Selecting and Working with Centrifuges
Centrifuges rotate to create a centrifugal force used to separate particles, liquids and other substances that have different densities. More dense parts move away from the centrifuge's axis while less dense parts move toward the axis.
Particles at the bottom of the sample are the precipitate. The liquid that stays at the top is called the supernate or supernatant liquid. Depending on the procedure, centrifugation may be performed to capture either the precipitate or the supernate.
- RPM: Revolutions per minute the rotor spins.
- RCF: Relative centrifugal force (also known as g–force). It's a function of the rotor radius and the square of the rotational speed. Two centrifuges with comparable RCF values provide the same separation power and is a better way to compare centrifuges than RPM, which is simply a function of how fast the motor spins. Calculators can determine RCF by inputting the RPM and rotor radius.
- Rotor: The rotating portion inside a centrifuge. These generally can be removed for cleaning, or for changing out rotors for different size tubes/samples or rotor configurations.
- Fixed Angle Rotor: Rotor with tube shields installed in a fixed angle. The tubes are in an inclined position during spinning, resulting in an angled separation line between the precipitate and supernate.
- Horizontal/Swing–Out/Swinging Bucket Rotor: This rotor swings out horizontally during the spinning process, generating a flat separation line between the precipitate and supernate.
An important piece of lab equipment, it is crucial to make sure you are using your centrifuge properly and safely.
- First, make sure your centrifuge is in proper working order. It should be calibrated and serviced annually by the manufacturer or trained service center. Use a tachometer throughout the year to verify the rotor is moving at the set speed.
- Always operate your centrifuge on a smooth level surface with firm footing.
- Always balance the tubes in your rotor. Unbalanced tubes moving at high speeds can damage the unit, cause tubes to potentially break and/or cause the centrifuge to wobble or "walk" on working surfaces, leading to product failure. Balance the tubes using the same material tubes in opposite sides of the rotor and balance using mass, not volume, if using materials of different densities for balancing purposes.
- If the centrifuge wobbles or shakes during spinning, pull the plug and begin troubleshooting. Make sure the sample load is balanced. If using adaptors, ensure all are in place, complete and balanced. Remove the rotor to check for debris or broken glass at the bottom of the unit. If these checks don't fix the wobbling, contact the manufacturer for repair or service options.
- Never open the lid of a centrifuge while the rotor is in motion. Many models have a safety shut off that won't allow them to be opened while in use. On some models, however, you can still open the unit after it has been turned off but the rotor continues to spin. A spinning rotor may present a safety issue involving potential hand injuries or unintended projectiles.
- Wear goggles for eye protection against splash/impact injuries when working around a centrifuge. Depending on the chemicals in use, a face shield adds secondary face protection against chemical burns. Although a centrifuge that's running smoothly on your counter may seem safe, protect your face and eyes from any splash or impacts that could happen if the tubes or the rotor were to fail.
- Place the centrifuge where it's safe from others bumping it while it's running. Move the cord out of traffic areas so the centrifuge cannot be pulled or tripped off the countertop.
Centrifuges are sold with or without the rotors and have many options for adaptors to fit the different size centrifuge tubes on the market. Microcentrifuges can hold tubes as small as 0.2mL while larger models hold 500mL bottles. Centrifuges are even available with refrigeration to keep samples cold during processing.
By choosing the correct model for your needs and working safely with it, a centrifuge can be a workhorse in your lab for years to come.