Technique J: Use of Centrifuge
SECTION 1: PURPOSE OF TECHNIQUE
This technique describes how to use a laboratory centrifuge.
SECTION 2: OPERATIONS OF CENTRIFUGES
There are many types of centrifuges available for laboratory use. Each centrifuge should have a specific manual for proper operation. If the manual is not available, an internet search for the manual under the specific model is advisable.
Centrifugation is used for the separation of solid-liquid mixtures that are stubborn to settle or difficult to filter. Centrifugal force is generated by rapidly spinning samples. Materials of higher density migrate solids to the bottom of the centrifuge tube.
A centrifuge can spin up to 10,000 rotations per minute. An unbalanced load will cause the centrifuge to knock and wobble. If severely unbalanced, the centrifuge can even wobble off the benchtop, potentially causing harm. To prevent wobbling, every sample in the centrifuge needs to be balanced by an equal mass of a vial in the opposite chamber in the centrifuge.
Special test tubes or centrifuge tubes must be used that exactly fit the width of the chambers in the centrifuge. Each tube should be filled to no greater than three-quarters full as the samples will be tilted in the centrifuge and could spill out.
SECTION 3: BALANCING A CENTRIFUGE
If only one sample is to be centrifuged, a tube of water containing an equal mass should be placed into an opposing position in the centrifuge to maintain balance. More than one sample can be centrifuged at a time, with the only requirement being that each opposing tube must have nearly the same mass.
If identical tubes are being used, filling the balance tube to the same level with water is usually close enough. If significantly different tubes are being used, or the centrifuge seems to be off-balance, weigh the individual tubes and add or subtract water from the balance tube until both are nearly the same mass.
SECTION 4: OPERATING A CENTRIFUGE
To operate the centrifuge, load the tubes, close the lid and turn on the centrifuge. Set the rotation speed if your instrument allows for it (a good general speed is 8,000 rotations per minute) and turn the dial to the recommended amount of time (set by your instructor or experimentation). Allow the system to spin for the designated time and turn it off.
Some centrifuges have an internal brake, but rapid breaking could stir up solids. It is better to simply let a centrifuge slow and come to a stop on its own. The liquid can then be decanted or pipetted off of the solid.
The time needed to separate solids from liquids in tubes varies on many factors, including but not limited to the density differences between the components and the size of particles of the solid. If the mixture is not separating after a reasonable amount of time, consider an alternative method.
SECTION 5: REFERENCES