges, this can be accomplished using identically-sized tubes, and filling blank tubes with water to the same level as in the sample tube.
- Precise balancing is less critical with a low-speed centrifuge than with a high-speed centrifuge.
- Place the tubes opposite one another to balance the load. Many symmetrical arrangements are possible. The buckets on most rotors will swing out when spinning. Close the door. A safety interlock usually requires that the door be completely closed.
- Set the control functions. Turn the power on. Set the rotor speed. Consult the table provided by the centrifuge manufacturer that relates the gravitational g-force to the selected speed, rotor, position, and tube size. Set the position of the electromagnetic brake. Set the time for the rotor to spin.
- Set the temperature. Not all centrifuges provide temperature control. In most laboratories, a centrifuge is left set at a particular temperature. Most often there are both room temperature and 4oC centrifuges.
- It usually takes some time (30-60 minutes) for a centrifuge to cool from room temperature to a cold temperature. Allow sufficient time for the rotor temperature to equilibrate when changing rotors. The rotor and the centrifuge should be at the same temperature before being put together. It is poor practice to place a warm rotor on a cold shaft since this binds very tightly when the rotor chills.
- Sometimes a centrifuge is placed in a cold room and operated there.
- Press the start button. The centrifuge will start and slowly reach the speed set for the run. The timer will turn off the rotor after the desired interval. An electromagnetic brake will slow the rotor. Allow the centrifuge to come to a complete stop before removing tubes. Most centrifuges have a safety interlock that prevents opening the door until the rotor has stopped moving. The low-speed centrifuge is used in many of the techniques described in this series. Review the techniques as necessary.